Temperature-dependent electron spin polarization of the triplet state of the primary donor in Rhodopseudomonas viridis

Abstract

The electron spin polarization (ESP) of the triplet of the primary donor ( 3P) in reaction centers of the photosynthetic bacterium Rhodopseudomonas viridis shows an anisotropic temperature dependence (Van Wijk, F.G.H. and Schaafsma, T.J. (1988) Biochim. Biophys. Acta 936, 236). The reported inversion of the initial electron spin polarization (IESP) for the canonical Y-direction of 3P at 100 K has been explained by means of coherent S-T ± mixing in the radical pair, due to a fast relaxing electron spin on the iron-quinone acceptor complex X (Hore, P.J., Hunter, D.A., Van Wijk, F.G.H., Schaafsma, T.J. and Hoff, A.J. (1988) Biochim. Biophys. Acta 936, 249). Using direct-detection EPR, we show that at 100 K the IESP in randomly oriented samples is not inverted for the canonical Y-direction of 3P. Furthermore, in single crystals the IESP at 100 K is shown to be almost zero for the complete YZ-plane of 3P. Since X ·− shows a strong g-anisotropy, the model of Hore et al., in which polarization-inversion only occurs when the effective g-value of X ·− is around g = 2.00, is inadequate to explain the temperature-dependent changes of the IESP. Therefore, we conclude that anisotropic fast spin-lattice relaxation in the radical pair triplet state is the origin of the temperature dependence of the ESP. The inversion for the canonical Y-direction under continuous illumination is the result of the interplay of spin-lattice relaxation in 3P and its triplet decay rates, in combination with changes in the IESP.