Magnetic resonance studies have greatly helped the study of the primary photochemistry of bacterial photosynthesis. The full power of magnetic resonance techniques for structure determination of paramagnetic species can only be realized, however, when measurements are made on fully oriented samples such as single crystals. Thus the recent crystallization of the reaction-centre protein from Rhodopseudomonas viridis1 has enhanced the amount of structural information about the reaction centre that can be obtained using these methods. It is well known that when the primary quinone acceptor of the reaction centre is prereduced, the primary photochemical charge separation lasts only about 20 ns before back electron transfer leads to the lowest excited triplet state of the primary donor2. We now report the electron paramagnetic resonance (EPR) spectra of this photochemically derived triplet state of the primary donor of R. viridis in a single reaction-centre crystal as a function of crystal orientation in the magnetic field. Analysis of these spectra allows us to assign the orientation of the triplet-state axis system of the primary donor relative to the crystal axis system.
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