1064-nm-excited Fourier transform Raman studies of bacteriochlorophyll- a in solid films and in a blue-green mutant of Rhodobacter sphaeroides

Abstract

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 C 2O and C 9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.