. Bacterial photosynthesis. Bacteria -- Physiology; Photosynthesis. 20 THE BACTERIAL PHOTOCHEMICAL APPARATUS The carotenoids of photosynthetic bacteria are, with a few excep- tions, aliphatic. They often carry tertiary hydroxy! and methoxyl groups located in the 1,1'-positions, and sometimes contain conjugated keto-groups. The chromophore, which consists of a variable number of conjugated double bonds, causing the yellow to pink-blue colour characteristic of these pigments, is often located rather unsymmetri- cally in the molecule. In addition, isolated double bonds may be present in agreement
. Bacterial photosynthesis. Bacteria -- Physiology; Photosynthesis. 20 THE BACTERIAL PHOTOCHEMICAL APPARATUS The carotenoids of photosynthetic bacteria are, with a few excep- tions, aliphatic. They often carry tertiary hydroxy! and methoxyl groups located in the 1,1'-positions, and sometimes contain conjugated keto-groups. The chromophore, which consists of a variable number of conjugated double bonds, causing the yellow to pink-blue colour characteristic of these pigments, is often located rather unsymmetri- cally in the molecule. In addition, isolated double bonds may be present in agreement with the formal composition of the carotenoids by com- bination of eight isoprene units followed by dehydrogenations. As an example is shown the assumed structure (4) of OH-spheroidenone (Formula I), (I) Table 1 gives a summary of the characteristic structural features of carotenoids in the photosynthetic units of various groups of organ- isms. The carotenoid pigments of the photosynthetic bacteria are distinguished from those of algae and higher plants by their aliphatic and frequently unsymmetric nature and the presence in the molecule of tertiary hydroxyl or methoxyl groups. The photosynthetic tissue of the algae and higher plants generally contain bicyclic carotenoids of the a- and /3-carotene type, often substituted in the 3-positions with sec- ondary hydroxyl groups; epoxy-carotenoids are here quite abundant. From photosynthetic bacteria there have been isolated to date 32 different carotenoids (including colourless forms). To the majority of these pigments fairly reliable chemical structures have been ascribed. In Table 2 is presented the distribution pattern of coloured carotenoids in 16 speciesof photosynthetic bacteria belonging to six different genera and four families. Aliphatic, hydroxylated and methoxylated carotenoids of what we shall call the normal spirilloxanthin series (involving the seven carotenoids participating in the transformation of lycopene to spiril
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