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 (FMN ox ) 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 ∼4 ps while for FMN ox 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-state 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.