THE LINKING TOGETHER OF AMINO ACIDS 23 



resulted when hippuryl-aspartic acid azide was combined with glycine 

 ester. 



In conjunction with Gumlich, Curtius has investigated the linking 

 of hippurazide with y9-araino-a-oxypropionic acid and with -amino- 

 butyric acid. With the former compound the combination took place 

 with the hydroxyl group instead of with the amino group, hippuryl-o- 

 oxy-$-aminopropionic acid, 



C 6 H B . CO . NH . CH, . CO O CH . (CH, . NH,) . COOH, 



being formed. With the latter compound by the usual series of reac- 

 tions they prepared hippuryl-/9-aminobutyric acid and hippuryl-/3- 

 aminobutyryl-$-aminobutyric acid. 



Curtius and Miiller have also prepared hippuryl-7-aminobutyric 

 acid and hippuryl-/9-phenyl-a-alanine, compounds of no great interest 

 since these amino acids do not occur in the protein molecule. They 

 show, however, that not only can a-amino acids be combined together 

 by the azide method, but also $- and 7- substituted amino acids. 



In order to build up chains containing the carbamic acid radicle, 

 NH . CO, just as Curtius and his co-workers have built up chains con- 

 taining glycyl, alanyl and asparagyl radicles, Curtius and Lenhard, 

 in 1904, proposed to make use of the azide of hippuryl-carbamic 



acid, 



C,H 9 . CO . NH . CH 2 . CO NH . CO . N,. 



This compound was not available, since sufficient quantities of 

 hippuryl urea, which Curtius had formerly prepared from hippuric 

 ester and urea, could not be obtained from hippurazide and urea. 

 They therefore attempted to make the azide of benzoylcarbamic acid 

 by the action of hydrazine on benzoylurea, but the only product which 

 they obtained was the hydrazide of benzoic acid. The benzoic acid 

 radicle is therefore very easily eliminated from the urea molecule, the 

 molecule of benzoylcarbamic acid hydrazide being hydrolysed accord- 

 ing to the equation : 



C 6 H 8 .CO.NH.CO.NH.NH, + H a O = C,H B . CO . NH. NH, + NH, + CO r 

 This non-success led them to attempt to combine phenylcarbamic 

 acid azide C 6 H 5 . NH . CO N,, which Curtius and Hofmann and 

 Curtius and Burkhardt had described in 1 896 and 1 898, with urea, but 

 again the desired result was not achieved, nor could a combination of 

 this compound with biuret be effected. 



It followed therefore that acid radicles cannot be combined with 

 urea by the acid azide reaction. 



When glycine was used instead of urea for combination with the 



