The role of adenine in fast excited-state deactivation of FAD: a femtosecond mid-IR transient absorption study.

We present a study of excited-state dynamics of two flavin cofactors: flavin-adenine dinucleotide (FAD) and flavin-mononucleotide (FMN). We used femtosecond mid-R transient absorption spectroscopy to study the effect of FAD conformation on its excited-state behavior. The conformation of FAD was modulated by changing the solvent polarity: in D(2)O, FAD is present predominantly in the "stacked" conformation, in which flavin and adenine moieties are in close proximity to each other, whereas the increased amount of DMSO led to an increased amount of the "open" conformer. FMN served as a model system which lacks adenine. We found that the "stacked" conformer undergoes an intramolecular photoinduced electron transfer from adenine to flavin with the forward electron transfer rate of k(f) = 1.9.10(11) s(-1) and the geminate recombination rate of k(b) = 1.1.10(11) s(-1). In the case of the "open" conformer, no intramolecular electron transfer was observed.