Abstract
Quantum mechanics/molecular mechanics calculations are employed to assign previously recorded experimental spectroscopic signatures of the intermediates occurring during the photo-induced repair of (6-4) photolesions by photolyases to specific molecular structures. Based on this close comparison of experiment and theory it is demonstrated that the acting repair mechanism involves proton transfer from the protonated His365 to the N3' nitrogen of the lesion, which proceeds simultaneously with intramolecular OH transfer along an oxetane-like transition state.