Reactions of hydroxyl radical (˙OH) and oxide radical anion (O˙⁻) with 2-aminopurine in aqueous medium

Abstract

Pulse radiolysis has been used to investigate the reaction of hydroxyl radical (·OH) and oxide radical anion (O^·-) with 2-aminopurine (2AP), a fluorescent analogue of adenine, in aqueous medium. The second-order rate constant for the reaction of ·OH with 2AP was determined to be 3 × 10⁹ dm³ mol^-1s^-1 and for the reaction of O·- it was 7.1 × 10⁸ dm³ mol^-1s^-1. The transient absorption spectrum obtained in the reaction of ·OH at pH 7 has absorption maxima at 370 and 470 nm. The spectrum undergoes a time-dependent transformation at higher time-scale. The intermediate species was found to react with N,N,N',N'-tetramethyl-p-phenylenediamine (TMPD). The yield of TMPD^·+ was calculated in terms of G(TMPD·+) to be 3.3 × 10^-7 mol J^-1 at pH 7. The ·OH reactions were also carried out at pH 10 and the transient absorption spectra have λ_max at 400 and 480 nm. The transient spectra obtained in the reaction of O·- at pH ≈14 have maxima at 400 and 480 nm. The transient intermediate species at pH 7 are assigned to the formation of 2AP-4-OH· (54%), 2AP-5-OH· (7%) and 2AP-8-OH· (39%) based on the spectral evidence and TMPD·+ build-up. Both 2AP-4-OH· and 2AP-5-OH· undergo OH^- elimination to form a radical cation. At higher pH (pH 10), the dehydration reaction of these OH-adducts leads to a N-centered radical (2AP-N(9)·, 71%). Formation of 2AP-8-OH· (29%) is also proposed at this pH. In the reaction of O·- with 2AP, it is proposed that a similar nitrogen centered 2AP-N(9)· radical is formed by an electron-transfer reaction at N(9).