Flavin-6-carboxylic acids as novel and simple flavoenzyme models. Nonenzymatic stabilization of the flavin semiquinone radical and the 4a-hydroperoxyflavin by intramolecular hydrogen bonding
Novel flavin derivatives, 10-ethyl-3-methylisoalloxazine-6-carboxylic acid (1) and 10-ethyl-3-methylisoalloxazine-6,8-dicarboxylic acid (2), which have a carboxyl group at C(6) position, were prepared. Even in the absence of metal cations and under aerobic condition, these flavin derivatives produced the corresponding stable semiquinone radicals by the dithionite reduction in sodium phosphate buffer