One-electron photoreduction of flavin mononucleotide: Time-resolved resonance Raman and absorption study

Resonance Raman and absorption spectra of the lowest excited triplet state T-1, and semiquinone neutral and anion radicals of flavin mononucleotide (FMN) have been obtained by time-resolved Raman spectroscopy and laser flash photolysis. Both spectra have revealed that photoreduction of FMN is initiated by the process in which the T-1 state of FMN abstracts an electron from substrates such as EDTA to generate the semiquinone anion radical FMN(-.) both in acidic and basic solutions over the pH range 5-9, and when the pH is smaller than 8.4, FMN(-.) is further converted into the semiquinone neutral radical FMNH(.) by combining with H+. The electron transformation is much faster in basic solutions than in acidic solutions. In more strongly acidic solutions pH <4.4, protonation of the T-1 state (3)FMN* occurs in less than 80 ns and the protonated T-1 state (3)[FMNH(+)]* abstracts an electron from the substrate to generate FMNH(.). No indication of the abstraction of a hydrogen atom by (3)FMN* to generate FMNH(.) was obtained.