Kinetics of the Competitive Degradation of Deoxyribose and other Biomolecules by Hydroxyl Radicals Produced by the Fenton Reaction

Abstract

The objective of this work is to reexamine the competitive degradation of deoxyribose by hydroxyl radicals (OH) produced by the reaction between H₂O₂ and Fe²⁺-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], [Fe^{2 +} -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 + /t_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 by determining the variations of V_D/V_D^S as a function of [S] and [D]. By taking k_D=1.9 × 10⁹ M^−ls⁻¹, a value of k_S=1.9 × 10⁹ M⁻¹s⁻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 × 10 M⁻¹s⁻¹ and alanine, k = 3.3 × 10⁸ M^−I 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.

Key Words:

• Hydroxyl radical
• Fenton reaction
• Deoxyribose
• Mannitol
• Kinetics.