THE VISUAL PIGMENTS 



meta-rhodopsin, rather than with lumi-rhodopsin or with a mixture 

 of the two. 



A stable, photosensitive form of transient orange. Before leaving 

 the subject of the initial photoproduct of visual purple, mention 

 should be made of some singular observations by berger and segal 

 (SEGAL, 1950; BERGER and SEGAL, 1950a, b). These authors 

 reported that in their experiments the transient' orange, formed by 

 bleaching (frog) visual purple solution at room temperature, was 

 thermally stable. They attributed this to a minor modification in 

 their method of preparing visual purple, namely to their practice of 

 extracting the pigment with buffered digitonin solution. Most 

 workers use unbuffered digitonin solutions and adjust the pH of the 

 resulting extract afterwards, berger and segal (1950) state that 

 when solutions of visual purple, which had been prepared at pH 7 in 

 their way, were exposed to Ught, the 'transient' orange so formed 

 could be kept in darkness for more than half-an-hour without 

 suffering appreciable decomposition. They further state that the 

 transient' orange was photosensitive and, consequently, that unless 

 the light used for the original bleaching of the visual purple was of 

 long wavelength, some 'transient' orange was photochemically 

 transformed into indicator yellow. 



The data given by berger and segal for the density spectrum of 

 their 'transient' orange agree with the data of other workers. Thus 

 the maximum density (i.e. at 480 m^) of 'transient' orange produced 

 from a visual purple solution of maximum density 1-0 (i.e. at 500 mju) 

 was 0-8 (cf. Figs. 2.7 and 2.10). In particular their data agree well 

 with the density spectrum of meta-rhodopsin, if allowance is made 

 for the fact that their solutions were contaminated with yellow 

 impurities. There is httle doubt that the substance they describe is 

 closely related to, if not identical with transient orange. It is the 

 more surprising, therefore, that the properties of their initial photo- 

 product differ so markedly, viz. in thermal stability and in light- 

 sensitivity, from those accorded to transient orange by other workers. 

 Transient orange is certainly not light-sensitive at very low tempera- 

 tures, nor when in a dry film, but this may be because water is neces- 

 sary for the photochemical (as for the thermal) degradation to 

 indicator yellow. Unfortunately, lythgoe and quilliam's experi- 

 ments in fluid solution at 3°C give no information on the Ught- 

 sensitivity (if any) of transient orange when in contact with water, for 

 these authors studied the decomposition only in darkness (Fig. 2.7). 



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