Multiple pathways for ultrafast transduction of light energy in the photosynthetic reaction center of Rhodobacter sphaeroides

A pathway of electron transfer is described that operates in the wild-type reaction center (RC) of the photosynthetic bacterium Rhodobacter sphaeroide s, The pathway does not involve the excited state of the special pair dimer of bacteriochlorophylls (P*), but instead is driven by the excited state o f the monomeric bacteriochlorophyll (B-A*) present in the active branch of pigments along which electron transfer occurs. Pump-probe experiments were performed at 77 K on membrane-bound RCs by using different excitation wavel engths, to investigate the formation of the charge separated state P+HA-. I n experiments in which P or B-A was selectively excited at 880 nm or 796 nm , respectively, the formation of P+HA- was associated with similar time con stants of 1.5 ps and 1.7 ps, However, the spectral changes associated with the two time constants are very different. Global analysis of the transient spectra shows that a mixture of P+BA- and P* is formed in parallel from B- A* on a subpicosecond time scale. In contrast, excitation of the inactive b ranch monomeric bacteriochlorophyll (B-B) and the high exciton component of P (P+) resulted in electron transfer only after relaxation to P*. The mult iple pathways for primary electron transfer in the bacterial RC are discuss ed with regard to the mechanism of charge separation in the RC of photosyst em II from higher plants.