PKUl'EKTIES OF THE CAKO'IENOIDS 



049 



The greatest effect on absorption in the c/s-peak region occurs with mono- 

 cia isomers in which the new Hnkage is in the middle of the molecule, i.e., in 

 the center of the conjugated double bond system. The greater the distance 

 from this center at which a mono-c/s linkage occurs on either side, the less 

 pronounced will the resulting absorption be in the c/s-peak region. In the 

 case of all-c/.s' compounds, the m-peak region is practically as flat as it is in 

 the case of the sdl-trans compound.-^- This is in line with the idea that a 

 bending of the al\-trans molecule, especially in the middle of the conjugated 

 system, is mainly responsible for the appearance of the high cis peak. 



(d) Effect of Isomerization of cis Coiwpounds on Spectral Characteristics. 

 The reversibility of the trans-cis rearrangement is beautifully illustrated by 

 the effect on the spectral properties when isomerization is produced, starting 

 with a neo isomer. Because this will result in the production of a substan- 

 tial amount of the sdl-trans form, the absorption maxima of such an equilib- 

 rium mixture of isomers is shifted to the right, i.e., to higher wave lengths, 

 the extinction coefficients of such maxima are also increased, and the cis- 

 peak effect is markedly lessened. The effect of iodine catalysis on the 

 molecular extinction curves of such natural poly-a's isomers as prolycopene 

 and pro-7-carotene, as well as all-as-lycopene, is illustrated in Figures 

 27-29. 



g. Structural Configuration of Stereoisomers. Thermal and photo- 

 cliemical isomerization methods, as well as data on absorption spectra. 



300 400 



WAVE LENGTH IN MJIX 



Fig. 27. Molecular extinction curves of prolycopene in hexane: ( — ) fresh 

 solution of prolycopene; (--) mixture of stereoisomers in proljxopene solution 

 after iodine catalysis at room temperature in light.^^^ 



