Redox chemistry of 8-azaadenine: a pulse radiolysis study

Abstract

Aza derivatives of purines and pyrimidines are important class of compounds, which are known for their cytotoxic, antimicrobial, and mutagenic activities. The redox chemistry of 8-azaadenine (8AA) has been investigated using pulse radiolysis technique. The oxidation reactions were studied using hydroxyl radical (^•OH), oxide radical anion (O^•- and sulfate radical anion (SO₄^•-), and the reduction reactions were studied using hydrated electron (e_aq^-) and hydrogen radical (H·). In the reaction of ^•OH, a bimolecular rate constant of 3.8× 10⁸dm³mol^-1s^-1 was determined at pH 6.0. The transient spectrum obtained for the reaction of ·OH at pH 6 has an absorption maximum around 340nm and is assigned to the formation of 8AA-4OH.. The charge population density was calculated theoretically (using Gaussian 98) and it showed that the fourth carbon atom (C(4)) is the most probable site for the attack of ·OH. The oxidizing nature of this radical is demonstrated by its reaction with N,N,N',N'-tetramethyl-p-phenylenediamine (TMPD). The existence of this species is further supported by theoretical calculations where the absorption maximum of this radical is calculated as 338nm. The yield of 8AA-4OH· is estimated as around 85%. At pH 10.2, the transient spectrum with maxima at 300 and 350nm is attributed to the dehydrated 8AA-4OH·, which is an N-centered radical of the type 8AA-N(9)·. In the reaction of O^•- (pH≈14) a transient spectrum with similar features is observed. Therefore this is also assigned to 8AA-N(9)·. A bimolecular rate constant for this reaction is determined as 4.2× 10⁸dm³mol^-1s^-1. In the reaction of SO₄^•- at pH 6, the transient spectrum having λ _max at 320nm is attributed to the formation of a neutral radical of 8AA (8AA-N(6)·), which is formed by the deprotonation of the initially formed radical cation. But at pH 10.2, the spectrum is found to be similar to the one observed in the reaction of O^•- and hence it is assigned to the formation of the nitrogen-centered radical 8AA-N(9)·. In the reaction of e_aq^-, a second-order rate constant of 1.8× 10¹⁰dm³mol^-1s^-1 is determined at pH 6 and the transient absorption spectrum with λ _max at 330nm is assigned to the protonated electron adduct of 8AA (8AA(NH·)). The reducing nature of this intermediate is confirmed by the formation of methyl viologen radical cation (MV^•+) from its reaction with MV²⁺. The transient intermediate in the case of the reaction of H. is proposed as 8AA-C2(H)N(3). at pH 1.