THE VISUAL PIGMENTS 



its density spectrum measured (meta-rhodopsin). Finally the solu- 

 tion, still in darkness, was warmed to room temperature and its 

 density spectrum measured once more (final product). The final 

 product was a mixture of alkaline indicator yellow (Amax 370 m/i) 

 and regenerated photosensitive pigment (Amax c. 487 m.fi). 



300 



400 600 



wo ve length - m/j 



600 



Fig. 2.9. Bleaching of cattle rhodopsin (visual purple) at low tempera- 

 tures in a glycerol-water mixture. Curves give the density spectra of the 

 preparation after the following treatments. 'Rhodopsin, 23°,' at 23° 

 before bleaching; 'rhodopsin, — 45°,' at — 45° before bleaching; 

 'lumi-rhodopsin, — 40°,' after complete bleaching at the low tempera- 

 ture; 'meta-rhodopsin, — 55°,' after warming to — 15° in darkness, 

 allowing all changes to complete and then cooling to — 55°; 'final 

 product, 23°,' after allowing it to warm up to room temperature in 

 darkness. All spectra corrected for changes in solvent volume due to 

 changing temperatures. 

 {_Wald, Durell and St. George (1950)) 



In another experiment wald, durell and ST. george {ibid.) 

 showed that the same sequence of reactions took place at room 

 temperature. To record the initial changes, which are normally very 

 transient at room temperature, they took advantage of the fact that 

 in the dry state rhodopsin does not bleach beyond the transient 

 orange stage. They therefore studied the bleaching of rhodopsin in a 

 dried gelatine film. The original density spectrum of their film is 

 shown in Fig. 2.10 ('rhodopsin'). It was then exposed to the very 

 brief illumination of a photoflash lamp and the density spectrum 



48 



