Biochemical and spectroscopic characterization of the B800-850 light-harvesting complex from Rhodobacter sulphidophilus and its B800-830 spectral form.

We demonstrate that the B800-830 spectral form of the B800-850 peripheral light-harvesting complex of Rhodobacter sulphidophilus, which is formed at low ionic strengths in the presence of the zwitterionic detergent LDAO, results from a local modification of the bacteriochlorophyll binding site and not the dissociation of the complex. This perturbation does not result in significant changes to the interactions between the pigments as studied by circular dichroism or fluorescence spectroscopy; however, modifications in the pigment binding sites are inferred from changes in the preresonance Raman spectrum. Specifically, an alteration of the hydrogen bonding of the 2-acetyl group of at least one of the bacteriochlorophyll groups that make up the 850 nm absorbing pair is observed. This implies an alteration in the conformation of the C-terminal domain of the alpha-polypeptide, in which are located the two tyrosyl residues that are believed to act as H-bond donors to these groups, induced by the protein-bound detergent in the absence of bound cations. We suggest that the ability of this complex to form an 800-830 complex is linked to the presence of an aspartyl residue immediately upstream of the tyrosyl residues. This study therefore provides a further illustration of the importance of hydrogen bonds to the 2-acetyl group of the bacteriochlorophyll in the determination of its spectral properties; furthermore, we provide a description of a conformational change that is able to modulate chromophore binding in these complexes.