G3G VI. CAROTENOIDS AND RELATED COMPOUNDS 



produced by iodine is the reversibility of the process. As shown in Table 

 2G, the mixtures which result are in the same ratio, irrespective of whether 

 one starts with the all-^7'ans form or with any of a number of the cis isomers. 



The speed at which equilibrium is reached depends upon several factors. 

 When the weight of the pigment is about 1/5000 molar (0.1 milligram per 

 milliliter of petroleum ether or benzene), and the weight of iodine is 1-3% of 

 the pigment, equilibrium is reached in 15 to GO minutes at 25°C. Zech- 

 meister^^^ reports that when iodine is present in the following percentages of 

 prolycopene, the extent of the isomerization complete at one minute is as 

 follows: 0.0013, 2%; 0.013, 37%; and 0.13, 93%. Another factor which 

 influences the rate of the trans-cis reaction is temperature, the higher 

 temperatures obviously favoring the reaction. Light, which by itself will 

 promote the formation of stereoisomers, seems to promote the action of 

 iodine to a marked extent. It has been stated^^^ that, for most practical 

 purposes, daylight or illumination with a Mazda lamp is necessary. How- 

 ever, over-exposure may cause destruction. Lycopene is especially sensi- 

 tive to such a change when the effects of iodine and light are combined. 

 Iodine itself may cause a certain amount of destruction in addition to the 

 stereoisomeric changes; this is usually negligible, but in the case of prolyco- 

 pene, losses as high as 30% have been reported. ^^^ The composition of the 

 equilibrium mixtures when the a\\-trans or cis forms of a- or /3-carotene, 

 cry ptoxan thin, lutein, and zeaxanthin are subjected to iodine catalysis is 

 summarized in Table 26. 



(d) cis-trans Isomerization Produced hy Acid Catalysis. When carote- 

 noids are subjected to the action of strong acids at ordinary temperatures, a 

 stereoisomeric mixture is produced. Thus, when a /3-carotene solution in 

 benzene or petroleum ether was treated with commercial concentrated 

 hydrochloric acid under cai'bon dioxide for 30 minutes, during which the 

 mixture was constantly shaken, Polgar and Zechmeister^'^^ found little 

 destruction of the carotenoid; the resulting mixture of isomers was as 

 follows: all-trans, 50, neo U, 23, neo B, 23, neo E, 3, labile isomer, 1. 



However, certain acids such as hydriodic acid may cause considerable or 

 complete destruction of the carotenoid without producing the stereoisomers. 

 Commercial concentrated hydriodic acid in the cold causes a reduction of 

 the carotenoids, while the use of boric acid melt results in the conversion of 

 luteins to desoxy luteins.^"" Xanthophylls and hydroxyketones are more 

 sensitive to concentrated acids than are the hydrocarbon polyenes; the 

 dark blue coloration after treatment with strong hydrochloric acid is a test 

 for violaxanthin, fucoxanthin, capsanthin, and capsorubin. Lutein is veiy 

 sensitive to organic acids when methanol solutions are refluxed.^^^ The 

 optical rotation decreases as the action of the acid is prolonged. Strain*"® 



605 A. Polgdr and L. Zechmeister, J. Am. Chem. Soc, 64, 1856-1861 (1942). 

 "6 H. H. Strain, J. Am. Chem. Soc, 63, 3448-3452 (1941). 



