644 VI. CAROTENOIDS AND RELATED COMPOUNDS 



Table 29 

 Visually Observed Absorption Maxima of Some Members of the Methylbixin 

 Set, Arranged in Order of Decreasing Affinities, and of the Dimethylcrocetin 



Set" 



Natural methylbixin (labile) 485,453.5 503,470 



All-/ra«.s-methylbixin (stable) 490,457 508,5,475 



Neomethylbixin A 485,454 502.5,469 



Neomethyllnxin B 471,444.5 491,458 



Neomethylbixin C 479.5,449 496,463 



" Data from L. Zechmeister, Chem. Revs., 34, 267-344 (1944), pp. 324, 334. 



tween 250 and 300 m/i.'*''' When a trans-^cis isomerization occurs, the 

 intensity of the extinction values in the lo\ver ultraviolet area is decreased 

 to the same degree as in the visible portion of the spectrum. However, 

 there is no corresponding shift of the extinction maxima of the ultraviolet 

 area to shorter wave lengths such as occurs with the maxima in the visible 

 portion of the spectrum. 



(c) The cis-Peak Effect. The area between 320 and 380 m/x has practi- 

 cally no inflection in the aW-trans carotenoids. However, Zechmeister and 

 Polgar'*''^ discovered that trans— ^cis changes were associated with the forma- 

 tion of maxima of considerable magnitude in this area, the intensity of 

 which was proportional to the extent to which isomerization had occurred. 

 This new maximum has been referred to as the "cf.s peak," and the range of 

 wave lengths at which it appears is called the "m-peak region." Figure 25 

 furnishes an example of the nature of the cis peak and the rapidity with 

 which it develops. 



It has also been demonstrated by Zechmeister and Polgar'*^^ that the 

 position of the maximum absorption of the cis peak bears a definite rela- 

 tionship to the extinction maxima in the visible spectrum. The difference 

 between the wave length of the cis peak and that of the extinction maximum 

 having the longest wave length of the all-^rans isomer was found to be 

 142 ± 2 mju, in hexane. The extraordinary uniformity of this value is 

 illustrated by the data recorded in Table 30. 



As illustrated in Figure 25, the as-peak effect may develop almost instan- 

 taneously when the solution containing the aW-trans compound is exposed 

 to light. Thus, with lycopene in hexane, 92% of the maximum effect is 



