Detection of a reduced-flavin triplet state in free flavins and DNA photolyase

A transient species was observed upon laser excitation (lambda exc = 355 nm) of reduced flavins (cFIH2) under anaerobic conditions. The transient decayed with a lifetime of about 1 microseconds, was quenched by iodide ions and was assigned as the reduced flavin triplet state 3cFIH2. The triplet-state absorption spectrum assumed three forms, depending on the pH of the solution, corresponding to 3cFIH2 and its monoprotonated and monodeprotonated forms. The triplet-state lifetime was not sensitive to the presence of pyrimidine cyclobutane dimers, apparently ruling out its involvement in the dimer splitting reaction observed with model systems of reduced flavins. An analogous species is apparently formed in the reduced form of Escherichia coli DNA photolyase.