STRUCTURAL PECULIARITIES AND CONFORMATIONAL TRANSITIONS OF THE B800-850 COMPLEX FROM CHROMATIUM-MINUTISSIMUM

Effects of various agents (detergents, solvents, salts, etc.) on the s pectral characteristics (conformational transitions) of light-harvesti ng B800-850 complex from the purple sulfur photosynthetic bacterium Ch romatium minutissimum were studied. It was determined that the complex comprises two dimeric bacteriochlorophylls: long- and short-wavelengt h. Changing the electrostatic and hydrophobic interactions inside the complex affects only the long-wavelength dimer of bacteriochlorophyll, this being revealed as a hypsochromic shift of the long-wavelength ab sorption band from 855 to 815-845 nm. This is accompanied by a corresp onding shift in the fluorescence emission band and decreasing circular dichroism (CD) signal in this region, while the state of both the sho rt-wavelength dimer (absorption band at 800 nm) and carotenoids do not change significantly. The effect is fully reversible, e.g., after the agent is removed, the previous state of the complex is restored. It i s supposed that a native conformation of the B800-850 complex is essen tial for its interactions with other complexes. The B800-850/B880-RC a ssociate was isolated from Chromatium minutissimum membranes under con ditions stabilizing the native conformation of the B800-850 complex. A model is discussed describing the observed conformational changes of the B800-850 complex, the role of the pigments in the stabilization of its structure, and the regulation of energy transfer from the periphe ral antenna to the reaction center via this changes.