THE NATURE OF THE PRIMARY ELECTRON-ACCEPTOR IN GREEN SULFUR BACTERIA

It was shown previously (Van de Meent, E.J., Kobayashi, M., Erkelens, C., Van Veelen, P.A., Amesz, J. and Watanabe, T. (1991) Biochim. Biophys. Acta 1058, 356-362) by means of HPLC, NMR and optica and mass spectroscopy that the primary electron acceptor of heliobacteria is 8(1)-hydroxychlorophyll (Chl) a. In view of the spectral and functional similarities between this pigment and the primary electron acceptor of green sulfur bacteria, we have applied the same methods to various species of green sulfur bacteria (Prosthecochloris aestuarii, Chlorobium limicola, C. limicola f thiosulfatophilum, C. vibrioforme and C phaeovibrioides) in order to study the identity and the occurrence of the latter pigment. It was already shown from flash spectroscopic and reversed phase HPLC experiments on isolated membranes and solubilized membrane fractions of P. aestuarii that the most likely candidate for the primary acceptor is a pigment named bacteriochlorophyll (BChl) 663, which had been tentatively identified as a lipophilic form of BChl c. In this communication we will show by means of optical spectroscopy, Cf-252-plasma desorption mass spectroscopy and H-1-NMR that BChl 663 is an isomer of Chl a. This result again emphasizes the similarities between the reaction centers of green sulfur bacteria, heliobacteria and Photosystem I. By means of normal-phase HPLC analysis of the five species of green sulfur bacteria it is shown that BChl 663 is universally present and in comparable quantities in this group of photosynthetic bacteria. No other pigments with similar spectroscopic properties were detected.