Thiyl radicals abstract hydrogen atoms from carbohydrates: reactivity and selectivity

Free radical damage of DNA is a well-known process affecting biological tissue under conditions of oxidative stress. Though carbohydrate-derived radicals are generally "repaired" by hydrogen transfer from thiols, the reverse possibility, namely hydrogen abstraction by thiyl radicals from carbohydrates, exists. The biological relevance of this process has been discussed controversially, especially because of the lack of rate constants. Therefore, we have measured rate constants for the hydrogen transfer reaction between thiyl radicals from cysteine and selected carbohydrates, 2-deoxy-D-ribose (dRib), 2-deoxy-D-glucose (dGls), alpha-D-glucose (Gls), and inositol (Ino). Rate constants are on the order of 10(4) M(-1)s(-1), with the highest average value for dRib, (2.7 +/- 1.0) x 10(4) M(-1)s(-1), and the lowest average value for dGls, (1.6 +/- 0.2) x 10(4) M(-1)s(-1), based on two ways of kinetic analysis, standard competition kinetics and stochastic simulation of the experimental results, respectively. In general, thiyl radicals attack preferentially the C(1)-H bond of the carbohydrates, to an extent of ca. 72% in dRib and 90% in dGls. Kinetic measurements were possible through a specifically designed competition system measuring the reaction of thiyl radicals with either the C-H bonds of the carbohydrates or the C(alpha)-H bond of cysteine under conditions where the extent of other competitive reactions of the thiyl radicals were minimized.