632 VI. CAROTENOIDS AND RELATED COMPOUNDS 



(o) cis-trans I somerization Produced hy Melting Crystalline Carotenoids. 

 When ciystals of a carotenoid are kept at a temperature a few degrees 

 above their melting point, both reversible and irreversible changes occur. 

 The change is manifested by a loss in color, which may amount to more than 

 50% of the original value. Simultaneously fluorescent compounds appear 

 which are decomposition products resulting from irreversible changes. 

 The amount of such destruction is less in the cases in which the carotenoid 

 melts at a lower temperature. It is also decreased when the time during 

 which the carotenoid is subjected to the elevated temperature is reduced 

 to a minimum. The inclusion of naphthalene to lower the temperature of 

 the fusion mixture also has the effect of reducing the extent of destruction 

 of the carotenoid molecule. 



In practice, the carotenoid is placed in a sealed tube with carbon dioxide, 

 heated to the desired temperature and maintained for five or ten minutes or 

 longer, after which it is rapidly cooled. Following solution in petroleum 

 ether, the stereoisomers may be separated from the original carotenoid and 

 from the decomposition products, by chromatography. In contradistinc- 

 tion to the results obtained by other procedures for producing stereoisomeric 

 changes, true equilibrium mixtures do not result from the fusion method. 

 Results which have been obtained by the application of melting crystals 

 are summarized in Table 23. 



In some cases not only a trans-cis rearrangement results on heating of 

 the crystalline carotenoids, but other more profound changes take place in 

 the molecule. Thus, Zechmeister and Sease*"" have reported that when 

 lutein prepared from the Aztec marigold {Tagetes erecta) was melted with 

 naphthalene, several new polyenes containing one less oxygen were formed. 

 These are referred to as desoxyluteins; trans-cis isomerization was proved 

 to have occurred in the case of desoxylutein I. 



(b) Thermal cis-trans Isomerization in Solution. Spontaneous isomeriza- 

 tion starts as soon as carotenoids are dissolved in a solvent; it proceeds 



Table 24 



Percentage of |(3-Carotene Stereoisomers with Decreased Adsorption 



Affinity Formed at Several Temperatures in Benzene-Petroleum Ether 



Solution" 



Temp., 



Per cent isomerized after following periods 



°C. 1 hr. 3 hrs. 24 hrs. 7 days 49 days 



20 — — <1 5.5 11.1 



40 4.0 5.4 11.2 — — 



60 7.5 9.7 — — — 



80 8.5 31.9 34.1 — — 



« Data from G. P. Carter and A. E. Gillam, Biochem. J., 33, 1325-1331 (1939). 



600 L. Zechmeister and J. W. Sease, /. Am. Chem. Soc, 65, 1951-1955 (1943). 



