THE VISUAL PIGMENTS AND THEIR PHOTOPRODUCTS 



the difference between the initial transient and the final stable pro- 

 duct. This curve has a maximum at c. 515 m^ (cf. 480 m^ for the 

 transient orange of visual purple). This experiment provides the 

 only data we have on the transient of visual violet (porphyropsin). 

 The overall difference spectrum, A-C in Fig. 2.8, gives the changes 

 that are normally measured, viz. the difference between the density 

 spectrum of the original visual pigment and that of its final stable 

 product. 



BRODA and GOODEVE (1941) found that a large proportion of 

 glycerol could be added to solutions of visual purple without effect, 

 other than that of dilution. By working with solutions consisting of 

 three volumes of glycerol to one of water, they were able to extend 

 the range of experiments on visual purple down to — 73°C. At this 

 temperature the solutions assumed a glass-like consistency and 

 remained perfectly clear. On exposure to light the solutions 

 *bleached' to an initial photoproduct (transient orange) which was 

 quite stable at — 73°C. The density spectrum of the transient orange 

 was independent of the hydrogen ion concentration between pH 6 

 and pH 9. On warming the bleached solutions, the transient orange 

 decomposed to secondary products. 



At — 73°C the visual purple density spectrum was narrower and 

 higher than at room temperature and its maximum was shifted from 

 505 m/< to 515 m//. The density spectrum of transient orange at 

 — 73°C was similarly affected, the maximum being at 510 m/z. The 

 spectral interval between the maximum at room temperature and 

 that at — 73°C was thus much greater with transient orange, about 

 30 mfi, than with visual purple, about 10 m//. Consequently at low 

 temperatures the change in colour, when solutions were exposed to 

 light, was so slight, as to give the visual impression that the visual 

 purple was photostable. 



Lumi- and meta-rhodopsin. Recently, wald, durell and ST. 

 GEORGE (1950) have described experiments which indicate a further 

 complexity in the initial stages of bleaching. The first experiment 

 (see Fig. 2.9) was carried out at low temperatures with a 2: 1 glycerol- 

 water solution of rhodopsin (visual purple). The density spectrum of 

 this solution was measured at 23°C and at — 45°C. Whilst still at 

 the low temperature the solution was then exposed to intense white 

 light until all changes were complete (curve labelled 'lumi-rhodopsin' 

 in Fig. 2.9). The solution was then warmed to — 15°C in darkness 

 and, when all changes had ceased, it was cooled again to — 55°C and 



47 



