Matrix-isolation of H·-induced Free Radicals from Purines in Acidic Glasses

Summary

H·(D·)-atoms produced by photo-oxidation (λ = 254 nm) of Fe²⁺ in acidic glasses (6 mol dm⁻³ H₂SO₄/H₂O or D₂SO₄/D₂O) at 77 K were allowed to react with the purine bases adenine, guanine, hypoxanthine and xanthine as well as with ribo- and deoxyribosides (-tides) of adenine and guanine by annealing to 110–130 K. The ensuing free radicals were studied by electron spin resonance (ESR) spectroscopy at 77 K. Individual radical species were assigned by simulation of patterns isolated from thermal annealing studies up to 180 K. It is shown that H·-atom reaction with the bases produces C2- and C8-addition radicals for adenine and C8-addition species for guanine. Guanine is also photo-oxidized directly in the glass, producing a cation or its deprotonated successor species. In the nucleosides (-tides) of both bases, H· atoms were found to abstract hydrogen from carbon sites C1′, C2′ or C3′, and C5′ for ribosides and C1′, C2′, and C5′ for deoxyribosides (-tides), respectively.