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 tr ipler state (P-3) in bacterial reaction centers from Rhodobacter sphaeroide s R26 and Rhodopseudomonas viridis is reported. With direct excitation of t he primary donor, this approach provides the orientation (spherical coordin ates delta, colatitude, and gamma, longitude) of the excited optical transi tion moment in the principal triplet axis system. It is essentially free fr om the effects of spin-lattice relaxation of P-3, enabling magnetophotosele ction measurements over a wide temperature range. Two independently measure d triplet-state spectra, excited with light polarized parallel and perpendi cular to the EPR magnetic field, are simulated with the same set of paramet ers. This procedure results in a high precision (ca. +/-5 degrees with suff icient signal/noise ratio) of the obtained spherical coordinates of the opt ical transition moment vector. We find delta = 80 +/- 5 degrees, gamma = 70 +/- 5 degrees and delta = 75 +/- 5 degrees, gamma = 70 +/- 5 degrees for R b. sphaeroides R26 and Rps. viridis, respectively. We demonstrate that exci tation of the sample with nonpolarized light is essentially nonisotropic, N eglect of this effect in spectral simulations of light-induced signals may lead to considerable error in the parameters determined.