FREEZE-FRACTURE STUDIES OF REACTION CENTERS FROM RHODOSPIRILLUM-RUBRUM IN CHROMATOPHORES AND LIPOSOMES

In freeze-fractures of chromatophores of R. rubrum the reaction centers are seen as hexagonal arranged particles of 13 nm diameter with a density of around 5500 particles/.mu.m2. Similar regions on the cytoplasmic membrane suggest that these parts are the prospective invagination sites. Isolated reaction centers are easily incorporated into liposomes. In freeze fractures of liposomes particles similar in shape and size, although less dense as in chromatophores are observed. In negative staining much smaller units of only 5 nm in diameter are found indicating that reaction centers occur in the membrane as tri- or tetramers. There is a strong correlation between particle density in chromatophores and titratable reaction centers remaining in these membranes after extraction of reaction centers by detergents; both values are in good agreement with the yield of reaction centers at a given detergent concentration.