Abstract

The initial separation of electric charges, which constitutes the key process of photosynthesis, occurs in a transmembrane chlorophyllprotein complex named the reaction center. Reaction centers from photosynthetic bacteria can be isolated in a functionally intact state and contain three polypeptides, four bacteriochlorophylls, two bacteriopheophytins and at least one quinone (Q_A). the case of the reaction center from Rps. sphaeroides R-26, the main absorption bands of the pigments are located at 865, 800, 760, 600 and 540 nm. The 865-nm band, which bleaches upon (photo)oxidation of the reaction center, is ascribed to the primary donor (P), a dimer of bacteriochlorophyll. The 800-nm band is assigned to the Q_Y transition of the two other "accessory" bacteriochlorophylls (B) while the 600-nm band corresponds to the Q_X transition of all four bacteriochlorophylls. The 760 and 540-nm bands are attributed to the Q_Y and Q_X transitions of the two bacteriopheophytins (H), respectively. The X-ray structure of the reaction center from a related bacterium (Rps. viridis) shows that the two bacteriochlorophylls constituting P, the two B and the two H molecules are organized with C₂ symmetry, thus defining two "branches" of pigments extending from P, with only one of them directed towards Q_A. We will denote B_A and H_A the B and H molecules associated with the latter branch.

© 1986 Optical Society of America