KINETICS OF THE COMPETITIVE DEGRADATION OF DEOXYRIBOSE AND OTHER BIOMOLECULES BY HYDROXYL RADICALS PRODUCED BY THE FENTON REACTION

The objective of this work is to reexamine the competitive degradation of deoxyribose by hydroxyl radicals (.OH) produced by the reaction between H2O2 and Fe+2-EDTA. The .OH radicals produced attack deoxyribose (D, rate constant k(D)) and eventually an .OH scavenger (S, rate constant k(S)) First, we examined the effect of [D], [H2O2], [Fe2+-EDTA], [EDTA]/[Fe2+] ratio and reaction time on the rate of D degradation, measured as the absorbance of the chromogen formed between the product of the reaction D+.OH (malondialdehyde) and thiobarbituric acid. In particular, it was showed that under our experimental conditions ([D] = 3 mM, [H2O2] = 0.85 mM, [Fe2+] = 0.13 mM), the rate of overall process is first order in Fe2+, zero order in H2O2 and is maximal for a ratio [EDTA]/[Fe2+] = 1.1. Second, the kinetics of .OH radical reaction in competition experiments between D and S (mannitol) was investigated. The results show that the ratio of the rates of D degradation in the absence (V-D) and in the presence (V-D(S)) of S should be represented by V-D/V-D(S) = 1 + k(S)[S]/(k(D)[D] + k(x)) where k(x) accounts for the rate of .OH reactions with other reagents such as Fe2+-EDTA, H2O2 etc ... After having determined k(x) for each set of experimental conditions, we obtained the values of k(S)/k(D) be determining the variations of V-D/V-D(S) as a function of [S] and [D]. By taking k(D) = 1.9 x 10(9) M(-1) s(-1) a value of k(S) = 1.9 x 10(9) M(-1) s(-1) was obtained, very close to that obtained by pulse radiolysis. Finally, the validity of the established relation was confirmed for other biomolecules (methionine, k = 5.6 x 10(9) M(-1) s(-1) and alanine, k = 3.3 x 10(8) M(-1) s(-1)). By constrast, it was not applicable to cysteine, thiourea and mercaptoethanol which was attributed to an interaction of the latter scavengers with Fe2+ and/or H2O2.