THE VISUAL PIGMENTS 



green — the absorption bands being centred respectively in the violet, 

 blue, green and red regions of the spectrum. 



Roughly speaking the addition of one conjugated ethylene group 

 ( — CH=CH — ) to poleyene structures causes a shift in the density 

 spectrum of about 20-25 m/bt towards longer wavelengths. Now 

 vitamin Aj, which has an absorption maximum at about 330 m//, 

 has five conjugated double bonds, while retinene^ which has, in 

 addition, a conjugated carbonyl group ( — C=0) has its maximum 

 at 380 mju. The y4-band of visual purple is maximal at 500 m/u, i.e. 

 120 mju further on. To account for this shift in terms of an increase 

 in length of the conjugated chain would require the addition of 5-6 

 double bonds, equivalent to the addition of another vitamin A^ 

 structure. 



Arguing along these lines wald (1949) suggested that the chromo- 

 phore of visual purple was derived from two molecules of vitamin A^ 

 or retinencj. Similarly, the chromophore of visual violet (porphy- 

 ropsin) was supposed to be derived from two molecules of vitamin Ag 

 or retinencg. 



Now the vitamin Ag molecule contains one conjugated bond more 

 than vitamin A^. This would account for the displacement of its 

 density spectrum to longer wavelengths. But the difference in spec- 

 tral position between the rhodopsin and porphyropsin bands is no 

 greater than the separation of the bands of retinene^ and retinencg, 

 or of vitamin Aj and vitamin Ag (Fig. 2.2). If the visual pigments 

 were derived from two molecules of vitamin A one would expect a 

 difference between them, corresponding to two conjugated double 

 bonds. 



A further difficulty is the fact that in nearly all the carotenoids — 

 which are based on a 'double vitamin A' structure — the main absorp- 

 tion band is characterized by three or more sharp subsidiary peaks. 



An exception to this general rule is the carotenoid astacene which, 

 like visual purple, has a single smooth absorption band in the visible. 



The spectra of astacene and visual purple are compared in Fig. 4.2. 

 Although both spectra have a maximum at c. 500 m/j, and are of 



102 



