54 RELATIONSHIPS BETWEEN COLOUR AND CONSTITUTION VII 



not yet possible at the present time to predict the ultra-violet absorption of a 

 carotenoid from its structure. As an example of a typical carotenoid spectrum 

 the absorption curve of xanthophyll is shown above. 



Most natural carotenoids, and also their degradation products such as 

 ^-apo-2-carotenal, /5-apo-2-carotenol, ^S-citraurin, /5-carotenone, etc., exhibit 

 absorption curves similar to the one shown here. 



Investigations by different workers have shown that the following empirical 

 relationships exist between the constitution and absorption spectrum of 

 polyenes : 



1. The addition of a conjugated double bond without other changes in 

 structure results in a displacement of the visible absorption bands towards 

 longer wavelengths by 20-22 m^ (in carbon disulphide solution). The removal 

 of a conjugated double bond has the opposite effect. 



Example: 



Crocetin, 7 conjugated ethylenic bonds, longest wavelength maximum in CSj at 



482 mjx. 

 Bixin, 9 conjugated ethylenic bonds, longest wavelength maximum in CSj at 



523.5 m/i. 



2. If an ethylenic bond in a six-membered ring is moved from a conjugated 

 to an isolated position the maxima are displaced by 9-1 1 m[x towards shorter 

 wavelengths. 



Examples: 



y3-Carotene, 11 conjugated ethylenic bonds, longest wavelength maximum in CSg at 



520 m/z. 

 a-Carotene, 10 conjugated and 1 isolated ethylenic bond, longest wavelength 



maximum in CSj at 509 m//. 

 Zeaxanthin, 11 conjugated ethylenic bonds, longest wavelength maximum in CSg 



at 517 vsif.1. 

 Xanthophyll, 10 conjugated and 1 isolated ethylenic bond, longest wavelength 



maximum in CSg at 508 m/t. 



3. If a terminal conjugated double bond is replaced by an epoxide group, 

 the maxima are displaced by 6-9 m^ towards the blue end of the spectrum, 



Examples: 



a-Carotene, 10 conjugated and 1 isolated ethylenic bond, longest wavelength 



maximum in CSj at 509 m/<. 

 a-Carotene epoxide, 9 conjugated and 1 isolated ethylenic bond and 1 epoxide 



group, longest wavelength maximum in CSj at 503 m/^. 

 Xanthophyll, 10 conjugated and 1 isolated ethylenic bond, longest wavelength 



maximum in CSj at 508 m//. 

 Xanthophyll epoxide, 9 conjugated and 1 isolated double bond and 1 epoxide 



group, longest wavelength maximum in CSj at 501.5 m^. 

 /3-Carotene, 11 conjugated ethylenic bonds, longest wavelength maximum in CSg 



at 520 Ta.fl. 



References p. 59. 



