24 THE BACTERIAL PHOTOCHEMICAL APPARATUS 



TABLE 3. 



The categories of step -reactions operative in the biosynthesis of coloured 

 carotenoids in photosynthetic bacteria 



Type no. Reaction type Structural features 



1 Dehydrogenation 



2 Dehydrogenation 



3 Hydration 



4 Methylation 



5 Oxidation (aerobic) 



6 Hydroxylation 



7 Oxidation (anaerobic) 



8 Cyclization 



in washed cell suspensions of R. nihrum (6), the effect of oxygen on 

 anaerobic cultures of Rps.spheroides (25,26), and finally the structural 

 determinations of the carotenoids involved (27,7). The biochemical 

 reactions participating in these transformations were grouped into five 

 different categories (23,7), namely, two types of dehydrogenations, a 

 hydration reaction consisting of addition of water to an isopropylidene 

 double bond, methylation of a tertiary hydroxyl group, and finally a 

 type of oxidation involving introduction of a conjugated keto- group. 

 These reaction types are represented by numbers 1-5, depicted in 

 Table 3. 



Tentative chemical structures, (XIII), (XV) and (XVII a or b), in 

 agreement with the limited data available for these trace compounds, 

 were assigned to the three carotenoids designated by Goodwin as 

 hydroxy- Y, hydroxy-R and P512 (5). The carotenoidP512, now referred 

 to as P518 (11,4), exhibits absorption maxima at longer wavelengths 

 than any other known carotenoid. This carotenoid has now been isolated 

 from Rps. spheroides and Rps. gelatinosa, and its structure appears 

 very likely to be that of a 2-keto-spirilloxanthin (XVIII) (11), 



