THE VISUAL PIGMENTS 



has a resultant dipole at right angles to the chain which gives rise to 

 the cis peak or 'half chromophore' band. 



The second overtone band in the far ultraviolet (c. 280 m^) results 

 from an induced concentration of 7r-electrons alternately in the first 

 and third and in the second and fourth quarters of the chain. These 

 quarter oscillations confer an overall dipole moment on the molecule 

 and one which is maximal for an 2i\\-trans isomer and less for any 

 c/5-form. 



A cis bond has its greatest effect when situated at the centre of the 

 chain for then it reduces the total chromophore length by a factor, of 

 cos 30° and hence decreases the intensity of the visible or full chromo- 

 phore band by cos^ 30 = 0-75. The cz j-peak or half-chromophore 

 band on the other hand is then maximal. 



This discussion can be summarized in tabular form as follows : 



Absorption characteristics of the polyenes 



Band 



Spectral* 

 location 



Visible 



Intensity of 



absorption in 



fl//-trans 



isomers 



Very great 



Near 

 ultraviolet 



Very small 



Full 

 chromophore 



Half 

 chromophore 



Quarter 

 chromophore 



* These spectral locations refer to polyenes containing about 12 conjugated double 

 bonds in the chain. 



Far ultraviolet Moderate 



Intensity of 



absorption 



after 



isomerization 



Decreased 



(maximum 



decrease 25 %) 



Increased to 



moderate 



intensity 



Decreased 



INTERCONVERSION OF CIS-TRANS ISOMERS 



Isomerization, that is the conversion of a specimen of a given 

 configuration into a mixture of other cis-trans forms, can be eff'ected 

 in a number of ways: for example, by reflux distillation of a solution 

 of the carotenoid, by melting the crystals, by treatment with iodine or 

 acids, and also by illumination. Separation of the mixture of isomers 

 can then be carried out on the chromatograph column, for the strength 

 of adsorption of the isomers varies considerably. 



What is the mechanism of isomerization? A conjugated chain is 

 potentially capable of interchanging its double and single bonds. 

 This event, followed by a return to the original state could result in 



134 



