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 been used to determine the rate constants for reactions of the .OH radical 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 investigated the kinetics of deoxyribose degradation by .OH radicals generated by the Fenton reaction in the presence of ascorbic acid, and observed that the inclusion of ascorbic acid in the Fenton system greatly increased the rate of .OH radical generation. As a result, the interaction between some scavengers and the Fenton reagents became negligeable and linear competition plots of A degree/A vs scavenger concentrations were obtained. The effects of experimental conditions such as, the concentrations of ascorbic acid, deoxyribose, H2O2 and Fe(2+)-EDTA, the EDTA/Fe2+ ratio as well as the incubation time, on the deoxyribose degradation and the determination of the rate constant for mercaptoethanol chosen as a reference compound were studied. The small standard error, (6.76 +/- 0.21) x 10(9) M-1s-1, observed for the rate constant values for mercaptoethanol determined under 13 different experimental conditions, indicates the latter did not influence the rate constant determination. This is in fact assured by introducing a term, kx, into the kinetic equation. This term represents the rate of .OH reactions with other reagents such as ascorbic acid, Fe(2+)-EDTA, H2O2 etc. The agreement of the rate constants obtained in this work with that determined by pulse radiolysis techniques for cysteine, thiourea and many other scavengers, suggests that this simple competition method is applicable to a wide range of compounds, including those which react with the Fenton reagents and those whose solubility in water is low.