Spectroscopic characterization of B820 subunits from light-harvesting complex I of Rhodospirillum rubrum and Rhodobacter sphaeroides prepared with the detergent n-octyl- rac-2,3-dipropylsulfoxide

Abstract

B820 subunits from the light-harvesting complex 1 (LH1) were prepared by treatment of chromatophore membranes from Rhodospirillum rubrum with the detergent n-octyl- rac-2,3-dipropysulfoxide (ODPS). Similar subunits were obtained by dialysis of purified LH1 complexes from Rb. sphaeroides mutant M2192 against a buffer containing the same detergent. In contrast to other methods, no prior extraction of carotenoids is necessary for the preparation of these subunits when using the detergent ODPS. Reassociation of the B820(ODPS) subunits resulted in an apparently native, but carotenoid depleted LH1 complex. The presence of a fourth spectral component, with an absorbance maximum at 845 nm, was demonstrated by fitting the absorbance difference spectra with a sum of gaussians. Results from absorbance, fluorescence polarization, circular dichroism and triplet-singlet spectroscopy indicate that the subunits from LH1 obtained by this method are very similar to the B820 subunit isolated from carotenoid depleted light-harvesting complex 1 by octyl glucoside titration. These results imply that the B820 subunit structure created by detergent treatment is inherent to the structure of the functional LH1 antenna both for Rs. rubrum and Rb. sphaeroides.