One-electron redox reactions of free radicals in solution. Rate of electron transfer processes to quinones

Abstract

A large number of biologically-important organic and inorganic free radicals have been produced in aqueous solutions, using the fast-reaction technique of pulse radiolysis and kinetic absorption spectrophotometry. The reactions of these free radicals with menaquinone (vitamin K 3, E 0 = 0.42 V) were followed by observing the formation kinetics of the semiquinone radical anion of menaquinone, •MK −. The absorption spectrum of •MK − has maxima at 395 nm and 300 nm, with extinction coefficients of 1.1·10 4 and 1.25·10 4 M −1·cm −1, respectively. The p K a of the radical •MK −-H + is 4.6±0.1. The free radicals were produced by a one-electron oxidation or reduction of various compounds by hydroxyl radicals and solvated electrons, e aq −. Alcohols, sugars, carboxylic acids, amino acids, peptides, aliphatic amines and amides, aromatic and heterocyclic molecules, pyridine derivatives (nicotinamide, NAD +), and transition metal ions have been examined. Significant differences have been observed in both the efficiency (expressed in percentage) and the rate constants of the electron transfer reactions from these free radicals to menaquinone. Absolute rates of electron transfer from approx. 5·10 8–5·10 9 M −1·s −1 have been observed for most of the free radicals studied. Information relating to the nature of the radicals and the acid-base properties of these radicals for effective one-electron redox reactions with quinones is indicated.