Spectroscopic properties of antenna complexes of Rhodobacter sphaeroides in vivo

Abstract

Intact membranes of antenna mutants of Rhodobacter sphaeroides obtained by chemical mutagenesis containing only the B800–850 or the B875 reaction center complex were used to study the spectral properties of these antenna complexes separately in vivo. Wild-type spectral characteristics were restored to each mutant, following complementation by the relevant gene. It is shown that the absorption spectra of recombinant strains and of wild-type Rhodobacter sphaeroides can be analyzed in terms of those of the separate complexes as observed in the mutants. Distinct differences occur between the spectra of the antenna complexes isolated by means of detergent solubilization of the membrane and those of the mutants. Measurements of absorption and flash-induced absorption difference spectra and of linear dichroism and fluorescence polarization spectra at low temperature indicate that in the intact membrane the previously characterized bacteriochlorophyll Q y absorption bands near 800, 850 and 875 nm display an optical inhomogeneity and that they all contain a relatively weak transition at longer wavelength, the orientation of which is more parallel to the membrane plane than the orientation of the main transitions. Rapid and efficient energy transfer to the long-wave component (BChl 870) in the B800–850 complex could be demonstrated. Some of the long-wave transitions are also observable at room temperature. They may reflect the mode of aggregation of the complexes in their lipid environment and, by increased overlap between donor emission and acceptor absorbance, serve to facilitate energy transfer within the antenna system.