Applications of ESE spectroscopy in the study of electron spin polarization in bacterial photosynthesis
Absfrucf - In photosynthesis the photon energy of light is converted into chemical potential in the so-called Reaction Center (RC), a pigmented membrane protein that upon excitation gives rise to a charged radical pair. The radicals are spin correlated and in an external magnetic field the interactions (dipolar, exchange) between the two electron spins give rise to non-Boltmann populations of their magnetic sublevels (spin polarization), which is probed by time-resolved EPR. An overview is given of polarization phenomena in RC of bacterial origin, and recent theoretical interpretations highlighted. It is demonstrated that in all likelihood the exchange interaction of the primary radical pair in the RC of purple bacteria is positive. The implications of this result for our understanding of the initial processes of solar energy conversion are discussed. 1. PHOTOSYNTHETIC PHOTOCHEMISTRY Photosynthesis is the conversion of the quantum energy of light by algae, higher plants and photosynthetic bacteria into the chemical energy of complex organic molecules and organized cellular structures. The light is absorbed by so-called antenna pigment-protein complexes, containing (bacterio)chlorophyll as their major pigment. The resulting electronic excited state is transferred among the antenna pigments by Forster resonant energy transfer until it is trapped by a specialized complex of (bacterio)chlorophylI, (B)Chl, molecules in a special protein, the reaction center 0. In a few picoseconds, photochemistry on the excited specialized complex results in the donation of an electron to an electron acceptor, thus creating a pair of separated, charged radicals. Subsequent dark electron transport at either the donor or the acceptor side (or both) stabilizes the charges at the expense of Cree energy ("downhill" electron transport). Schematically: hu Antenna -.Antenna* -D*A-D+-4