Distinction of 2e- and le- Reduction Modes of the Flavin Chromophore as Studied by Flash Photolysis

Evidence is given for the fact that the excited flavin triplet (3Flox) exhibits competitive 1e- and 2e- transfer chemistry, depending on the nature of the photosubstrate. As an 'external' photo-reductant, the 2e- donor borohydride has been investigated. Borohydride is found to compete effectively with the 'internal' 1e- donors, namely excess starting flavin in the ground state (Flox) and, as primary product, (alkyl)dihydroflavin (RFlredH). It will be shown that some of the flavin radicals, observed by earlier authors in the reaction of 3Flox with CH or C-COO- substrates or in the autophotolytic side-chain cleavage of riboflavin, are due to the dye-dye reaction (I): 3Flox + Flox H+ leads to HFl + Fl+ -H+ leads to 2Flox. (I) In contrast flavin photoreduction by borohydride or hydrocarbon substrates need not involve radicals, but may in fact be a hydride or 'carbanion-plus-proton' addition towards the highly polar and considerably basic (pK = 4.4) acceptor triplet (cf. reaction II) (see text: Formula). The products are much more photoreactive than the starting substrates, which leads to the secondary photocomproportionation (III): 3Flox + R-FlredH leads to HFl + RFl, (R = alkyl or H). (III) This latter reaction is the second source of radicals in the system. This (cf. II) 'photohydrogenation' of flavin is mechanistically related to the biological reduction of flavin by CH substrates.