The Photoreduction of Flavins by Amino Acids and Edta. A Continuous and Flash Photolysis Study

Primary and secondary photochemical processes in oxygen‐free aqueous solution have been characterised for FMN alone and in the presence of EDTA and four amino acids using nanosecond and microsecond flash photolysis and continuous photolysis techniques. The relative contributions of oneelectron and two‐electron (group or hydride transfer) reactions to the deactivation of the triplet has been determined by comparing the radical concentration (560 nm) with the bleaching of the ground state (446 nm). It was concluded that one‐electron reactions (hydrogen atom or electron abstraction) are the major mode of reactivity of the flavin triplet state with all the suhstrates studied. The nature of the reactions of the flavin semiquinone radical have been studied quantitatively by microsecond flash photolysis. These secondary reactions consist of either a ‘back reaction’ between the flavin and substrate radicals (tryptophan or glycyl‐tyrosine) or the transfer of a second electron (or hydrogen atom) from the substrate radical to the flavin radical (EDTA, methionine and possibly cysteine) to form reduced flavin and oxidised substrate. From a comparison of the quantum yields of formation of reduced flavin using ‘flash’ and continuous irradiation, an additional pathway for the decay of the flavin radical is suggested to occur at low light intensities in the presence of glycyl‐tyrosine or histidine.