8 THE CHEMICAL CONSTITUTION OF THE PROTEINS 



The Linking Together of Amino Acids. 



By the action of benzoyl chloride upon the silver salt of glycine, 

 Curtius, in 1881, obtained in addition to the expected hippuric acid 



C 6 H 5 CO . Cl + H 2 N . CH 2 . COOH = HC1 + C 6 H 5 . CO . NH . CH 2 . COOH 



two other acids of higher molecular weight. One of these was hippuryl- 

 glycine or benzoyl-glycyl-glycine, 



C 6 H 5 CO . HN . CH 2 . CO NH . CH 2 . COOH, 



as was proved by the study of its salts, its ethyl ester and amide, and 

 by its hydrolysis into hippuric acid and glycine. It was the first 

 definite compound known which contained two amino acid residues 

 combined together. 



The constitution of the other acid, called the 7-acid, could not be 

 determined ; it had the formula C 10 H 12 N 3 O 4 , was soluble with difficulty 

 in water and gave the biuret reaction. It was formed in larger quantities 

 when hippuric ester was fused with glycine, but under these conditions 

 another compound benzoyl-bisglycyl-glycine was formed at the same 

 time, so that in this way a series of compounds resulted, each succeeding 

 member containing a glycyl- NH 2 . CH 2 . CO- group more than the pre- 

 ceding one. 



Further investigations upon the constitution of these compounds 

 were only carried out in 1904 by Curtius and Benrath, who found that 

 the 7-acid from the analysis of its silver salt, ester, etc., had the formula 

 C 19 H 24 N 6 O 8 , and that it was benzoyl-pentaglycyl-glycine, 



C 6 H 5 . CO (NH . CH 2 . C0) 5 NH . CH 2 . COOH, 



its ester being identical with the compound synthesised by Curtius and 

 Wiistenfeld (see below). 



Two other compounds hippuryl -glycine and benzoyl- triglycyl- 

 glycine were found to be formed by fusing together hippuric ester 

 and glycine, but not the previously isolated benzoyl-diglycyl-glycine. 

 Longer chains than the six membered 7-acid are not believed by 

 Curtius and Benrath to be formed in this reaction. 



In 1890 Curtius, by the action of hydrazine upon benzoyl chloride, 

 benzamide or benzoic ester, obtained benzoylhydrazine, 



C 6 H 5 . COOC 2 H 5 + H 2 N . NH 2 = C 6 H 5 . CO . HN . NH 2 + HOC 2 H 5 , 



which, when treated with nitrous acid, gave benzoylazoimide, 



/N 

 C 6 H 5 . CO . HN . NH 2 + HNO 2 = 2H 2 O + C 6 H 5 . CO N<f ||| 



