Synthesis in Photosynthetic Bacteria

Synthesis Photosynthctic Bactois.—A characteristic of the carotenoids of purple photosynthetic bacteria is the hydration of the terminal 1 and 1 double bonds. This is frequently followed by methylation of the hydroxyls at C-1 and C-1. Rarely does cyclization occur, but -carotene is formed in Rhodo-microbium vanneilii alongside the main pigment 1-hydroxy-1,2-dihydrolyco-pene (rhodopin). In the presence of 1 mmol 1 nicotine, synthesis of both pigments is inhibited and that of lycopene stimulated by a concomitant amount so nicotine not only inhibits cyclization, which involves the terminal 

In the presence of nicotine (7.5 mmol ), Rps. spheroides accumulates neurosporene in place of the two main pigments. - On re-suspending cells which had been washed free from nicotine, neurosporene disappeared and the normal pigments were synthesized. Experiments with Rps. gelatinosa showed that the same pathway existed in this organism, and emphasized the precursor-product relationship between spheroidene (151) and spheroidenone (153). These observations, combined with the characterization of the previously undetected demethylspheroidene (154) from cultures of both species, provide strong direct support for the biosynthetic pathway just indicated. No 3,4-didehydro-derivative of neurosporene (155) was detected in nicotine-inhibited cells, which suggests that the insertion of the double bond at C-3 does not normally occur before the hydration of the double bond at C-1. 

Ben-Aziz, H. C. Malhotra, R. K. Singh, G. Britton, and T. W. Goodwin, unpublished results. 

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