Flash photolysis of flavins. V. Oxidation and disproportionation of flavin radicals.

Abstract

Flash photolysis techniques have been utilized to investigate the reactivity patterns of flavin radical species. Rate constants for disproportionation were found to be la the following order: lumiflavin>FMN>FAD and neutral radicals>anionic radicals. Neutral flavin radicals react with oxygen at a rate which is at least 104 times slower than the anionic species. No evidence for an intermediate complex or adduct is obtained in this reaction. The pK values for the ionization of the neutral flavin radicals are in the order FAD>FMN>riboflavin=limiflavin. The rates of reaction of ferricyanide with flavin radicals are essentially independent of pH, whereas benzoquinone reacts slightly more slowly (≅5 times) with the neutral flavin radical than with the anionic form. Cytochromec reacts at least ten times more slowly with flavin radicals than does ferricyanide.