HYDROGEN-BONDING AND CIRCULAR-DICHROISM OF BACTERIOCHLOROPHYLLS IN THE RHODOBACTER-CAPSULATUS LIGHT-HARVESTING 2 COMPLEX ALTERED BY COMBINATORIAL MUTAGENESIS

We have investigated the spectroscopic properties of two classes of li ght-harvesting 2 (LH2 B800-850) mutants of Rhodobacter capsulatus obta ined by combinatorial mutagenesis to the C-terminal half of the beta-a poprotein: a pseudoLH2 (pLH2) class, in which the 800-nm absorption wa s 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 ban d and showed 850-nm absorption red-shifts of up to 30 nm. In several o f 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 investigat ed, the B850 bacteriochlorophyll a binding site remained intact, conse rving both the hydrogen bonding environment of the chromophores and th eir conformation and liganding, In contrast, the intensity of the CD s pectra of pLH1 complexes was considerably reduced, relative to that of wild-type or pLH2 complexes, consistent with alterations in the inter actions between pigments and in their relative orientation. Elevated f luorescence polarization over the red wing of the B850 band in the pLH 2 complexes indicated a reduction of exciton mobility within the ring of BChl molecules. Possible structural alterations governing the spect ral properties of the different mutants are discussed.