156 COMPOUNDS OF THE PROTEINS 



it is exactly equal to the proportion of COHN groups which the 



unhydrolyzed molecule contains; precisely similar results were obtained 

 with Gliadin. 



Direct proof on the other hand, that free NH 2 -groups are not 

 responsible for any appreciable proportion of the acid-combining 

 capacity of proteins has been furnished by the experiments of Blasel 

 and Matula, and of Pauli and Hirschfeld. These investigators prepared 

 Deaminized Gelatin by acting upon gelatin with nitrous acid, thus 

 destroying all the free NH 2 -groups in the molecule. They then 

 compared, with the aid of the hydrogen electrode, the acid-combining 

 capacity of the deaminized gelatin with that of normal gelatin. They 

 found that the combining-capacity of deaminized gelatin for acids is 

 only very slightly inferior to that of normal gelatin, indicating, beyond 

 any question, that the combining-capacity of gelatin for acids is, in 

 very large proportion, attributable to elements of the molecule other 

 than free NH 2 -groups. Since nitrogen atoms must certainly be the 

 agents through which union of acids with protein is brought about, 

 the inference is unavoidable that the elements of the molecule which 

 actually accomplish the binding of acids by protein are, in very large 

 proportion the COHN -groups within the body of the protein 

 molecule. 



To account for both the acid- and the base-combining capacity of 

 the proteins we must therefore look, not to the small proportion of 

 free NH 2 or COOH-groups which the proteins afford, but to the 



COHN groups within the body of the molecule. Now two varieties 



of this linkage can be, conceived, between which it has not proved 

 possible as yet, to decide by any direct method of analysis. Thus 

 Glycyl-glycine may conceivably be either: 



Keto-Form. 

 H 2 N.CH 2 .CO HN.CH 2 .COOH 



or: 



Enol-Form. 

 H 2 N.CH 2 .C(OH) =N.CH 2 .COOH 



and our analytical data, and the modes of decomposition and synthesis 

 of the proteins and peptides do not enable us, with any degree of cer- 

 tainty, to distinguish between them. Neither form is therefore incon- 

 sistent with our present knowledge of the synthesis and hydrolysis of 

 proteins and polypeptides, but while the keto-form of the COHN 

 -group would conceivably possess the power of neutralizing acids, it 

 offers no probable point of union for bases. The enol-form, on the 

 contrary, would provide a point of union for either acids or bases. 



According to Werner's theory of valencies, the nitrogen in either of 

 these types of linkage contains two latent valencies, positive and 

 negative, which, while the nitrogen remains trivalent, neutralize one 

 another, but when the nitrogen becomes pentavalent are capable, 

 respectively, of neutralizing a negative and a positive radical. The 



