Near-infrared excited Fourier-transform Raman spectra have been obtained from different spectral forms of Rhodobacter sulfidophilus light-harvesting II complexes. This complex, when isolated in lauryldimethylamine oxide, exists in a 805-828 nm form, which can be reversibly converted to the native 805-851 nm form upon addition of salt. The FT-Raman spectra predominantly show contributions of the carotenoid in the light-harvesting complex, with small but significant contributions of the bacteriochlorophylls excited in preresonance in the Qy transition. One strongly and one weakly interacting 2a acetyl C=O group as well as one moderately strong interacting and one non-interacting 9-keto C=O carbonyl modes of the bacteriochlorophylls can be discerned for the 805-828 nm form. Changes of relative band intensities caused by different resonance conditions for the different spectral forms lead to an assignment of the strongly interacting 2a acetyl C=O and the moderately strong interacting 9 keto C=O to bacteriochlorophylls organized in the 828 pigment moiety. Shifts of these bands to higher frequencies upon the salt-induced transition indicate a perturbation of the pigment-protein interaction, probably caused by a local protein conformational change.
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