THE STRUCTURE OF VISUAL PURPLE (rHODOPSIN) 



phore (A- and 5-bands at c. 500 m/^ and 350 m//, respectively) and 

 that due to protein (C-band at 280 mfi). 



The nature of the protein moiety probably varies from species to 

 species. Such variation might account for the slight differences, 

 observed in different animals, for the wavelength position of the 

 y^-band maximum. For example frog's visual purple has an ^-band 

 maximum at 502-503 m/u ; rat, ox and sheep visual purples at 

 498-500 m/i (collins and morton, 1950a); human visual purple 

 at 497 m^ (crescitelli and dartnall, 1953). 



THE CHROMOPHORE OF THE VISUAL 

 PURPLE MOLECULE 



Of more immediate interest is the structure of the chromophore 

 group and the nature of its linkage to the protein moiety. Informa- 

 tion about this has come mainly from a study of the simpler break- 

 down products of visual purple. 



THE visual pigments AND THEIR BREAKDOWN PRODUCTS, THE 

 RETINENES and VITAMINS A 



WALD (see Chap. 2) showed that the chromophore of visual 

 purple is related to the carotenoids, a large class of fat-soluble, 

 highly unsaturated pigments found in animals and plants. According 

 to WALD (1949): 'when rhodopsin is exposed to Hght in the retina, 

 two major changes occur; the carotenoid is cleaved from protein, 

 and is degraded through orange intermediates, first to the yellow 

 retinenci, then to colourless vitamin Aj. The retina can also re- 

 synthesize rhodopsin in two ways: rapidly by reversion from 

 retinenci, and relatively slowly from vitamin A^.' Porphyropsin (or 

 visual violet) the photosensitive pigment of fresh- water fish was shown 

 by WALD to be similarly related to corresponding substances, 

 retinencg and vitamin Ag. The absorption spectra of the visual 

 pigments (in 1 per cent aqueous digitonin) and of their corresponding 

 retinenes and vitamins A (in chloroform) are shown in Fig. 2.2. 



The structural formula for vitamin A^ (C19H27.CH2OH) is 



H3C^ /CH3 CH3 CH3 



HX 



^'\c/'"Vh/%ch/'%ch/'Sch/^ 



^2 



99 



