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 τ1=3.5±0.4 ps
and τ2=1.2±0.3 ps at T=300 K decrease continously with temperature
to values of τ1=1.4±0.3 ps and τ2=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.