Charge Redistribution in Oxidized and Semiquinone E. coli DNA Photolyase upon Photoexcitation: Stark Spectroscopy Reveals a Rationale for the Position of Trp382

The electronic structure of the two lowest excited electronic states of FAD and FADH(.) in folate-depleted E coli DNA photolyase (PLOX and PLSQ, respectively) was measured using absorption Stark spectroscopy. The experimental analysis was supported by TDDFT calculations of both the charge redistribution and the difference dipole moments for the transitions of both oxidation states using lumiflavin as a model. The difference dipole moments and polarizabilities for PLOX are similar to those obtained in our previous work for flavins in simple solvents and in an FMN-containing flavoprotein. No such comparison can be made for PLSQ, as we believe this to be the first experimental report of the direction and magnitude of excited-state charge redistribution in any flavosemiquinone. The picture that emerges from these studies is discussed in the context of electron transfer in photolyase, particularly for the semiquinone photoreduction process, which involves nearby tryptophan residues as electron donors. The direction of charge displacement derived from an analysis of the Stark spectra rationalizes the positioning of the critical Trp382 residue relative to the flavin for efficient vectorial electron transfer leading to photoreduction. The ramifications of vectorial charge redistribution are discussed in the context of the wider class of flavoprotein blue light photoreceptors.