Energetics of the primary electron transfer reaction revealed by ultrafast spectroscopy on modified bacterial reaction centers

Abstract

The modification of reaction centers from Rhodobacter sphaeroides by the introduction of pheophytins instead of bacteriopheophytins leads to interesting changes in the primary photosynthetic reaction: long-living populations of the excited electronic state of the special pair P* and the bacteriochlorophyll anion Bi show up. The data allow the determination of the energetics in the reaction center. The free energy of the first intermediate P+Br , where the electron has reached the accessory bacteriochlorophyll BA lies = 450 cm-' below the initially excited special pair P*.

Radicals and Radical Pairs in Photosynthesis

“…The controversy about the monomeric BChl on the A-side (the ET-active side) participating actively in the primary charge separation is still not resolved (16,17,(174)(175)(176). If the intermediate B,.…”

Temperature Dependence of Electron Transfer to the M-Side Bacteriopheophytin in Rhodobacter capsulatus Reaction Centers

“…[55][56][57] Additionally, calculations generally place the P + B -state higher in free energy than the P + H -state on both L and M branches by 0.15-0.25 eV, consistent with the difference in redox potentials of bacteriochlorophyll and bacteriopheophytin in vitro. [58][59][60][61] Calculations and experiments have provided estimates or bracketed ranges for the free energies of the charge-separated states in the wild-type RC: P + B L -0.05-0.1 eV below P*; 29,50,51,[62][63][64][65][66][67][68][69][70] P + H L -∼0.25 eV below P* when relaxed; [71][72][73][74][75] P + B M -0.1-0.2 eV above P* and P + H M -below P* by no more than ∼0.15 eV and probably within 0.1 eV. [50][51][52][76][77][78][79] Systematic efforts to manipulate the free energy differences of the L-and M-branch charge-separated states by site-directed mutagenesis of key amino acids, including those near B M and B L , have led to mutant RCs in which electron transfer to the M branch competes effectively with charge separation to the L branch, yielding P + H M -(reviewed in ref 80).…”

Long-time quantum simulation of the primary charge separation in bacterial photosynthesis.

“…Recent experiments by Zinth and coworkers (24,25) on modified reaction centers (where the bacteriopheophytin has been replaced by a pheophytin of much higher free energy) have reported significant (-30%) long-lived populations of the reduced accessory chlorophyll. The additional information obtained from these experiments is perhaps sufficient to allow unambiguous characterization of the mechanism of bacterial photosynthesis.…”

“…These conclusions are in accord with the theoretical analysis of Marcus (19,20) and with the data interpretation of ref. 24 according to a simple two-step kinetic model.…”