Analysis of the kinetics of P+ HA- recombination in membrane-embedded wild-type and mutant Rhodobacter sphaeroides reaction centers between 298 and 77 K indicates that the adjacent negatively charged QA ubiquinone modulates the free energy of P+ HA- and may influence the rate of the protein dielectric response.

Time-resolved spectroscopic studies of recombination of the P+HA- radical pair in photosynthetic reaction centers (RCs) from Rhodobacter sphaeroides give an opportunity to study protein dynamics triggered by light and occurring over the lifetime of P+HA-. The state P+HA- is formed after the ultrafast light-induced electron transfer from the primary donor pair of bacteriochlorophylls (P) to the acceptor bacteriopheophytin (H-A). In order to increase the lifetime of this state, and thus increase the temporal window for the examination of protein dynamics, it is possible to block forward electron transfer from H-A(-) to the secondary electron acceptor Q(A). In this contribution, the dynamics of P+HA- recombination were compared at a range of temperatures from 77 K to room temperature, electron transfer from H-A(-) to Q(A) being blocked either by prereduction of Q(A) or by genetic removal of Q(A). The observed P+HA- charge recombination was significantly slower in the Q(A)-deficient RCs, and in both types of complexes, lowering the temperature from RT to 77 K led to a slowing of charge recombination. The effects are explained in the frame of a model in which charge recombination occurs via competing pathways, one of which is thermally activated and includes transient formation of a higher-energy state, P+BA-. An internal electrostatic field supplied by the negative charge on Q(A) increases the free energy levels of the state P+HA-, thus decreasing its energetic distance to the state P+BA-. In addition, the dielectric response of the protein environment to the appearance of the state P+HA- is accelerated from similar to 50-100 ns in the Q(A)-deficient mutant RCs to similar to 1-16 ns in WT RCs with a negatively charged Q(A)(-). In both cases, the temperature dependence of the protein dynamics is weak.