TRANSFER OF LIGHT-INDUCED ELECTRON-SPIN POLARIZATION IN BACTERIAL PHOTOSYNTHETIC REACTION CENTERS

In reaction centers of photosynthetic bacteria strong light-induced emissive ESR signals have been found, not only after a flash but also under continuous illumination. The signal, with g = 2.0048 and ΔHpp = 8.0 G, is only present under reducing conditions in material in which the primary acceptors, ubiquinone (U) and its associated high-spin ferrous ion, are magnetically uncoupled. Its amplitude under continuous illumination is strongly dependent on light intensity and on microwave power. The emissive signal is attributed to the prereduced primary acceptor, U-, which becomes polarized through transfer of spin polarization by a magnetic exchange interaction with the photoreduced, spin-polarized intermediary acceptor, I-. A kinetic model is presented which explains the observed dependence of the emissivity on light intensity and microwave power. Applying this analysis to the light saturation data of the emissive signal, a value for the exchange interaction, J(I-U-), between I- and U- of 3.5 G and between P+ and I-, J(P+I-), of 1-5 G is derived. The implications of the low value of the latter interaction for the early photochemistry in the reaction center are discussed. It is anticipated that transfer of spin polarization is not confined to photosynthetic material but will generally occur whenever radicals are photoproduced in a solid matrix in the vicinity of a stable paramagnetic species. © 1979 American Chemical Society.