PRECIPITATION OF PROTEINS BY INORGANIC SALTS 169 



hydration intermediate between these extremes the protein may be 

 soluble. Dehydration, partial or complete, leading to resolution or to 

 coagulation may be induced by heat, by non-electrolytes possessing 

 an affinity for water, or by concentrated electrolytes. 



The importance of a high degree of dehydration in the production of 

 Coagula, irresistibly suggests that this phenomenon is dependent upon 

 the formation of anhydrides, analogous to the Diketopiperazines which 

 may be formed from the amino-acids and polypeptides by dehy- 

 drating-agents, and of the general formula: 



NH xlNXl.V^V^V 



R< | or R<^ 



?O.HN' 



Such bodies may exist either in the keto-form, illustrated by the 

 above formulae, or in the enol-form, such as: 



,N.(HO)Cx 



Coagulation by mineral salts appears invariably to be accompanied 

 or preceded by chemical interaction of the coagulating-salt and the 

 protein salt of an acid or base which preexisted in solution before 

 the coagulant was added. The coagulated protein in these instances, 

 therefore, does not represent the unaltered protein salt as it existed 

 in solution before the coagulant was added. When Alcohol is used as 

 the coagulant however, it is found, at least in the case of the Caseinates 

 of the Alkaline Earths, that the protein salt as such is coagulated, 

 carrying down with it the amount of mineral base with which it was 

 combined before the coagulant was employed, so that after washing 

 out the alcohol with ether, and absorbing the ether by desiccation over 

 sulphuric acid, calcium caseinate is obtained in the form of a dry 

 powder which is soluble in water, whereas free casein is insoluble in 

 water. If coagulation by alcohol is attributable to dehydration of the 

 protein, the elements of water must be contributed chiefly by the 

 interaction of free amino- and carboxyl-groups with the formation of 

 ring-anhydrides, and that this should be possible without disintegration 

 of the compounds with bases affords another indication that free 

 carboxyl-groups are not responsible for the union of proteins with bases. 

 The same considerations probably will be found to apply to the 

 coagulation by alcohol of the compounds of proteins with acids, but as 

 yet these compounds have not been so thoroughly investigated from 

 this standpoint as the compounds of proteins with bases. 



