PROTEINS — PROPERTIES. 19 



agents, the most important, both in character and amount, are 

 amino-acids ; it has been found that nearly, if not quite, all 

 the amino-acids obtained from proteins are a-conipounds of the 

 type — 



NHa 



I XH.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 were called " an- 

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

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

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

 compounds are formed, thus — 



NH., COOH NH— CO 



R— CH< " -I- /CH— i:, = i:{^CH< >CH— Rj + 20H.,. 



COOH NH, CO— NH 



By boiling with concentrated hydrochloric acid the hetero- 

 cyclic compound is converted into an open chain, thus — 



NH,, CdOH 



NH— CO 1 I 



R— CH< .CH— l!i + OH., = i;— CH— CO— NH— CH— R,. 



CO— NH' 



These compounds form the simplest polypeptides. By the 

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

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

 by Curtius, it was possible to link up three or moie amino-acids ; 

 or bv 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-uronp of another amino-acid, 

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

 group and substitutin.t; an amidogen radicle for the halogen 

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

 group of another amino-acid. 



Bv these and other means polypeptides containing two. three, 

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

 s()me of these have been identified with products of hydrolysis 

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



NH3— 000 

 * Or possibly R— C< \ 



H COO— HNC— R. 

 H 



