MENAQUINONE-7 IN THE REACTION-CENTER COMPLEX OF THE GREEN SULFUR BACTERIUM CHLOROBIUM-VIBRIOFORME FUNCTIONS AS THE ELECTRON-ACCEPTOR A(1)

Photosynthetically active reaction center complexes were prepared from the green sulfur bacterium Chlorobium vibrioforme NCIMB 8327, and the content of quinones was determined by extraction and high-performance liquid chromatography. The analysis showed a stoichiometry of 1.7 mol ecules of menaquinone-7/reaction center. No other quinones were detect ed in the isolated reaction centers, whereas membrane preparations als o contained chlorobiumquinone. The possible involvement of quinones in electron transport was investigated by electron paramagnetic resonanc e (EPR) spectroscopy. A highly anisotropic radical was detected by Q-b and EPR spectroscopy in both membranes and isolated reaction centers f ollowing dark reduction with sodium dithionite and photoaccumulation a t 205 K. At 34 GHz, the EPR spectrum is characterized by a g tensor wi th g(xx) = 2.0063, g(yy) = 2.0052, g(zz) = 2.0020 and Delta B of 0.7 m T, consistent with its identification as a quinone. This spectrum is h ighly similar in terms of g values and line widths to photoaccumulated A(1)(-) in photosystem I of Synechococcus sp. PCC 7002. The results i ndicate that menaquinone-7 in the green sulfur bacterial reaction cent er is analogous to phylloquinone in photosystem I.