THE HOMOGENEITY OF VISUAL PIGMENT SOLUTIONS 



We may define a homogeneous photopigment as one which yields 

 the same results (difference spectrum) when bleached — no matter 

 what the spectral composition of the bleaching Hght, nor what 

 proportion of the photopigment is affected. Every instalment of 

 photo-chemical bleaching — however caused — must be identical, after 

 allowance for the magnitude of the instalment has been made, 

 e.g. by expressing the difference spectra on a percentage basis. 



The results of a typical experiment which illustrates this test of 

 homogeneity are shown in Fig. 6.3 (A). Curve 1 represents the 

 original density spectrum of an unexposed extract prepared from 

 carp retinae. After 3 hr exposure to violet light (430 m/u) from a 

 monochromator, the density spectrum changed to curve 2. At this 

 stage in the experiment the solution was left undisturbed in darkness 

 for 18 hr, but this resulted in no significant changes. The solution 

 was then exposed to red light (630 m^) for 1 hr which caused bleach- 

 ing to curve 3. This was followed by a 1-hr exposure to green light 

 (530 mju) which caused further bleaching to curve 4. At this stage 

 the experiment was interrupted once more, the solution remaining in 

 darkness for 22 hr. During this period considerable regeneration of 

 the photopigment took place (curve 5). Finally the solution was 

 exposed to white light to complete the bleaching (curve 6). Differ- 

 ence spectra constructed from these results are shown in Fig. 6.3 (B). 

 They form a family of similarly-shaped curves. Several experiments 

 of this type were carried out. In all cases the difference spectra 

 obtained — when plotted with their maxima equally scaled — were 

 practically identical, showing that the extracts contained only one 

 photosensitive pigment. 



In this instance it is fairly easy to interpret the results obtained. 

 The same applies, also, in a case, Uke that of the tench, where there is 

 a mixture of two components absorbing in widely separated parts of 

 the sjDectrum. It is then possible to subject the component which 

 absorbs in the longer wavelength regions to the partial bleaching test 

 of homogeneity, without introducing serious complications due to 

 concurrent bleaching of the other component. When all of the red- 

 sensitive component has been removed, the other component can be 

 likewise examined. 



It may happen, however, that in a binary mixture there is a con- 

 siderable ^overlap' between the density spectra of the two photo- 

 pigments. It would thus be impossible to bleach one pigment without 

 affecting the other. In such a case a sharp 'separation' cannot be 



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