302 PROPERTIES OF AMINO-ACIDS AND PEPTIDES 



The following conclusions may be drawn from the results given in 

 Table I and shown in the curves. 



(a) The results for water, urea, acetamide, and acetone are prac- 

 tically identical. There is therefore at the dilutions used no com- 

 bination involving the amino groups of urea or acetamide and 

 hydrochloric acid, or enolization of urea, acetamide, or acetone 

 accompanied by reaction with alkali which would be manifested by a 

 change in hydrogen ion concentration. 



(b) The results for the four amino-acids are nearly the same, 

 showing only minor differences. Thus, on the acid side, alanine and 

 leucine are identical, glycine and a-amino butyric acid differing slightly 

 and in opposite directions; on the alkaline side glycine and alanine 

 are the same differing slightly from the other two. There is a broad 

 zone near the point at which no acid or alkali was added where practic- 

 ally no buffer action is shown. 



(c) The results for the dipeptides, except for a few apparently 

 minor differences, are strikingly similar throughout. 



(d) As compared with the amino-acids, there was a very much 

 smaller range where little or no buffer action was shown by the dipep- 

 tides. Aside from this, the curves for the two series were practically 

 parallel. The vertical differences between the two sets of curves show 

 the differences in the amounts of acid and of alkali required to bring 

 the substances to the same pH values. Much more acid or alkali 

 was required for the dipeptides than for the amino-acids, the differ- 

 ence being due evidently to the — CO— NH— group of the dipeptides. 

 The differences increase with increasing quantities of acid or alkali 

 added, reach a maximum, and then decrease again. The chemical 

 nature of the — CO — NH — group readily accounts for these proper- 

 ties, acid combining with the — CO — NH— group, alkali bringing 

 about enol-lactim rearrangement and accompanying reaction. 



(e) Aceturic acid on the acid addition side showed the same results 

 as acetic acid and water. The influence of the highly ionized hydro- 

 chloric acid predominated over any effects of the weaker acids. On 

 the alkaline side, the pH values found for aceturic acid were smaller 

 than those for acetic acid with the same amount of alkali. This 

 difference is evidently connected with the presence of the — CO — NH — 

 group and analogous to the difference between dipeptides and amino- 



