TEMPERATURE-DEPENDENCE OF THE PRIMARY ELECTRON-TRANSFER IN PHOTOSYNTHETIC REACTION CENTERS FROM RHODOBACTER-SPHAEROIDES

The primary electron transfer (ET) in reaction centers (RC) of Rhodobacter sphaeroides is investigated as a function of temperature with femtosecond time resolution. For temperatures from 300 to 25 K the ET to the bacteriopheophytin is characterized by a biphasic time dependence. The two time constants of tau-1 = 3.5 +/- 0.4 ps and tau-2 = 1.2 +/- 0.3 ps at T = 300 K decrease continously with temperature to values of tau-1 = 1.4 +/- 0.3 ps and tau-2 = 0.3 +/- 0.15 ps at 25 K. The experimental results indicate that the ET is not thermally activated and that the same ET mechanisms are active at room and low temperatures. All observations are readily rationalized by a two-step ET model with the monomeric bacteriochlorophyll as a real electron carrier.