Kinetics of Electron Returns in Successive Two-Photon DNA Repair by (6-4) Photolyase

(6-4) photolyase is a flavoenzyme that catalyzes the chemically challenging repair of carcinogenic (6-4) photoproduct lesions in DNA using sunlight. Nature may have overcome the challenge by combining two distinct successive photoreactions, each starting with electron transfer from an excited flavin to the lesion. We herewith report a transient absorption study of electron returns to flavin following the chemical transformations of the lesion. We have successfully dissected the kinetics of the electron returns that finalize the first and second photoreactions (similar to 40 mu s and similar to 200 ns, respectively) in the (6-4) photolyase from Xenopus laevis, corroborating and detailing the two-photon reaction model.