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PROTEINS 



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 



CH 2 CO 

 NH 2 CH 2 COOH / \ 



= NH NH + 2H 2 O 



COOHCH 2 NH 2 \ / 



CO CH 2 



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 2 CO 



NH 



\ 



NH + HJ3 = NH 



CH 2 COOH 



\ 



CO CH 2 



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 : 



CH 2 C1COC1+NH 2 CH 2 CONHCH 2 COOC 2 H5=CH2C1CONHCH2CONHCH 2 COOC2H5+HC1 



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

 ammonia, yields a tripeptide, 



CH 2 ClCONHCH2CONHCH 2 COOH4-NH3=CH 2 NH 2 CONHCH 2 cqNHCH 2 COOH+HCl 



Diglycylglycine 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 R^HNHgCOCl. This 

 latter is then allowed to act upon the amino group of a second 

 acid as follows : 



R, R. 



RjCHNHaCOCl + NH 2 CH-COOH = RjCHNHaCONH-CHCOOH + HC1 



The resulting polypeptide may, of course, be of considerable 

 complexity, according to the nature of R x and R 2 . 



