Temperature dependence of the primary electron transfer in photosynthetic reaction centers from Rhodobacter sphaeroides

Abstract

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 τ 1=3.5±0.4 ps and τ 2=1.2±0.3 ps at T=300 K decrease continously with temperature to values of τ 1=1.4±0.3 ps and τ 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.