Reduction, oxidation, and addition reactions between free radicals and flavins

Abstract

Flavins and reduced flavins were reacted with a variety of free radicals produced in dilute aqueous solution at pH 7 +/- 0.1 by radiation chemical methods. The radical .CH2C(CH3)2OH and the aliphatic beta, gamma, and delta radicals of ethanol, 2-propanol, and 1-butanol added to the radical form of flavin adenine dinucleotide (FAD) (FH.) to yield products that could not be reoxidized to flavin by oxygen. The first radical also added to FAD but with a much lower efficiency. In contrast, the alpha-carbon radicals .CH(OH)CH2OH, CH3(-3) CHOH, and (CH3)2COH appeared to undergo two reactions: FH. + RR'COH leads to FH- + RR'C==O FH. + RR'COH leads to FH- + RR'C==O + H+ The formate radical anion .CO2(-) reacted similarly, producing stoichiometric two-electron reduction of riboflavin and lumiflavin as well as FAD. While eaq- also seemed capable of this, it was found to react irreversibly with FADH2, which makes it a poor reagent for producing the dihydroflavin. The dihydro form of FAD was reoxidized to FAD by the species RS. and .BR2(-). In contrast to FAD, the alloxazine lumichrome underwent only one-electron reduction and oxidation by .CO2(-) and .Br2(-), respectively.