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 (pH 6.89). Hyperfine electron spin resonance (ESR) spectra for the flavin semiquinone radicals have been obtained and were stable even after 1 day under ambient circumstances. Further characterization was obtained for these semiquinone radicals by UV-visible spectroscopy. The quantum chemical calculations have shown that the highest spin densities are located at N(5) position, and ESR experiments in D2O established that the exchangeable protons are attached to N(5). The flavin-6-carboxylic acids 1 and 2 also activated H2O2 oxidizing thioanisoles to their sulfoxides. These remarkable reactivities of the flavin derivatives 1 and 2 were ascribed to the intramolecular hydrogen bonding between N(5) and carboxyl group at C(6) position of the flavin nucleus.