PROPERTIES OF THE CAROTENOIDS 625 



intermediates. In line with this idea, Schroeder^^^ showed that if the buds 

 and stems of the monkey -flowers {Mimulus longijlorus Grant) are placed in 

 water and allowed to open in diffuse light, substantial amounts of pro- 

 lycopene and pro-7-carotene are present, while only lycopene and 7-caro- 

 tene result when the buds are allowed to develop on the intact plant in the 

 open. Zechmeister"*^^ also calls attention to another similar example, i.e., 

 that some flowers show a little yellow color if there is a reduced amount of 

 sunshine during their developmental period, while the usual deep orange 

 color characteristic of the all-trans carotenoids develops only during pro- 

 longed clear weather. Probably such changes in situ are related to the 

 external light available. However, it is known that the cis structures can 

 be maintained for long periods in the vegetable tissues exposed to intense 

 sunlight, although the same pigments would be extremely sensitive when 

 insolated in solutions. 



d. Possibilities for Steric Changes in the Polyenes. As recently as 



1931, it was stated by Wittig and Wiemer^^^ that when the number of con- 

 jugated double bonds increases, the phenomenon of cis-trans isomerism be- 

 comes less important and finally disappears as a result of the increased 

 mobility of the valence electrons. This hypothesis w^as questioned by 

 Kuhn,^^^ who stated: "The fact that the higher diphenylpolyenes are 

 known only in one spatial foiTn is due to the inadequacy of the preparative 

 methods." Zechmeister^" concluded that the "length of the conjugated 

 double bond system does not affect its ability to assume various cis-trans 

 configurations; in some carotenoid sets a dozen stereoisomers have been 

 observed." 



Pauling^^^ demonstrated that only certain of the double bonds are avail- 

 able for spatial rearrangement. The unsaturated linkages are thus divided 

 into two groups, the first of which is "stereochemically effective" and the 

 second of which is "stereochemically ineffective." In the C40 carotenoid 

 which is composed of dehj^drogenated isoprene residues, it can be shown 

 that only one double bond in each C5 unit in the aliphatic chain is able to 

 assume a cis configuration. Only those double bonds having hydrogen 

 atoms on the two carbon atoms adjacent to the carbons joined by the 

 double bonds are capable of existing in cis fonn. In a chain with the 

 stmcture, CX — CR=CR' — CX, the cis form would assume the following 

 spatial arrangement: 



R r' 



X x' 



"8 G. Wittig and W. Wiemer, Ami., 483, 144-156 (1930). 



"' R. Kuhn, in K. Freudenbcrg, SUrcocheinie, Vol. II, Dcuticke, Leipzig, and Vienna. 



1932. pp. 915-920. 



