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

Abstract

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 3FMN∗ 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 3FMN∗ to generate FMNH · was obtained.