The triplet state of the primary donor of the photosynthetic bacterium Rhodopseudomonas viridis

The absorbance-detected magnetic-resonance technique has been applied to the study of the triplet state of the primary donor in chromatophores of the photosynthetic bacterium Rps. viridis. The results confirm the triplet-minus-singlet absorbance-difference spectrum and its interpretation as previously obtained for isolated reaction centers (Den Blanken, H.J. and Hoff, A.J. (1982) Biochim. Biophys. Acta 681, 365–374). Our present results affirm that the primary donor is a bacteriochlorophyll b dimer, and that there is no blue exciton band at 850 nm. We show that the reaction centers are not identical, but have a small heterogeneity in their properties. In chromatophores and sometimes in isolated reaction centers a shoulder is observed in the long-wavelength absorbance-difference band of the primary donor. This shoulder is possibly caused by charge transfer interaction of the donor with an adjacent chromophore (Vermeglio, A. and Paillotin. G. (1982) Biochim. Biophys. Acta 681, 32–40; Maslov, V.G., Klevanik, A.V., Ismailov, M.A. and Shuvalov, V.A. (1983) Doklady Akad. Nauk. SSSR 269, 1217–1221) or it reflects a slight heterogeneity in the reaction-center geometry, which cannot be removed with the selection offered by the magnetic resonance technique. The zero-field triplet-ESR spectrum and the sublevel decay rates of the triplet state of the primary donor are presented, as detected in whole cells at the antenna fluorescence, and in chromatophores and isolated reaction centers at the absorbance-difference band at 838 nm. We do not observe the expected reversal of the sign of the ESR transitions monitored with the two techniques. A tentative explanation is given in terms of energy transfer from unrelaxed excited states of the antenna pigments to the reaction center.