86 THE PROTEIN SUBSTANCES. 



position that the amino-acids are here also combined together in the 

 imide binding. 



The large number of amino-acids isolated from the proteins make 

 a large number of bindings possible. The number of possible combina- 

 tions is still further increased by the fact that all the amino-acids with 

 the exception of glycocoll contain at least one asymmetric carbon atom, 

 and this leads to the possible formation of stereochemically different 

 peptides. Thus in order to give a simple example, from two optically 

 active amino acids, four different isomeric forms of dipeptides may occur,, 

 namely (if we designate the optical antipodes by d- and Z-) dd, II, dl and Id. 

 Of these forms two can form a racemic dipeptide, thus d-alanyl-cZ-leucine + 

 Z-alanyl-Z-leucine and d-alanyl-Z-leucine + Z-alanyl-d-leucine. As the pro- 

 teins are optically active and on hydrolysis yield chiefly optically active 

 amino-acids, those polypeptides which can be built up from the natural 

 amino-acids of the proteins are of special importance in the study of the 

 constitution of the proteins. 



Most of the artificial polypeptides are constructed from monamino- 

 mono-carboxylic acids, but polypeptides have also been prepared which 

 contain diamino-acids or amino-dicarboxylic acids, and in this way the 

 number of possible polypeptides becomes still greater. With an aminodi- 

 carboxylic acid such as aspartic acid, other amino-acids can be bound 

 with one carboxyl group or with both, but also, if we start with aspara- 

 gine, they can be anchored with the amide group. If we start from the 

 acid amides we can also obtain a peptide which still contains the CONH 2 

 and on total hydrolysis yields NH 3 , like most proteins. A polypeptide 

 of this kind is the tripeptide glycyl-Z-asparaginyl-Z-leucine prepared by 

 E. FISCHER and KOENIGS. 



NH 2 CH 2 CO.NHCHCO.NHCH(C 4 H 9 )COOH 



CH 2 CONH 2 



The methods used by E. FISCHER in the synthetical preparation of 

 polypeptides are chiefly as follows: 



The first dipeptide prepared by him, glycylglycine, he obtained from 

 glycocoll ethyl ester which in water is transformed into a diketopiper- 

 azine, glycine anhydride, according to the following equation: 



/CH2.COV 



2(NH 2 CH 2 CO.O.C 2 H 5 ) = 2C 2 H 6 OH + NH< >NH. 



X CO.CH 2 / 



By the action of dilute alkali upon this anhydride with the taking up 

 of water the glycylglycine NH 2 CH 2 CO.NHCH 2 COOH is formed, and 

 according to this principle other dipeptides can also be prepared. 



