Formation and Structure of Radicals from ᴅ-Ribose and 2-Deoxy- ᴅ-ribose by Reactions with SO4·̅ Radicals in Aqueous Solution. An in-situ Electron Spin Resonance Study

Abstract

Abstract ESR spectroscopy has been used to analyse the conformation of the radicals produced by the reaction of SO4·̅ with ᴅ-ribose (1), and 2-deoxy-ᴅ-ribose (6), at pH 1.3-5. From ribose three different types of radicals formed by H abstraction at C-1, C-2 and C-3 followed by a regio-selective α,β-water elimination have been identified: the 2-deoxy-ribonolacton-2-yl (3), the 1-deoxy-pentopyranos-2-ulos-1-yl (4). and the 4-deoxy-pentopyranos-3-ulos-4-yl (2). Using deoxyribose two radicals of similar type, formed by H abstraction at C-3 and C-4 followed by water elimination, have been observed: the 2,4-dideoxy-3-ulos-4-yl (7) and the 2,3-dideoxy-4-ulos-3-yl (8). In addition, from both sugars an a-hydroxyalkyl radical has been identified based in part on the timing of their conformational motions: the ribos-3-yl (5) (the precursor of 2) and the 2-deoxy-ribos-1-yl (9), respectively. For radical 5 the rate constant k(e) for the water elimination and hence transformation into radical 2 was estimated. From the analysis of selective line broadening the frequencies of conformational changes of radicals 2 and 7 have been estimated. For 7 the frequencies of exchange of the two methylene groups were found to differ by more than 3 orders of magnitude.