Electron Hopping through the 15 Å Triple Tryptophan Molecular Wire in DNA Photolyase Occurs within 30 ps

DNA photolyase is a photoactive flavoprotein that contains three tryptophan residues between the FAD cofactor and the protein surface, the solvent-exposed Trp being located 14.8 angstrom from the flavin. Photoreduction of the neutral radical FADH(center dot) form to the catalytically active FADH(-) form occurs via electron transfer through this chain. The first step in this chain takes 30 ps, the second less than 4 ps. Using a combination of site-directed mutagenesis and femtosecond polarization spectroscopy to discriminate the spectroscopically indistinguishable Trp residues, we show that the third step occurs in less than 30 ps. This implies that the first photoreduction step is rate limiting and that the Trp chain effectively acts as molecular ''wire'' ensuring.