40 PROTEINS. 



NH, 



| NH,H* 



R CH COOH, or, better, R C< \ 



H COO. 



Amino -acids of this type have also formed the starting point 

 in the synthesis of compounds having many of the characteristic 

 reactions of proteins, and identical with those formed by their 

 hydrolysis the polypeptides. 



These acids condense readily to form what are called " an- 

 hydrides," but which really have no anhydride grouping ; the 

 condensation takes place between the amino-group of one mole- 

 cule and the carboxyl-group of another, and di-keto-piperazine 

 compounds are formed, thus 



NH 2 COOH NH-CO 



R-CH< + >CH-R 1 = R-CH< >CH R^OK*. 



COOH NH 2 CO-NH 



By boiling with concentrated hydrochloric acid the hetero- 

 cyclic compound is converted into an open chain, thus 



NH 2 COOH 



NH CO | | 



R CH< >CH Rj + OH 3 = R CH CO NH CH R,. 



CO NH 



These compounds form the simplest polypeptides. By the 

 action of an acid chloride on the silver salt of a polypeptide, or 

 by the condensation of acid-azides with amino -acids, employed 

 by Curtius, it is possible to link up three or more amino -acids ; 

 or by protecting the amino-group of a polypeptide by the intro- 

 duction of a carboxethyl-group Fischer was able to condense 

 the carboxyl-group with an amino-group of another amino-acid, 

 and by introducing an a-halogen acid chloride into the amino- 

 group and substituting an amidogen radicle for the halogen 

 he was able to condense the amino-group with the carboxyl- 

 group of another amino-acid. 



By these and other means polypeptides containing two, three, 

 four, and five amino-acid radicles have been synthesised, and 

 some of these have been identified with products of hydrolysis 

 of proteins, or with their optical isomerides. There is little 

 doubt that the grouping CH NH CO CH is characteristic 

 of proteins ; Fischer also considers it proved that the di-keto- 

 piperazine group also occurs. 



Fischer has prepared a polypeptide containing 18 molecules 

 of amino -acids ; it was Z-leucyl-triglycyl-Z-leucyl-triglycyl-Z- 



NH 3 OOC 

 * Or possibly R-C< \ 



H COO HoNC R. 

 H 



