98 PHYSIOLOGY 



By the action of nitrous acid the terminal NH 2 groups are split off and 

 replaced by OH. When proteins are treated with nitrous acid only a 

 small proportion of the total nitrogen is split off in this way. The 

 linking of the amino groups must therefore take place by means of the 

 nitrogen, i.e. by NH groups. Synthetic experiments have fully con- 

 firmed this hypothesis. In 1883 Curtius obtained a substance giving 

 the biuret reaction, the so-called ' biuret base,' by the spontaneous 

 polymerisation of glycocoll ester. This base has been shown by recent 

 researches to consist of four glycine molecules arranged together in 

 an open chain. The clue to the structure of this base was given by 

 Fischer, who has devised a number of ingenious methods for combining 

 together amino-acids of any character and in any number. Thus from 

 two molecules of glycine we may obtain the compound glycyl glycine, 

 as follows : 



NH 2 . CH 2 . COOH + HNH . CH 2 . COOH - - H,0 = 

 NH 2 . CH 2 .CO.NH.CH 2 . COOH 



This may be prepared in various ways. In one method glycine is converted 

 into its ester CH 2 .NH 2 .CO.OCH 3 . In a watery solution this undergoes spon- 

 taneous conversion into glycine anhydride, which belongs to the class of bodies 

 known as diketopiperazins, as follows : 



/CH 2 C0\ 



2NH 2 .CH,CO.OCH 3 = 2CH 3 OH + NH< /NH 



methyl alcohol N'CO CH/ 



On treating this with dilute alkali it takes up water, splitting in the situation 

 of the dotted line and forming glycyl glycine, NH 2 CH 2 CO.NH.CH 2 COOH. 



More general methods have been devised by Fischer for the same purpose, 

 depending on the use of the halogen acyl chlorides. 



Thus chloracetylchloride and alanine yield chloracetalanine : 



C1.CH 2 .COC1 + NH 2 .CH(CH 3 ).COOH = 

 C1.CH 2 .CO - NH.CH(CH 3 )COOH + HC1. 



By the subsequent action of ammonia, the halogen group is replaced by the 

 amino group, and a dipeptide results : 



C1.CH 2 .CO - NH.CH(CH 3 )COOH + 2NH 3 = 

 NH 2 .CH 2 .CO - NH.CH(CH 3 )COOH + NH 4 C1. 



Different halogen acyl chlorides are used for introducing the various amino- 

 acid radicals, e.g. chloracetylchloride for glycyl, o-bromopropionylchloride for 

 alanyl, &c. 



By various such methods Fischer has succeeded in combining 

 compounds containing as many as eighteen amino-acids, e.g. alanyl 

 leucine, glycyl tyrosine, dialanyl cystine, dileucyl cystine, leucyl 

 pentaglycyl glycine, and so on. The last named would be built up 

 out of one molecule of leucine and six molecules of glycine. These 

 compounds have been designated by Fischer as polypeptides, to sig- 

 nify their close connection with the peptones produced by the agency 



