DERIVATIVES 447 



Such a compound would give the biuret reaction and contain 

 but few free carboxyl groups or amino groups, which is 

 entirely in agreement with the properties of proteins. Acting 

 on this assumption, Fischer has synthesized a number of 

 compounds containing such a structure, with the object of 

 studying their properties and comparing them, if possible, 

 with natural proteins. To these synthetic substances he has 

 given the general name of Polypeptides. 



The simplest polypeptide known is glycylglycine ; this 

 substance was obtained as follows : — 



Glycine, when kept for some time in aqueous solution, 

 loses water from two molecules, giving an anhydride — 



CH2— CO 



NHXHjCOOH / \ 



= NH NH + 2HP 



COOHCH2NH2 \ / 



CO— CH2 

 Glycine anhydride or 

 diketopiperazine 



This substance, when boiled with hydrochloric acid, is hydro- 

 lysed, the ring being opened with the formation of the di- 

 peptide glycylglycine — 



CH,— CO CH2— COOH 



/ " \ / 



NH NH + HjO = NH 



\ / \ 



CO— CHg COCH2NH2 



To give anything like a complete account of the methods 

 employed in the synthesis of polypeptides is outside the 

 province of this book. It may, however, be mentioned that 

 a very fruitful method of synthesizing these substances con- 

 sists in acting on an amino acid, or a polypeptide with chlor- 

 acetyl chloride, thus : — 



CHjClCOCl + NHoCH2CONHCH2COOC2H5 = CH2ClCONHCH2CONHCH2COOC2H5 + HCl 



The latter, after conversion into the acid, and treatment with 

 ammonia, yields a tripeptide — 



CH2CICONHCH2CONHCH2COOH + NH3 = CHaNHaCONHCHjCONHCH^COOH + HCl 



Diglycylglyciiie a Tripeptide 



Another valuable method consists in treating an amino 

 acid suspended in acetyl chloride with phosphorus pentachlo- 

 ride and so obtaining an acid chloride R1CHNH2COCI. This 



