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

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 × 10 9 dm 3 mol −1 s −1 and for the reaction of O ˙− it was 7.1 × 10 8 dm 3 mol −1 s −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).