PROPERTIES OF THE CAROTENOIDS 609 



The effect of solvent on the color of the carotenoid solution is especially 

 marked where the polyene carbonyl or carboxyl group exists in conjugation 

 with the carbon-to-carl)on double bond system. For example, rhodoxanthin 

 exhibits a pure red color in alcohol solution, while it appears orange yellow 

 when dissolved in petroleum ether. The absorption maxima of the longest 

 wave lengths are 538 and 521 m^, respectively, in the two solvents. Kuhn 

 and Brockmann^^^ explain these variations by suggesting a reciprocal effect 

 between the polar carbonyl group of the carotenoid and the polar alcohol 

 molecule. Such an effect cannot occur in a non-polar solvent such as 

 petroleum ether. On the other hand, when two hydrogens are added to 

 rhodoxanthin, the resulting dihydrorhodoxanthin no longer shows this 

 anomaly, as the carbonyl groups are no longer conjugated. The latter 

 conditions also explain the failure of zeaxanthin to exhibit such divergent 

 absorption spectra in different solvents. 



Marked changes result in the form of the absorption curves when trans -^ 

 cis changes occur. The absorption maxima occur at the longest wave 

 lengths in the natural all-/7'ans carotenoids. With the increase in the num- 

 ber of cis linkages, the color of the carotenoid is decreased, since the absorp- 

 tion maxima are shifted toward the ultraviolet end of the spectrum. One 

 of the most characteristic changes which occurs with an increase in cis 

 linkages is the development of the so-called "cis peak" in the ultraviolet 

 region. Practically no absorption takes place in this area in the case of the 

 all-^rans compounds, but it may become an area in which the extinction 

 coefficient is of considerable importance after isomerization. The location 

 and intensity of this cis peak is of great value in establishing the position 

 and number of the cis bonds. A correction for this effect is important 

 when one is determining vitamin A in the presence of carotenoids, as the 

 maximum absorption for the cis peak is frequently in close proximity to 

 that employed for the estimation of vitamin A. A further discussion of 

 this effect is included in the section on stereoisomerism of the carotenoids 

 (see pages 621 ff.). 



The absorption maxima of the more common C40 polyene hydrocarbons 

 and of some of their derivatives in several solvents are included in Table 15, 

 while Table 16 summarizes the same information for the simpler carote- 

 noids. 



Not only are the points of maximum absorption of considerable impor- 

 tance in establishing the identity of a substance, but likewise the relative 

 intensities of the various maxima are of great help. In addition, it is fre- 

 quently of as much value to know the points of minimum absorption. All 

 these data are available in the molecular extinction curves. *™~^^' Data 



^'» L. Zechmeister and A. Polgdr, J. Am. Cheni. Soc, 65, 1522-1528 (1943). 

 *^ H. J. Deuel, Jr., C. Johnston, E. R. Meserve, A. Polgdr, and L. Zechmeister, Arch. 

 Biochem., 7, 247-255 (1945). 



*^ P. Karrer and E. Wiirgler, Helv. Chim. Acta, 26, 116-121 (1943). 



«i A. Polgdr and L. Zechmeister, J. Am. Chem. Soc, 66, 186-190 (1944). 



