Reactivity of Semiquinones with Ascorbate and the Ascorbate Radical as Studied by Pulse Radiolysis

• Published in: Archives of biochemistry and biophysics 2000-12, Vol.384 (1), p.74-80
• Main Authors: Roginsky, Vitaly, Michel, Christa, Bors, Wolf
• Format: Article
• Language: English
• Subjects: ascorbate; ascorbate radical; Ascorbic Acid - chemistry; Benzoquinones - chemistry; hydroquinones; Hydroquinones - chemistry; kinetic spectroscopy; Kinetics; Oxidation-Reduction; pulse radiolysis; Pulse Radiolysis - methods; rate constants; redox potentials; semiquinones
• ISSN: 0003-9861; 1096-0384
• DOI: 10.1006/abbi.2000.2050

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, Q•−, 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 methoxy-substituted 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 × 108 to 2.5 × 108 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) × 105 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).