Energetics of Primary Charge Separation in Bacterial Photosynthetic Reaction Center Mutants: Triplet Decay in Large Magnetic Fields

Abstract

The triplet state of aromatic molecules forms and decays by intersystem crossing, as originally demonstrated by Kasha and Lewis. By contrast, the triplet state of the primary electron donor, 3P, in photosynthetic reaction centers is formed exclusively by spin- and magnetic-field-dependent charge recombination of the initially formed radical ion pair. 3P decays by intersystem crossing at low temperatures; however, at higher temperatures, it can also decay by activated re-formation of the radical ion pair from which it was born, followed by a spin- and magnetic-field-dependent pathway that leads ultimately to the ground state. The discovery of this activated decay pathway leads to an approach for obtaining information on the relative energies of the radical pair and 3P state (Chidsey et al. Proc. Natl. Acad. Sci. U.S.A. 1985, 82, 6850−6854); with knowledge of the absolute energy of 3P from its phosphorescence, the energy of the initial charge separation reaction can be obtained. In this paper, we present the first data on the temperature and magnetic field dependence of the formation and decay of 3P for Rb. sphaeroides reaction center mutants in a background that contains no carotenoid. The mutations have been studied in other contexts and were designed to perturb the redox potential of the primary electron donor or acceptor. The measured trends are in the same direction as expected from chemical intuition; however, the quantitative changes are typically smaller than expected. Possible reasons for this finding are discussed. Improved values are obtained for the enthalpy and free energy change associated with primary charge separation in wild-type reaction centers.