THE STRUCTURE OF VISUAL PURPLE (RHODOPSIN) 



took place, the alkaline solutions had an absorption band maximum 

 at 360-370 m^. On acidification, this band was replaced by another 

 having its maximum at 435-460 mju. When no reaction occurred 

 (i.e. with dimethylamine, urea and formamide) the solutions showed 

 merely the characteristic band of uncombined retinenci (Amax = 

 385-390 m^); this was unaflfected by pH. 



Although a large excess of amino compound over retinene^ was 

 required in these experiments the authors considered that the reac- 

 tion must involve two molecules of retinenci to one of the amine 

 'since one molecule of retinencj with one molecule of aliphatic 

 amine could scarcely give the indicator yellow type of spectrum. 

 This implies that conjugated proteins resembhng and including 

 indicator yellow itself possess two retinenci molecules attached to 

 the same amino nitrogen atom.' (ball, collins, dalvi and 

 MORTON, 1949.) 



As a result of this work, collins and morton (1950b) suggested 

 the following as the most Hkely formula for the condensation product 

 of retinenci and methylamine in alkahne solution. 



In this formulation there is a break in the conjugation at the N-group 

 and the structure, having two sets of six conjugated double bonds, 

 may be compared with two unconjugated vitamin Ag residues 

 (^max = 350 mju). Addition of acid would then give 



with *fuir conjugation restored. 



The first structure corresponds to alkaline indicator yellow and 

 might be expected to have an absorption band maximum at 365 m//. 

 The second structure corresponds to acid indicator yellow with 

 ^max at 440 mfi. 



The methylamine-retinenci compound was, of course, proposed 

 only as an analogue to indicator yellow. In the latter substance the 

 protein was supposed to be attached to the central nitrogen atom in 

 place of the methyl (— CH3) group. 



107 



