POLYPEPTIDES. 87 



leucylhistidine; tripeptides: di-glycylglycine, alanylglycyltyrosine, leu- 

 cyltryptophylglutamic acid; tetrapeptides: glycylglutamyldiglycine, dileu- 

 cylglycylglycine; pentapeptides : tetraglycylgly cine and leucyltriglycylgly- 

 cine; hexa- and heptapeptides: leucyltetraglycylglycine and leucylpentagly- 

 cylglycine. The most complex polypeptide thus far prepared is an 

 octadecapeptide with 15 glycocoll and 3 leucine residues namely: Weucyl- 

 triglycyl-Z-leucyltriglycyl-/-leucyl-octaglycylglycine = 



NH 2 CH(C 4 H9)CO.[NHCH 2 CO]3.NHCH(C4H9)CO. 

 [NHCH 2 CO]3.NHCH(C 4 H9)CO.[NHCH2CO]8.NHCH 2 COOH. 



with the supposition 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 1-) dd, II, dl and Id. 

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

 Z-alanyl-Z-leucine and d-alanyl-Meucine+/-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 CONH2 

 group and on total hydrolysis yields NHs, like most proteins. A poly- 

 peptide of this kind is the tripeptide, glycl-/-asparaginyl-Z-leucine prepared 

 by E. FISCHER and KOENIGS. 



NH 2 CH2CO.NHCHCO.NHCH(C4H 9 )COOH 



CH 2 CONH 2 



In consideration of the form of binding of the sulphur in the proteins 

 it is interesting to consider the preparation of thiopolypeptids as performed 



