Acetylenic carotenoids function

Functional groups include allene, acetylene, hydroxy, methoxy, epoxy, keto, aldehyde, carboxylate, lactone, acyl ester, glycoside, glycosyl ester and sulphate. Carotenoids with elements other than oxygen directly attached to the carbon skeleton have not been found in Nature. 

Two closely related carotenoid sulphates have been recently isolated from ophiuroids. Ophioxant-hin (266) is the major pigment from both Ophioderma longicaudum (6) and Ophiocomina nigra (183), of which the latter has also furnished dehydroophioxanthin (267), with an acetylenic functionality. 

Most carotenoids are xanthophylls, not carotenes. These compounds are formed by the introduction of one or more oxygen functions into the molecule. Other modifications of carotenoids involve the formation of allenic and acetylenic groups, additions to the polyene chain, and the loss of carbon atoms. Little is known concerning the biosynthesis of these compounds. 

Ketocarotenoids such as astaxanthin (28) and its acetylenic derivatives (34) and (80) occur in coloured carotenoprotein complexes examples of which are crustacyanin (52, 123), ovoverdin (48), ovorubin (50), allo-porin (151) and asteriarubin (65). It has been suggested that the carotenoids may function by stabilizing the tertiary structure of the proteins involved (51). Recombination experiments between the colourless apoproteins and selected carotenoids have demonstrated the need for conjugated 4,4 -keto groups for the formation of these complexes (151,180). However, detailed studies for instance with (3S,3 5)-astaxanthin (28), its 3R,3 R enantiomer and the mesoform for elucidation of the steric requirements for the complexing carotenoid have not yet been reported. 

Acetylenic retinoids (439) with a cyclogeranyl ring containing a functional group were prepared and used as building blocks in the synthesis of the carotenoid canthaxanthin (Rosenberger et al., 1979). 

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