THE STRUCTURE OF VISUAL PURPLE (RHODOPSIN) 



with a half-period of 25 min at 18°C. In the case of indicator yellow, 

 a similar hydrolysis to retinene and protein would be expected. 



In one experiment, 1 ml of visual purple solution was mixed with 

 3 ml of 40 per cent formaldehyde to which buffer had been added to 

 bring the pH to 9. The optical density of the solution at 500 m^ was 

 0-760. The solution was then bleached with intense light. The 



<&o 



300 



350 



450 



400 

 Wavelength in mp 



Fig. 4.4. The hydrolysis of alkaline indicator yellow to retinene at pH 9 

 in the presence of 10 M formaldehyde. Curve A^ density spectrum of 

 freshly bleached visual purple solution; dashed curves, later measure- 

 ments; curve B, after 60 min. 

 {Redrawn after Collins, 1953) 



difference in density at 500 m// before and after bleaching was 0-728. 

 Taking the molecular extinction coefficient of retinene as 48,000, 

 these measurements show that the solution of visual purple was 

 15-2 X 10~^ molar in potential retinene. Immediately after bleach- 

 ing, the density spectrum of the solution was measured at frequent 

 intervals, for an hour, over the range 300-460 m^. The results 

 (Fig. 4.4) showed a well-defined isosbestic point indicating the 

 transformation of one substance into another. The final density 

 spectrum (curve B, Fig. 4.4) corresponds closely to free retinene, the 

 first spectrum (curve A, Fig. 4.4) to a mixture of retinene and alkaUne 

 indicator yellow. The molecular extinction coefficient (aqueous) of 



113 



