Chemical Constituents of the Animal Body. 229 



condensation of two similar ammo-acids, but a dipeptide 

 can also be formed from two different acids. Thus, by the 

 condensation of glycine and alanine, two dipeptides can be 

 formed, namely, glycyl alanine 



NH 2 CH 2 CO-NH CH(CH 3 )-COOH 

 and alanyl glycine 



NH 2 .CH(CH 3 )-CO NHCH^COOH 



Both these products will yield on hydrolysis a mixture of 

 equimolecular proportions of glycine and alanine. By a 

 similar scheme of condensation tri- and poly-peptides can be 

 formed by the condensation of three or more ammo-acids. 

 As examples of tripeptides the following may be cited : 

 Glycyl glycyl glycine 



NH 2 CHa-CO-NH CH 2 -CO-NH CH 2 -COOH 



which yields on hydrolysis only glycine. 

 Leucyl alanyl glycine 



NH 2 .CH(C 4 H 9 ).CO.NH.CH(CH 3 ).CO-NH.CH 2 .COOH 



which yields on hydrolysis a mixture of leucine, alanine, 

 and glycine.* Many complex polypeptides have been pre- 

 pared synthetically, and those with higher molecular 

 weights have, in many respects, properties not unlike 

 those of the proteins, or certain of the more complex 

 derivatives of the latter, obtained therefrom by the scission 

 of only a portion of the ammo-acids from the molecule. 

 Such derivatives are known as proteoses (albumoses) and 

 peptones, and are produced from proteins by partial hydro- 

 lysis under the influence of certain ferments. These will 

 be considered in greater detail later. In general, a protein 



* The methods of synthesizing polypeptides have been elaborated by Emil 

 Fischer and his pupils. They are described in detail in his monograph on the 

 subject, and also by R. H. A. Plimmer in one of a series of biochemical 

 monographs. (Longmans.) 



