complexity in chemistry ; 
 reaction networks , coupled equilibria , 
 spatiotemporal compartmentalization , or feedback loops often result 
 in emergent behavior characterized by responsiveness , adaptivity and 
 nonlinearity ; life being the prime example .
even though our creations can not yet rival those of nature , the 
 rise of interest in systems chemistry gives hope that the gap will 
 decrease as we gather understanding and devise new mechanisms to create 
 and control complexity .
one of 
 the more successful methods to generate complex ( supra)molecular 
 systems is dynamic combinatorial chemistry ( dcc ) . in this approach ,
a few small building blocks react reversibly with 
 each other , giving rise to mixtures of much more complex library members , 
 together constituting a dynamic combinatorial library ( dcl ) . in its 
 relatively short history
, it has led to practical outcomes , such as 
 discoveries of biologically active compounds or responsive materials , as well as to discoveries of fundamental value , such as emergence 
 of self - replicating molecules or complex reaction networks and cascades .
addition of a second type of reversible 
 chemistry not only adds another layer of complexity , but also provides 
 an additional handle to control it .
however , only a small fraction 
 of reported work takes advantage of this strategy , and only a handful describe three or more dynamic covalent chemistries 
 in a single system .
this situation stands in a stark contrast with supramolecular systems , where several different interaction 
 motifs are often used simultaneously .
combined combinatorial 
 chemistries can be orthogonal ( scheme 1a ) , when one functional 
 group can only be involved in formation of one covalent bond type , 
 or promiscuous ( scheme 1b ) , which means that some of the functionalities can form more than 
 one type of dynamic covalent bonds .
disulfide 
 and hydrazone exchange are a pair of orthogonal chemistries , as in 
 aqueous solution thiols do not form stable adducts either with aldehydes , 
 or with hydrazides . in such cases the two chemistries 

operate completely independently , unless they are coupled by an independent 
 interaction , e.g. , noncovalent bonds . on the other hand , libraries 
 based on thiol
 disulfide exchange can easily communicate with 
 thioester - based libraries , as both reactions involve promiscuous thiol 
 building blocks . depending on exact chemistries 

used , reaction conditions may be tuned in such a way that exclusively 
 one type of exchange is active , or that two or more chemistries operate 
 simultaneously ( scheme 1a ) .
this is however more often defined by the nature of the exchange 
 chemistries , than by the intentions of the experimenters . in the case 
 of disulfides and thioesters , for example , the two chemistries tend 
 to only work simultaneously at mildly basic ph . in contrast ,
hydrazone 
 exchange , which normally operates at moderately to strongly acidic 
 ph , was only active simultaneously with disulfide exchange at the 
 cost of both reactions being slow .
different exchange types are 
 represented by bond and building block ( bb ) shapes , whereas colors 
 denote bb identity . for clarity
( a ) orthogonal chemistries : under conditions ( 1 ) only one type of 
 exchange is active , under conditions ( 2 ) only the other , whereas under 
 conditions ( 3 ) both exchanges operate simultaneously . under all conditions 
 bbs exchange only within the same type of chemistry .
however , the 
 number of components involved in each exchange pool remains constant . 

( c ) antiparallel chemistries ( this work ) : bbs participate in both 
 chemistries , but the ratio of the two chemistries can be tuned by 
 altering the system conditions . combining 
 exchange chemistries that can communicate leads to another 
 interesting possibility :
if the two exchange pools share a building 
 block , increase of its amount in one pool necessarily depletes it 
 in the other one . in other words ,
the distribution of covalent bond 
 types is reflected by the composition of the dcl , leading to the concept 
 of antiparallel chemistries ( scheme 1c ) . the term  antiparallel  reflects 
 that both reactions can take place at the same time ( thus parallel ) but occur at each other s expense ( hence anti ) . in a system that comprises two parts which share 
 a constituent that can be shifted from one to another by an external 
 parameter , a new level of control emerges . together with the thermodynamic 
 control inherent to the dcl itself
, its composition now also depends 
 on the external parameter , which is in the hands of the experimenter . 

thus , in antiparallel chemistries the two reactions can occur simultaneously 
 and communicate through a common reactant , which is distinct from 
 the situation in orthogonal chemistries in which all reactions operate 
 independently of each other . in our design of antiparallel chemistries 
 we decided to combine 
 thiol  disulfide and thio - michael exchange ( figure 1 ) .
the choice 
 stems from the fact that both chemistries involve thiols , but the 
 library members themselves require sulfur atoms to be in different 
 oxidation states .
disulfides form from thiols by oxidation , whereas 
 formation of thio - michael adducts does not result in oxidation of 
 thiols .
therefore , the oxidation state of the library controls the 
 disulfide / thio - michael ratio . in a fully reduced library
there can 
 be only thio - michael adducts , while oxidation increases the amount 
 of disulfides at the expense of the thio - michael adducts until the 
 library is fully oxidized , and the thio - michael adducts are replaced 
 by disulfides .
such antiparallelism of these two chemistries is possible 
 only because the thio - michael reaction has different number of thiols 
 on both sides of the equilibrium , allowing for depletion of its reaction 
 pool by thiol removal .
such operation would not be possible with e.g. , 
 thioester exchange , where both sides of the equilibrium contain the 
 same number of each species .
for the thiol 
 building block we chose dithiol a , already 
 known to form a series of macrocycles upon oxidation , while instead of a classical michael acceptor we decided 
 to use bc , previously reported by joshi and anslyn ( figure 2a ) . the latter , being a michael acceptor with one thiol group 
 already present ( and unable to dissociate into thiol and alkyne ) , 
 is in fact bivalent , which means that in combination with a it can give rise to a mixture of linear and macrocyclic compounds , 
 thus being a promising starting material for making diverse dcls . 

upon mixing the starting materials in the absence of oxidants we expected 
 a mixture of linear and macrocyclic thio - michael mono- and bis - adducts 
 would form . mixing fully oxidized a with bc , on the other hand , should not lead to any changes , as disulfides 
 do not form adducts with michael acceptors , while at intermediate 
 oxidation levels the system should contain the thio - michael adducts , 
 disulfides and possibly a number of species containing both types 
 of bonds .
( note that in fully oxidized libraries , building block c has to be present as a single michael adduct as it can not 
 undergo a -elimination .
thus , such dcls will contain 2/3 of 
 their a , b content in the form of disulfides 
 and 1/3 as single michael adducts .
note that for michael acceptors 
 that can undergo complete -elimination , fully oxidized dcls 
 will not contain any michael adducts or free thiols . )
( a ) building blocks 
 ( above ) and characteristic representatives of thio - michael adducts 
 ( left ) , disulfides ( right ) , and intermediate species ( middle ) ; ( b ) 
 chromatograms of the antiparallel dcls at different oxidation levels : 
 fully reduced ( bottom ) , 50% oxidized ( middle ) , and fully oxidized 
 ( top ) ; ( c ) heat map plot showing the abundances of the library constituents 
 depending on the oxidation level ( shade represents the peak area normalized 
 to the maximum amount the particular species reach ; the numbers next 
 to the species show the red / ox ratio of the sulfur atoms in their 
 structures ) . to test our hypotheses , 
 we performed two series of experiments , 
 the first to see the outcome of mixing of bc with a at various levels of oxidation , and the second to see whether 
 the system can be reversibly reduced and oxidized . in the first series 
 we investigated libraries initially containing equimolar amounts of a and bc ( both 2.5 mm ) , at oxidation levels ranging 
 from
we prepared 
 these libraries by mixing 5.0 mm solutions of a and fully 
 oxidized a ( an ) to obtain the 
 desired redox level , followed by the addition of an equal amount of 
 5.0 mm bc ( all components were dissolved in an aqueous 
 borate buffer , ph = 8.2 ) . after mixing ,
until equilibrated ( kinetic experiments 
 showed no changes after 24 h , except for the fully oxidized library ) , 
 and subsequently analyzed by uplc , while the library members were 
 identified by uplc - ms .
control experiments revealed that the uv response 
 is a linear function of the concentrations of the various library 
 members ( see supporting information ( si ) 
 section 3 ) .
the results show that upon mixing a and bc a diverse library is formed rapidly , containing 
 20 different detectable 
 species .
the expected linear or macrocyclic thio - michael adducts accounted 
 for a large part of the library ( the dominant species are b and ac , as visible in figure 2b , bottom ) . in the fully oxidized library 
 ( figure 2b ,
top ) , also 
 expectedly , the disulfides stemming from a and the initial 
 michael acceptor bc dominate , while the presence of exchange 
 products ( b2 , cac , and bab ) can be explained by traces of thiols remaining after 
 oxidation of a. due to low exchange rates , the data for 
 the fully oxidized library may differ from what would be present at 
 equilibrium , but , as later analysis will show , the difference is small 
 in the worst case and the general trends hold at all oxidation levels . 
 solutions at the intermediate oxidation levels ( e.g. , 50% , as shown 
 in figure 2b , middle ) , 
 together with the thio - michael adducts and the disulfides , also contain 
 a number of species which contain both kinds of covalent bonds , altogether 
 forming libraries of over 30 different compounds . plotting the 
 normalized peak areas of the library constituents 
 against the oxidation level ( figure 2c ) shows
the thiols and the michael adducts generally 
 reach their maximum concentrations when the system is fully reduced , 
 and gradually diminish as the oxidation level increases . disulfides 
 and michael acceptors ( cac and bc ) follow 
 exactly the opposite pattern , and reach their maximum concentrations 
 at high oxidation levels . the intermediate species , which contain 
 both thio - michael and disulfide linkage , are nearly absent at the 
 two extremes . altogether
, these observations confirm the initial hypothesis 
 that , in a system comprising thiols and michael acceptors , the distribution 
 of the bond types and therefore the composition of the library depends 
 on the oxidation level . in the second series of experiments 
 we tested the redox reversibility 
 of the system ; i.e. , whether the library composition can be controlled 
 by an external input , in this case reducing or oxidizing agents .
for 
 that purpose we prepared fully oxidized and fully reduced libraries 
 of a and bc , in a similar way as described 
 for the first series .
after 48 h equilibration , which led to the same 
 compositions as described previously , samples of the fully reduced 
 library were oxidized by nabo3 or i2 , and samples 
 of the fully oxidized library were reduced by tcep or dtt . for each 
 reagent , 0.3 , 0.5 , 0.7 , or 1.0 equiv
the libraries were again left to equilibrate for 48 h and 
 then analyzed by uplc .
the results ( figure 3 ) show that the system is indeed redox reversible 
 and also that the reactions proceed without any side products , as 
 no new peaks appeared in the chromatograms .
therefore , the library 
 can be switched to any intermediate oxidation level and the corresponding 
 composition by simply adding redox agents , allowing for easy external 
 control .
only the fully oxidized library does not equilibrate readily 
 because the disulfide exchange is catalyzed by thiolate anions , which 
 are absent under these conditions .
( a ) comparison of 50% oxidized libraries obtained in five different 
 ways ( from top to bottom ) : addition of reducing agents ( tcep or dtt ) 
 to 100% oxidized dcl ; mixing half - oxidized a with bc ; addition of oxidizing agents ( i3 or nabo3 ) to a 0% oxidized dcl .
( b ) heat map showing 
 the distribution of the library constituents in the 50% oxidized libraries .
our attempts to rationalize the 
 behavior of the system revealed 
 an interesting phenomenon : as shown above , an macrocycles and bc dominate at 100% 
 oxidation , whereas b and ac adducts are 
 main species at low oxidation levels ( figure 4 ) .
however , we can imagine the opposite scenario , 
 where b2 and cac would be the 
 main species in a fully oxidized dcl , with a and bcb dominating the unoxidized library .
their effect becomes clear 
 as we analyze the equilibria for oxidized ( eq 1 ) and reduced ( eq 2 ) dcls , connecting the two alternative scenarios:12 three - dimensional plot showing abundances of representative 
 dcl 
 member families coupled by antagonistic relations , as a function of 
 the oxidation level of the library .
the diameter of the circles represents 
 the summed peak area of library members connected by a vertical edge .
as we can see , the left sides 
 of both equilibra have 2n molecules , whereas there 
 are 2n + 1 molecules on 
 the right sides .
thus , it is entropically preferred to shift the equilibrium 
 to the right side , i.e. , in favor of , respectively , an and bc , and b and ( ac)n .
interestingly , 
 the composition of the system does not follow monotonically 
 from one oxidation extreme to the other .
for example , b2 and cac reach their maximum concentrations 
 at partial reduction while they are both fully oxidized species ( figure 2c ) . to better understand 
 this counterintuitive behavior , we found it informative to plot different 
 library member families onto a single three - dimensional graph , represented 
 as a cube ( figure 4 ) .
one axis of the plot corresponds to a : b ratio within a library member , another to the c content . 

the third axis corresponds to the oxidation level : as it increases , 
 the thiol : disulfide ratio decreases and double thio - michael adducts 
 become single adducts .
species sharing an edge of the cube are antagonists , as they compete for 
 the same building blocks or oxidation state .
the 
 latter results from the wiring of the network , which makes the antagonistic 
 effects distinct from previous reports based solely on the competition 
 between library members for common building blocks involving only 
 one type of chemistry .
library members can be mapped onto the 
 cube as a function of their 
 composition and oxidation state of their sulfur atoms . plotting the 
 sum of all library members corresponding to different oxidation levels 
 for the four composition extremes as a function of library oxidation 
 reveals that the equilibria 1 and 2 dominate only at the extreme 
 oxidation levels .
in fact , the diagonals connecting the agonistic 
 species at the favored sides of these equilibria are perpendicular 
 to each other .
therefore , compounds like b2 that , at full oxidation , suffer from antagonism by entropically 
 favored compounds ( equilibrium 1 ) start to benefit from agonism by compounds that become entropically 
 favored at lower oxidation levels . hence , despite being fully oxidized 
 themselves they benefit from partial reduction of the mixture .
thus , 
 the concentration of library members is a complex function of the 
 structure of the building blocks , the wiring of the molecular network , 
 and the experimental conditions .
while the design of the thio - michael 
 system shown in figure 2 is somewhat unconventional , 
 we also performed similar experiments on a classical michael acceptor d ( ( e)-4-phenylbut-3-en-2-one ) , while retaining 
 the dithiol a. the results ( si , pages s39s50 ) show that the concept also applies to more 
 traditional thio - michael additions , in which only a single thiol adds 
 to the michael acceptor .
to conclude , we have 
 designed a system in which two chemistries , 
 namely thiol  disulfide exchange and thio - michael exchange , 
 operate simultaneously , giving rise to diverse dcls . as both chemistries 
 use the same building blocks , as one exchange pool grows ,
furthermore , as the two pools require sulfur atoms at different oxidation 
 level , external control of their ratio is possible using reducing 
 and oxidizing agents .
we envisage that dynamic covalent antiparallelism 
 should be applicable to other pairs of dynamic covalent chemistries , 
 e.g. , disulfide / thiazolidine , or the 
 recently developed dithioacetal / disulfide system .
especially exciting should be a combination of antiparallel , orthogonal , 
 and communicating chemistries , allowing for complex and addressable 
 feedback between different subsystems .
the particular system studied 
 has also shown how the antiparallelism of the two chemistries , combined 
 with opposing entropic effects , gives rise to a complex network of 
 interactions , resulting in nonlinear changes in the library composition 
 in response to the external stimulus . externally addressable complexity 
 achieved in such way
should prove useful in functional screening of 
 dcls , where the library can be biased toward a desired connectivity 
 type , rather than just building block composition .
from the 
 systems chemistry perspective , we are excited to see how 
 antiparallelism creates molecular systems that can adapt to environmental 
 changes by switching to the type of chemistry better fitted for the 
 new conditions .
this emergent behavior to some extent resembles homeostatic 
 processes in living organisms , or switching between aerobic and anaerobic 
 metabolisms .
water was doubly distilled prior 
 to use . 4-mercaptobenzoic acid ( technical grade , 90% ) and
3-butyn-2-one 
 ( 96% ) used for the synthesis of bc were purchased from 
 sigma - aldrich and acros organics , respectively , and used without further 
 purification .
boric acid and potassium hydroxide utilized for the 
 preparation of buffers and ph adjustment were obtained from acros 
 organics and merck chemicals , respectively .
sodium perborate , potassium 
 iodide , dithiothreitol ( dtt ) , and tris(2-carboxyethyl)phosphine ( tcep ) 
 used for the reduction / oxidation of a and libraries were 
 purchased from sigma - aldrich .
acetonitrile ( ulc / ms grade ) and water 
 ( ulc / ms grade ) were obtained from biosolve bv .
the 50 mm borate buffer 
 was prepared from boric acid dissolved in doubly distilled water , 
 and adjusted with 1.0 m koh to ph 8.2 .
afterward , it was degassed 
 by nitrogen purging under reduced pressure for 60 min .
libraries were 
 prepared in clear hplc glass vials ( 12  32 mm ) closed with teflon - lined 
 snap caps purchased from jaytee .
library solutions were stirred using 
 teflon - coated microstirrer bars on a magnetic stirrer at 1100 rpm . 

we prepared 
 a 5.0 mm stock solution of bc , a 10 mm stock solution 
 of a , and a 10 mm stock solution of nabo3 . 

equal volumes of a and nabo3 solutions were 
 mixed to obtain 5.0 mm oxidized a , and left stirring 
 for 3 h before further use .
simultaneously , a was diluted 
 twice with buffer solution to obtain 5.0 mm unoxidized a. libraries were prepared by mixing adequate volumes of bc , reduced a , and oxidized a ( as 
 listed 
 in table s1 ) .
for uplc and uplc - ms analyses , 3 l 
 samples were drawn from 
 solutions and diluted with 6 l of dmso prior to injection .
a 6.11 mm stock solution of bc was prepared ; 5.0 mm stock solutions of reduced and oxidized a were used from previous experiments .
solutions were prepared 
 by mixing 550 l of either reduced or oxidized a and 450 l of bc to give equimolar mixtures with 
 a final concentration of 2.75 mm ( of each building block ) .
solutions of redox agents , 
 dithiothreitol ( dtt ) , tris(2-carboxyethyl)phosphine ( tcep ) , sodium 
 perborate ( nabo3 ) , and iodine in potassium iodide ( ki + 
 i2 ) ( 25 mm each ) , were prepared right before use .
they 
 were mixed with the above solutions ( reduced or oxidized ) of a and bc in proper ratios ( as listed in tables s5 and s6 ) to obtain dcls with oxidation 
 level set as 0% , 30% , 50% , 70% , or 100% , and left for 2 days to equilibrate . 

 the ability to design reaction networks 
 with high , but addressable 
 complexity is a necessary prerequisite to make advanced functional 
 chemical systems . 
 dynamic combinatorial chemistry has proven to be 
 a useful tool in achieving complexity , however with some limitations 
 in controlling it . 
 herein we introduce the concept of antiparallel 
 chemistries , in which the same functional group can be channeled into 
 one of two reversible chemistries depending on a controllable parameter . 
 
 such systems allow both for achieving complexity , by combinatorial 
 chemistry , and addressing it , by switching from one chemistry to another 
 by controlling an external parameter . in our design 
 the two antiparallel 
 chemistries are thiol  disulfide exchange and thio - michael addition , 
 sharing the thiol as the common building block . by means of oxidation 
 and reduction 
 the system can be reversibly switched from predominantly 
 thio - michael chemistry to predominantly disulfide chemistry , as well 
 as to any intermediate state . 
 both chemistries operate in water , at 
 room temperature , and at mildly basic ph , which makes them a suitable 
 platform for further development of systems chemistry . 
Introduction
Results and Discussion
Conclusions
Experimental Section