THE VISUAL PIGMENTS 



It also seems to be firmly established that indicator yellow is 

 retinene bound to protein through a nitrogen atom. The only 

 evidence which might appear to contradict this conclusion is wald 

 and brown's observation (p. 117) that the synthesis of rhodopsin is 

 most readily accomplished at pH 6, a hydrogen ion concentration at 

 which retinene will readily condense with sulph-hydryl-, but not with 

 amino-groups. But the syntheses were carried out with retinal 

 extracts. In other words the solutions contained not only opsin and 

 retinene but also other substances of retinal origin, some of which 

 might act as catalysts. Thus the pH-activity curve (Fig. 4.5) may 

 merely express the pH-activity of an enzyme : it can hardly be used 

 as evidence against an aldehyde-amino condensation on the grounds 

 that this reaction — when no catalyst is present — ^requires alkaline 

 conditions. 



On the other hand, the results of the amperometric titrations imply 

 that sulph-hydryl groups as well are involved in the formation of 

 rhodopsin from retinene and opsin. Since the aldehydic group of 

 retinene is already bespoken for condensation with a protein amino- 

 group, it seems Hkely that the protein sulph-hydryl groups must react 

 with another part of the retinene molecule. 



Taken as a whole, therefore, the evidence suggests that the chromo- 

 phore of visual purple is bound to protein at a number of points. 

 One bond is through a nitrogen atom, as in indicator yellow, 



) Protern / 



Indicator Yellow (see p. 109) 



In addition the visual purple chromophore is held to the protein 

 molecule by forces which involve the protein sulph-hydryl groups, 

 and which are relaxed when visual purple 'bleaches' to indicator 

 yellow. We have seen (p. 101) that the 7r-electrons of a conjugated 

 polyene chain form orbitals extending over the whole length of the 

 chain. We may suppose that, under certain conditions, viz. when the 

 chromophore is of a suitable shape to fit the protein, the cloud of 

 TT-electrons overlaps, and consequently interacts with, the electrons 

 of the opposing parts of the protein molecule to form new orbitals. 



122 



