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

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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−1 s−1 and for the reaction of O˙− it was 7.1 × 108 dm3 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 λmax 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).


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