THE VISUAL PIGMENTS AND THEIR PHOTOPRODUCTS 



density spectrum of transient orange. We now know from the work 

 of COLLINS and morton (1950c) that when a cold visual purple 

 solution is bleached to the transient orange stage, and then allowed 

 to stand in darkness, some of the transient orange breaks down to 

 indicator yellow and some takes part in the formation of a photo- 

 sensitive pigment ('iso-rhodopsin'). (This is well shown in Fig. 2.9.) 

 Thus the curve 77 in Fig. 2.7 is for a mixture of indicator yellow 

 and 'regenerated' pigment — not a pure indicator yellow curve as 

 lythgoe and quilliam supposed. 



LYTHGOE and QUILLIAM repeated this experiment over a range of 

 hydrogen ion concentrations (pH 4-9-9-25). They obtained sub- 

 stantially the same results for transient orange, though in markedly 

 acid or alkaline solutions the early changes in the density spectra — 

 following exposure of the solutions — were too rapid for their slow 

 apparatus, lythgoe and quilliam estimated the Amax of transient 

 orange to be at 470 m/u. They also found a quantitative relation 

 between the fading of transient orange and the formation of the 

 j&nal product (indicator yellow). This showed that indicator yellow 

 was formed via transient orange and was not, for example, an 

 alternative initial product of the bleaching. 



WALD (1938) also obtained essentially similar results for the 

 *dark process' that followed irradiation of a visual purple solution 

 (prepared from bull frogs), wald's apparatus, which recorded an 

 absorption spectrum in 2 min., was admirably suited for the study of 

 unstable intermediates, but he lost some of this advantage through 

 working at a high laboratory temperature (26°C). Consequently 

 wald's *dark process,' maximal at c. 480 m/u, amounted only to 

 some 20 per cent of the visual purple maximum — compared with 

 nearly 90 per cent found by lythgoe and quilliam (Fig. 2.7). 

 Thus over 70 per cent of the transient had already faded before 

 WALD had measured the first curve, a minute or two after bleaching. 



WALD (1939b) carried out a similar experiment with a solution of 

 porphyropsin (visual violet) prepared from the white perch {M or one 

 americana). In the upper part of Fig. 2.8, curve A gives the original 

 density spectrum of the solution, curve B that after 30 sec. exposure 

 to a bright light and curve C that about an hour later, the solu- 

 tion having been kept in darkness meanwhile. The experiment was 

 carried out at room temperature. Consequently, as in the previous 

 experiment, and in spite of the shortness of the exposure time, 

 considerable fading of the initial photoproduct must have occurred 



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