SALTS OF ALBUMIN AND ACIDS 87 



quantity of acid combined shows that several molecules of acid 

 have reacted with one molecule of the protein. Hausmann 

 finds for the ammo-nitrogen of gelatin the value of 35-8 per cent, 

 of the total nitrogen, which is far too high ; in more complete 

 later determinations about 26 per cent, amino-nitrogen was 

 found. Electrometric measurements show that the basic 

 valency is reduced by 22 per cent, by removal of the amino- 

 groups. 



The NH-group of the peptide linkage is the first obvious 

 direction in which to look for the origin of the combination of 

 the desamino-proteins with acids. The acid can be added on 

 at this point in just the same way as on to a terminal amino- 

 group. 



...C N .. . + HC1 >... C NHC1 



II I II I 



OH OH 



As will be discussed when treating of the combination of 

 proteins with alkalis, the peptide linkage may change from the 

 lactam (I.) into the lactim (II.) form, thus : 



(I.) C N ^1=^ C = N (II.) 



It must, therefore, be emphasised that only the lactam form 

 appears suitable for linkage with acids. This agrees with the 

 determinations of M. Siegfried * according to which only the 

 peptide linkage of polypeptides in which the lactam form is 

 present can enter into combination with carbonic acid. Expe- 

 rience of the alkali proteins (see Chapter VIII.) indicates that the 

 change from the lactam to the lactim form is a change requiring 

 time ; consequently there is an increase in the combination of 

 protein and alkali with time. The combination of albumin 

 with acids, on the other hand, shows no notable alteration with 

 time, at least in the concentrations usually employed. We can 

 therefore conclude that this process occurs at lactam groupings 

 which originally exist in the peptide linkages. 



* R. H. A. Plimmer, " Chemical Constitution of the Proteins." 

 Longmans. See Part II., p. 58, for the literature. 



