THE VISUAL PIGMENTS 



irradiation, thawing in darkness, and measurement of density spec- 

 trum — was repeated a number of times. 



The results of an experiment with frogs' visual purple are given in 

 Fig. 3.7. The 'regenerated' pigment (iso-rhodopsin) differs sHghtly 

 from the original one. This is shown by the different density maxi- 

 mum (at 493 mju in place of 502 m/i) and by the fact that the curves 

 for the first, second, third and fourth regeneration stages, and for the 

 final bleached solution all pass through an isosbestic point — at about 



300 



400 



500 



600 



Fig. 3.7. Formation of an isomer of visual purple. Heavy continuous 

 curve is the density spectrum of a frog's visual purple solution. This was 

 completely bleached at — 70°C and then allowed to reach room- 

 temperature equilibrium in darkness ( ). Succeeding curves 



show the results obtained when this procedure was repeated three times. 



The final continuous curve was obtained after complete bleaching at 



room temperature. Note that the >?.max for the 'regenerated' pigment 



(iso-rhodopsin) differs from the original visual purple. 



{Collins and Morton, 1950b) 



415 m// — which does not lie on the curve for the original pigment. 

 Similar results were obtained with the visual purple (Amax = 497- 

 500 m/0 of rats and of oxen, except that the regenerated iso-rhodopsin 

 had ;.max at 487-488 m/<. 



As Fig. 3.7 shows, the amount of pigment regenerated in each 

 darkness period was about 50 per cent of that originally present. 

 This was confirmed by wald, durell and ST. george (1950) who 

 showed, in addition, that similar results were obtained at room 

 temperature when visual purple in a dry gelatine film was bleached, 

 and then moistened in darkness (see Figs. 2.9 and 2.10). 



To explain the 50 per cent regeneration, collins and MORTON 



80 



