Pattern of hydroxyl radical reaction with 6- and 9-substituted purines: effect of substituents on the rates and activation parameters of the unimolecular transformation reactions of two isomeric hydroxyl adducts

Abstract

via a polar transition state (p' = -0.9) to give isomeric radicals by attachment to C-4 and C-8 and, possibly, to other positions. The adduct to C-4 (A-4-OH) undergoes a unimolecular dehydration reaction to give a radical with oxidizing properties. Substituents at C-6 exert a strong influence on the rate of this dehydration (p' = -3). The adduct to C-8 (A-8-OH) also undergoes a unimolecular transformation reaction which is assigned to opening of the imidazole ring. Substituents at C-6 have only a small influence on the rate of the ring-opening reaction (p' = -0.3). A ribose substituent at N(9) decreases the rates of both dehydration and ring opening. The transformation reactions of A-4-OH and A-8-OH can be distinguished not only by their different response to substituents at C-6 but also by their activation parameters: E A and AS* values for dehydration of A-4-OH are typically 2 9 kcal mol-' and -6 eu, respectively, whereas they are =6 kcal mol-' and -19 eu for ring opening of A-8-OH. and conductance detection. The electrophilic OH reacts with the purines by addition (k = 1.3 X lo8 to 8.4 X lo9 M-l S-I )