2D0 DYEING 



General intermolecular attraction (van der Waals forces) are 

 thought sometimes to aid the indiscriminate anchoring of dyes, 

 provided that close enough approximation to the substrate can 

 somehow be attained. 



I 



NH . . . NH, 



A possible hydrogen bond (...) between a peptide group of a protein chain 



and an amino-group of a dye 



Although these kinds of bonding must be kept in mind, yet 

 there is at present no strong evidence that they play a dominant 

 role in microtechnical dyeing. The biologist has one advantage 

 over the textile chemist in judging the forces that bind a dye to its 

 substrate. The tissues of plants and animals, as studied in the 

 biological laboratory, provide us with very obvious visible indica- 

 tions as to whether a dye is acting in conformity with the electric 

 charge on its ions or not. It is true that the cortex of wool is some- 

 what acidophil and the medulla somew^hat basiphil; ^-^ but the 

 contrast is not very sharp, and anyhow textile chemists do not 

 devote a very great deal of study to the appearance of dyed wool 

 under the microscope. When a biologist looks at a microscopical 

 preparation of the most ordinary kind, every cell suggests the 

 supremacy of ionic forces, for the acidic chromatin is coloured by 

 the basic dye and the ground cytoplasm in a contrasting colour by 

 the acid dye. 



If reactions between oppositely charged groups play a major 

 part in dyeing, one would expect a stoichiometric relation between 

 the amount of protein dyed and the maximum amount of dye that 

 could be taken up. This is a subject on which there is conflicting 

 evidence. One can measure the amount of inorganic acid with 

 which a particular protein will combine, and then compare this 

 with the amount of acid dye that can be taken up. It is claimed 

 that the number of arginine, lysine, and histidine groups in the 

 protein account for the amount of acid or acid dye that will com- 

 bine, and that there is therefore no reason to suppose that the 

 peptide groups of the main protein chain participate in the process 

 of dyeing. ^^^ Singer and Morrison, *^^ however, found that at pH 2 



