n8 ALTERATIONS IN STATE OF THE 



with alkali. These are the first to be protected by methylation of 

 the protein, but even then a marked capacity for combination 

 with alkali persists. For, besides the terminal carboxyl groups, 

 the peptide linkages can also react as acid valencies. There is 

 no doubt that by transformation into the lactim form (II.), 

 thus :- 



r TSJ_ r = N 



I. Lactamor fT j V Lactim or II. 



keto form ^ ^- Q'JJ enol form 



it can function as an acid group with a hydrogen atom replace- 

 able by metals. It is quite unknown how far the peptide 

 linkages in the various proteins are originally in form I. or 

 form II., but there is a certain amount of evidence that the enol 

 form of some part of the peptide complex arises only as a 

 secondary effect of the addition of alkali. This view is 

 supported by the changes of hydration and conductivity with 

 time, which, ceteris paribus, distinguish the alkali proteins from 

 the acid proteins and which appear to be essentially a conse- 

 quence of increased combination with alkali, which in turn is 

 accomplished by rearrangement into the lactim form. In this 

 way albumin can be regarded as a pseudo-acid as defined by 

 Hantzsch. 



The occurrence of isomerism by transformation of the peptide 

 linkage has been recognised for some time in the case of the 

 di- and tripeptides. H. Leuchs and W. Manasse * showed, in 

 1907, that the ester of carbethoxyglycylglycine on hydrolysis 

 with alkalis passes from the lactam form I. into the lactim 

 form II. Here by the action of alkalis, therefore, the resulting 

 glycylglycincarboxylic acid on esterification gives an ester 

 isomeric with the original substance. 



Leuchs and La Forge f a year later found the same effect with 

 diglycylglycin ester, in which both peptide linkages undergo 

 the change. 



In the supposed transformation of the peptide linkage in 

 higher polypeptides with which we are concerned, no asym- 

 metrical carbon atom is directly engaged, and we were in fact 



* Ber., 1907, 4-0, 3235. 

 t Ber., 1908, 41, 2586. 



