DNA (6−4) Photolesion Repair Occurs in the Electronic Ground State of the TT Dinucleotide Dimer Radical Anion

(6−4) Photolyases are enzymes utilizing light to repair DNA pyrimidine−pyrimidone photolesions. The first step of enzyme function involves absorption of a photon and electron transfer from a flavin adenine dinucleotide anion (FADH − ) to the (6−4) lesion-inducing repair, that is, splitting of the covalently bound nucleobase dimer. Here we demonstrate that the repair mechanism occurs in the electronic ground state of the lesion radical anion because the initially absorbed photon energy is not sufficient to initiate electron transfer and to excite electronically the radical anion of the (6−4) lesion simultaneously.