Ultrafast excited state dynamics of oxidized flavins: Direct observations of quenching by purines

We have measured the excited-state dynamics and spectral evolution of oxidized flavin-adenine dinucleotide (FAD(ox)) and flavin mononucleotide (FMNox)in simple solvents with subpicosecond time resolution by one-color and white-light continuum transient absorption spectroscopy. In water, the FAD(ox) transient shows significant quenching of the excited state with a lifetime of similar to 4 ps while for FMNox this component is of much lower amplitude. However, when caffeine is added to the FMN solution the amplitude of the fast component is recovered. Additionally, the excited-state dynamics of FAD in neat formamide, a solvent that breaks up stacking interactions in FAD, is similar to that obtained for FMN in water. These measurements provide conclusive evidence that the excited-stale quenching observed in FAD or flavin-purine complexes occurs in less than 5 ps. The observed spectral evolution shows that internal conversion from S-2 to S-1 occurs in less than 100 fs.