THE STRUCTURE OF VISUAL PURPLE (RHODOPSIN) 



According to these ideas, the formula' for visual purple may be 

 written, 



]s. 



in which the vertical dotted Hnes represent the 7r-electron bonding 

 between the polyene chain and protein and d+ and 6— the electrical 

 tension which, because of electron displacements, is set up between 

 them. This structure has a formal resemblance to that proposed by 

 KUHN and SORENSEN (sec KARRER and JUCKER, 1950) for ovoverdin, 

 the blue-green chromoprotein of lobster shell. 



f*rotein + + 



Ovoverdin (chromophore attached at both 

 ends to the same protein molecule) 



Astaxanthene, the chromophore of ovoverdin has Amax about 

 500 m^ while ovoverdin itself has an absorption band centred at 

 640 mfi (stern and Salomon, 1937). The spectral interval between 

 the 2max of visual purple (500 m/^) and of retinene, its chromophore 

 (385 m/i), is similar. 



As described in the next chapter, only those retinenes of certain 

 cis-trans conformations will react with opsin (visual purple protein) 

 to form photosensitive pigments. When bleached, however, these 

 pigments yield an inactive dill-trans isomer of retinene. Thus when 

 the chromophore of visual purple absorbs a quantum of energy, the 

 consequent raising of the electronic levels must cause an alteration to 

 the shape of the chromophore so that it no longer has the 'lock and 

 key' correspondence to protein. The effect of this is twofold : the 

 electric tension between chromophore and protein disappears (giving 

 rise, perhaps, to excitation processes in the retinal rod) and the parts 

 of the protein molecule which were 'covered' by the chromophore are 

 now revealed (sulph-hydryl groups). 



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