Simultaneous electrochemical and EPR studies of flavin radical equilibria

The comproportionation reaction of the oxidized and reduced forms of both riboflavin and lumiflavin to form their respective radicals is studied over a range of pHs in aqueous solution and in the aprotic solvent methylene chloride. The comproportionation equilibrium constant is reported for basic solutions reduced at an amalgamated Au electrode, where adsorption does not interfere with equilibrium measurements, with both Na+ and Ca2+ as the counterion. The basic and nonaqueous solution voltammetries of riboflavin and lumiflavin are shown to be similar by measuring the radical concentration by electron paramagnetic resonance (EPR) during an in situ electrolysis as a function of potential. In both solvent systems the first of the two reduction waves corresponds to a two‐electron reduction with the radical present formed by comproportionation.