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Nondestructive Structural Analysis of Photosynthetic Pigments in Living Rhodobacter Sphaeroides Mutants by Near-Infrared Fourier Transform Raman Spectroscopy

The 1064-nm excited Fourier transform Raman spectra have been measured for two kinds of whole living cells of photosynthetic bacteria, Rhodobacter sphaeroides G1C and R26 mutants, to investigate in situ structures of photosynthetic pigments, bacteriochlorophyll-a (BChl-a) and neurosporene (the only carotenoid included in the G1C mutant). The 1064-nm excited spectra consist of contributions from both neurosporene and BChl-a in the light harvesting (LH) complexes (G1C) or from BChl-a alone in the LH complexes (R26). The pattern of the 1064-nm excited spectrum of BChl-a in the LH complexes, whose Raman bands are pre-resonance enhanced via its Qy band, is dramatically different from that of its 355-nm excited spectrum, whose Raman bands are resonance enhanced via the B bands; for example, a band at 1606 cm−1 due to a Ca=Cm stretching mode of BChl-a, which is the most intense in the 355-nm excited spectrum, is barely observed in the 1064-nm excited spectrum. The frequency of the above band indicates that BChl-a in the LH complexes is five-coordinate. Bands due to C=O stretching modes of the 9-keto and 2-acetyl groups of BChl-a appear clearly near 1665 and 1640 cm−1, respectively, in the 1064-nm excited spectra. The frequencies of these C=O stretching bands suggest that most of the 9-keto and 2-acetyl groups of BChl-a in the complexes are involved in intermolecular interaction with the proteins. Bands assignable to Ca=N and Ca=Cb stretching modes, which are, in general, very weak in the 355-nm excited spectra, appear strongly in the 1064-nm excited spectra, implying that their bond lengths are changed sizably in the electronic transitions corresponding to the Qy bands.