Double complementarity principle

The reactions of polyhydric alcohols with the hydroxyl radical in aqueous solution have been extensively studied (e.g. in radiolytic and biomimetic systems), mainly because of their suitability as models for more complicated carbohydrate substrates [55] or enzymatic systems involving glycol-type radicals [56, 57]. Because there are no double bonds to which OH could add, only H-abstraction reactions are possible. Because the C-H bond energy is significantly lower than the 0-H bond energy, it is the carbons from which H are abstracted and not the alcohol function. In this type of reaction, a,yS-dihydroxyalkyl radicals are formed. The same radicals could, in principle, be produced by addition of "OH to enols, see Scheme 2, lower part. This shows the complementarity of H-abstraction and OH-addition and thereby the relevance of the former to one-electron oxidation of olefinic bonds (Scheme 2). 

The double-stranded DNA consists of two parallel chains folded into an a-helix. The chains are joined by hydrogen bonds that follow the rules of complementarity. In accordance with these rules, adenine is always bound with thymine of the other chain and guanine is always bound to cytosine (see the following scheme). The principles of molecular complementarity do not apply only to nucleic acids. Rather, they represent general rules for the formation of various supramolecular structures. If nucleotides are labeled by their first letters, the complementary pairs would be AT and GC, respectively. The structural key for complementarity is the number of hydrogen bonds. There are two such bonds in the AT pair and three in... 

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