TIME-RESOLVED SERS STUDY OF DIRECT PHOTOCHEMICAL CHARGE-TRANSFER BETWEEN FMN AND A AG ELECTRODE

The direct photoinduced charge transfer from adsorbed flavin mononucle otide (FMN) to a Ag electrode was observed by time-resolved surface-en hanced Raman scattering (TRSERS), electrochemistry, and photogalvanic effect studies. Two short-lived photoproduct radical ion intermediates were observed by TRSERS of FMN and confirmed by normal mode calculati ons as enol-keto forms of a photo-oxidized flavin monocation radical i n aqueous and deuterated solution. The enol configuration of the FMN c ation forms by electron injection to the metal and intramolecular prot on transfer from N3-H to the C2=0 keto group on ring III at times fast er than 75 ns upon 350 or 337 nm excitation. By 775 ns the proton tran sfers from the C2-OH enol group back to the N3 atom of ring III of the FMN cation. Photo-oxidation to the keto form of the radical cation oc curs on excitation with Light from 450 to 610 nm. The radical cation d ecays back to FMN by 1.5 mu s. Surface concentrations of adsorbed FMN on the Ag electrode were estimated by linear sweep voltammetry and wer e found to be less than a monolayer. New mechanisms of photoinduced ch arge transfer between FMN and a Ag electrode and the photogalvanic eff ect on a dye-modified metal electrode are proposed as a result of the experiments.