NITROGENOUS COMPOUNDS 61 



!(GIycyl-glycine.) 



NH 3 CH(CH 3 ) CO 



NH 

 H-CH 2 CH(CH 3 ) 2 



(Alanyl-leucine.) 



By a continuation of the same process, three, four, or 

 more molecules of amino-acids may be linked up together, 

 forming tripeptides, tetra-peptides, and so on. Those which 

 contain only two molecules of amino-acid, e.g. glycyl-glycine, 

 are called dipeptides. Theoretically there are no limits to 

 the formation of such polypeptides. Compounds contain- 

 ing eighteen molecules of amino-acid have already been 

 prepared. 



The formula for glycyl-alanyl-glycyl-tyrosine a tetra- 

 peptide is as follows : 



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

 NH< f* 



X CH(CH 2 .C 6 H 4 .OH) --- CO.OH 



The polypeptides are resolved into amino-acids on hydro- 

 lytic treatment. They are of special importance from their 

 close relation to the proteins. The peptides may be regarded 

 as intermediate between the peptones the simplest proteins 

 and amino-acids. They are, in fact, only excluded from the 

 protein group by the arbitrary test of the biuret reaction. 



