THE VISUAL PIGMENTS 



y4-band, maximal at about 500 mjn. After descending to a minimum 

 at about 400 mju the spectrum rises again to a small peak — the 

 ^-band — at about 350 m/u. Absorption in this band, as goodeve, 

 LYTHGOE and Schneider's results showed (Chap. 3) is also effective 

 in bleaching visual purple. Beyond the 5-band in the ultra-violet the 

 spectrum again rises to a sharp, high maximum at about 278 m/n. 

 This, the C-band, is due to the protein moiety of the visual purple 

 molecule and is principally determined by the aromatic amino-acids 

 which form part of the protein structure. Light absorbed in this band 

 does not bleach the molecule. When visual purple is bleached, the 

 A- and ^-bands disappear and are replaced by one maximal in the 

 near ultra-violet. This band is due to the product of bleaching 

 (indicator yellow) : its spectral position depends on the pH of the 

 solution. The C- or protein-band is unaffected by bleaching. 



THE PROTEIN MOIETY OF THE VISUAL 

 PURPLE MOLECULE 



Since measurements of photosensitivity (<xy) by the method of 

 photometric curves are not affected by the presence of stable im- 

 purities, it is instructive to compare goodeve, lythgoe and 

 SCHNEIDER'S rcsults for ay with the density values (proportional 

 to a) for a pure — or nearly pure — solution of visual purple, rather 

 than with the density values for the somewhat contaminated solu- 

 tions that these authors perforce used (Fig. 3-6). This has been done 

 in Fig. 4.1 from which it can be seen that the photosensitivity and 

 optical density data agree approximately at all wavelengths above 

 320 m/Li. Below 320 m^ they diverge. This divergence is due to the 

 non-photosensitive protein band at about 280 m/u, absorption in 

 which is not effective in bleaching visual purple. The difference 

 between the two curves within the spectral range of 250-350 mjn can 

 be simulated by a suitable mixture of tyrosine and tryptophan. 

 According to Collins, love and morton (1952) the approximate 

 amounts of tyrosine and tryptophan required to account for the 

 protein band are 6 per cent and 3 per cent, respectively, of the dry 

 weight of visual purple. These percentages are normal for animal 

 proteins, e.g. casein (6-6 and 1-2), haemoglobin (3-2 and 1-3) and 

 cattle fibrin (6-5 and 3-0). The whole density spectrum of visual 

 purple within the wavelength range of 250-600 m/i can thus be 

 accounted for in terms of that due to the photosensitive chromo- 



98 



