622 VI. CAROTENOIDS AND RELATED COMPOUNDS 



for the separation of the components. The marked effect of heat in pro- 

 ducing cis-irans isomerism was demonstrated by Strain, ^^^ while the use of 

 iodine as a catalyst of cis-trans changes for the C40 carotenoids was dis- 

 covered by Zechmeister and Tuzson.^^" The subject of cis-trans isomeriza- 

 tion and stereochemistry of the carotenoids has recently been reviewed by 

 Zechmeister.'*^^ 



b. Nomenclature. Although considerable confusion previously ex- 

 isted in the terminology of various stereoisomers of the carotenoids, it 

 would seem that such difficulties can now be avoided by the application of 

 the nomenclature employed by Zechmeister.^" This system is a logical 

 one, since it is based upon the structural relationship of the various stereo- 

 isomers. 



According to the Zechmeister usage, when the pigment is referred to by 

 the usual name, for example, jS-carotene, this designation refers to the all- 

 trans form in which eveiy double bond possesses a irajis configuration. The 

 all-a's form is applied to compounds in which all the double bonds, which 

 are spatially unhindered, have assumed the cis arrangement; however, 

 these aW-cis compounds will still possess a number of trans linkages which 

 cannot be changed to the cis fonn because of spatial considerations. In 

 addition to these two tj^pes of stereoisomers, there are a great number of 

 isomers in which one or more of the double bonds have assumed the cis 

 arrangement. Each double bond in the chromophore system is numbered 

 serially, and these numbers are italicized to avoid confusion with the num- 

 bering of the carbon atoms. The lowest number is applied to the unsatu- 

 rated linkage nearest the /3-ionone ring. If such a cyclic system is absent, 

 the lowest number is assigned to the linkage adjacent to an a-ionone group. 

 The conjugated linkages in or near open groups or terminal aliphatic groups 

 are given the highest numbers. Special rules apply in each individual case 

 for numbering the double bonds in entirely aliphatic or unsymmetrical 

 molecules. 



In addition to the terminology based upon the known configuration of 

 the double bonds, Zechmeister has employed a temporary nomenclature for 

 the differentiation of the isomers, based upon relative ease of adsorbability 

 on the chromatographic column. The isomers which have a weaker adsorb- 

 ability than the all-trans form are referred to as neo-isomers A, B, C, etc. 

 Thus, neo-/3-carotene A will appear on the chromatographic column 

 directly under all-<rans-/3-carotene; the isomer having weaker adsorption 

 than A and appearing directly under it in the chromatographic column is 

 called neo-j8-carotene B. The isomers with progressively weaker adsorba- 

 bility are called C, D, E, etc. In the cases in which the stereoisomers are 

 more readily adsorbed than the aW-trans form, those with increased affinity 



«i L. Zechmeister und A. Polgdr, J. Am. Chein. Soc, 66, 137-144 (1944). 



