ONE-ELECTRON PHOTOOXIDATION OF REDUCED DESULFOVIBRIO-VULGARIS FLAVODOXIN ON LASER EXCITATION AT 355 NM

Electron ejection from the reduced flavin in flavodoxin from Desulfovi brio vulgaris was obtained on exposure of the protein to the third har monic radiation (354.7 nm) generated from a pulsed Nd/YAG laser. The r esults indicate that the reaction is due to stepwise two-photon excita tion of the reduced flavin via the excited singlet state. The absorpti on spectrum of the neutral flavosemiquinone radical formed in this pro cess was obtained. This spectrum remains stable over the time of study (0.2 ms) in the pH range studied, except for a slight evolution durin g the first microseconds, attributed to conformational readjustments o f the active site. This two-photon excitation method provides a conven ient means of generating the flavosemiquinone for ultrafast kinetic st udies.