48 FIXATION 



respect, and indeed if the polypeptide chains are bound together 

 into a gel, there need not be any terminal amino-acids. The chief 

 hydrophil constituents are the carboxyl, hydroxyl, and amino- 

 groups of the side-chains, and the carbonyl of the backbone; the 

 imino-groups of several amino-acids, the amido-group of aspara- 

 gine, and the sulphydryl of cysteine can also associate with water. 



Part of a protein chain. The hydrophil groups are shozvn in bold letters. 



The most striking effect of a coagulant fixative on a protein sol 

 or gel is an alteration in the effects of these hydrophil groups, 

 which formerly held water so firmly that it could not be separated 

 by powerful mechanical force, but now can be squeezed out by 

 hand (p. 42). The effect is irreversible, for the protein loses its 

 power to imbibe water and resume its former properties. 



To the chemist the most significant result of denaturation is the 

 increase in reactivity. Certain reactive constituents of the protein 

 were previously present, wholly or partly, in latent form: now they 

 exhibit themselves and respond freely to tests for their presence. 

 An increased digestibility by enzymes results, and indeed accounts 

 for the fact that we cook our protein food. Two views have been 

 held about the nature of the latency of the reactive groups before 

 denaturation. It has been suggested that they do not exist in the 

 natural protein, but originate during the process of denaturation 

 (and also in proteolysis). Most students of the subject, however, 

 consider that the reactive groups are present in the original pro- 

 tein, but something hinders the access of test-reagents to them. 

 They may be involved in linkages between protein chains or 



