Oxidation of pyrimidine and purine base components of nucleic acids by hexavalent manganese in aqueous alkaline medium: A kinetic and mechanistic study

Abstract

Oxidation of pyrimidine bases viz, uracil, thymine and cytosine and purine bases viz., adenine and guanine by MnO<img src='http://www.niscair.res.in/jinfo/four1.gif' border=0> in aqueous alkali shows first order dependence on [MnO<img src='http://www.niscair.res.in/jinfo/four1.gif' border=0>] and fractional order dependence on [nucleic acid base]. The effect of ionic strength is negligible on the rate of oxidation. 5,6- Dihydroxylated compounds have been found to be the products of oxidation in the case of pyrimidines while purines yield the 5-formamidopyriInidines. A mechanism is proposed involving formation of a 5-membered cyclic complex between the substrate and MnO<img src='http://www.niscair.res.in/jinfo/four1.gif' border=0><i>- </i> in a fast step which subsequently disproportionates to give the final products. The formation constants of the intermediate complex and the rates of its disproportionation have been calculated from the derived rate law. The rate of oxidation of pyrimidines is in the order: thymine > uracil > cytosine while in the case of purines the order is guanine > adenine.