THE STRUCTURE OF VISUAL PURPLE (RHODOPSIN) 



means of a salt bridge to another half cell in which a rotating platinum 

 electrode was in contact with an ammoniacal solution (pH 9) of 

 rhodopsin in 2 per cent digitonin. The electrical circuit was com- 

 pleted through a galvanometer. Silver nitrate (0-001 M) was titrated 

 into the visual pigment solution, the galvanometer readings being 

 recorded for each 0- 1 ml increment of silver nitrate. The results of a 



60- 



50 



C 40 

 



I 



zo- 



X 



E 



o 20 

 



> 



10 



T — I — I — I — I — 1 — I — T — r- 



Tiiraiion of caii/e rhodopsin 



1 — I — r 



1 — I — I — I — r 



J ! 1 L 



0.1 



mt. silver ni-ira-6e''0.00/M 



Fig. 4.6. Amperometric titration of visual purple with silver nitrate. 



The horizontal distances (marked 0-19 ml and 0-20 ml) give the silver 



ion equivalent of the sulph-hydryl groups liberated on bleaching (at the 



arrow labelled 'light'). 



{Wald and Brown, 1952) 



typical experiment are shown in Fig. 4.6. The titration was begun in 

 dim red hght to avoid bleaching the visual pigment. No current 

 flowed until 0-2 ml of silver nitrate solution had been added, this 

 amount being absorbed by the free sulph-hydryl groups present in the 

 preparation. A further 0-4 ml of silver nitrate solution was then 

 added. This produced a Hnear increase of current in the circuit. The 

 rhodopsin was then bleached by white light. The current fell (see 

 Fig. 4.6) because the — SH groups exposed as a result of the bleaching 

 removed silver ions from the solution. Addition of more silver 

 nitrate ion brought the current back to its former value and beyond. 

 From this experiment, the silver ion equivalent of the — SH groups 



119 



