Abstract

The 1064-nm excited Fourier transform Raman spectra have been measured for two kinds of whole living cells of photosynthetic bacteria, <i>Rhodobacter sphaeroides</i> G1C and R26 mutants, to investigate <i>in situ</i> structures of photosynthetic pigments, bacteriochlorophyll-<i>a</i> (BChl-<i>a</i>) and neurosporene (the only carotenoid included in the G1C mutant). The 1064-nm excited spectra consist of contributions from both neurosporene and BChl-<i>a</i> in the light harvesting (LH) complexes (G1C) or from BChl-<i>a</i> alone in the LH complexes (R26). The pattern of the 1064-nm excited spectrum of BChl-<i>a</i> in the LH complexes, whose Raman bands are pre-resonance enhanced via its <i>Qy</i> band, is dramatically different from that of its 355-nm excited spectrum, whose Raman bands are resonance enhanced via the <i>B</i> bands; for example, a band at 1606 cm⁻¹ due to a Ca<u>--</u>Cm stretching mode of BChl-<i>a</i>, 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-<i>a</i> in the LH complexes is five-coordinate. Bands due to C=O stretching modes of the 9-keto and 2-acetyl groups of BChl-<i>a</i> 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-<i>a</i> in the complexes are involved in intermolecular interaction with the proteins. Bands assignable to Ca<u>--</u>N and Ca<u>--</u>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 <i>Qy</i> bands.

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