Quantum Yield Measurements of Short-Lived Photoactivation Intermediates in DNA Photolyase: Toward a Detailed Understanding of the Triple Tryptophan Electron Transfer Chain

The light-dependent DNA repair enzyme photolyase contains a unique evolutionary conserved triple tryptophan electron transfer chain (W382-W359-W306 in photolyase from E. coli) that bridges the similar to 15 angstrom distance between the buried flavin adenine dinucleotide (FAD) cofactor and the surface of the protein. Upon excitation of the semireduced flavin (FADH degrees), electron transfer through the chain leads to formation of fully reduced flavin (FADH(-), required for DNA repair) and oxidation of the most remote tryptophan residue W306, followed by its deprotonation. The thus-formed tryptophanyl radical W306 degrees(+) is reduced either by an extrinsic reductant or by reverse electron transfer from FADH(-). Altogether the kinetics of these charge transfer reactions span 10 orders of magnitude, from a few picoseconds to tens of milliseconds. We investigated electron transfer processes in the picosecond-nanosecond time window bridging the time domains covered by ultrafast pump-probe and "classical" continuous probe techniques. Using a recent dedicated setup, we directly show that virtually no absorption change between 300 ps and 10 ns occurs in wild-type photolyase, implying that no charge recombination takes place in this time window. In contrast, W306F mutant photolyase showed a partial absorption recovery with a time constant of 0.85 ns. In wild-type photolyase, the quantum yield of FADH(-) W306 degrees(+) was found at 19 +/- 4%, in reference to the established quantum yield of the long-lived excited state of [Ru(bpy)(3)](2+). With this yield, the optical spectrum of the excited state of FADH degrees can be constructed from ultrafast spectroscopic data; this spectrum is dominated by excited state absorption extending from below 450 to 850 nm. The new experimental results, taken together with previous data, allow us to propose a detailed kinetic and energetic scheme of the electron transfer chain.