Calcium sparks

A calcium spark is the microscopic release of calcium (Ca2+) from a store known as the sarcoplasmic reticulum (SR), located within muscle cells. This release occurs through an ion channel within the membrane of the SR, known as a ryanodine receptor (RyR), which opens upon activation. This process is important as it helps to maintain Ca2+ concentration within the cell. It also initiates muscle contraction in skeletal and cardiac muscles and muscle relaxation in smooth muscles. Ca2+ sparks are important in physiology as they show how Ca2+ can be used at a subcellular level, to signal both local changes, known as local control, as well as whole cell changes.
Activation.
As mentioned above, Ca2+ sparks depend on the opening of ryanodine receptors, of which there are three types:
Opening of the channel allows Ca2+ to pass from the SR, into the cell. This increases the local Ca2+ concentration around the RyR, by a factor of 10. Calcium sparks can either be evoked or spontaneous, as described below.
Evoked.
Electrical impulses, known as action potentials, travel along the cell membrane (sarcolemma) of muscle cells. Located in the sarcolemma of smooth muscle cells are receptors, called dihydropyridine receptors (DHPR). In skeletal and cardiac muscle cells, however, these receptors are located within structures known as T-tubules, that are extensions of the plasma membrane penetrating deep into the cell (see figure 1). These DHPRs are located directly opposite to the ryanodine receptors, located on the sarcoplasmic reticulum and activation, by the action potential causes the DHPRs to change shape.
In cardiac and smooth muscle, activation of the DHPR results in it forming an ion channel. This allows Ca2+ to pass into the cell, increasing the local Ca2+ concentration, around the RyR. When four Ca2+ molecules bind to the RyR, it opens, resulting in a larger release of Ca2+, from the SR . This process, of using Ca2+ to activate release of Ca2+ from the SR is known as calcium-induced calcium release.
However, in skeletal muscle the DHPR touches the RyR. Therefore, the shape change of the DHPR activates the RyR directly, without the need for Ca2+ to flood into the cell first. This causes the RyR to open, allowing Ca2+ to be released from the SR.
Spontaneous.
Ca2+ sparks can also occur in cells at rest (i.e. cells that have not been stimulated by an action potential). This occurs roughly 100 times every second in each cell and is a result of Ca2+ concentration being too high. An increase in Ca2+ within the SR is thought to bind to Ca2+ sensitive sites on the inside of the RyR causing the channel to open. As well as this, a protein called calsequestrin (found within the SR) detaches from the RyR, when calcium concentration is too high, again allowing the channel to open (see sarcoplasmic reticulum for more details). Similarly, a decrease in Ca2+ concentration within the SR has also proven to lower RyR sensitivity. This is thought to be due to the calsequestrin binding more strongly to the RyR, preventing it from opening and decreasing the likelihood of a spontaneous spark.
Calcium after release.
There are roughly 10,000 clusters of ryanodine receptors within a single cardiac cell, with each cluster containing around 100 ryanodine receptors. During a single spontaneous spark, when Ca2+ is released from the SR, the Ca2+ begins to move around the cell, in the same way that the smell of perfume spreads across a room when sprayed (see diffusion for more details). As the RyRs in the heart are activated by Ca2+, the movement of the Ca2+ released during a spontaneous spark, can activate other neighbouring RyRs within the same cluster. However, there usually isn't enough Ca2+ present in a single spark to reach a neighbouring cluster of receptors. The calcium can, however, signal back to the DHPR causing it to close and preventing further influx of calcium. This is known as negative feedback.
An increase in Ca2+ concentration within the cell or the production of a larger spark, can lead to a large enough calcium released that the neighbouring cluster can be activated by the first. This is known as spark-induced spark activation and can lead to a Ca2+ wave of calcium release spreading across the cell.
During evoked Ca2+ sparks, all clusters of ryanodine receptors, throughout the cell are activated at almost exactly the same time. This produces an increase in Ca2+ concentration across the whole cell (not just locally) and is known as a whole cell Ca2+ transient. This Ca2+ then binds to a protein, called troponin, initiating contraction, through a group of proteins known as myofilaments.
In smooth muscle cells, the Ca2+ released during a spark is used for muscle relaxation. This is because, the Ca2+ that enters the cell via the DHPR in response to the action potential, stimulates both muscle contraction and calcium release from the SR. The Ca2+ released during the spark, then activates two other ion channels on the membrane. One channel allows potassium ions to enter the cell, whereas the other allows chloride ions to leave the cell. The result of this movement of ions, is that the membrane voltage becomes more negative. This deactivates the DHPR (which was activated by the positive membrane potential produced by the action potential), causing it to close and stopping the flow of Ca2+into the cell, leading to relaxation.
Termination.
The mechanism by which SR Ca2+ release terminates is still not fully understood. Current main theories are outlined below:
Local depletion of SR Ca2+.
This theory suggests that during a calcium spark, as calcium flows out of the SR, the concentration of Ca2+ within the SR becomes too low. However, this is not believed to be the case for spontaneous sparks. This is because an average spark lasts around 200 milliseconds (one fifth of a second), however researchers have produced sparks lasting longer than 200 milliseconds, therefore showing that there is still enough Ca2+ left within the SR after a 'normal' (200ms) spark. During the activation of a large number of ryanodine receptors however, as is the case during evoked sparks, the entire SR is depleted of Ca2+ and therefore this mechanism could still play a part in the termination of evoked calcium sparks.
Stochastic attrition.
Despite the complicated name, this idea simply suggests that all ryanodine receptors in a cluster, and the associated dihydropyridine receptors happen to randomly close at the same time. This would not only prevent calcium release from the SR, but it would also stop the stimulus for calcium release (i.e. the flow of calcium through the DHPR). However, due to the large numbers of RyRs and DHPRs in a single cell, this theory seems to be unrealistic, as there is a very small probability that they would all close together at exactly the same time.
Inactivation/adaptation.
This theory suggests that after activation of the RyR and the subsequent release of Ca2+, the channel closes briefly to recover. During this time, either the channel cannot be reopened, even if calcium is present (i.e. the RyR is inactivated) or the channel can be reopened, however more calcium is required to activate it than usual (i.e. the RyR is in an adaptation phase). This would mean that one-by-one the RyRs would close, thus ending the spark.
Sticky cluster theory.
This theory suggests that the above three theories all play a role in preventing calcium release.
Discovery.
Spontaneous Ca2+ sparks were discovered in cardiac muscle cells, of rats, in 1992 by Peace Cheng and Mark B. Cannell in Jon Lederer's laboratory at the University of Maryland, Baltimore, U.S.A.
Initially the idea was rejected by the scientific journal, Nature, who believed that the sparks were only present under laboratory conditions (i.e. they were artifacts), and so wouldn't occur naturally within the body. However they were quickly recognised as being of fundamental importance to muscle physiology, playing a huge role in excitation-contraction coupling.
The discovery was made possible due to improvements in confocal microscopes. This allowed for the detection of the release of Ca2+, which were highlighted using a substance known as fluo-3, which caused the Ca2+ to glow. Ca2+ “sparks” were so called because of the spontaneous, localised nature of the Ca2+ release as well as the fact that they are the initiation event of excitation-contraction coupling.
Detection and analysis.
Because of the importance of Ca2+ sparks in explaining the gating properties of ryanodine receptors in situ (within the body), many studies have focused on improving their detectability in the hope that by accurately and reliably detecting all Ca2+ spark events, their true properties can finally help us to answer the unsolved mystery of spark termination.
External links.
Software


Pixie Hollow Bake Off

Pixie Hollow Bake Off is a six-minute short, based on the Disney Fairies franchise, produced by DisneyToon Studios. It aired in the United Kingdom on October 20, 2013, on Disney Channel. It is based loosely on J. M. Barrie's Peter Pan stories, by way of Disney's animated adaptation.
Plot.
Tink challenges Gelata (Lisa Faulkner in the UK version and by Giada De Laurentiis in the US version) to see who can bake the best cake for the queen's party. There are a group of baking fairies, the main one being Gelata. The baking fairies always make exactly the same cake for Queen Clarion's Arrival Day party, for the last 399 years.
Tinker Bell has an idea for a different kind of cake, which Gelata takes as a challenge... hence, the bake off. Tink and her friends compete against the baking fairies. In the end, Tink's cake looks very appealing, but finds out looks aren't everything.
Release.
The special debuted in the United Kingdom on The Disney Channel on October 20, 2013. It was later featured as an exclusive bonus disc accompanying the Blu-ray release of "The Pirate Fairy" in 2014, which included 10 extra mini-shorts of around 1–2 minutes. The special was released widely through Disney Movies Anywhere.


Gretel Campi

Gretel Sara Campi Pérez (born 20 January 1984) is a biologist and a retired footballer who played as a midfielder.
Born in Cuba, she moved to Ecuador and has been a member of the Ecuador women's national team.
International career.
Campi played for Ecuador at senior level in two South American Women's Football Championship editions (2003 and 2006).
International goals.
"Scores and results list Ecuador's goal tally first"


Alicyclobacillus macrosporangiidus

Alicyclobacillus macrosporangiidus is a species of Gram positive, strictly aerobic, bacterium. The bacteria are acidophilic and produce endospores. It was first isolated from soil in a crop field in Fujieda, Japan. The species was first described in 2007, and the name is derived from the Latin "macros" (big) and "sporangium" (sporangia), referring to the large spores produced by the organism.
The optimum growth temperature for "A. macrosporangiidus" is 50-55 °C, and can grow in the 35-60 °C range. The optimum pH is 4.0-4.5, and cannot grow at pH 3.0 or pH 6.5.
"A. macrosporangiidus" was found during a Japanese survey of various beverages and environments, which also discovered 5 other species of "Alicyclobacillus": "A. contaminans", "A. fastidiosus", "A. kakegawensis", "A. sacchari", and "A. shizuokensis".


Mycobacterium palustre

Mycobacterium palustre is a slowly growing mycobacterium first isolated from an environmental source in Finland. It is potentially pathogenic, and has been isolated from human and veterinary clinical specimens.


Dragonhaven

Dragonhaven is a fantasy novel written by Robin McKinley, published by Putnam in 2007.
Plot summary.
The story is set in the Smokehill National Park, a wildlife preserve for the preservation and study of dragons. The dragons are elusive; evidence of their existence can be found everywhere, but the dragons themselves remain hidden. Young Jake Mendoza, who lives with his father, the owner and director of the park, goes out for his first overnight solo and comes across a dying dragon. The dragon has been fatally injured by a poacher who has breached the security of the wildlife preserve.
The fact that a dragon has killed a human, even a poacher, will make life very complicated for Smokehill National Park, which exists in a tough political climate, due to the controversial nature of keeping dragons alive. But what makes life even more complicated for Jake is that he discovers that the dying dragon had been a mother, and that one of her dragonlets is still alive. It is illegal to save the dragon's life, but Jake, having discovered the baby dragon, cannot leave it to die. He takes the dragon home and raises it.
However, this creates a controversy. The family of the dead poacher want the dragons at Dragonhaven killed. Jake and the other rangers are trying their best to convince those against the preservation of dragons that the creatures are really peaceful and friendly.
The bulk of the story involves Jake's growing relationship with the young dragon and other dragons, all the responsibilities that come along with caring for an orphaned wild animal, and his own maturation from child to young adult. The novel is written in a childish style at first, but Jake's writing style matures as he matures.
In the end, Dragonhaven is saved by Jake and his dragon "friends," as they slowly learn how their two species can communicate with each other by their mind, in the process proving that dragons are as intelligent as humans and wish to be at peace with them.
Characters.
Jake Mendoza - main character
Lois - the dragon-let that Jake finds
Martha - a friend of Jake's (marries Jake later on)
Eleanor - Martha's younger sister
Billy - the Head Ranger at SmokeHill National Park
Frank Mendoza- the top person at SmokeHill National Park, runs the whole park (Jake's father)
Eric - Jake's "enemy" at DragonHaven. Dislikes Jake for unknown reasons
Snark - Jake's deceased dog
Bud and Gulp - two dragons Jake befriends due to Lois
Grace - Billy's wife
Katie - Martha and Eleanor's Mother
Poacher - A bad guy who kills Lois's Mom


Papillomaviridae

Papillomaviridae is a family of non-enveloped DNA viruses whose members are known as papillomaviruses. Several hundred species of papillomaviruses, traditionally referred to as "types", have been identified infecting all carefully inspected mammals, but also other vertebrates such as birds, snakes, turtles and fish. Infection by most papillomavirus types, depending on the type, is either asymptomatic (e.g. most Beta-PVs) or causes small benign tumors, known as papillomas or warts (e.g. human papillomavirus 1, HPV6 or HPV11). Papillomas caused by some types, however, such as human papillomaviruses 16 and 18, carry a risk of becoming cancerous.
Papillomaviruses are usually considered as highly host- and tissue-tropic, and are thought to rarely be transmitted between species. Papillomaviruses replicate exclusively in the basal layer of the body surface tissues. All known papillomavirus types infect a particular body surface, typically the skin or mucosal epithelium of the genitals, anus, mouth, or airways. For example, human papillomavirus (HPV) type 1 tends to infect the soles of the feet, and HPV type 2 the palms of the hands, where they may cause warts. Additionally, there are descriptions of the presence of papillomavirus DNA in the blood and in the peripheral blood mononuclear cells.
Papillomaviruses were first identified in the early 20th century, when it was shown that skin warts, or papillomas, could be transmitted between individuals by a filterable infectious agent. In 1935 Francis Peyton Rous, who had previously demonstrated the existence of a cancer-causing sarcoma virus in chickens, went on to show that a papillomavirus could cause skin cancer in infected rabbits. This was the first demonstration that a virus could cause cancer in mammals.
Taxonomy of papillomaviruses.
There are over 100 species of papillomavirus recognised, though the ICTV officially recognizes a smaller number, categorized into 53 genera, as of 2019. All papillomaviruses (PVs) have similar genomic organizations, and any pair of PVs contains at least five homologous genes, although the nucleotide sequence may diverge by more than 50%. Phylogenetic algorithms that permit the comparison of homologies led to phylogenetic trees that have a similar topology, independent of the gene analyzed.
Phylogenetic studies strongly suggest that PVs normally evolve together with their mammalian and bird host species, but adaptive radiations, occasional zoonotic events and recombinations may also impact their diversification. Their basic genomic organization appears maintained for a period exceeding 100 million years, and these sequence comparisons have laid the foundation for a PV taxonomy, which is now officially recognized by the International Committee on Taxonomy of Viruses. All PVs form the family "Papillomaviridae", which is distinct from the "Polyomaviridae" thus eliminating the term "Papovaviridae". Major branches of the phylogenetic tree of PVs are considered genera, which are identified by Greek letters. Minor branches are considered species and unite PV types that are genomically distinct without exhibiting known biological differences. This new taxonomic system does not affect the traditional identification and characterization of PV "types" and their independent isolates with minor genomic differences, referred to as "subtypes" and "variants", all of which are taxa below the level of "species". Additionally, phylogenetic groupings at higher taxonomic level have been proposed.
This classification may need revision in the light of the existence of papilloma–polyoma virus recombinants. Additional species have also been described. Sparus aurata papillomavirus 1 has been isolated from fish.
Human papillomaviruses.
Over 170 human papillomavirus types have been completely sequenced. They have been divided into 5 genera: Alphapapillomavirus, Betapapillomavirus, Gammapapillomavirus, Mupapillomavirus and Nupapillomavirus. At least 200 additional viruses have been identified that await sequencing and classification.
Animal papillomaviruses.
Individual papillomavirus types tend to be highly adapted to replication in a single animal species. In one study, researchers swabbed the forehead skin of a variety of zoo animals and used PCR to amplify any papillomavirus DNA that might be present. Although a wide variety of papillomavirus sequences were identified in the study, the authors found little evidence for inter-species transmission. One zookeeper was found to be transiently positive for a chimpanzee-specific papillomavirus sequence. However, the authors note that the chimpanzee-specific papillomavirus sequence could have been the result of surface contamination of the zookeeper's skin, as opposed to productive infection.
Cottontail rabbit papillomavirus (CRPV) can cause protuberant warts in its native host, the North American rabbit genus "Sylvilagus". These horn-like warts may be the original basis for the urban legends of the American antlered rabbit the Jackalope and European "Wolpertinger". European domestic rabbits (genus "Oryctolagus") can be transiently infected with CRPV in a laboratory setting. However, since European domestic rabbits do not produce infectious progeny virus, they are considered an incidental or "dead-end" host for CRPV.
Inter-species transmission has also been documented for bovine papillomavirus (BPV) type 1. In its natural host (cattle), BPV-1 induces large fibrous skin warts. BPV-1 infection of horses, which are an incidental host for the virus, can lead to the development of benign tumors known as sarcoids. The agricultural significance of BPV-1 spurred a successful effort to develop a vaccine against the virus.
A few reports have identified papillomaviruses in smaller rodents, such as Syrian hamsters, the African multimammate rat and the Eurasian harvest mouse. However, there are no papillomaviruses known to be capable of infecting laboratory mice. The lack of a tractable mouse model for papillomavirus infection has been a major limitation for laboratory investigation of papillomaviruses.
Four papillomaviruses are known to infect birds: Fringilla coelebs papillomavirus 1, Francolinus leucoscepus papillomavirus 1, Psittacus erithacus papillomavirus 1 and Pygoscelis adeliae papillomavirus 1. All these species have a gene (E9) of unknown function, suggesting a common origin.
Evolution.
The evolution of papillomaviruses is thought to be slow compared to many other virus types, but there are no experimental measurements currently available. This is probably because the papillomavirus genome is composed of genetically stable double-stranded DNA that is replicated with high fidelity by the host cell's DNA replication machinery.
It is believed that papillomaviruses generally co-evolve with a particular species of host animal over many years, although there are strong evidences against the hypothesis of coevolution. In a particularly speedy example, HPV-16 has evolved slightly as human populations have expanded across the globe and now varies in different geographic regions in a way that probably reflects the history of human migration. Cutaneotropic HPV types are occasionally exchanged between family members during the entire lifetime, but other donors should also be considered in viral transmission.
Other HPV types, such as HPV-13, vary relatively little in different human populations. In fact, the sequence of HPV-13 closely resembles a papillomavirus of bonobos (also known as pygmy chimpanzees). It is not clear whether this similarity is due to recent transmission between species or because HPV-13 has simply changed very little in the six or so million years since humans and bonobos diverged.
The most recent common ancestor of this group of viruses has been estimated to have existed .
There are five main genera infecting humans (Alpha, Beta, Gamma, Mu and Nu). The most recent common ancestor of these genera evolved -. The most recent ancestor of the gamma genus was estimated to have evolved between and .
Structure.
Papillomaviruses are non-enveloped, meaning that the outer shell or capsid of the virus is not covered by a lipid membrane. A single viral protein, known as L1, is necessary and sufficient for formation of a 55–60 nanometer capsid composed of 72 star-shaped capsomers (see figure). Like most non-enveloped viruses, the capsid is geometrically regular and presents icosahedral symmetry. Self-assembled virus-like particles composed of L1 are the basis of a successful group of prophylactic HPV vaccines designed to elicit virus-neutralizing antibodies that protect against initial HPV infection. As such, papillomaviridæ are stable in the environment.
The papillomavirus genome is a double-stranded circular DNA molecule ~8,000 base pairs in length. It is packaged within the L1 shell along with cellular histone proteins, which serve to wrap and condense DNA.
The papillomavirus capsid also contains a viral protein known as L2, which is less abundant. Although not clear how L2 is arranged within the virion, it is known to perform several important functions, including facilitating the packaging of the viral genome into nascent virions as well as the infectious entry of the virus into new host cells. L2 is of interest as a possible target for more broadly protective HPV vaccines.
The viral capsid consists of 72 capsomeres of which 12 are five-coordinated and 60 are six-coordinated capsomeres, arranged on a T = 7d icosahedral surface lattice.
Tissue specificity.
Papillomaviruses replicate exclusively in keratinocytes. Keratinocytes form the outermost layers of the skin, as well as some mucosal surfaces, such as the inside of the cheek or the walls of the vagina. These surface tissues, which are known as stratified squamous epithelia, are composed of stacked layers of flattened cells. The cell layers are formed through a process known as cellular differentiation, in which keratinocytes gradually become specialized, eventually forming a hard, crosslinked surface that prevents moisture loss and acts as a barrier against pathogens. Less-differentiated keratinocyte stem cells, replenished on the surface layer, are thought to be the initial target of productive papillomavirus infections. Subsequent steps in the viral life cycle are strictly dependent on the process of keratinocyte differentiation. As a result, papillomaviruses can only replicate in body surface tissues.
Life cycle.
Infectious entry.
Papillomaviruses gain access to keratinocyte stem cells through small wounds, known as microtraumas, in the skin or mucosal surface. Interactions between L1 and sulfated sugars on the cell surface promote initial attachment of the virus. The virus is then able to get inside from the cell surface via interaction with a specific receptor, likely via the alpha-6 beta-4 integrin, and transported to membrane-enclosed vesicles called endosomes. The capsid protein L2 disrupts the membrane of the endosome through a cationic cell-penetrating peptide, allowing the viral genome to escape and traffic, along with L2, to the cell nucleus.
Viral persistence.
After successful infection of a keratinocyte, the virus expresses E1 and E2 proteins, which are for replicating and maintaining the viral DNA as a circular episome. The viral oncogenes E6 and E7 promote cell growth by inactivating the tumor suppressor proteins p53 and pRb. Keratinocyte stem cells in the epithelial basement layer can maintain papillomavirus genomes for decades.
Production of progeny virus.
The expression of the viral late genes, L1 and L2, is exclusively restricted to differentiating keratinocytes in the outermost layers of the skin or mucosal surface. The increased expression of L1 and L2 is typically correlated with a dramatic increase in the number of copies of the viral genome. Since the outer layers of stratified squamous epithelia are subject to relatively limited surveillance by cells of the immune system, it is thought that this restriction of viral late gene expression represents a form of immune evasion.
New infectious progeny viruses are assembled in the cell nucleus. Papillomaviruses have evolved a mechanism for releasing virions into the environment. Other kinds of non-enveloped animal viruses utilize an active lytic process to kill the host cell, allowing release of progeny virus particles. Often this lytic process is associated with inflammation, which might trigger immune attack against the virus. Papillomaviruses exploit desquamation as a stealthy, non-inflammatory release mechanism.
Association with cancer.
Although some papillomavirus types can cause cancer in the epithelial tissues they inhabit, cancer is not a typical outcome of infection. The development of papillomavirus-induced cancers typically occurs over the course of many years. Papillomaviruses have been associated with the development of cervical cancer, penile cancer and oral cancers. An association with vulval cancer and urothelial carcinoma with squamous differentiation in patients with neurogenic bladder has also been noted. There are cancer causing papillomavirus genome that encodes two small proteins called E6 and E7 that mimic cancer causing oncogenes. The way they work is that they stimulate unnatural growth of cells and block their natural defenses. Also they act on many signaling proteins that control proliferation and apoptosis.
Laboratory study.
The fact that the papillomavirus life cycle strictly requires keratinocyte differentiation has posed a substantial barrier to the study of papillomaviruses in the laboratory, since it has precluded the use of conventional cell lines to grow the viruses. Because infectious BPV-1 virions can be extracted from the large warts the virus induces on cattle, it has been a workhorse model papillomavirus type for many years. CRPV, rabbit oral papillomavirus (ROPV) and canine oral papillomavirus (COPV) have also been used extensively for laboratory studies. As soon as researchers discovered that these viruses cause cancer, they worked together to find a vaccine to it. Currently, the most effective way to go about it is to mimic a virus that is composed of L1 protein but lack the DNA. Basically, our immune system builds defenses against infections, but if these infections do not cause disease they can be used as a vaccine. PDB entry 6bt3 shows how antibodies surfaces attack the surface of the virus to disable it.
Some sexually transmitted HPV types have been propagated using a mouse "xenograft" system, in which HPV-infected human cells are implanted into immunodeficient mice. More recently, some groups have succeeded in isolating infectious HPV-16 from human cervical lesions. However, isolation of infectious virions using this technique is arduous and the yield of infectious virus is very low.
The differentiation of keratinocytes can be mimicked "in vitro" by exposing cultured keratinocytes to an air/liquid interface. The adaptation of such "raft culture" systems to the study of papillomaviruses was a significant breakthrough for "in vitro" study of the viral life cycle. However, raft culture systems are relatively cumbersome and the yield of infectious HPVs can be low.
The development of a yeast-based system that allows stable episomal HPV replication provides a convenient, rapid and inexpensive means to study several aspects of the HPV lifecycle (Angeletti 2002). For example, E2-dependent transcription, genome amplification and efficient encapsidation of full-length HPV DNAs can be easily recreated in yeast (Angeletti 2005).
Recently, transient high-yield methods for producing HPV pseudoviruses carrying reporter genes has been developed. Although pseudoviruses are not suitable for studying certain aspects of the viral life cycle, initial studies suggest that their structure and initial infectious entry into cells is probably similar in many ways to authentic papillomaviruses.
Human papillomavirus binds to heparin molecules on the surface of the cells that it infects. Studies have shown that the crystal of isolated L1 capsomeres has the heparin chains recognized by lysines lines grooves on the surface of the virus. Also those with the antibodies show that they can block this recognition.
Genetic organization and gene expression.
The papillomavirus genome is divided into an early region (E), encoding six open reading frames (ORF) (E1, E2, E4, E5, E6, and E7) that are expressed immediately after initial infection of a host cell, and a late region (L) encoding a major capsid protein L1 and a minor capsid protein L2. All viral ORFs are encoded on one DNA strand (see figure). This represents a dramatic difference between papillomaviruses and polyomaviruses, since the latter virus type expresses its early and late genes by bi-directional transcription of both DNA strands. This difference was a major factor in establishment of the consensus that papillomaviruses and polyomaviruses probably never shared a common ancestor, despite the striking similarities in the structures of their virions.
After the host cell is infected, HPV16 early promoter is activated and a polycistronic primary RNA containing all six early ORFs is transcribed. This polycistronic RNA contains three exons and two introns and undergoes active RNA splicing to generate multiple isoforms of mRNAs. One of the spliced isoform RNAs, E6*I, serves as an E7 mRNA to translate E7 oncoprotein. In contrast, an intron in the E6 ORF that remains intact without splicing is necessary for translation of E6 oncoprotein. However, viral early transcription subjects to viral E2 regulation and high E2 levels repress the transcription. HPV genomes integrate into host genome by disruption of E2 ORF, preventing E2 repression on E6 and E7. Thus, viral genome integration into host DNA genome increases E6 and E7 expression to promote cellular proliferation and the chance of malignancy.
A major viral late promoter in viral early region becomes active only in differentiated cells and its activity can be highly enhanced by viral DNA replication. The late transcript is also a polycistronic RNA which contains two introns and three exons. Alternative RNA Splicing of this late transcript is essential for L1 and L2 expression and can be regulated by RNA cis-elements and host splicing factors.
Technical discussion of papillomavirus gene functions.
Genes within the papillomavirus genome are usually identified after similarity with other previously identified genes. However, some spurious open reading frames might have been mistaken as genes simply after their position in the genome, and might not be true genes. This applies specially to certain E3, E4, E5 and E8 open reading frames.
E1.
Encodes a protein that binds to the viral origin of replication in the long control region of the viral genome. E1 uses ATP to exert a helicase activity that forces apart the DNA strands, thus preparing the viral genome for replication by cellular DNA replication factors.
E2.
The E2 protein serves as a master transcriptional regulator for viral promoters located primarily in the long control region. The protein has a transactivation domain linked by a relatively unstructured hinge region to a well-characterized DNA binding domain. E2 facilitates the binding of E1 to the viral origin of replication. E2 also utilizes a cellular protein known as Bromodomain-4 (Brd4) to tether the viral genome to cellular chromosomes. This tethering to the cell's nuclear matrix ensures faithful distribution of viral genomes to each daughter cell after cell division. It is thought that E2 serves as a negative regulator of expression for the oncogenes E6 and E7 in latently HPV-infected basal layer keratinocytes. Genetic changes, such as integration of the viral DNA into a host cell chromosome, that inactivate E2 expression tend to increase the expression of the E6 and E7 oncogenes, resulting in cellular transformation and possibly further genetic destabilization.
E3.
This small putative gene exists only in a few papillomavirus types. The gene is not known to be expressed as a protein and does not appear to serve any function.
E4.
Although E4 proteins are expressed at low levels during the early phase of viral infection, expression of E4 increases dramatically during the late phase of infection. In other words, its "E" appellation may be something of a misnomer. In the case of HPV-1, E4 can account for up to 30% of the total protein at the surface of a wart. The E4 protein of many papillomavirus types is thought to facilitate virion release into the environment by disrupting intermediate filaments of the keratinocyte cytoskeleton. Viral mutants incapable of expressing E4 do not support high-level replication of the viral DNA, but it is not yet clear how E4 facilitates DNA replication. E4 has also been shown to participate in arresting cells in the G2 phase of the cell cycle.
E5.
The E5 are small, very hydrophobic proteins that destabilise the function of many membrane proteins in the infected cell. The E5 protein of some animal papillomavirus types (mainly bovine papillomavirus type 1) functions as an oncogene primarily by activating the cell growth-promoting signaling of platelet-derived growth factor receptors. The E5 proteins of human papillomaviruses associated to cancer, however, seem to activate the signal cascade initiated by epidermal growth factor upon ligand binding. HPV16 E5 and HPV2 E5 have also been shown to down-regulate the surface expression of major histocompatibility complex class I proteins, which may prevent the infected cell from being eliminated by killer T cells.
E6.
E6 is a 151 amino-acid peptide that incorporates a type 1 motif with a consensus sequence –(T/S)-(X)-(V/I)-COOH. It also has two zinc finger motifs.
E6 is of particular interest because it appears to have multiple roles in the cell and to interact with many other proteins. Its major role, however, is to mediate the degradation of p53, a major tumor suppressor protein, reducing the cell's ability to respond to DNA damage.
E6 has also been shown to target other cellular proteins, thereby altering several metabolic pathways. One such target is NFX1-91, which normally represses production of telomerase, a protein that allows cells to divide an unlimited number of times. When NFX1-91 is degraded by E6, telomerase levels increase, inactivating a major mechanism keeping cell growth in check. Additionally, E6 can act as a transcriptional cofactor—specifically, a transcription activator—when interacting with the cellular transcription factor, E2F1/DP1.
E6 can also bind to PDZ-domains, short sequences which are often found in signaling proteins. E6's structural motif allows for interaction with PDZ domains on DLG (discs large) and hDLG (Drosophila large) tumor suppressor genes. Binding at these locations causes transformation of the DLG protein and disruption of its suppressor function. E6 proteins also interact with the MAGUK (membrane-associated guanylate kinase family) proteins. These proteins, including MAGI-1, MAGI-2, and MAGI-3 are usually structural proteins, and can help with signaling. More significantly, they are believed to be involved with DLG's suppression activity. When E6 complexes with the PDZ domains on the MAGI proteins, it distorts their shape and thereby impedes their function. Overall, the E6 protein serves to impede normal protein activity in such a way as to allow a cell to grow and multiply at the increased rate characteristic of cancer.
Since the expression of E6 is strictly required for maintenance of a malignant phenotype in HPV-induced cancers, it is an appealing target of therapeutic HPV vaccines designed to eradicate established cervical cancer tumors.
E7.
In most papillomavirus types, the primary function of the E7 protein is to inactivate members of the pRb family of tumor suppressor proteins. Together with E6, E7 serves to prevent cell death (apoptosis) and promote cell cycle progression, thus priming the cell for replication of the viral DNA. E7 also participates in immortalization of infected cells by activating cellular telomerase. Like E6, E7 is the subject of intense research interest and is believed to exert a wide variety of other effects on infected cells. As with E6, the ongoing expression of E7 is required for survival of cancer cell lines, such as HeLa, that are derived from HPV-induced tumors.
E8.
Only a few papillomavirus types encode a short protein from the E8 gene. In the case of BPV-4 (papillomavirus genus "Xi"), the E8 open reading frame may substitute for the E6 open reading frame, which is absent in this papillomavirus genus. These E8 genes are chemically and functionally similar to the E5 genes from some human papillomaviruses, and are also called E5/E8.
L1.
L1 spontaneously self-assembles into pentameric capsomers. Purified capsomers can go on to form capsids, which are stabilized by disulfide bonds between neighboring L1 molecules. L1 capsids assembled "in vitro" are the basis of prophylactic vaccines against several HPV types. Compared to other papillomavirus genes, the amino acid sequences of most portions of L1 are well-conserved between types. However, the surface loops of L1 can differ substantially, even for different members of a particular papillomavirus species. This probably reflects a mechanism for evasion of neutralizing antibody responses elicited by previous papillomavirus infections.
L2.
L2 exists in an oxidized state within the papillomavirus virion, with the two conserved cysteine residues forming an intramolecular disulfide bond. In addition to cooperating with L1 to package the viral DNA into the virion, L2 has been shown to interact with a number of cellular proteins during the infectious entry process. After the initial binding of the virion to the cell, L2 must be cleaved by the cellular protease furin. The virion is internalized, probably through a clathrin-mediated process, into an endosome, where acidic conditions are thought to lead to exposure of membrane-destabilizing portions of L2. The cellular proteins beta-actin and syntaxin-18 may also participate in L2-mediated entry events. After endosome escape, L2 and the viral genome are imported into the cell nucleus where they traffic to a sub-nuclear domain known as an ND-10 body that is rich in transcription factors. Small portions of L2 are well-conserved between different papillomavirus types, and experimental vaccines targeting these conserved domains may offer protection against a broad range of HPV types.


Bourdonia

"For the plant genus, see Chaetopappa".
Bourdonia is a genus of isopod crustaceans in the family Cabiropidae.
"Bourdonia tridentata" is a hyperparasite of "Bopyroides hippolytes" from the shrimp "Pandalus borealis".


Stenotrophomonas

Stenotrophomonas is a genus of Gram-negative bacteria, comprising at least ten species. The main reservoirs of Stenotrophomonas are soil and plants. "Stenotrophomonas" species range from common soil organisms ("S. nitritireducens") to opportunistic human pathogens ("S. maltophilia"), the molecular taxonomy of the genus is still somewhat unclear.
Importance.
The most common species, "S. maltophilia" is very versatile and can be beneficial for plant growth and health, can be used in agriculture, biocontrol, bioremediation and phytoremediation strategies as well as the production of biomolecules of economic value. On the other hand, some of "S. maltophilia" strains are pathogenic to humans with multidrug resistant profile. "S. indologenes" can also cause or be part of polymicrobial infections in humans, especially small children. "Stenotrophomonas" can also be phytopathogenic unlike closely related genera "Xylella" and "Xanthomonas". Members of the genus "Stenotrophomonas" have an important ecological role in the nitrogen and sulphur cycles. "Stenotrophomonas" species, especially "S. maltophilia" and "S. rhizophila", are often found in association with plants, such as cucumber, oilseed rape, potato, strawberry, alfalfa, sunflower, maize, rice, wheat, various weeds, willow and poplar. "Stenotrophomonas" can be isolated from the rhizosphere or from internal plant tissues, particularly from the vascular tissues of the root and stem.
History.
The first species described was "S. maltophila" by Hugh and Ryschenko in 1961. At the time it was named "Pseudomonas maltophilia", but later transferred to the genus "Xanthomonas" before it was given its own genus. The genus name (from the Greek ‘stenos’, meaning narrow, ‘trophus’, meaning one who feeds and ‘monas’, meaning unit) was intended to highlight the limited nutritional range of the bacterium. However, several studies subsequently demonstrated that the genus is capable of great metabolic versatility and intraspecific heterogeneity.
Genetics.
Full genome sequence of an environmental isolate, "S. maltophilia" R551‑3, and a clinical isolate, "S. maltophilia" K279a, are available. Both strains contain genes that encode type I pili, which have been implicated in adhesion and the early stages of biofilm formation, and type IV pili, which have been implicated in adherence, auto-aggregation, twitching motility and biofilm formation. Conserved distribution of pili-coding gene clusters in sequenced genomes may indicate similarities in the plant and animal colonization strategies. The identification of "Stenotrophomonas spp". is problematic, as these bacteria show no activities in most of the standard metabolism-based phenotyping panels. Additionally, the species are genotypically similar, with 95.7–99.6% 16S rRNA gene sequence similarities. One of the housekeeping genes gyrB, encoding the B-subunit of the DNA gyrase, has successfully employed for typing. Moreover, gyrB sequence comparisons, indicate that strains identified as S. maltophilia may represent distinct new species.
Small palindromic elements that carry tetranucleotide GTAG at one terminus were found to be widespread in the genome of "Stenotrophomonas maltophilia". The repeats are species-specific variants of the superfamily of repetitive extragenic palindromes (REPs). Hundreds of genes are immediately flanked by these repeats and they likely function as RNA control sequences by the folding of the repeats in the mRNA and either stabilizing upstream transcripts or favoring their degradation.
Metabolism.
"Stenotrophomonas" spp. can efficiently colonize such different biotopes as plants, humans and marine environments. "Stenotrophomonas" spp. metabolize a large range of organic compounds present in the rhizosphere, including phenolic compounds found in plant root exudates. "S. maltophilia" can degrade "p"‑nitrophenol and 4‑chlorophenol, polycyclic aromatic hydrocarbons, selenium compounds, benzene, toluene, ethylbenzene and xenobiotics. "Stenotrophomonas" spp. produces plant growth hormone indole‑3‑acetic acid (IAA), it can also promote plant growth due to nitrogen fixation and the oxidation of elemental sulphur, which in turn provides sulphate for the plants. Many "S. maltophilia" strains have intrinsic resistance to various heavy metals. Most "S. maltophilia" isolates produce antifungal compounds, such as maltophilin and xanthobaccin or volatile organic compounds with antifungal activity. "S. maltophilia" strains have an extraordinarily high hydrolytic potential; they produce diverse proteases, chitinases, glucanases, DNases, RNases, lipases and laccases. "S. maltophilia" are equipped for iron uptake, as they produce the siderophore enterobactin and many TonB‑dependent receptors (TBDRs) used for the active transport of iron–siderophore complexes.


Acaulospora splendida

Acaulospora splendida is a species of fungi in the family Acaulosporaceae. Originally reported from Costa Rica in 1988, it forms arbuscular mycorrhiza and vesicles in roots.


Acaulospora nicolsonii

Acaulospora nicolsonii is a species of fungi in the family Acaulosporaceae. It forms arbuscular mycorrhiza and vesicles in roots.


Santaworld

Santaworld () is a Santa Claus-based theme park on the Gesunda Mountain south of Mora, Sweden. It was opened on 8 December 1984.
It featured prominently in the music video to Shakin' Stevens's 1985 single "Merry Christmas Everyone".


Brazilian Archives of Biology and Technology

The Brazilian Archives of Biology and Technology is a bimonthly peer-reviewed scientific journal covering all areas of fundamental and applied biology. It was established in 1946 as the "Arquivos de Biologia e Tecnologia", publishing in Portuguese. It obtained its current name in 1998, when it switched to publication in English only. It is published on SciELO by the .
Abstracting and indexing.
The journal is abstracted and indexed in:
According to the "Journal Citation Reports", the journal has a 2014 impact factor of 0.546.


Picobirnavirus

Picobirnavirus is a genus of double-stranded RNA viruses. It is the only genus the family Picobirnaviridae. Although amniotes, especially mammals, were thought to serve as hosts, it has been recently suggested that these viruses might infect bacteria and possibly some other invertebrates. There are three species in this genus. Associated symptoms include gastroenteritis in animals and humans, though the disease association is unclear.
Etymology.
Picobirnavirus is a small ("pico", Spanish for "small"), bisegmented ("bi", Latin for "two"), double-stranded RNA virus. Picobirnaviruses were initially considered to be birna-like viruses, and the name was derived from birnavirus (bisegmented RNA), but the virions are much smaller (diameter 35 nm vs. 65 nm). Picobirnaviruses were first detected in humans and black-footed pigmy rice rats in 1988.
Structure.
Viruses in the genus are non-enveloped, with icosahedral geometries, and T=3 symmetry. The diameter is around 35–40 nm.
Genome.
The genome is linear, bipartite, and composed of double-stranded RNA. It includes a "segment 1" which is 2.2–2.7 kilobases (kb) in length and a "segment 2" which is 1.2–1.9 kb in length. The genome codes for three to four proteins. The capsid protein gene is encoded by the second open reading frame of the larger genomic segment 1.
Picobirnaviruses are divided into two genogroups on the basis of the sequence of "segment 2". The RNA-dependent RNA polymerase (RdRp) is encoded by "segment 2". The group viruses have been isolated from humans and other mammals, as well as birds and squamates.
Life cycle.
Picobirnaviruses target intestine tissue. Entry into the host cell is achieved by penetration into the host cell. Once in the host cell, viral replication is cytoplasmic. Replication follows the double-stranded RNA virus replication model. Double-stranded RNA virus transcription is the method of transcription. The viruses are released from the cell by budding. Mammals serve as the natural host. Transmission routes are fecal-oral.
Taxonomy.
Picobirnaviruses were initially thought to belong to the family "Birnaviridae", but later were confirmed to differ with respect to host, virion size, capsid, RNA-dependent RNA polymerase, genome size, and organization. The family "Picobirnaviridae" is now classified distinctly and composed of one genus "Picobirnavirus", which has three species:


Elfpunk


Bartonellaceae


Kieserite

Kieserite is the magnesium sulfate mineral (MgSO4·H2O) and is named after Dietrich Georg von Kieser (Jena, Germany 1862). It has a vitreous luster and it is colorless, grayish-white or yellowish. Its hardness is 3.5 and crystallizes in the monoclinic crystal system. Gunningite is the zinc member of the kieserite group of minerals.
Occurrence.
Kieserite commonly occurs in marine evaporites and rarely in volcanic environments as a sublimate. It occurs in association with halite, carnallite, polyhalite, anhydrite, boracite, sulfoborite, leonite, epsomite and celestine.
Mars.
In early 2005, Mars Express, a European Space Agency orbiter, discovered evidence of kieserite in patches of Valles Marineris (the largest canyon on Mars), along with gypsum and polyhydrated sulfates. This is direct evidence of Mars's watery past and augments similar discoveries made by the Mars Exploration Rover Opportunity in 2004.
Uses.
It is used in the production of Epsom salt and as a fertilizer, the overall global annual usage in agriculture in the mid 1970s was 2.3 million tons.
Kieserite is also used for cleaning hard water deposits from tiles, stones, and other pool and fountain lining materials. Due to its hardness, which is greater than hard water deposits but less than tiles and other water feature linings, it is blasted at the hard water deposits to remove them.


Kampfite

Kampfite is a rare barium silicate–carbonate–halide mineral with the chemical formula . Discovered in 1964 and described in 2001, it is named after Anthony R. Kampf. The mineral is known only from Fresno County, California.
Description.
Kampfite is translucent and light blue-gray in color. Specimens occur as irregular masses up to in size.
Composition and structure.
When first described, the formula was identified as . Subsequent work on analyzing the crystal structure led to a revised formula of . The structural noted in the original description was not found in the later work.
The structure consists of double layers of tetrahedra connected by barium-containing polyhedra. It is this layering which causes kampfite's one perfect cleavage. Kampfite shares chemical and structural similarities with cymrite. , it is the only known barium silicate carbonate aside from fencooperite.
Occurrence.
Kampfite has been found in association with celsian, fresnoite, macdonaldite, pyrrhotite, titantaramellite, traskite, and witherite. The mineral is known only from locations in Fresno County, California. Kampfite is found in quartz-rich regions of sanbornite rocks within gneiss which has undergone contact metamorphism associated with an adjacent granodiorite intrusive.
History.
The mineral discovered in 1964 by Robert Walstrom from the Esquire #1 claim of the Rush Creek deposit in eastern Fresno County. It was described in 2001 in "The Canadian Mineralogist". The mineral was named "kampfite" in honor of Anthony Robert Kampf (1948–), curator of the Los Angeles County Museum of Natural History, for his crystallography work on new and rare minerals.
Co-type specimens are held by the University of British Columbia and the Geological Survey of Canada.


Santite

Santite (KB5O8·4H2O) is a hydrated borate mineral of potassium found in Tuscany, Italy. It is named for Georgi Santi (1746–1823), a former director of the Museum of Natural History, Italy.


Unquadtrium


Isotopes of astatine

Astatine (85At) has 39 known isotopes, all of which are radioactive; the range of their mass numbers is from 191 to 229. There are also 24 known metastable excited states. The longest-lived isotope is 210At, which has a half-life of 8.1 hours; the longest-lived isotope existing in naturally occurring decay chains is 219At with a half-life of 56 seconds.
Alpha decay.
Astatine has 23 nuclear isomers (nuclei with one or more nucleons – protons or neutrons – in an excited state). A nuclear isomer may also be called a "meta-state"; this means the system has more internal energy than the "ground state" (the state with the lowest possible internal energy), making the former likely to decay into the latter. There may be more than one isomer for each isotope. The most stable of them is astatine-202m1, which has a half-life of about 3 minutes; this is longer than those of all ground states except those of isotopes 203–211 and 220. The least stable one is astatine-214m1; its half-life of 265 ns is shorter than those of all ground states except that of astatine-213.
Alpha decay energy follows the same trend as for other heavy elements. Lighter astatine isotopes have quite high energies of alpha decay, which become lower as the nuclei become heavier. However, astatine-211 has a significantly higher energy than the previous isotope; it has a nucleus with 126 neutrons, and 126 is a magic number (corresponding to a filled neutron shell). Despite having a similar half-life time as the previous isotope (8.1 hours for astatine-210 and 7.2 hours for astatine-211), the alpha decay probability is much higher for the latter: 41.8 percent versus just 0.18 percent. The two following isotopes release even more energy, with astatine-213 releasing the highest amount of energy of all astatine isotopes. For this reason, it is the shortest-lived astatine isotope. Even though heavier astatine isotopes release less energy, no long-lived astatine isotope exists; this happens due to the increasing role of beta decay. This decay mode is especially important for astatine: as early as 1950, it was postulated that the element has no beta-stable isotopes (i.e. ones that do not undergo beta decay at all), though nuclear mass measurements reveal that 215At is in fact beta-stable, as it has the lowest mass of all isobars with "A" = 215. A beta decay mode has been found for all other astatine isotopes except for astatine-213, astatine-214, and astatine-216m. Among other isotopes: astatine-210 and the lighter isotopes decay by positron emission; astatine-216 and the heavier isotopes undergo beta decay; astatine-212 can decay either way; and astatine-211 decays by electron capture instead.
The most stable isotope of astatine is astatine-210, which has a half-life of about 8.1 hours. This isotope's primary decay mode is positron emission to the relatively long-lived alpha emitter, polonium-210. In total, only five isotopes of astatine have half-lives exceeding one hour: those between 207 and 211. The least stable ground state isotope is astatine-213, with a half-life of about 125 nanoseconds. It undergoes alpha decay to the extremely long-lived (in practice, stable) isotope bismuth-209.


Ammonium hexachloroplatinate

Ammonium hexachloroplatinate, also known as ammonium chloroplatinate, is the inorganic compound with the formula (NH4)2[PtCl6]. It is a rare example of a soluble platinum(IV) salt that is not hygroscopic. It forms intensely yellow solutions in water. In the presence of 1M NH4Cl, its solubility is only 0.0028 g/100 mL.
Preparation and structure.
The compound consists of separate tetrahedral ammonium cations and octahedral [PtCl6]2− anions. It is usually generated as a fine yellow precipitate by treating a solution of hexachloroplatinic acid with a solution of an ammonium salt. The complex is so poorly soluble that this step is employed in the isolation of platinum from ores and recycled residues.
As analyzed by X-ray crystallography, the salt crystallizes in a cubic motif reminiscent of the fluorite structure. The [PtCl6]2− centers are octahedral. The NH4+ centers are hydrogen bonded to the chloride ligands.
Uses and reactions.
Ammonium hexachloroplatinate is used in platinum plating. Heating (NH4)2[PtCl6] under a stream of hydrogen at 200 °C produces platinum sponge. Treating this with chlorine gives H2[PtCl6].


Rhodocene

Rhodocene is a chemical compound with the formula [Rh(C5H5)2]. Each molecule contains an atom of rhodium bound between two planar aromatic systems of five carbon atoms known as cyclopentadienyl rings in a sandwich arrangement. It is an organometallic compound as it has (haptic) covalent rhodium–carbon bonds. The [Rh(C5H5)2] radical is found above 150 °C or when trapped by cooling to liquid nitrogen temperatures (−196 °C). At room temperature, pairs of these radicals join via their cyclopentadienyl rings to form a dimer, a yellow solid.
The history of organometallic chemistry includes the 19th-century discoveries of Zeise's salt and nickel tetracarbonyl. These compounds posed a challenge to chemists as the compounds did not fit with existing chemical bonding models. A further challenge arose with the discovery of ferrocene, the iron analogue of rhodocene and the first of the class of compounds now known as metallocenes. Ferrocene was found to be unusually chemically stable, as were analogous chemical structures including rhodocenium, the unipositive cation of rhodocene and its cobalt and iridium counterparts. The study of organometallic species including these ultimately led to the development of new bonding models that explained their formation and stability. Work on sandwich compounds, including the rhodocenium-rhodocene system, earned Geoffrey Wilkinson and Ernst Otto Fischer the 1973 Nobel Prize for Chemistry.
Owing to their stability and relative ease of preparation, rhodocenium salts are the usual starting material for preparing rhodocene and substituted rhodocenes, all of which are unstable. The original synthesis used a cyclopentadienyl anion and tris(acetylacetonato)rhodium(III); numerous other approaches have since been reported, including gas-phase redox transmetalation and using half-sandwich precursors. Octaphenylrhodocene (a derivative with eight phenyl groups attached) was the first substituted rhodocene to be isolated at room temperature, though it decomposes rapidly in air. X-ray crystallography confirmed that octaphenylrhodocene has a sandwich structure with a staggered conformation. Unlike cobaltocene, which has become a useful one-electron reducing agent in research, no rhodocene derivative yet discovered is stable enough for such applications.
Biomedical researchers have examined the applications of rhodium compounds and their derivatives in medicine and reported one potential application for a rhodocene derivative as a radiopharmaceutical to treat small cancers. Rhodocene derivatives are used to synthesise linked metallocenes so that metal–metal interactions can be studied; potential applications of these derivatives include molecular electronics and research into the mechanisms of catalysis. The value of rhodocenes tends to be in the insights they provide into the bonding and dynamics of novel chemical systems, rather than their applications.
History.
Discoveries in organometallic chemistry have led to important insights into chemical bonding. Zeise's salt, K[PtCl3(C2H4)]·H2O, was reported in 1831 and Mond's discovery of Ni(CO)4 occurred in 1888. Each contained a bond between a metal centre and small molecule, ethylene in the case of Zeise's salt and carbon monoxide in the case of nickel tetracarbonyl. The space-filling model of the anion of Zeise's salt (image at left) shows direct bonding between the platinum metal centre (shown in blue) and the carbon atoms (shown in black) of the ethylene ligand; such metal–carbon bonds are the defining characteristic of organometallic species. Bonding models were unable to explain the nature of such metal–alkene bonds until the Dewar–Chatt–Duncanson model was proposed in the 1950s. The original formulation covered only metal–alkene bonds but the model was expanded over time to cover systems like metal carbonyls (including [Ni(CO)4]) where π backbonding is important.
Ferrocene, [Fe(C5H5)2], was first synthesised in 1951 during an attempt to prepare the fulvalene (C10H8) by oxidative dimerization of cyclopentadiene; the resultant product was found to have molecular formula C10H10Fe and reported to exhibit "remarkable stability". The discovery sparked substantial interest in the field of organometallic chemistry, in part because the structure proposed by Pauson and Kealy (shown at right) was inconsistent with then-existing bonding models and did not explain its unexpected stability. Consequently, the initial challenge was to definitively determine the structure of ferrocene in the hope that its bonding and properties would then be understood. The sandwich structure was deduced and reported independently by three groups in 1952: Robert Burns Woodward and Geoffrey Wilkinson investigated the reactivity in order to determine the structure and demonstrated that ferrocene undergoes similar reactions to a typical aromatic molecule (such as benzene), Ernst Otto Fischer deduced the sandwich structure and also began synthesising other metallocenes including cobaltocene; Eiland and Pepinsky provided X-ray crystallographic confirmation of the sandwich structure. Applying valence bond theory to ferrocene by considering an Fe2+ centre and two cyclopentadienide anions (C5H5−), which are known to be aromatic according to Hückel's rule and hence highly stable, allowed correct prediction of the geometry of the molecule. Once molecular orbital theory was successfully applied, the reasons for ferrocene's remarkable stability became clear.
The properties of cobaltocene reported by Wilkinson and Fischer demonstrated that the unipositive cobalticinium cation [Co(C5H5)2]+ exhibited stability similar to that of ferrocene itself. This observation is not unexpected given that the cobalticinium cation and ferrocene are isoelectronic, although the bonding was not understood at the time. Nevertheless, the observation led Wilkinson and F. Albert Cotton to attempt the synthesis of rhodocenium and iridocenium salts. They reported the synthesis of numerous rhodocenium salts, including those containing the tribromide ([Rh(C5H5)2]Br3), perchlorate ([Rh(C5H5)2]ClO4), and reineckate ([Rh(C5H5)2] [Cr(NCS)4(NH3)2]·H2O) anions, and found that the addition of dipicrylamine produced a compound of composition [Rh(C5H5)2] [N(C6H2N3O6)2]. In each case, the rhodocenium cation was found to possess high stability. Wilkinson and Fischer went on to share the 1973 Nobel Prize for Chemistry "for their pioneering work, performed independently, on the chemistry of the organometallic, so called sandwich compounds".
The stability of metallocenes can be directly compared by looking at the reduction potentials of the one-electron reduction of the unipositive cation. The following data are presented relative to the saturated calomel electrode (SCE) in acetonitrile:
These data clearly indicate the stability of neutral ferrocene and the cobaltocenium and rhodocenium cations. Rhodocene is ca. 500 mV more reducing than cobaltocene, indicating that it is more readily oxidised and hence less stable. An earlier polarographic investigation of rhodocenium perchlorate at neutral pH showed a cathodic wave peak at −1.53 V (versus SCE) at the dropping mercury electrode, corresponding to the formation rhodocene in solution, but the researchers were unable to isolate the neutral product from solution. In the same study, attempts to detect iridocene by exposing iridocenium salts to oxidising conditions were unsuccessful even at elevated pH. These data are consistent with rhodocene being highly unstable and may indicate that iridocene is even more unstable still.
Speciation.
The 18-electron rule is the equivalent of the octet rule in main group chemistry and provides a useful guide for predicting the stability of organometallic compounds. It predicts that organometallic species "in which the sum of the metal valence electrons plus the electrons donated by the ligand groups total 18 are likely to be stable." This helps to explain the unusually high stability observed for ferrocene and for the cobalticinium and rhodocenium cations – all three species have analogous geometries and are isoelectronic 18-valence electron structures. The instability of rhodocene and cobaltocene are also understandable in terms of the 18-electron rule, in that both are 19-valence electron structures; this explains early difficulties in isolating rhodocene from rhodocenium solutions. The chemistry of rhodocene is dominated by the drive to attain an 18-electron configuration.
Rhodocene exists as [Rh(C5H5)2], a paramagnetic 19-valence electron radical monomer only at or below −196 °C (liquid nitrogen temperatures) or above 150 °C in the gas phase. It is this monomeric form that displays the typical staggered metallocene sandwich structure. At room temperature (25 °C), the lifetime of the monomeric form in acetonitrile is less than two seconds; and rhodocene forms [Rh(C5H5)2]2, a diamagnetic 18-valence electron bridged dimeric "ansa"-metallocene structure. Electron spin resonance (ESR), nuclear magnetic resonance (NMR) and infrared spectroscopic (IR) measurements point to the presence of an equilibrium interconverting the monomeric and dimeric forms. ESR evidence confirms that the monomer possesses a high order axis of symmetry (C"n", "n" > 2) with a mirror plane (σ) perpendicular to it as symmetry elements; this experimentally demonstrates that the monomer does possess the typical sandwich structure of a metallocene although the interpretation of the ESR data has been questioned. The decomposition pathway of the monomer has also been studied by mass spectrometry. The dimerisation is a redox process; the dimer is a rhodium(I) species and the monomer has a rhodium(II) centre. Rhodium typically occupies oxidation states +I or +III in its stable compounds.
This dimerisation process has the overall effect of decreasing the electron count around the rhodium centre from 19 to 18. This occurs because the oxidative coupling of the two cyclopentadienyl ligands produces a new ligand with lower hapticity and which donates fewer electrons to the metal centre. The term hapticity is used to indicate the "number of carbon (or other) atoms through which [a ligand] binds ("n")" to a metal centre and is symbolised as η"n". For example, the ethylene ligand in Zeise's salt is bound to the platinum centre through both carbon atoms, and it hence formally has the formula K[PtCl3(η2-C2H4)]·H2O. The carbonyl ligands in nickel tetracarbonyl are each bound through only a carbon atom and are hence described as monohapto ligands, but η1-notations are typically omitted in formulae. The cyclopentadienyl ligands in many metallocene and half-sandwich compounds are pentahapto ligands, hence the formula [Rh(η5-C5H5)2] for the rhodocene monomer. In the rhodocene dimer, the coupled cyclopentadienyl ligands are 4-electron tetrahapto donors to each rhodium(I) metal centre, in contrast to the 6-electron pentahapto cyclopentadienyl donors. The increased stability of the 18-valence electron rhodium(I) dimer species as compared to the 19-valence electron rhodium(II) monomer likely explains why the monomer is only detected under extreme conditions.
Cotton and Wilkinson demonstrated that the 18-valence electron rhodium(III) rhodocenium cation [Rh(η5-C5H5)2]+ can be reduced in aqueous solution to the monomeric form; they were unable to isolate the neutral product as not only can it dimerise, the rhodium(II) radical monomer can also spontaneously form the mixed-hapticity stable rhodium(I) species [(η5-C5H5)Rh(η4-C5H6)]. The differences between rhodocene and this derivative are found in two areas: (1) One of the bound cyclopentadienyl ligands has formally gained a hydrogen atom to become cyclopentadiene, which remains bound to the metal centre but now as a 4-electron η4- donor. (2) The rhodium(II) metal centre has been reduced to rhodium(I). These two changes make the derivative an 18-valence electron species. Fischer and colleagues hypothesised that the formation of this rhodocene derivative might occur in separate protonation and reduction steps, but published no evidence to support this suggestion. (η4-Cyclopentadiene)(η5-cyclopentadienyl)rhodium(I), the resulting compound, is an unusual organometallic complex in that it has both a cyclopentadienyl anion and cyclopentadiene itself as ligands. It has been shown that this compound can also be prepared by sodium borohydride reduction of a rhodocenium solution in aqueous ethanol; the researchers who made this discovery characterised the product as biscyclopentadienylrhodium hydride.
Fischer and co-workers also studied the chemistry of iridocene, the third transition series analogue of rhodocene and cobaltocene, finding the chemistry of rhodocene and iridocene are generally similar. The synthesis of numerous iridocenium salts including the tribromide and hexafluorophosphate have been described. Just as with rhodocene, iridocene dimerises at room temperature but a monomer form can be detected at low temperatures and in gas phase and IR, NMR, and ESR measurements indicate a chemical equilibrium is present and confirm the sandwich structure of the iridocene monomer. The complex [(η5-C5H5)Ir(η4-C5H6)], the analogue of rhodocene derivative reported by Fischer, has also been studied and demonstrates properties consistent with a greater degree of π-backbonding in iridium(I) systems than is found in the analogous cobalt(I) or rhodium(I) cases.
Synthesis.
Rhodocenium salts were first reported within two years of the discovery of ferrocene. These salts were prepared by reacting the carbanion Grignard reagent cyclopentadienylmagnesium bromide (C5H5MgBr) with tris(acetylacetonato)rhodium(III) (Rh(acac)3). More recently, gas-phase rhodocenium cations have been generated by a redox transmetalation reaction of rhodium(I) ions with ferrocene or nickelocene.
Modern microwave synthetic methods have also been reported. Rhodocenium hexafluorophosphate forms after reaction of cyclopentadiene and rhodium(III) chloride hydrate in methanol following work-up with methanolic ammonium hexafluorophosphate; the reaction yield exceeds 60% with only 30 seconds of exposure to microwave radiation.
Rhodocene itself is then formed by reduction of rhodocenium salts with molten sodium. If a rhodocenium containing melt is treated with sodium or potassium metals and then sublimed onto a liquid nitrogen-cooled cold finger, a black polycrystalline material results. Warming this material to room temperature produces a yellow solid which has been confirmed as the rhodocene dimer. A similar method can be used to prepare the iridocene dimer.
Substituted rhodocenes and rhodocenium salts.
The [(η5-C5"t"Bu3H2)Rh(η5-C5H5)]+ cation.
Novel approaches to synthesising substituted cyclopentadienyl complexes have been developed using substituted vinylcyclopropene starting materials. Ring-enlarging vinylcyclopropane rearrangement reactions to produce cyclopentenes are well known and serve as precedent for vinylcyclopropenes rearranging to cyclopentadienes. The [(η5-C5"t"Bu3H2)Rh(η5-C5H5)]+ cation has been generated by a reaction sequence beginning with addition of the chlorobisethylenerhodium(I) dimer, [(η2-C2H4)2Rh(μ-Cl)]2, to 1,2,3-tri-"tert"-butyl-3-vinyl-1-cyclopropene followed by reaction with thallium cyclopentadienide:
The 18-valence electron rhodium(III) pentadienediyl species generated by this reaction demonstrates again the instability of the rhodocene moiety, in that it can be refluxed in toluene for months without 1,2,3-tri-"tert"-butylrhodocene forming but in oxidising conditions the 1,2,3-tri-"tert"-butylrhodocenium cation forms rapidly. Cyclic voltammetry has been used to investigate this and similar processes in detail. The mechanism of the reaction has been shown to involve a loss of one electron from the pentadienediyl ligand followed by a fast rearrangement (with loss of a hydrogen atom) to form the 1,2,3-tri-"tert"-butylrhodocenium cation. Both the tetrafluoroborate and hexafluorophosphate salts of this cation have been structurally characterised by X-ray crystallography.
[(η5-C5"t"Bu3H2)Rh(η5-C5H5)]BF4 forms a colourless centrosymmetric monoclinic crystal belonging to the "P"21/c space group, and with a density of 1.486 g cm−3. Looking at the ORTEP diagram of the structure of the cation (at right), it is evident that it possesses the typical geometry expected of a rhodocene or rhodocenium cation. The two cyclopentadienyl rings are close to parallel (the centroid–Rh–centroid angle is 177.2°) and the rhodium centre is slightly closer to the substituted cyclopentadienyl ring (Rh–centroid distances are 1.819 Å and 1.795 Å), an observation attributed to the greater inductive effect of the "tert"-butyl groups on the substituted ligand. The ORTEP diagram shows that the cation adopts an eclipsed conformation in the solid state. The crystal structure of the hexafluorophosphate salt shows three crystallographically independent cations, one eclipsed, one staggered, and one which is rotationally disordered. This suggests that the conformation adopted is dependent on the anion present and also that the energy barrier to rotation is low – in ferrocene, the rotational energy barrier is known to be ~5 kJ mol−1 in both solution and gas phase.
The diagram above shows the rhodium–carbon ( inside pentagons on the left) and carbon–carbon ( outside pentagons on the left) bond distances for both ligands, along with the bond angles ( inside pentagons on the right) within each cyclopentadienyl ring. The atom labels used are the same as those shown in the crystal structure above. Within the unsubstituted cyclopentadienyl ligand, the carbon–carbon bond lengths vary between 1.35 Å and 1.40 Å and the internal bond angles vary between 107° and 109°. For comparison, the internal angle at each vertex of a regular pentagon is 108°. The rhodium–carbon bond lengths vary between 2.16 Å and 2.18 Å. These results are consistent with η5-coordination of the ligand to the metal centre. In the case of the substituted cyclopentadienyl ligand, there is somewhat greater variation: carbon–carbon bond lengths vary between 1.39 Å and 1.48 Å, the internal bond angles vary between 106° and 111°, and the rhodium–carbon bond lengths vary between 2.14 Å and 2.20 Å. The greater variation in the substituted ligand is attributed to the distortions necessary to relieve the steric strain imposed by neighbouring "tert"-butyl substituents; despite these variations, the data demonstrate that the substituted cyclopentadienyl is also η5-coordinated.
The stability of metallocenes changes with ring substitution. Comparing the reduction potentials of the cobaltocenium and decamethylcobaltocenium cations shows that the decamethyl species is ca. 600 mV more reducing than its parent metallocene, a situation also observed in the ferrocene and rhodocene systems. The following data are presented relative to the ferrocenium / ferrocene redox couple:
The differences in reduction potentials is attributed in the cobaltocenium system to the inductive effect of the alkyl groups, further stabilising the 18-valence electron species. A similar effect is seen in the rhodocenium data shown above, again consistent with inductive effects. In the substituted iridocenium system, cyclic voltammetry investigations shows irreversible reductions at temperatures as low as −60 °C; by comparison, the reduction of the corresponding rhodocenes is quasi-reversible at room temperature and fully reversible at −35 °C. The irreversibility of the substituted iridocenium reductions is attributed to the extremely rapid dimerisation of the resulting 19-valence electron species, which further illustrates that iridocenes are less stable than their corresponding rhodocenes.
Pentasubstituted cyclopentadienyl ligands.
The body of knowledge concerning compounds with penta-substituted cyclopentadienyl ligands is extensive, with organometallic complexes of the pentamethylcyclopentadienyl and pentaphenylcyclopentadienyl ligands being well-known. Substitutions on the cyclopentadienyl rings of rhodocenes and rhodocenium salts produce compounds of higher stability as they allow for the increased delocalisation of positive charge or electron density and also provide steric hindrance against other species approaching the metal centre. Various mono- and di-substituted rhodocenium species are known, but substantial stabilisation is not achieved without greater substitutions. Known highly substituted rhodocenium salts include decamethylrhodocenium hexafluorophosphate [(η5-C5Me5)2Rh]PF6, decaisopropylrhodocenium hexafluorophosphate [(η5-C5"i"Pr5)2Rh]PF6, and octaphenylrhodocenium hexafluorophosphate [(η5-C5Ph4H)2Rh]PF6. Decamethylrhodocenium tetrafluoroborate can be synthesised from the tris(acetone) complex [(η5-C5Me5)Rh(Me2CO)3](BF4)2 by reaction with pentamethylcyclopentadiene, and the analogous iridium synthesis is also known. Decaisopropylrhodicnium hexafluorophosphate was synthesised in 1,2-dimethoxyethane (solvent) in an unusual one-pot synthesis that involves the formation of 20 carbon–carbon bonds:
In a similar reaction, pentaisopropylrhodocenium hexafluorophosphate [(η5-C5"i"Pr5)Rh(η5-C5H5)]PF6 can be synthesised from pentamethylrhodocenium hexafluorophosphate [(η5-C5Me5)Rh(η5-C5H5)]PF6 in 80% yield. These reactions demonstrate that the acidity of the methyl hydrogens in a pentamethylcyclopentadienyl complex can be considerably increased by the presence of the metal centre. Mechanistically, the reaction proceeds with potassium hydroxide deprotonating a methyl group and the resulting carbanion undergoing nucleophilic substitution with methyl iodide to form a new carbon–carbon bond.
The compounds pentaphenylrhodocenium tetrafluoroborate [(η5-C5Ph5)Rh(η5-C5H5)]BF4, and pentamethylpentaphenylrhodocenium tetrafluoroborate [(η5-C5Ph5)Rh(η5-C5Me5)]BF4 have also been reported. They demonstrate that rhodium sandwich compounds can be prepared from half-sandwich precursors. For example, in an approach broadly similar to the tris(acetone) synthesis of decamethylrhodocenium tetrafluoroborate, pentaphenylrhodocenium tetrafluoroborate has been synthesised from the tris(acetonitrile) salt [(η5-C5Ph5)Rh(CH3CN)3](BF4)2 by reaction with sodium cyclopentadienide:
Octaphenylrhodocene, [(η5-C5Ph4H)2Rh], is the first rhodocene derivative to be isolated at room temperature. Its olive green crystals decompose rapidly in solution, and within minutes in air, demonstrating a dramatically greater air sensitivity than the analogous cobalt complex, although it is significantly more stable than rhodocene itself. This difference is attributed to the relatively lower stability of the rhodium(II) state as compared to the cobalt(II) state. The reduction potential for the [(η5-C5Ph4H)2Rh]+ cation (measured in dimethylformamide relative the ferrocenium / ferrocene couple) is −1.44 V, consistent with the greater thermodynamic stabilisation of the rhodocene by the C5HPh4 ligand compared with the C5H5 or C5Me5 ligands. Cobaltocene is a useful one-electron reducing agent in the research laboratory as it is soluble in non-polar organic solvents, and its redox couple is sufficiently well behaved that it may be used as an internal standard in cyclic voltammetry. No substituted rhodocene yet prepared has demonstrated sufficient stability to be used in a similar way.
The synthesis of octaphenylrhodocene proceeds in three steps, with a diglyme reflux followed by workup with hexafluorophosphoric acid, then a sodium amalgam reduction in tetrahydrofuran:
The crystal structure of octaphenylrhodocene shows a staggered conformation (similar to that of ferrocene, and in contrast to the eclipsed conformation of ruthenocene). The rhodium–centroid distance is 1.904 Å and the rhodium–carbon bond lengths average 2.26 Å; the carbon–carbon bond lengths average 1.44 Å. These distances are all similar to those found in the 1,2,3-tri-"tert"-butylrhodocenium cation described above, with the one difference that the effective size of the rhodium centre appears larger, an observation consistent with the expanded ionic radius of rhodium(II) compared with rhodium(III).
Applications.
Biomedical use of a derivative.
There has been extensive research into metallopharmaceuticals, including discussion of rhodium compounds in medicine. A substantial body of research has examined using metallocene derivatives of ruthenium and iron as metallopharmaceuticals. One area of such research has utilised metallocenes in place of the fluorophenyl group in haloperidol, which is a pharmaceutical classified as a typical antipsychotic. The ferrocenyl–haloperidol compound investigated has structure (C5H5)Fe(C5H4)–C(=O)–(CH2)3–N(CH2CH2)2C(OH)–C6H4Cl and can be converted to the ruthenium analog via a transmetalation reaction. Using the radioactive isotope 103Ru produces a ruthenocenyl–haloperidol radiopharmaceutical with a high affinity for lung but not brain tissue in mice and rats. Beta-decay of 103Ru produces the metastable isotope 103"m"Rh in a rhodocenyl–haloperidol compound. This compound, like other rhodocene derivatives, has an unstable 19-valence electron configuration and rapidly oxidises to the expected cationic rhodocenium–haloperidol species. The separation of the ruthenocenyl–haloperidol and the rhodocenium–haloperidol species and the distributions of each amongst bodily organs has been studied. 103"m"Rh has a half-life of 56 min and emits a gamma ray of energy 39.8 keV, so the gamma-decay of the rhodium isotope should follow soon after the beta-decay of the ruthenium isotope. Beta- and gamma-emitting radionuclides used medically include 131I, 59Fe, and 47Ca, and 103"m"Rh has been proposed for use in radiotherapy for small tumours.
Metal–metal interactions in linked metallocenes.
The original motivation for research investigations of the rhodocene system was to understand the nature of and bonding within the metallocene class of compounds. In more recent times, interest has been rekindled by the desire to explore and apply the metal–metal interactions that occur when metallocene systems are linked. Potential applications for such systems include molecular electronics, semi-conducting (and possibly ferromagnetic) metallocene polymers (an example of a molecular wire), and exploring the threshold between heterogeneous and homogeneous catalysis. Examples of known bimetallocenes and termetallocenes that possess the rhodocenyl moiety include the hexafluorophosphate salts of rhodocenylferrocene, 1,1'-dirhodocenylferrocene, and 1-cobaltocenyl-1'-rhodocenylferrocene, each shown at right. Linked metallocenes can also be formed by introducing several metallocenyl substituents onto a single cyclopentadienyl ligand.
Structural studies of termetallocene systems have shown they typically adopt an "eclipsed double transoid" "crankshaft" geometry. Taking as an example the 1-cobaltocenyl-1'-rhodocenylferrocene cation shown above, this means that the cobaltocenyl and rhodocenyl moieties are eclipsed, and thus carbon atoms 1 and 1' on the central ferrocene core are as close to vertically aligned as is possible given the staggered conformation of the cyclopentadienyl rings within each metallocene unit. Viewed from side-on, this means termetallocenes resemble the down–up–down pattern of a crankshaft. The synthesis of this termetallocene involves the combining of rhodocenium and cobaltocenium solutions with 1,1'-dilithioferrocene. This produces an uncharged intermediate with linked cyclopentadienyl–cyclopentadiene ligands whose bonding resembles that found in the rhodocene dimer. These ligands then react with the triphenylmethyl carbocation to generate the termetallocene salt, [(η5-C5H5)Rh(μ-η5:η5-C5H4–C5H4)Fe(μ-η5:η5-C5H4–C5H4)Co(η5-C5H5)](PF6)2. This synthetic pathway is illustrated below:
Rhodocenium-containing polymers.
The first rhodocenium-containing side-chain polymers were prepared through controlled polymerization techniques such as reversible addition−fragmentation chain-transfer polymerization (RAFT) and ring-opening metathesis polymerisation (ROMP).


Potassium 2-ethylhexanoate

Potassium 2-ethylhexanoate, also known as potassium "iso"-octanoate, is a chemical used to convert the "tert"-butylammmonium salt of clavulanic acid into potassium clavulanate or clavulanate potassium.. It is also used as a corrosion inhibitor in automotive antifreeze.


Diphenyl ditelluride

Diphenylditelluride is the chemical compound with the formula (C6H5Te)2, abbreviated Ph2Te2 This orange-coloured solid is the oxidized derivative of the unstable benzenetellurol, PhTeH. Ph2Te2 is used as a source of the PhTe unit in organic synthesis and as a catalyst for redox reactions.
Preparation.
Ph2Te2 is prepared by the oxidation of tellurophenolate, which is generated via the Grignard reagent:
The molecule has "C2" symmetry.


Celestine (mineral)

Celestine (the IMA-accepted name) or celestite is a mineral consisting of strontium sulfate (SrSO4). The mineral is named for its occasional delicate blue color. Celestine and the carbonate mineral strontianite are the principal sources of the element strontium, commonly used in fireworks and in various metal alloys.
Etymology.
Celestine derives its name from the Latin word "caelestis" meaning celestial, which in turn is derived from the Latin word "caelum" meaning sky or heaven.
Occurrence.
Celestine occurs as crystals, and also in compact massive and fibrous forms. It is mostly found in sedimentary rocks, often associated with the minerals gypsum, anhydrite, and halite.
The mineral is found worldwide, usually in small quantities. Pale blue crystal specimens are found in Madagascar. White and orange variants also occurred at Yate, Bristol, UK, where it was extracted for commercial purposes until the mid-20th century.
The skeletons of the protozoan "Acantharea" are made of celestine, unlike those of other radiolarians which are made of silica.
In carbonate marine sediments, burial dissolution is a recognised mechanism of celestine precipitation. It is sometimes used as a gemstone.
Geodes.
Celestine crystals are found in some geodes. The world's largest known geode, a celestine geode in diameter at its widest point, is located near the village of Put-in-Bay, Ohio, on South Bass Island in Lake Erie. The geode has been converted into a viewing cave, Crystal Cave, with the crystals which once composed the floor of the geode removed. The geode has celestine crystals as wide as across, estimated to weigh up to each.
Celestine geodes are understood to form by replacement of alabaster nodules consisting of the calcium sulfates gypsum or anhydrite. Calcium sulfate is sparingly soluble, but strontium sulfate is mostly insoluble. Strontium-bearing solutions that come into contact with calcium sulfate nodules dissolve the calcium away, leaving a cavity. The strontium is immediately precipitated as celestine, with the crystals growing into the newly-formed cavity.


Sodium tetraphenylborate

Sodium tetraphenylborate is the organic compound with the formula NaB(C6H5)4. It is a salt, wherein the anion consists of four phenyl rings bonded to boron. This white crystalline solid is used to prepare other tetraphenylborate salts, which are often highly soluble in organic solvents. The compound is used in inorganic and organometallic chemistry as a precipitating agent for potassium, ammonium, rubidium, and cesium ions, and some organic nitrogen compounds.
Synthesis and structure.
Sodium tetraphenylborate is synthesized by the reaction between sodium tetrafluoroborate and phenylmagnesium bromide:
A related synthesis involves the use of phenylsodium in place of the Grignard reagent.
Unlike smaller counteranions, such as nitrate and the halides, tetraphenylborate confers lipophilicity to its salts. Many analogous tetraarylborates have been synthesized, containing both electron-rich and electron-deficient aryl groups.
The anhydrous salt adopts a polymeric structure in the solid state consisting of Na+-phenyl interactions. As such the salt could be classified as an organosodium compound.
Use in chemical synthesis.
Preparation of "N"-acylammonium salts.
Addition of sodium tetraphenylborate to a solution of a tertiary amine and an acid chloride in acetonitrile gives the acylonium salt by precipitating NaCl from the reaction mixture. This method has a broad scope:
Sodium tetraphenylborate is also employed as a phenyl donor in palladium-catalyzed cross-coupling reactions involving vinyl and aryl triflates to give arylalkenes and biaryl compounds in good yields and under mild conditions, respectively.
Use in coordination chemistry.
Tetraphenylborates are often studied in organometallic chemistry because of their favorable solubility in nonpolar solvents and their crystallinity. For example, the homoleptic trimethylphosphite complexes {M[P(OCH3)3]5}2+ (Ni, Pd, and Pt) have been prepared as their tetraphenylborate salts. Similarly, sodium tetraphenylborate has been used to isolate complexes containing dinitrogen ligands. In the reaction below, sodium tetraphenylborate allows N2 to displace the chloride ligand, which is removed from solution as a precipitate of sodium chloride:
The use of tetraphenylborate is limited to non-acidic cations. With strong acids, the anion undergoes protonolysis to give triphenylborane and benzene:
Related tetraorganoborates.
Weakly coordinating anions often are based on tetraarylborates, with electronegative substituents. Examples include B(C6F5)4− and Brookhart's acid containing the tetrakis[3,5-bis(trifluoromethyl)phenyl]borate anion.