142 PHYSIOLOGY OF ALIMENTATION. 



terest, and whether they actually represent combinations of 

 amino-acids such as are produced in the action of ferments 

 or acids on proteins of various kinds. 



That such is the case can be shown first of all by certain 

 chemical tests. In the analysis of proteins we encounter a 

 group of substances ordinarily called peptones, which are 

 characterized by their power of giving certain reactions such 

 as the biuret test. It is of great interest, therefore, that 

 many of the peptides which have been produced synthetically 

 also give the biuret test. While the dipeptide glycyl-glycin 

 and the tripeptide triglycin give no biuret reaction, this is 

 positive with the tetrapeptide tetraglycin. Dialanylcystin 

 gives a biuret reaction , while the higher peptides containing 

 seven and more amino-acids in the molecule, such as leucyl- 

 pemaglycin, give a red biuret test which seems identical 

 with that given by the peptones derived from silk. Phos- 

 photungstic acid, which is abundantly used as a precipitant 

 for the simple digestion-products of proteins, will also precipi- 

 tate many of the synthetic peptides. Certain of the amino- 

 acids which are only difficultly soluble in water yield peptides 

 which are readily soluble. The reverse is often observed in 

 the analysis of proteins. When, moreover, amino-acids hav- 

 ing a sweet taste are joined together chemically they yield 

 substances having a bitter taste. It is a well-known fact 

 that the ordinary peptones have a bitter taste. 



Certain of the peptides which have been artificially produced 

 can be split into amino-acids by pancreatic juice in the same 

 way as the peptides formed in an ordinary digestion mixture. 1 



With these remarks on the synthesis from amino-acids of 

 substances which seem to agree in their chemical and biologi- 

 cal character with the peptones we will try to see if there is 

 any experimental evidence at hand to indicate that from 

 substances standing close to the class of peptones others 

 giving the reactions of the albumins may be obtained. It 



1 ABDERHALDEN: 1. c., p. 201. 



