Kinetics and thermodynamics of the P870 +Q −A → P870 +Q −B reaction in isolated reaction centers from the photosynthetic bacterium Rhodopseudomonas sphaeroides

Abstract

The temperature dependences of the P870 +Q − A → P870Q A and P870 +Q − B → P870Q B recombination reactions were measured in reaction centers from Rhodopseudomonas sphaeroides. The data indicate that the P870 +Q − B state decays by thermal repopulation of the P870 +Q − A state, followed by recombination. ΔG° for the P870 +Q − A → P870 +Q − B reaction is −6.89 kJ · mol −1, while ΔH° = −14.45 kJ · mol −1 and − TΔS° = + 7.53 kJ · mol −1. The activation ethalpy, H ‡ , for the P870 +Q − A Δ P870 +Q − B reaction is +56.9 kJ · mol −1, while the activation entropy is near zero. The results permit an estimate of the shape of the potential energy curve for the P870 +Q − A → P870 +Q − B electron transfer reaction.