Hydrogen bonding and circular dichroism of bacteriochlorophylls in the Rhodobacter capsulatus light-harvesting 2 complex altered by combinatorial mutagenesis.

Abstract

We have investigated the spectroscopic properties of two classes of light-harvesting 2 (LH2, B800-850) mutants of Rhodobacter capsulatus obtained by combinatorial mutagenesis to the C-terminal half of the beta-apoprotein: a pseudoLH2 (pLH2) class, in which the 800-nm absorption was normal but the 850-nm peak was blue-shifted by up to 14 nm, and the other a pseudoLH1 (pLH1) class, which lacked the 800-nm absorption band and showed 850-nm absorption red-shifts of up to 30 nm. In several of the pLH1 antennae, carotenoid depletion contributed to the phenotype, while in the pLH2 complexes there was some carotenoid enrichment. A number of mutants from each class have also been characterized by low-temperature absorption and fluorescence spectroscopy, resonance Raman spectroscopy, and circular dichroism. In all of the mutants investigated, the B850 bacteriochlorophyll a binding site remained intact, conserving both the hydrogen bonding environment of the chromophores and their conformation and liganding. In contrast, the intensity of the CD spectra of pLH1 complexes was considerably reduced, relative to that of wild-type or pLH2 complexes, consistent with alterations in the interactions between pigments and in their relative orientation. Elevated fluorescence polarization over the red wing of the B850 band in the pLH2 complexes indicated a reduction of exciton mobility within the ring of BChl molecules. Possible structural alterations governing the spectral properties of the different mutants are discussed.