PROPEKTIES OF THE CAROTENOIDS 639 



t.ioiis, therefore, which accompanies a trans-^cis rotation is the sliifting of 

 the absorption curve toward the ultraviolet end of the spectrum. This is 

 also accompanied by a decrease in the extinction values at the maxima, 

 since the actual intensity of the absorption of the cis form is less than that 

 of the corresponding all-^rans isomer. The change in position of the 

 maxima and of the height of the extinction curve is so pronounced as to be 

 visible to the naked eye when the solution of the all-^rans carotenoid is 

 sufficiently concentrated to be visible. The addition of a drop of iodine to 

 catalyze the stereoisomeric change is immediately followed by a noticeable 

 decrease in intensity of the color of the solution. 



On the other hand, when stereoisomeric changes are produced in carote- 

 noids having cis bonds, a definite increase in color intensity is noted. 

 This results from the reversible reaction whereby some of the cis isomer is 

 changed back to the all-^rans compound. However, because such solutions 

 always consist of an equilibrium mixture which contains the various cis 

 isomers as well as the all-^rans carotenoid, the color intensity produced by 

 catalytic action on a cis isomer never reaches the intensity of that given by a 

 corresponding amount of the oXX-trans form. The accomplishment of a 

 cis^^trans rotation results not only in the shift of absorption maxima away 

 from the ultraviolet region toward the longer wave lengths, but also in an 

 increase in the intensity of such absorption maxima. It is, therefore, an 

 exact reversal of the phenomena which accompany a trans— ^cis rotation. 

 Although the single bonds in a conjugated double bond system like that 

 present in a carotenoid attempt to maintain a coplanar arrangement, there 

 is some overlapping of the hydrogen atoms at the cis linkages which tends 

 to force the cis isomer out of coplanarity. 



The first evidence of the relationship of structure to spectroscopic behavior 

 was given by Gillam and El Ridi.^*^ Many data on the spectral charac- 

 teristics of natural and isomerized carotenoids down to a wave length of 

 380 m/i have been reported by Strain, i^^-"^-^^' Beadle and Zscheile,^"^ 

 White, Zscheile, and Brunson,^^" and White, Brunson, and Zscheile.^"* 



The effects of isomerization of several all-^raws carotenoids with a cata- 

 lytic amount of iodine on the absorption curves are shown in Figures 17-24. 

 Similar but less pronounced alterations are produced in each case by reflux- 

 ing in darkness for 45 minutes. The curve for the isomeric mixture pro- 

 duced when all-i7"ans-7-carotene is so treated is included in Figure 19. 



The comparative spectra maxima of some members of the methylbixin 

 and of the dimethylcrocetin sets are included in Table 29 (see page 644). 



In addition to the absorption in the visible portion of the spectrum, all- 

 Irans carotenoids exhibit extinction maxima in the ultraviolet region be- 



60' B. W. Beadle and F. P. Zscheile, J. Biol. Chem., 144, 21-33 (1942). 

 *•>* J. W. White, A. M. Brunson, and F. P. Zscheile, Ind. Eng. Chem., Anal. Ed., 1 i, 

 798-801 (1942). 



