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Abstract

The primary charge separation in photosynthesis is mediated by a membrane protein pigment complex, the photosynthetic reaction center (RC). The RC from the purple bacterium Rhodopseudomonas (Rps). viridis has been crystallized and the subsequent X-ray structure analysis provided a complete picture of protein structure and pigment arrangement (see 1). The RC consists of four protein subunits, the H (high), M (medium) and L (low) subunits, according to their apparent molecular weights from SDS-PAGE, and a tightly bound cytochrome subunit. Four heme groups are covalently bound to the cytochrome subunit, whereas the other pigments are embedded into the L and M subunits. There are four bacteriochlorophyll b molecules, two of them form the special pair (the primary electron donor P), two bacteriophaeophytin b molecules, two chemically different quinones, Q_A (primary quinone, a menaquinone-9) and Q_B (secondary quinone, a ubiquinone-9, see 2), one carotenoid molecule (a dihydroneurosporene) and a non-heme iron. These pigments are arranged in a highly symmetric manner in two branches extending from the special pair to the non-heme iron (3). However, only one branch is active in electron transfer.