Analog Effects of Low Temperature and Lipid Rigidity on the Distribution of Two Conformational States of the Reaction Centers

Abstract

A main step for our understanding of the way the reaction center stabilizes the electromagnetic energy coming from the antenna has been accomplished with its crystallization and X-ray structural analysis. This was done for the reaction centers from Rhodopseudomonas viridis [1–3] and Rhodobacter sphaerpides [4–8]. If the 3D structure is known with a high resolution (better than 3 Å), however our knowledge of the relationship between the protein dynamics and the electron transfer process is still very incomplete. In particular, the influences of surrounding lipids, of the contacts between the antenna proteins and the reaction center or between reaction centers, and of protein fluctuations have been poorly investigated. In the view of collected data on the biphasicity of the charge recombination decays between the oxidized primary electron donor, P⁺, and the quinones acceptors, Q^{A −} and Q^{B −}, in Rps. viridis RC’s, and in Rb. sphaeroides RC’s where anthraquinones are acting as Q_A we suggest that two reaction center states exist in the dark. Since they are also observed in Rps. viridis chromatophores, they probably are of importance in the charge separation process. We discuss the effects of pH, low temperature and lipid rigidity on the relative distribution of these states.