Magnetic field-stimulated luminescence and a matrix model for energy transfer. A new method for determining the redox state of the first quinone acceptor in the reaction center of whole cells of Rhodospirillum rubrum

In whole cells of Rhodospirillum rubrum the light-induced absorbance difference spectrum of the reduction of the first quinone electron acceptor Q₁ was determined in order to relate the emission yield ф and the magnetic field-induced emission increase Δф to the redox state of Q₁. It was found that Δф/ф² is a linear function of the number of reaction centers, in which Q₁ is reduced, independent of the fraction of reaction centers in the oxidized state. The emission yield is a hyperbolic function of the fraction of reaction centers closed, either by reduction of the acceptor Q₁ or by oxidation of the primary electron donor P. Apparently, in whole cells of R. rubrum a matrix model for energy transfer between various photosynthetic units can be applied. A model is presented, which is a generalization of theoretical considerations reported before (Duysens, L.N.M. (1978) in Chlorophyll Organization and Energy Transfer in Photosynthesis, Ciba Found. Symp. 61 (New Series), pp. 323–340, Elsevier/North-Holland, Amsterdam) and which is in excellent agreement with the experiments. From simultaneous measurements of Δф and ф the redox state of the reaction center can relatively easily be determined. So far, this is the only method for simultaneously measuring the fractions P⁺ and Q^?₁ in intact cells under steady-state conditions.