B850 pigment-protein complex of Rhodopseudomonas sphaeroides: Extinction coefficients, circular dichroism, and the reversible binding of bacteriochlorophyll

Abstract

Chromatophores of Rhodopseudomonas sphaeroides yield the antenna complex B850 in either of two states, depending on the method of isolation. Methods using dodecyl (= lauryl) dimethylamine oxide yield B850 with an absorption spectrum like that in vivo: the bands at 800 and 850 nm, due to the bacteriochlorophyll (Bchl) components Bchl-800 and Bchl-850, are in ratio A(800)/A(850) = 0.65 +/- 0.05. When B850 is isolated by methods using dodecyl sulfate, the Bchl-800 is attenuated or absent. Bchl assays of these materials and of the isolated antenna complex B875 yielded the following extinction coefficients, +/-SD, on the basis of the molarity of Bchl: For B875, epsilon(875) = 126 +/- 8 mM(-1) cm(-1). For B850 in the normal (high-Bchl-800) state, epsilon(850) = 132 +/- 10 mM(-1) cm(-1). For the individual components of Bchl in B850, epsilon(850) of Bchl-850 = 184 +/- 13 mM(-1) cm(-1) and epsilon(800) of Bchl-800 = 213 +/- 28 mM(-1) cm(-1). With these coefficients the molecular ratio of Bchl-850 to Bchl-800 equals 1.8 +/- 0.4 for B850 in the high-Bchl-800 state. Starting with B850 depleted of Bchl-800, the addition of dodecyldimethylamine oxide restored the 800-nm absorption band. The 850-nm band became shifted toward the blue, narrowed, and slightly attenuated, and its associated circular dichroism became 20% more intense. Free Bchl added with dodecyldimethylamine oxide accelerated the restoration of Bchl-800 and retarded the attenuation of Bchl-850. We conclude that free Bchl can interact reversibly with a binding site for Bchl-800 in the B850 complex, with dodecyl sulfate favoring dissociation and dodecyldimethylamine oxide promoting association. Thus the reversible dissociation of a native chlorophyll-protein complex has now been demonstrated.