PROPERTIES OF THE CAROTENOIDS 635 



without the aid of a catalyst and in the absence of hght. This phenomenon 

 was first observed by Zechmeister and Tuzson'**^ for lycopene, cryptoxan- 

 thin, and /3-carotene, while the behavior of zeaxanthin and lutein, under 

 similar conditions, was described bj^ Strain^^^ as well as by Zechmeister 

 ei a/. 490.50 1,502 'pj^g speed at which isomerization takes place was found to 

 depend upon the solvent employed but most of all upon the temperature at 

 which the solution was maintained. While some spontaneous changes 

 occur at room temperature, such reactions are extremely slow. Zech- 

 meister^^' reports the following per cent of isomerization after 24 hours in a 

 benzene or petroleum ether solution: a-carotene, /3-carotene, ciyptoxan- 

 thin, and capsanthin, 1-2; gazaniaxanthin and zeaxanthin, 4-5; capso- 

 rubin, 8; and lycopene, 10. If /3-carotene solutions are kept at — 2°C. in 

 the dark and protected from air, it has been reported^°^ that less than 3-4% 

 will undergo trans-cis rearrangement in the course of 3 months. The 

 effect of temperature on the rate of isomerization is given in Table 24, while 

 the comparative effects of refluxing solutions of different carotenoids are 

 recorded in Table 25 (see pages 632 and 633). 



When all-^?'ans compounds are refluxed in a dilute benzene or petroleum 

 ether (b.p., 60-80°C.) solution, equilibrium is reached in from 15 to 60 

 minutes. Compounds with some cis linkages have varying stability. For 

 example, prolycopene and pro-7-carotene are so theraiostable that their 

 absorption curves show no alteration after refluxing in petroleum ether for 

 30 minutes."^- The rate of isomerization of j3-carotene has been shown by 

 Hunter et ai.*"^ to be greater in the non-polar solvent, toluene, than in the 

 polar solvent nitromethane. 



(c) cis-trans Isomerization Produced by Iodine Catalysis at Room Tempera- 

 ture. The effect of iodine on the stereoisomerism was first established by 

 Zechmeister and his associates"^'^^"-^"^ with a large number of C40 carote- 

 noids. ^\^len iodine is added to a solution of the aW-ti'ans carotenoid in 

 amounts approximating 1 to 2% of the amount of pigment, an equilibrium 

 mixture of stereoisomers results. Not all the theoretically possible iso- 

 mers are produced; in practice only from 2 to 12 of the more stable ones 

 are formed in detectable quantities. 



The equilibrium mixture is different from that produced when such pro- 

 cedures as fusion or refluxing of the solutions are used to produce trans-cis 

 rearrangements. The differences are not only quantitative but possibly 

 also qualitative, since some isomers may be formed — as a result of iodine 

 catalysis — which are completely absent when the thermal methods are 

 employed. The most important characteristic of the equilibrium mixture 



"1 L. Zechmoistor, L. v. C'holnoky, and A. Polgdr, Ber., 72, 1678-1685 (1939). 

 "2 L. Zechmeister, L. v. Cholnoky, and A. Polgdr, Ber., 72, 2039-2041 (1939). 

 "^ G. P. Carter and A. !•:. Gillam, Biorhem. J., 33, 1325-1331 (1939). 

 "^ R. F. Hunter, A. D. Scott, and .1. II. Edi.sbury, Biochcm. J., 36, 697-702 (1942). 



