THE STRUCTURE OF VISUAL PURPLE (RHODOPSIN) 



1948) appears to be identical with the natural retinene^ which can be 

 extracted from bleached visual purple solutions by a mixed solvent 

 consisting of acetone, ethanol and hght petroleum. Thus, in chloro- 

 form, both synthetic and natural retinenej have density maxima at 

 380-385 mfx and both give the same colour test (^^max = 664) with 

 the antimony trichloride reagent. 



When synthetic retinene^ is dissolved in 1 per cent aqueous 

 digitonin its density spectrum (Amax = 380 m/^) is not affected by pH. 



400 500 



Wavelength (m/^.) 



600 



Fig. 4.3. Imitation of the reactions of indicator yellow. Curve A, 



density spectrum of compound formed by the reaction of retinenci and 



methylamine in alkaline solution; curve B, the same after acidification. 



{Ball, Collins, Dalvi and Morton, 1949) 



The density spectrum of a bleached visual purple solution, on 

 the other hand, is dependent on the pH: in alkaline solution the 

 colour is pale yellow whilst in acid solution it is deep chrome 

 yellow. LYTHGOE (1937) gave the name 'indicator yellow' to 

 the substance responsible for these changes. Indicator yellow has 

 ^max = 365 m/^ in alkaline solution and Amax = 440 m/^ in acid 

 solution. 



The probable relationship between retinenci and indicator yellow 

 has now been estabhshed by the briUiant investigations of morton 

 and his colleagues at Liverpool (ball, collins, morton and 

 STUBBS, 1948; ball, collins, dalvi and morton, 1949; collins, 

 1953). These authors studied the interaction between retinenci and 

 amino compounds. The procedure was to mix an aqueous solution 

 of the amino compound with an alcoholic solution of crystalline 



105 



