RELATIONSHIPS BETWEEN COLOUR AND CONSTITUTION 57 



TABLE 12 



DISPLACEMENT OF THE POSITION OF MAXIMA BY DIFFERENT SOLVENTS* 



* Only the longest wavelength maxima are given. Some of the data in the third column 

 were obtained using petroleum ether instead of hexane as a solvent. 



carotenal, etc.), relationships are more complicated, probably owing to the 

 interaction which can occur between the pigment and certain solvents, e.g. 

 alcohol. Thus, the spectrum of capsanthin in alcohol is completely blurred. The 

 same behaviour is exhibited by y5-apo-2-carotenal and other polyene ketones 

 containing carbonyl groups conjugated with the system of ethylenic bonds. 

 If the conjugation between the ethylenic bonds and the carbonyl group is 

 broken, however, the absorption maxima are as sharp as usual^. 



The empirical relationships here described between the constitution of 

 carotenoids and their light absorption properties have general validity. The 

 exceptions which are occasionally encountered may often be ascribed to a lack 

 of stability of the pigments involved and do not detract from the great value of 

 spectroscopy in carotenoid research. 



The theoretical interpretation of the visible and ultraviolet absorption 

 spectra of natural and synthetic polyenes has also attracted considerable 

 attention within recent years. The absorption of light in this region of the 

 spectrum is thought to give rise to electronic oscillations along the axis of the 

 polyene chain, and it can be predicted, on this basis, that the wavelengths and 

 intensities of the maxima will increase with the number of conjugated ethylenic 

 bonds^. 



References p. sg. 



