THE STRUCTURE OF VISUAL PURPLE (RHODOPSIN) 



Spectra would probably be displaced to shorter wavelengths, i.e. in 

 the opposite direction to the observed shift which accompanies 

 rhodopsin synthesis. Moreover, the higher values for the sulph- 

 hydryl : retinene ratio obtained in the amended calculations pose a 

 difficulty. WALD and brown (1953) therefore suggested that some 

 sulph-hydryl may have had a different origin, i.e. that the bleaching of 

 rhodopsin might be a denaturation process, resulting in the exposure 

 of *sulf hydryl and other groups on the protein, which had previously 

 been unavailable to reagents.' 



CONCLUSION 



The work of the Liverpool school indicates that the first stable 

 product formed when rhodopsin bleaches is indicator yellow, and 

 that in this substance a carbon-nitrogen bond Hnks retinene to 

 protein. The work of the Harvard school, on the other hand, 

 suggests that, in rhodopsin, retinene is coupled to protein via a 

 sulphide or disulphide linkage. 



In a recent paper, Collins, green and morton (1954) have 

 endeavoured to reach a compromise. Since the evidence from the 

 syntheses (wald and brown) suggests that rhodopsin is 'retinene — 

 S — opsin' while the analytic evidence (morton and colleagues) 

 indicates that it is 'retinene — N — opsin,' collins, green and 

 MORTON have proposed that freshly formed 'retinene — S — opsin' 

 undergoes an intra-molecular rearrangement to 'retinene — N — opsin.' 

 In their view the N-compound, since it would absorb at longer wave- 

 lengths than the S-compound, represents a more Ukely formulation 

 for rhodopsin. 



But this proposal allows that wald and brown have provided 

 adequate evidence for the involvement of the aldehydic group of 

 retinene in the reaction with the sulph-hydryl groups of opsin. How- 

 ever, for reasons to be given shortly, the writer beheves that this is 

 not the case and that, in consequence, the results of the two schools 

 are not mutually exclusive, as might at first sight appear. 



assessment of the evidence 



The work of morton and his colleagues leaves Httle room 

 for doubting that indicator yellow is a key to the structure of 

 rhodopsin. 



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