The mechanism of energy transfer in the bacterial photosynthetic reaction center

In the accompanying paper (Scholes, G. D.; Jordanides, X. J.; Fleming, G. R . J. Phys. Chem. 2001, 105, 1640), a generalization of Forster theory is de veloped to calculate electronic energy transfer (EET) in molecular aggregat es. Here we apply the theory to wild-type and mutant photosynthetic reactio n centers (RCs) from Rb. sphaeroides, as well as to the wild-type RC from R ps. viridis. Experimental information from the X-ray crystallographic struc ture, resonance Raman excitation profiles, and hole-burning measurements ar e integrated with calculated electronic couplings to model the EET dynamics within the RC complex. Optical absorption and circular dichroism spectra a re calculated at various temperatures between 10 K and room temperature, an d compare well with the experimentally observed spectra. The calculated ris e time of the population of the lower exciton state of P, P-, as a result o f energy transfer from the accessory bacteriochlorophyll, B, to the special pair, P, in Rb. sphaeroides (Rps. viridis) wild-type at 298 K is 193 fs (2 39 fs), and is in satisfactory agreement with experimental results. Our cal culations, which employ a weak-coupling mechanism suggest that the upper ex citon state of P, P+ plays a central role in trapping excitation from B. Ou r ability to predict the experimental rates is partly attributed to a prope r calculation of the spectral overlap J(partial derivative alpha)(epsilon) using the vibronic progressions. The main advance we have made, however, is to calculate the electronic couplings V-partial derivative alpha in terms of the molecular composition of donor and/or acceptor aggregates, rather th an treating the accepters P+ and P- as point dipoles associated with each s pectroscopic band. Thus, we believe our electronic couplings capture the es sence of the many-body interactions within the RC. Calculations for EET in two mutants, (M)L214H (the beta mutant) and (M)H202L (the heterodimer), are in reasonable agreement with experimental results. In the case of the hete rodimer the agreement depends on a decrease in the electronic couplings bet ween D-M and the rest of the pigments.