COAGULANT PRIMARY FIXATIVES 35 



si::^h groups in two previously separate protein chains and thus 

 hold them together. The mercury thus adds itself to the protein in 

 the form of an ion. The reaction is promoted by the presence of 

 sodium chloride, since added chlorides increase the amount of the 

 reactive ion. 



In the neighbourhood of the iso-electric point, the -NHo groups 



I 

 NH 



I 

 HC(CH,)4NH., 



I 



c=o 



I 



Lysine forming part of a protein chain, in a solution 

 at or near the iso-electric point 



of the basic amino-acids are not ionized. It is now the undissoci- 

 ated salt, HgCls, that reacts. Mercury can accept extra electrons 

 from electron-donor atoms, such as nitrogen. The compound 

 formed resembles the 'ammine' formed by the combination of 

 ammonia with mercuric chloride.^' ^ The lysine/mercuric chloride 



H3N CI 



\ / 

 Hg 



/ \ 

 H3N CI 



77?^ compound of ammonia 

 with mercuric chloride 



bond formed in this way is a very loose one. The reaction is 

 opposed by sodium chloride, and the coagulum produced may 

 indeed be dissolved (and fixation thus undone) by addition of this 

 salt. 



In alkaline solutions the mercuric ion, Hg++, reacts with the 

 -COOH groups of acidic proteins; but fixatives are rarely 

 alkaline. 



Mercuric chloride can also react with the cysteine groups of 

 proteins, forming a bridge that connects these groups in previ- 

 ously separate protein chains. 



