Interpretation of excitation-energy transfer to the special pair from the core antenna as quantum-mechanical virtual mediation to charge separation in the reaction center in purple bacterium Rhodopseudomonas viridis

Abstract

Excitation energies in the antenna system decrease toward the reaction center, comprising a funnel structure. They increase, however, at the funnel center, from the core antenna to the special pair in the reaction center, by ~150 and ~350 cm-1 in Rhodobacter sphaeroides and Rhodopseudomonas viridis, respectively. The excitation energy in the pair rapidly produce charge separation to the accessory bacteriochlorophyll. The excitation-energy transfer from the core antenna to the reaction center (more exactly, the charge-separated state therein) is the bottleneck in the energy harvesting process, taking ~50 ps at room temperature. In Rb. sphaeroides, the time for it is nearly temperature independent until ~77 K although no data has been reported below it. In Rp. viridis, on the other hand, the time increases with decreasing temperature, reaching ~1.3 ns at 6 K. If the special pair is really excited in Rp. viridis, we should expect from the law of chemical equilibrium that the time would increase to ~105 s at 6 K, considering that the special pair is thermally ~150 cm-1 high from the core antenna. This astronomical discrepancy can be removed by considering that the excited special pair is passed as a quantum-mechanical virtual state in Rp. viridis at low temperatures, although really passed in Rb. sphaeroides at all temperatures with the thermal height of ¿50 cm-1. A theory to reproduce the observation is presented on the basis of a unified treatment of sequential and superexchange (virtual) mediation at the midway state.