B-side electron transfer promoted by absorbance of multiple photons in Rhodobacter sphaeroides R-26 reaction centers

Femtosecond transient absorbance spectra of quinone-depleted Rhodobacter sphaeroides R-26 reaction centers in the QX transition region have been measured at 15 K under various excitation conditions. This study focuses on the excitation wavelength dependence and excitation intensity dependence of the formation of charge-separated states on the A- and B-side of the reaction center, judging from the bleaching of the 533 nm (B-side) and 544 nm (A-side) ground-state transitions of the reaction center bacteriopheophytins (HA and HB). Upon low-intensity selective excitation directly into the bacteriopheophytin QY transitions (near 760 nm), bleaching of both ground-state bacteriopheophytin QX transitions (533 and 544 nm) appeared immediately, showing that initially either the A- or B-side bacteriopheophytin could be excited. However, both excited states ultimately resulted in P+HA- formation under these conditions. Low-intensity excitation at any of the various wavelengths (595, 750, 760, and 880 nm) showed no di...