Photophysics of photosynthesis. Structure and spectroscopy of reaction centers of purple bacteria

External magnetic field and magnetic isotope effects on the dynamic behavior of radical pairs in solutions have been studied theoretically, where the relaxation of their electron spins was taken into account. The decay observed with some radical pairs in micellar solutions under high magnetic fields is successfully interpreted in terms of the relaxation mechanism, and the magnetic field dependence of the relaxation rates is calculated for a model system.

Theoretical study of relaxation mechanism in magnetic field effects on chemical reactions.

When light impinges on photosynthetic material – a plant leaf, an alga or a photosynthetic bacterium – it is absorbed by an array of lightcollecting pigments. Through resonant energy transfer the absorbed quantum of light is transported to a trap, the reaction centre. Within such a trap, a specialized (bacterio)chlorophyll complex is able to eject from its excited state an electron. This electron is ‘captured’ by an adjacent acceptor, which in turn donates the electron to a second acceptor, and so on. Thus, light energy is converted into chemical energy which is ultimately used in the metabolic processes of the cell.

Magnetic field effects on photosynthetic reactions.

(1976). Density matrix description of spin-selective radical pair reactions. Molecular Physics: Vol. 32, No. 5, pp. 1491-1493.

Density matrix description of spin-selective radical pair reactions

Pairs of radical ions generated in polar solvents by photoinduced electron transfer either recombine within a few nanoseconds or separate. The (geminate) recombination process is governed by a hyperfine‐coupling‐induced coherent motion of the unpaired electron spins which can be modulated by weak external magnetic fields. The process which also generates the well‐known CIDNP and CIDEP effects is described theoretically by a stochastic Liouville equation comprising for realistic systems a large set of coupled diffusion equations. For the integration of these equations a finite‐difference algorithm with space and time discretization is developed. By comparison with exact solutions of the Liouville equation for model systems, it is demonstrated that an approximate Liouville equation which entails only two coupled diffusion equations for singlet and triplet radical pairs, respectively, suffices to predict the geminate recombination yields accurately. The approximate Liouville equation is employed then to stud...

Theory of the magnetic field modulated geminate recombination of radical ion pairs in polar solvents: Application to the pyrene–N,N‐dimethylaniline system

Measuring transient absorptions on the nanosecond time range, we observed the magnetic field ( H = 40 Oe) modulation of the pyrene triplet and pyrene anion concentrations formed through quenching of singlet excited pyrene molecules by N,N -diethylaniline in methanol at room temperature. We interpret this effect as being due to a magnetic field dependent triplet production in the geminate radical ion pair, induced by hyperfine interaction of the unpaired electrons. The discussion is based on a simple model related to the radical pair model in CIDNP theory. Moreover, the sign of the magnetic field modulation of triplets and radical ions allows to infer on the relative probability for the formation of excited and ground state molecules in ion recombination processes.

W. Bube

Papers

Magnetic field modulation of geminate recombination of radical ions in a polar solvent