Magnetophotoselection study of the lowest excited triplet state of the primary donor in photosynthetic bacteria

The first time-resolved magnetophotoselection study of the primary donor tripler state (P-3) in bacterial reaction centers from Rhodobacter sphaeroides R26 and Rhodopseudomonas viridis is reported. With direct excitation of the primary donor, this approach provides the orientation (spherical coordinates delta, colatitude, and gamma, longitude) of the excited optical transition moment in the principal triplet axis system. It is essentially free from the effects of spin-lattice relaxation of P-3, enabling magnetophotoselection measurements over a wide temperature range. Two independently measured triplet-state spectra, excited with light polarized parallel and perpendicular to the EPR magnetic field, are simulated with the same set of parameters. This procedure results in a high precision (ca. +/-5 degrees with sufficient signal/noise ratio) of the obtained spherical coordinates of the optical transition moment vector. We find delta = 80 +/- 5 degrees, gamma = 70 +/- 5 degrees and delta = 75 +/- 5 degrees, gamma = 70 +/- 5 degrees for Rb. sphaeroides R26 and Rps. viridis, respectively. We demonstrate that excitation of the sample with nonpolarized light is essentially nonisotropic, Neglect of this effect in spectral simulations of light-induced signals may lead to considerable error in the parameters determined.