THE DIRECT ATTACHMENT OF DYES TO TISSUES I9I 



The degree of ionization of the carboxyl and amino-groups 

 depends on the pH. When this is pushed to the acid side of the 

 iso-electric point, the dissociation of the acid groups is suppressed 

 and that of the amino-groups increased. The protein thus becomes 

 progressively more positively charged as more and more of the 

 amino-groups become ionized. They are all ionized when pH 2 is 

 reached, or thereabouts. 



NH 



HC(CH2).C( 



NH 



I + 



HC(CH2)4NH3 ionized amino-group 



^ 



=0 



Part of a protein chain on the acid side of the iso-electric point 



When the pH is on the less acid or more alkaline side of the 

 iso-electric point, the dissociation of the amino-groups is sup- 

 pressed, while that of the carboxyl groups is increased. Thus the 

 protein becomes more negatively charged as the pH increases, 

 until all the carboxyl groups are ionized at about pH 11. 



NH 



I ^O . . 



HC(CH,)2Cx ionized carboxxl group 



T " ^o- 



o 



? 



NH 

 HC(CH2)4NH2 



T 



Part of a protein chain on the less acid or more alkaline side of the iso-electric 



point 



Although the protein as a whole is neutral at the iso-electric 

 point, yet a few ionized carboxyl and amino-groups still exist at 

 this pH. This is important for dyeing. 



The chief amino-acid side-groups that can give negative charges 

 are aspartic, glutamic, and hydroxy-glutamic. The -OH group of 

 tyrosine and serine can also be ionized, and so can the terminal 

 carboxyl group of a protein chain. 



