CHAPTER FOUR 



The Structure of Visual Purple 

 (Rhodopsin) 



Visual purple has not yet been obtained in the crystalline state. 

 Indeed it is very doubtful whether pure solid specimens have ever 

 been prepared. Normally the pigment is studied in digitonin solution 

 and it is a difficult enough matter to ensure that such preparations 

 are free from impurities which absorb hght; to say nothing of those 

 which do not. The amount of the colourless digitonin present, for 

 example, is much greater than that of the visual pigment. No simple 

 solvent for visual purple is known. It will not dissolve in water or 

 petroleum hydrocarbons and is destroyed by such solvents as alcohol, 

 acetone and chloroform. 



Because of these difficulties and the fact that only very small 

 amounts of visual purple can be obtained from a reasonable number 

 of eyes, the usual chemical methods employed in working out 

 formulae have only a limited application. Nevertheless by ingenious 

 arguments from diverse data, and a leavening of inspired guesswork, 

 good progress has been made. 



Visual purple is a chromoprotein, that is its molecule consists of a 

 protein to which is attached one or more chromophores (i.e. molecu- 

 lar groups having the property of absorbing visible light). In this 

 respect visual purple is similar to such substances as haemoglobin, 

 cytochrome C, haemocyanin, ovoverdin, and so on. In a chromo- 

 protein it is normally the chromophore which is the reactive part of 

 the molecule, the protein acting mainly as a support. For example, 

 in haemoglobin the chromophore haeme is the carrier of oxygen. 

 Similarly with visual purple the chromophore is responsible for the 

 A- and 5-absorption bands in the 'visible' spectrum and is the initial 

 site of photochemical change. In most chromoproteins the chromo- 

 phore structure includes a metal — usually iron or copper. Visual 

 purple, however, is free from heavy metals and consequently is more 

 closely related to such chromoproteins as ovoverdin and yellow 



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