DISSIPATIVE 3-STATE SYSTEM AND THE PRIMARY ELECTRON-TRANSFER IN THE BACTERIAL PHOTOSYNTHETIC REACTION-CENTER

The mechanism of the ultrafast primary charge separation process in ba cterial photosynthesis has been examined with a general dissipative th ree-state tight-binding model. Using real-time path integrals, the tra nsient populations of the three electronic states (BChl(2)BChlBPh, BC hl(2)(+)BChl(-)BPh, BChl(2)(+)BChlBPh(-)) involved in the reaction hav e been computed by numerically exact quantum Monte Carlo techniques. T he simulations show that the dissipative three-state system can reprod uce many characteristic features of the initial charge separation in t he reaction center for at least two parameter regions. In both regions , the accessory bacteriochlorophyll population remains small throughou t the electron transfer and the transfer rate exhibits the experimenta lly observed inverse temperature dependence. The first region is assoc iated with a low-lying BChl(2)(+)BChl(-) state and weak electronic cou plings. In this region, the transient populations are predominantly mo noexponential, and the dynamics is consistent with a stepwise (incoher ent) mechanism with nonadiabatic electron-transfer rates. The other re gion is associated with a BChl(2)(+)BChl(-) state lying above the phot oexcited special pair (BChl(2)). In this region, the transient popula tions are predominantly biexponential, but the degree of nonexponentia lness depends heavily on temperature. The dynamics is qualitatively co nsistent with the superexchange (coherent) mechanism. In the region be tween these two, associated with a BChl(2)(+)BChl(-) state approximate ly isoenergetic with BChl(2), the transient populations do not agree with key experimental features.