reactive 
 oxygen species ( ros ) are essential for many biological 
 processes , including immune responses , cell signaling , neurotransmission , etc . despite the presence of enzymatic and nonenzymatic 
 antioxidant defense mechanisms ,
oxidative stress may persist in 
 cells as a result of endogenously and exogenously generated ros , which may lead to damage to macromolecules including 
 dna , proteins and lipids .
oxidative dna 
 damage is known to be associated with the onset of many human diseases 
 including cancer and neurodegeneration .
damage to dna 
 may compromise genomic integrity . a variety of dna 
 lesions including modified nucleobases and 2-deoxyribose , dna strand breaks , dna  dna cross - links , and dna  protein cross - links can be produced .
some 
 dna lesions like phosphoglycoaldehydes and abasic sites are chemically 
 reactive , whereas lesions like the 8,5-cyclopurine-2-deoxynucleosides 
 are not chemically reactive and exhibit elevated stabilities compared 
 to the corresponding unmodified nucleosides .
unrepaired dna lesions are known to compromise the fidelity and/or 
 efficiency of dna - templated processes , including dna replication and 
 transcription .
thus , identifying the dna lesions 
 generated , evaluating their impact on dna replication and transcription , 
 and investigating their repair are fundamental to gaining insights 
 into their implications in human health .
 pseudorepair  
 or  inverted  lesions , may arise from the improper chemical 
 repair of 2-deoxyribose radicals generated from ros attack .
specifically , 
 the hydroxyl radical can abstract a hydrogen atom from the 2-deoxyribose 
 moiety to yield carbon - centered radicals . in the presence of an h - atom donor , improper repair of these radicals 
 formed at the c1- , c3- , and c4-positions can 
 result in the inversion of stereochemical configuration at these carbons 
 ( scheme 1 ) . in this review , we will summarize 
 the research progress that has been made pertaining to the epimeric 
 lesions of 2-deoxyribose in dna . areas to be addressed
include the 
 known formation of these lesions , their impact on stability and structure 
 of duplex dna , and their biological consequences .
additionally , we 
 will discuss their potential therapeutic relevance , as well as future 
 research directions about these modified nucleosides .
the c1-epimers 
 of 2-deoxyadenosine ( da ) , 2-deoxyguanosine ( dg ) , and 
 2-deoxyuridine ( du ) were initially reported in nucleosides . 

exposure of aqueous solutions of da or dg to ionizing radiation under 
 anoxic conditions resulted in the generation of 9-(2-deoxy--d - erythro - pentofuranosyl)adenine ( -da ) 
 and 9-(2-deoxy--d - erythro - pentofuranosyl)guanine 
 ( -dg ) , respectively .
apart from these purine 
 nucleosides , 1-(2-deoxy--d - erythro - pentofuranosyl)uracil ( -du ) was found to form from the selectively 
 generated c1-radical of 2-deoxyuridine .
it is anticipated that both 1-(2-deoxy--d - erythro - pentofuranosyl)thymine ( -dt ) 
 and 1-(2-deoxy--d - erythro - pentofuranosyl)cytosine 
 ( -dc ) can also emanate from oxidative damage .
the inversion 
 from the - to -anomer at the c1-position of 
 da in dna was initially reported when poly(da ) , poly(da - dt ) , or salmon 
 testis dna was exposed to  rays in vitro . with the exception of the loss of adenine ( which 
 displayed a 3-fold higher level of occurrence relative to -da ) , 
 the formation of this c1-epimer was identified as a major 
 lesion of da in dna under anaerobic conditions .
yields ( based on the 
 total number of da residues subjected to damaging conditions ) of -da 
 were 1.31.5% for all sequences .
other lesions of da and their 
 respective yields are ( 5r)-8,5-cyclo-2-deoxyadenosine 
 ( 0.050.21% ) , 8-hydroxyadenine ( 0.030.10% ) , and 8-hydroxy-2-deoxyadenosine 
 ( 0.07% ) . considering that 8,5-cyclo-2-deoxyadenosine 
 has been observed previously in cells and tissues , it is reasonable 
 to predict that -dns should also be formed at appreciable levels in vivo .
intriguingly , lesiak and wheeler showed that -da 
 can occur readily in the absence of thiols , which was proposed to 
 result from either the addition of a hydrated electron followed by 
 protonation and/or disproportionation reactions .
relative to the dna damage induced by  rays , 
 exposure of 
 da and 2-deoxyadenosine-5-monophosphate with  
 rays resulted in significantly less amounts of -da relative 
 to odns , indicating that the polynucleotide structure 
 enhances the formation of -da .
apart from -da , -du 
 was identified in odns as a result of selective oxidative damage .
currently , the formation of -dg , -dc , or -dt 
 in dna remains to be determined .
the c1-epimers were 
 proposed to form from 2-deoxyribose 
 damage occurring at the c4- and/or c1-position ( scheme 2 ) . originally , mariaggi et al . proposed that -da results from the c4-radical 
 ( 2 ) through the hydroxyl radical - mediated hydrogen abstraction 
 from this carbon . through this mechanism ,
a schiff base ( 9 ) is formed via ring opening of the sugar moiety facilitated by the 
 protonation of the furanose oxygen .
this leads to the generation of 
 the c1-cation ( 10 ) , immediately followed by the 
 reconstitution of the pentofuranosyl moiety in the presence of an 
 h - atom donor , generating the two c1-epimers ( 1 and 7 ) .
this mechanism 
 for -da formation is supported by the presence of two other 
 da damage products also proposed to originate from the c4-radical , 
 9-(2-deoxy--d - erythro - pentopyranosyl)adenine 
 and 9-(2-deoxy--d - erythro - pentopyranosyl)adenine , 
 which require a free 5-hydroxyl group for the pentopyranosyl 
 ring formation
. however , selectively 
 generated c4-radicals of da and dt did not give rise to the 
 formation of -da or -dt , respectively , suggesting that the c4-radical is unlikely to be a precursor 
 for the formation of the c1-epimers .
instead , the epimerization 
 of the c1-position is more likely attributed to the formation 
 of the c1-radical ( 4 ) ; indeed , both -du and -du were observed to form from 
 the independently generated 2-deoxyurid-1-yl radical .
the impact of the -anomer on the thermodynamic stability of 
 duplex dna containing 2-deoxynucleosides with the normal 3-5 
 phosphodiester linkage has only been characterized for -da . 

this was accomplished by performing uv melting temperature measurements 
 of a 9-mer duplex with a site - specifically incorporated -da 
 being paired with the correct ( dt ) or incorrect nucleoside ( da , dg , 
 or dc ) . in this report ,
the measured tm values were employed to gauge the impact of 
 -da on g , h and s by assuming a two - state transition 
 model .
the impact of -da on duplex stability was found to vary 
 with the pairing nucleoside of -da .
the presence of a single 
 -dadt base pair was found to increase slightly the duplex 
 stability ( tm of 0.6 c ) relative 
 to the canonical dadt base pair , whereas the -dadt base pair was observed to be destabilizing 
 in other studies .
discrepancies among these findings 
 might be attributed to the different sequence contexts and/or duplex 
 lengths used in these studies .
this notion is supported by a 4.4 c 
 range ( 49.053.4 c ) in melting temperature ( i.e. , duplex 
 stability ) introduced by changing the nearest neighbor base - pairs . in the case of the incorrect base pairings of 
 -da ( -dada , -dadg , and -dadc ) , 
 significant destabilization was observed , as reflected by a drop in tm values by 7.211.0 c and a rise 
 in g25  c values by 1.672.64 
 kcal / mol .
the -dadg pairing 
 resulted in the largest degree of destabilization to duplex dna ( tm = 11.0 c , g25  c = 2.64 kcal / mol ) , whereas lower 
 extents of destabilization were found for the -dadc ( tm = 7.4 c , g25  c = 1.78 kcal / mol ) and -dada 
 ( tm = 7.2 c , g25  c = 1.67 kcal / mol ) mispairs . aside from -da , the impact of -dg , 
 -dt , and -dc on duplex stability was evaluated for duplexes 
 containing two -dns with unconventional phosphodiester linkages 
 ( 3-3 and 5-5 ) . in that study , -dc was found to be significantly 
 more destabilizing ( tm = 10.0 
 c ) than -da , -dg , and -dt , which all exhibit 
 similar effects ( average tm = 5.3 
 c ) .
the effect of -dg , -dc , 
 and -dt on duplex stability for the conventional 3-5 
 phosphodiester linkage remains to be determined .
circular dichroism 
 ( cd ) experiments demonstrated that the global conformation of duplex 
 dna harboring an individual -da remains in b - form ( figure 1a ) .
in addition , the 
 nmr solution structure of duplex dna containing an -dadt 
 pair revealed local changes in duplex structure ( figure 1b ) , where -da participates 
 in both reverse watson 
crick base - pairing with dt and base - stacking 
 interactions with the neighboring nucleobase ( figure 1c ) .
furthermore , -da was observed to introduce a kink 
 at the modified nucleoside site and result in an expansion of the 
 minor groove by 18. overall , these 
 structural perturbations were found to vary with the sequence context 
 ( figure 1d ) .
( a ) cd spectra of double - stranded dna revealing 
 the impact of the -da on dna secondary structure .
the global b - form dna conformation observed 
 for the unmodified duplex ( at ) is maintained for duplexes containing 
 the -da lesion being paired with the correct ( dt ) and incorrect 
 ( da , dc , and dg ) nucleosides ; -da is represented by . 
 ( b ) nmr solution structure of an -da - containing 10-mer duplex 
 dna demonstrating the b - form duplex conformation .
crick base pairing interactions of a and t normally 
 observed in b - form duplex dna and the reverse watson  crick 
 base pairing interaction observed between -da and dt in the 
 nmr solution structure .
( d ) structural 
 comparison highlighting the impact of the nearest - neighbor base pair 
 on the minor - groove width and kink angle introduced by -da .
duplexes ( red and orange ) containing the -da 
 lesion were solved by nmr , while the unmodified duplex ( blue ) is a 
 model . the minor groove is indicated by m , while the position of -da 
 is indicated by . the nearest - neighbor base pair altered is 
 highlighted in red , orange , or blue , respectively .
so far , replication 
 studies have been conducted only for -da . in this respect , 

-da was found to stall the klenow fragment in vitro , though bypass could be observed after prolonged incubation .
gilbert sequencing and primer extension 
 assays indicate that -da directs the incorporation of both 
 correct ( dtmp ) and incorrect ( dcmp , damp and dgmp ) nucleotides , with 
 the misincorporation of dgmp being significantly disfavored .
in addition , 
 promutagenic properties and bypass efficiency of -da were found 
 to be sequence - dependent , particularly by the nearest neighbor base 
 pair but independent of the exonuclease activity of the polymerase . 

in addition , similar results were obtained for t7 and taq dna polymerases 
 as well as a reverse transcriptase .
replication studies with the use of -da - bearing , single - stranded 
 m13 bacteriophage showed that -da constitutes a moderate block 
 to dna replication and introduces single nucleotide deletion in e. coli cells . although nucleotide 
 misincorporation was not found in vivo , the frequency 
 of single - nucleotide deletion
was observed to be sequence - dependent , 
 with mutation frequencies ranging from 1 to 26% .
a misinsertion - strand - slippage mechanism was thought to 
 contribute to the generation of single - nucleotide deletions .
the impact of -dt , -dg , and -dc 
 on dna replication , however , remains to be investigated .
the c1-epimeric lesions 
 were found to be substrates for repair enzymes of bacteria , yeast , 
 and humans . for e. coli ,
a wide range of repair enzymes 
 were evaluated , including multiple endonucleases ( i.e. , endonucleases 
 iii , iv , viii , and deoxyinosine-3-endonuclease ) , exonuclease 
 iii , and formamidopyrimidine n - glycosylase . among these enzymes ,
endonuclease iv was the 
 only repair enzyme capable of recognizing both -da and -dt .
in addition , this endonuclease activity was selective for dsdna 
 substrates , as the corresponding ssdna constructs could not be cleaved 
 by the enzyme .
apart from the e. coli enzymes , s. cerevisiae apurinic / apyrimidinic 
 ( ap ) endonuclease 1 ( apn1 ) and human ap endonuclease i ( ape1 ) could 
 also recognize -da and -dt .
together , these reports suggest that the repair pathway of -da 
 and -dt is conserved among species and that -dg and 
 -dc are likely substrates for the same repair pathway . on the 
 grounds that the -dns are not recognized by dna glycosylases 
 in the base excision repair ( ber ) pathway
, it has been proposed that these c1-epimeric lesions are 
 repaired by the nucleotide incision repair ( nir ) pathway , an alternative pathway to ber .
the inversion of configuration 
 at the c3-position results in the generation of the c3-epimeric 
 lesions , i.e. , 1-(2-deoxy - r - l - threo - pentofuranosyl)-containing nucleosides , commonly referred 
 to as xylose nucleosides .
these types of lesions were first observed 
 to form from the independently generated c3-radicals of nucleosides 
 in the presence of an h - atom donor .
the 
 c3-epimeric lesion was observed to form in equal amounts as 
 the properly chemically repaired c3-epimer of 2-deoxyribose .
in addition , 1-(2-deoxy - r - l - threo - pentofuranosyl)thymine ( dxt ) was 
 found to form in odns from the independently generated c3-radical 
 under anaerobic conditions , where dxt 
 was produced more readily in model replication - relevant architectures 
 ( ssdna and 5-overhang ) than in dsdna .
moreover , the formation of the c3-epimeric lesion competes 
 directly with strand break formation under anaerobic conditions ( scheme 3 ) .
future investigations about the in vivo formation of dxt and other dxns will provide valuable 
 insights into the physiological relevance of the c3-epimeric 
 lesions .
formation for the c3-epimeric 
 lesion and competing strand break pathway resulting from oxidation 
 ( [ o ] ) in aqueous solution .
alterations in both dna stability 
 and structure have been reported 
 for odns containing a single dxn lesion .
primarily , these studies 
 have been performed using uv melting analysis , circular dichroism 
 ( cd ) measurements , and molecular modeling .
it has been reported that 
 the presence of a single dxt in double - stranded dna affects both dna 
 stability and structure , though the duplex maintains b - dna geometry . in model dna replication - relevant architectures ( fork , 5-overhang , 
 and 3-overhang ) ,
dxt was more destabilizing and introduced 
 greater structural alterations when located closer to the 3-terminus 
 of the damaged odn .
the presence of an 
 individual dxc or dxg in duplex dna also led to a significant decrease 
 in duplex stability .
initial structural characterizations 
 of all four dxns individually , as well as the structural analysis 
 of fully xylose duplex dna , indicate that the sugar pucker of the 
 dxns adopts the c3-endo conformation , resembling that of ribonucleosides 
 ( figure 2 ) .
together , 
 these reports suggest that the presence of an isolated dxn may perturb 
 the stability and structure of duplex dna in vivo .
2-deoxyribose sugar puckers observed in duplex dna ( b and a form ) , rna , and for dxns .
investigations 
 about the impact of the c3-epimeric lesions on biological 
 processes are limited . along this
line , primer extension assays were 
 conducted to assess the ability of dna polymerases to incorporate 
 2-deoxyxyloadenosine triphosphate ( dxatp ) and/or 2-deoxyxylothymidine 
 triphosphate ( dxttp ) into dna in vitro .
it was found that several different dna polymerases 
 were capable of incorporating dxntps into the elongating strand .
notably , 
 complete primer extension was not observed , suggesting that the presence of the dxn may significantly inhibit 
 dna replication in vivo .
provided that the sugar 
 pucker of the dxns prefers the c3-endo conformation , it is 
 likely that the lack of complete primer extension is a result of the 
 structural effects of the dxn lesion .
future investigations seeking 
 to obtain detailed structural data , as well as experiments measuring 
 the bypass efficiencies of the dxn lesions , will significantly expand 
 the current knowledge about the impact of these lesions on dna replication .
endonuclease activity on dna 
 substrates containing dxc and/or dxt was observed to vary among enzymes 
 and positions of the dxn .
ecorii was incapable of 
 cleaving the dna backbone with the presence of dxns at the restriction 
 recognition site , while the activity of mvai was 
 variable and overall significantly more tolerant of the presence of 
 the lesion than ecorii .
the c4-epimeric 
 lesions of dt and da were found to be generated in vitro in single- and double - stranded dna from independently generated 
 radicals , while the c4-epimer 
 of dg could be induced in an aqueous solution of dg upon exposure 
 to -rays . similar to the c3-epimeric 
 lesion , the formation of the c4-epimer competes with strand 
 break generation ( scheme 4 ) .
the stereoselectivity of 5 versus 1 was observed to be directly influenced by the dna structure , with 
 significantly greater levels of 5 being generated in 
 ssdna than dsdna .
nevertheless , the levels of 5 were 
 reported not to be affected by either the source of h - atom donor or 
 the identity of the nucleoside ( dt or da ) from which the radical is 
 generated , suggesting that the c4-epimeric 
 lesions may be produced in dna at similar levels for the four canonical 
 2-deoxynucleosides .
oxidative damage in ssdna in the presence 
 of oxygen resulted in the epimeric mixtures of 5 and 1 , with yields ( 151% ) varying with the concentrations 
 of h - atom donor .
currently , the formation 
 of these c4-epimeric lesions in vivo remains 
 to be determined .
the formation for the c4-epimeric 
 lesion ( 5 ) and competing strand break pathway arising 
 from spontaneous strand scission .
investigations about the impact 
 of the c4-epimeric lesions 
 on dna stability are minimal , with no reports characterizing structural 
 changes .
this is likely due to the lack of facile synthetic routes 
 to obtaining these c4-epimers .
the presence of the c4-epimer 
 of da in the duplex portion of a dangling end substrate was reported 
 to be destabilizing , as reflected by a decrease in melting temperature 
 of 1.2 c .
crick base - pairing interactions 
 based on the fidelity of nucleotide incorporation opposite this lesion , 
 as discussed in section 4.3 below ; nevertheless , it remains to be assessed whether the same findings 
 can be extended to the c4-epimers of other nucleosides .
not much 
 is known about the biological consequences of the c4-epimeric 
 lesions with the exception of the studies assessing how this lesion 
 affects dna replication .
primer extension assay with the use of the 
 klenow fragment revealed that the c4-epimer of da introduced 
 a stop - site one nucleotide downstream of the lesion , though it could 
 be bypassed after prolonged incubation .
in addition , this bypass was independent of the exonuclease activity 
 of the polymerase .
intriguingly , the levels of incorporation of the 
 incorrect dntps were found to be decreased relative to the undamaged 
 substrate .
thus , the presence of the 
 c4-epimeric lesion of da was found not to be mutagenic in vitro but may be cytotoxic as reflected by the stalling 
 of the klenow fragment of dna polymerase i. with respect to dna repair , the ability of the ner pathway 
 to repair the c4-epimer of da was investigated in 
 vitro with the use of human cell extracts . in duplex dna ,
the lesion was found not to be a substrate 
 for human ner when it is paired with a dt ; however , it could be recognized 
 by the human ner machinery when it is present in mispairs .
therefore , these results suggest that the c4-epimer 
 of da formed in at base pairs is unlikely a substrate for the 
 ner pathway . it remains to be examined whether the c4-epimers 
 of this and other nucleosides can be repaired by other pathways including 
 ber and nir .
currently , none of the epimeric 2-deoxyribose lesions have been 
 detected in vivo . given the literature precedence 
 in identifying the formation of these lesions in vitro at all three stereocenters ( c1 , c3 , and c4 ) 
 of 2-deoxyribose
, it is highly likely that these purely structural 
 lesions can be generated in vivo from oxidative dna 
 damage .
the presence of an h - atom donor source facilitates the 
 formation 
 of these epimeric dna lesions . as observed at both the c1- 
 and c4-positions
, the nature of the h - atom donor does not 
 impact lesion formation . in cells
, there are 
 a variety of potential h - atom donors that are readily available , with 
 glutathione ( gsh ) being the major antioxidant in the nucleus , cytosol , 
 and mitochondria . upon the formation 
 of the 2-deoxyribose radicals , the presence of o2
readily 
 competes with h - atom donors via generating peroxyl radicals of 2-deoxyribose , 
 which can also be reduced by h - atom donors .
thus , it is anticipated 
 that the greatest levels of these epimeric lesions are generated under 
 conditions and/or microenvironments where o2 levels are 
 low and h - atom donors are prevalent .
for example , the nucleus is known 
 to contain relatively low concentrations of o2 and adequate concentrations ( 315 mm ) 
 of gsh , which was previously demonstrated 
 to facilitate the formation of these lesions . in the case of cancer tissues , which are 
 often hypoxic and resistant to radiation 
 and chemotherapeutic treatments
the potential for these epimeric 
 lesions to form under aerobic conditions in vivo is 
 also likely ( but expected to be produced at significantly lower levels ) 
 as demonstrated in vitro at the c4-position .
several anticancer and antiviral drugs , as well as artificial metal 
 nucleases , function by inducing strand breaks via h - atom abstraction 
 from the 2-deoxyribose moiety .
the same 2-deoxyribose 
 radicals were previously found to yield the epimeric lesions of 2-deoxyribose . 

the specific 2-deoxyribose radicals generated are dictated by both 
 dna structure and dna - oxidizer - binding interactions . in b - dna , both the c1 and c4 h - atoms reside 
 in the minor groove , whereas the c3 h - atom is located in the 
 major groove .
therefore , drugs that bind 
 to the minor groove and generate the c1- and/or c4-radical , 
 as demonstrated by some enediynes and bleomycins , may foster the inversion of stereochemistry 
 at the c1- and c4-positions ( figure 3a ) .
given that these drugs bind to the minor groove of dna , 
 accessibility of the h - atom donor to the  face of the c1- 
 and c4-radicals via the minor groove is likely inhibited , 
 potentially facilitating the improper chemical repair of the 2-deoxyribose 
 radicals by cellular thiols . to our knowledge , the formation of the 
 c1- and/or c4-epimers from enediynes or bleomycins 
 in the presence of thiols has not been reported .
similarly , major - groove 
 binders known to generate the c3-radical , such as rhodium 
 complexes , may induce the preferential formation 
 of the c3-epimeric lesions ( figure 3b ) . in this case , the h - atom donor may only have access to the  
 face of the c3-radical through the minor groove , while the 
 proper chemical repair from the  face may be inhibited by the 
 presence of the dna - binding agent targeting the major groove . potential formation 
 of the epimeric 2-deoxyribose lesions resulting 
 from dna binding agents .
( a ) proposed formation of the c1- 
 and c4-epimers resulting from the h - atom abstraction by minor - groove 
 binders .
( b ) proposed formation of the c3-epimers resulting 
 from the c3-radical generation by major - groove binders . in considering the potential generation 
 of these lesions by chemotherapy 
 or radiation therapy ,
regard , hypoxia is known to confer resistance to chemotherapeutics 
 and radiation therapy , where depleted o2 levels could decrease 
 directly the toxicity of many therapeutics and serve as a signal for 
 hypoxia - inducible transcription factors .
we reason that 
 the drug resistance manifested during hypoxia could also be attributed , 
 in part , to the formation of these epimeric 2-deoxyribose lesions 
 facilitated by cellular thiols , which competes directly with the formation 
 of the intended dna strand breaks . in this regard
, the formation of 
 these lesions would prevent the generation of the dna strand breaks , 
 which is a toxic effect of many therapeutic agents targeting cancer 
 cells .
consequently , any biological activity associated with the generation 
 of the epimeric 2-deoxyribose lesions may be masked by the already 
 abnormal activity of the cancer cells . as a result , the formation 
 of these epimeric lesions may be generated as an alternative route 
 for cancer cell survival during hypoxia , preventing lethal strand 
 scissions at the expense of the biological consequences introduced 
 by the epimeric lesions .
thus , the formation of these epimeric 2-deoxyribose 
 lesions in cells upon exposure to chemotherapeutic agents and ionizing 
 radiation under normoxic and hypoxic conditions merits future investigations . 

the outcome of such studies may provide molecular insights into hypoxia - induced 
 resistance to chemotherapy and radiation therapy .
additionally , such 
 studies may unveil the impact of the cellular environment on the generation 
 of these lesions .
the potential formation of the aforementioned epimeric dna lesions in vivo is well prefaced with the reported in vitro literature . demonstrating the formation of these lesions in vivo is essential for expanding the current scope of 
 their investigations .
this endeavor is challenged by the lack of alterations 
 in chemical functional groups introduced by these lesions , necessitating 
 a structural and/or enzymatic approach for their identification in vivo .
these antibodies , however , may not recognize a single isolated c1 
 -anomer as generated from oxidative dna damage .
thus , it would 
 be interesting to develop antibodies that can be employed for recognizing 
 an individual c1 -anomer or the c3 or c4 
 2-deoxyribose epimers in dna
. another approach would be to utilize 
 enzymatic digestions coupled with lc - ms / ms analysis .
this would necessitate 
 optimized methods to facilitate the chromatographic separation and/or 
 mass spectrometric differentiation of these epimeric lesions from 
 their unmodified counterparts .
regardless of the approach , method 
 development for assessing the formation of these lesions in 
 vivo will require them to be synthetically available .
the 
 syntheses of the c1- and c3-epimers have been reported 
 for all four dns . in the case of the c4-epimers , available synthetic routes 

are currently limited to da , which is generated utilizing a modified 
 radical precursor .
it should be noted 
 that the generation of epimeric nucleosides via radical intermediates 
 is accompanied with a high risk of byproduct formation . as a result , 
 this approach may offer the limited yield for the desired epimer . 

thus , the development of facile synthetic methods for generating the 
 full spectrum of c4-epimeric lesions will directly broaden 
 the experimental scope of future investigations and enable the assessment 
 about the in vivo formation of these epimeric lesions .
future studies evaluating the impact of these lesions on processes 
 such as dna replication , transcription , and repair will yield insightful 
 results on the biological consequences for the formation of these 
 lesions . currently , in vivo studies are limited to 
 the c1-epimeric lesion of da in the e. coli system . if these lesions are in fact 
 formed in vivo , understanding the repair of these 
 lesions and the effects of these lesions on dna replication and transcription 
 in mammalian cells should be pursued .
additionally , it is possible 
 that these lesions are formed in vivo under hypoxic 
 conditions , in which case they may be utilized as biomarkers of hypoxia .
in this review , we summarized our current 
 knowledge of the epimeric 
 2-deoxyribose lesions in dna , which are the improper chemical repair 
 products of the c1- , c3- , and c4-radicals 
 of 2-deoxyribose .
particularly , we have reviewed relevant literature 
 highlighting the known formation of these lesions , their impact on 
 the stability and structure of the dna double helix , and their biological 
 consequences . as discussed above , some of these epimeric 2-deoxyribose 
 lesions
have been observed to be generated in dna from either  
 radiation and/or independently generated radicals in vitro , while their in vivo presence remains elusive . 

in addition , it has been observed that the presence of an individual 
 epimeric 2-deoxyribose lesion in dna destabilizes the dna double helix 
 and introduces local structural alterations in dna , which has been 
 demonstrated to vary with dna sequence context and architecture .
some 
 of these lesions also compromise dna replication , being promutagenic 
 or potentially cytotoxic .
existing literature has offered significant 
 precedence for the 
 potential formation of the epimeric 2-deoxyribose lesions in vivo , as well as an initial understanding of the potential 
 biological consequences that they may exert , while much remains to 
 be learned for this family of dna lesions . in particular , determining 
 if these dna lesions form for all four nucleosides and measuring their 
 respective levels of formation in vitro and in vivo will not only broaden our scope of knowledge for 
 these dna lesions but also unveil whether and how the fate of the 
 2-deoxyribose radicals is influenced by the nature of the nucleobase . 

moreover , future studies addressing the impact of these lesions on 
 dna replication and transcription and how they are repaired in mammalian 
 cells will offer important knowledge for understanding the implications 
 of these lesions in human diseases .
 genomic 
 integrity is constantly challenged by dna damaging agents 
 such as reactive oxygen species ( ros ) . 
 consequently , dna damage can 
 compromise the fidelity and efficiency of essential dna metabolic 
 processes , including replication and transcription , which may contribute 
 significantly to the etiology of many human diseases . here , we review 
 one family of dna lesions , the epimeric 2-deoxyribose lesions , which 
 arise from the improper chemical repair of the 2-deoxyribose radicals . 
 unlike most other dna lesions , the epimeric 2-deoxyribose lesions 
 are indistinguishable from their corresponding unmodified nucleosides 
 in both molecular mass and chemical reactivity . 
 we placed our emphasis 
 of discussion on the formation of these lesions , their impact on the 
 structure and stability of duplex dna , their biological consequences , 
 their potential therapeutic relevance , and future research directions 
 about these modified nucleosides . 
Introduction
C1-Epimer
C3-Epimer
C4-Epimer
Physiological Relavance
Potential Therapeutic Implications
Prospects and Limitations
Conclusions