The
riboflavin (RF,
Vitamin B2) and
flavin adenine dinucleotide (
FAD)-sensitized photooxidation of
tryptophan (TrpH) and
tyrosine (TyrOH) were studied by
laser flash photolysis. TrpH and TyrOH quench triplet
flavin sensitizers to produce reduced
flavin radicals (FlH*) and oxidized radicals of TrpH or TyrOH (Trp* and TyrO*. Although Trp* and TyrO* cannot be observed directly by the
laser flash photolysis, N,N,N',N'-
tetramethyl-p-phenylenediamine (TMPD), as a probe, was added to the system to result in the formation of radical
cations of TMPD (TMPD*+) via quenching of Trp* and TyrO*, which provides more definitive proof of electron transfer in the
photosensitization process than only direct observation of reduced
flavin radicals. Electron transfer from TrpH and TyrOH to oxidized radicals of
riboflavin and
FAD with similar rate constants to the triplet
flavins was observed for the first time, which may be a new way of TrpH and TyrOH damage. These results may shed new light on future application of
flavins in
photodynamic therapy, and imply that
flavins might be applied potentially to
photosensitization of
oxygen deficiency or under high-intensity
pulsed laser irradiation.