Temperature dependence of the free energy, enthalpy, and entropy of P+Q(A)(-) charge recombination in Rhodobacter sphaeroides R-26 reaction centers

For reaction centers of photosynthetic bacteria reconstituted with low-pote ntial quinones in the QA site, the state P(+)Q(A)(-) formed by light activa tion decays to the ground state via a thermally activated route through the P+H- state. The rate of charge recombination by this thermal pathway is pr oportional to the equilibrium constant between P(+)Q(A)(-) and P+H-. Thus, the free energy difference between P(+)Q(A)(-) and P+H- can be determined b y measuring the charge recombination rate via the uphill route. The enthalp y and entropy change of the reaction can then be deduced from the temperatu re dependence of the charge recombination kinetics. The free energy, entrop y, and enthalpy changes between P(+)Q(A)(-) and P+H- were determined at tem peratures from 40 to 318 K for several low-potential quinones. From 200 K t o room temperature, Delta H degrees approximate to Delta G degrees, so the entropy changes are small. However, in the temperature range 80-200 K, a si gnificant entropy change is observed, and the free energy becomes strongly temperature-dependent. The newly formed P(+)Q(A)(-) State lives for millise conds. On this time scale at low temperature, the P(+)Q(A)(-) State appears to be trapped prior to charge recombination in a state similar to 200 meV (10 K) higher in free energy than the relaxed form found at room temperatur e.