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 

 spirilloxanthin in R. nibrum (6,7), are abundant in the genus Rhodo- 

 spirillum. This series occurs olso in some species oiRhodop seudo- 

 monas, Chromatium, Thio spirillum, 2ccid in Rhodomicrobiuni vanuielii. 

 Keto-carotenoids of the spheroidenone type are restricted to three 

 species of Rhodopseudomonas , whereas another type of keto-carot- 

 enoids is present in some Thiorhodaceae spp. Cyclic carotenoids have 

 not so far been found in pure cultures of any of the Athiorhodaceae or 

 Thiorhodaceae. Derivatives of the monocyclic y- carotene are, so far 

 as is known, restricted to the green bacteria, and y5-carotene has been 

 found only in Rhodomicrolmim vannielii. 



