KINETICS OF THE COMPETITIVE DEGRADATION OF DEOXYRIBOSE AND OTHER MOLECULES BY HYDROXYL RADICALS PRODUCED BY THE FENTON REACTION IN THE PRESENCE OF ASCORBIC-ACID

The competition method in which the Fenton reaction is employed as an . OH radical generator and deoxyribose as a detecting molecule, has be en used to determine the rate constants for reactions of the . OH radi cal with its scavengers. Nonlinear competition plots were obtained for those scavengers which reacted with the Fenton reagents (Fe2+ or H2O2 ). Ascorbic acid is believed to overcome this problem. We have investi gated the kinetics of deoxyribose degradation by . OH radicals generat ed by the Fenton reaction in the presence of ascorbic acid, and observ ed that the inclusion of ascorbic acid in the Fenton system greatly in creased the rate of . OH radical generation. As a result, the interact ion between some scavengers and the Fenton reagents became negligeable and linear competition plots of A(0)/A vs scavenger concentrations we re obtained. The effects of experimental conditions such as, the conce ntrations of ascorbic acid, deoxyribose, H2O2 and Fe2+-EDTA, the EDTA/ Fe2+ ratio as well as the incubation time, on the deoxyribose degradat ion and the determination of the rate constant for mercaptoethanol cho sen as a reference compound were studied. The small standard error, (6 .76 +/- 0.21) x 10(9) M(-1)s(-1), observed for the rate constant value s for mercaptoethanol determined under 13 different experimental condi tions, indicates the latter did not influence the rate constant determ ination. This is in fact assured by introducing a term, k(x), into the kinetic equation. This term represents the rate of . OH reactions wit h other reagents such as ascorbic acid, Fe2+-EDTA, H2O2 etc. The agree ment of the rate constants obtained in this work with that determined by pulse radiolysis techniques for cysteine, thiourea and many other s cavengers, suggests that this simple competition method is applicable to a wide range of compounds, including those which react with the Fen ton reagents and those whose solubility in water is low.