PROPERTIES OF THE CAROTENOIDS 623 



are designated as neo-T, U, V, etc. Some inconsistency does result, how- 

 ever, since the stereoisomers of lutein, zeaxanthin, and capsanthin which 

 are adsorbed more strongly than the all-^rans forms are assigned letters at 

 the beginning of the alphabet. 



In addition to the "neo"-ciS isomers, which are admittedly synthetic 

 products, certain partial cis derivatives have been found in nature. Such 

 compounds are designated by the prefix "pro" in place of "neo." Two 

 such compounds which have been reported in a number of natural products 

 are pro-7-carotene and prolycopene. 



c. Configuration of Natural Carotenoids. By far the largest propor- 

 tion of the carotenoids which occur in nature is of the a\\-irans variety. 

 Proof that such is the case has been established by x-ray studies on crystal- 

 line carotenoids, ^^2'^^* by interpretation of their absorption spectra, ^^^~^^^ 

 and most recently by an examination of solutions of such pigments on a 

 chromatographic column. A preponderance of the sdl-trans form would be 

 expected, since these isomers have the greatest stability and the lowest 

 energy content. 



A number of exceptions to the general rule of all-trans isomers have been 

 reported. One of the first observations was of the occurrence of both 

 trans- and as-crocetins in the gentiobiosides of the saffron stigma. Bixin, 

 which has been isolated from the seed hulls of the tropical anatto tree 

 (JBixa orellana L.), is the cis form. Herzig and Faltis^*^ also prepared the 

 trans product from orlean. 



In the case of C40 carotenoids, Zechmeister et al.^^^ were the first to obtain 

 qualitative evidence of the presence of a cw-lycopene (prolycopene) and a 

 ds-7-carotene (pro-7-carotene). The final proof that such carotenoids 

 exist in the plant was afforded by the isolation of the crystalline product 

 from several natural sources. While the absorption curves of the cis-trans 

 pairs of bixin and crocetin have visible spectral maxima only a few milli- 

 microns apart, the maxima of prolycopene and of pro-7-carotene are 35 mix 

 and 31 mju lower, respectively, than the corresponding all-trans isomers. 

 Prolycopene has been shown to contain 6 cis bonds, while pro-7-carotene 

 has 5 such linkages. 



The comparatively wide distribution of the cis carotenoids is indicated 

 by the fact that, in the 6 plants from which they have been isolated, 5 

 families are represented. There is also some indication that the procarot- 

 enoids may occur during development and that possibly they exist as 



«2 J. Hengstenberg and R. Kiihn, Z. Knjst. Mineral. Petrog., A75, 301-310 (1930); 

 A76, 174-176 (1931). 



«3 G. MacKinney, /. Am. Chem. Soc, 56, 488 (1934). 



*" R. S. Mulliken, Science, 87, 427-428 (1938). 



^« R. S. Mulliken, /. Chem. Phi/s., 7, 364-374 (1939). 



«« R. S. Mulliken, Rev. Modem Phys., I4, 265-274 (1942). 



«' L. Pauling, Fortschr. Chem. organ. Naturstoffe, S. 203-235 (1939). 



