Reconstitution of electrochromically active pigment-protein complexes from Rhodobacter sphaeroides into liposomes

Abstract

Light-harvesting complex II (B800/850, LHC II complexes) and reaction centers with both light-harvesting complex I and II (B875 and B800/850; i.e.,RC LHC I-LHC II complexes) have been isolated from Rhodobacter sphaeroides . Both complexes have been incorporated into liposomes made from phospholipids purified from Escherichia coli. The electrochromic band shift of carotenoids, present in these complexes, could stillbe observed in these liposomes upon generation of a potassium diffusion potential. For RC LHC I-LHC II complexes the characteristics of the liposomes were studied in more detail: the extent of the absorbance changes increased with the amount of pigment protein complex incorporated and was maximal at a ratio of 70 nmol bacteriochlorophyll per mg lipid. Higher amounts of incorporated complexes led to a decrease in carotenoid signal due to an increasing membrane leakage. The carotenoid absorbance change at 503–487 nm showed a linear dependency on the diffusion potentials both in the negative as well as in the positive potential range. The spectrum of the absorbance changes at a fixed diffusion potential for RC LHC I-LHC II liposomes had asimilar shape as the spectrum found for chromatophores of Rb. Sphaeroides , however, with shifted maxima and minima. The spectrum foundfor LHC II liposomes was the inverted spectrum of the RC LHC I-LHC II liposomes. Electron micrographs of both type of liposomes showed distinct protein particles with a diameter of 15 nmfor RC LHC I-LHC II and 11 nm for RC LHC II complexes.