198 DYEING 



thicken the objects for which they have an affinity, which would be 

 impossible if they only acted by permeation. 



It is also easy to gain the impression that basic dyes show a 

 considerable degree of specificity, while acid ones act diffusely. 

 This apparent contrast is not quite genuine. The difference lies in 

 the objects dyed rather than in the dyes. If every cell contained a 

 characteristic object as basic as chromatin is acidic, we should not 

 be so much struck by the diffuseness of acid dyes, for we could 

 arrange to colour the basic object differentially with them. The 

 eosinophil granules of certain leucocytes can indeed be dyed 

 sharply in this way. Often, however, we deliberately use the acid 

 dyes to colour the amphoteric cytoplasm feebly, in a colour 

 contrasting with that of the acidic objects, which have been 

 strongly coloured by a basic dye. 



Despite this, there is reason to believe that acid dyes really are 

 somewhat more diffuse in their action on tissues than basic ones, 

 and it is reasonable to look for a component of the protein chain 

 with which they might react, other than the specifically basic 

 residues of lysine, arginine, and histidine. The dyeing of the syn- 

 thetic fibre, nylon, is instructive here.^^* This substance does not 

 provide much opportunity for the kind of reaction with acid dyes 

 that we have been considering, because an amino-group only 

 occurs at one end of the long chain-molecule. When this has reacted 

 by salt-formation with the anion of an acid dye, an increase in 

 acidity will result in a further, sudden uptake of dye, which is 



? 



=0 

 NH 



Peptide group in nylon or protein 



attributed to the activation of the peptide groups that occur re- 

 peatedly in the nylon chain.^^^ It is supposed that hydrogen ions 

 are taken up by the peptide groups, which thus become positively 

 charged and therefore attract dye-anions. If nylon can behave in 

 this way, there would not seem to be any reason why acid dyes 

 should not attach themselves similarly to the protein chain, in acid 

 solution. It is a striking fact that wool becomes softened ('ten- 

 dered') when it has taken up more than a certain amount of an 

 acid dye. This suggests an alteration in the main chain of the pro- 

 tein. A reaction of this sort would be unrelated to the preponder- 



