The Nature of Excited States and Intermediates in Bacterial Photosynthesis

Abstract

In bacterial reaction centers (RCs) charge separation is initiated by excitation of the special pair primary electron donor (P) to its first excited singlet state (¹P). Within a few ps an electron moves from P to I, the intermediate electron acceptor (Woodbury et al., 1985; Breton et al., 1986; Martin et al., 1986). As described in the paper by Flemming, Martin, and Breton in this volume, the rate of this initial reaction increases as the temperature is lowered. The electron transfers from I⁻· to Q_A within about 200ps at room temperature and somewhat faster at lower temperature (Kirmaier and Holten, 1987). For most of the experiments described in this paper we have used quinone-depleted RCs when the species is Rhodobacter sphaeroides and Q⁻·-RCs when the species is Rhodopseudomonas viridis. In this case the fate of the initial charge-separated radical pair state, ¹(P⁺·I⁻·), is more complex, as illustrated in Figure 1. ¹(P⁺·I⁻·) can decay either by charge recombination to ¹PI or ground state PI, or the spin multiplicity of the radical pair can evolve to ³(P⁺·I⁻·). ³(P⁺·I⁻·) can decay by charge recombination to ³PI or the spin multiplicity can continue evolving back to ¹(P⁺·I⁻·). ³PI can decay by intersystem crossing or reform ³(P⁺·I⁻·). Interconversion between the singlet and triplet radical pair states is characterized by a parameter ω, whose value depends on the magnetic properties of the radicals P⁺· and I⁻· and on the externally applied magnetic field strength (Boxer et al., 1983).