1064-NM-EXCITED FOURIER-TRANSFORM RAMAN STUDIES OF BACTERIOCHLOROPHYLL-A IN SOLID FILMS AND IN A BLUE-GREEN MUTANT OF RHODOBACTER-SPHAEROIDES

1064-nm-excited Fourier transform Raman spectra of bacteriochlorophyll-a (BChl) in various solid films and in chromatophores from a blue-green mutant of Rhodobacter sphaeroides have been obtained. The observed Raman spectra are free from high fluorescence backgrounds and sample degradation. The observed intensities seem to be enhanced because of a pre-resonant effect between the exciting radiation at 1064 nm and the Q(y) absorption at 770-870 nm of BChl. The spectral features are substantially different from the Soret and Q(x) resonance Raman spectra extensively investigated so far; several bands in the wavenumber region lower than 1200 cm-1 are particularly enhanced in the Q(y) pre-resonance Raman spectra. Bands due to both the C2 = O and C9 = O stretches appear at 1700-1620 cm-1, providing structural information on these carbonyl groups. In the C = C stretching region (1620-1490 cm-1), the correlation between band positions and the co-ordination number of central magnesium, which was previously found in the Soret-excited Raman spectra, is preserved in the Q(y) pre-resonance Raman spectra as well. The relative intensities of strong bands in the 1200-1000 cm-1 region appear to be useful for characterizing the BChl state. By using these advantages of the Q(y) pre-resonance Raman spectra, molecular interactions and arrangements of BChl in hydrated films and in the B870 light-harvesting complex of R. sphaeroides are discussed.