Reactivity of semiquinones with ascorbate and the ascorbate radical as studied by pulse radiolysis.
Free-radical interactions between hydroquinones (QH2) and ascorbate (AscH-) have a profound impact in many biological situations. Despite the obvious biological significance, not much is known about the kinetics of reactions of QH2 and AscH- with their corresponding free radicals, i.e., semiquinones, Q1.-, and the ascorbate radical, Asc.-. Furthermore, a general approach to reliably measure rate constants for the above reactions is fraught with complications. In this work, the kinetic behavior of Q.- and Asc.-, after pulse radiolytic oxidation of mixtures of a series of alkyl- and methoxysubstituted hydroquinones and ascorbate by azide radicals in aqueous buffer, pH 7.40, was monitored in submillisecond range by time-resolved UV spectroscopy. Rate constants for reactions of Q.- with AscH-(reaction [1]) and Asc.- (reaction [2]) were directly determined by using new kinetic procedures which distinguished between reactions [1] and [2]. The results show that the rate constants for reaction [2] vary only within a narrow range from 1.2 x 10(8) to 2.5 x 10(8) M(-1) s(-1) and do not display any pronounced correlation with Q.- structures. In contrast, the value of k1 for nonsubstituted Q.- was found to be (1.8 +/- 0.2) x 10(5) M(-1) s(-1) and decreases with the number of alkyl and methoxy substituents as well as with the decrease of the one-electron reduction potential E(Q.-/QH2).
Roginsky
V
V
N.N. Semenov Institute of Chemical Physics, Russian Academy of Sciences, Moscow.
Michel
C
C
Bors
W
W
D001205:Ascorbic Acid / Q000737:chemistry*; D016227:Benzoquinones / Q000737:chemistry*; D006873:Hydroquinones / Q000737:chemistry*; D007700:Kinetics; D010084:Oxidation-Reduction; D011675:Pulse Radiolysis / Q000379:methods
D016227:Benzoquinones; D006873:Hydroquinones; C025232:semiquinone radicals; D001205:Ascorbic Acid