THE SYNTHESIS OF THE PROTEINS 37 



mixture of the two compounds, for the separation of mixed crystals of 

 similar compounds is of the greatest difficulty. 



Concerning the nomenclature of the two compounds where they 

 have been isolated, the more insoluble is called the A compound and 

 the more soluble the B compound. It has been possible to determine by 

 the action of trypsin, which only hydrolyses the compound containing 

 the naturally occurring amino acids, what combinations are present in 

 them ; thus, as alanyl-leucine A was hydrolysed by trypsin it must con- 

 tain d-alanyl-1-leucine, and the two compounds must therefore be 



d-alanyl-l-leucine\ d-alanyl-d-leucine \ 



l-alanyl-d-leucineJ 1-alanyl-l-leucine J 



This has been proved by the later work upon the optically active 

 polypeptides composed of these amino acids. 



One product only can result when the two components consisting of 

 the pure optically active amino acids are combined together, e.g. 



d-alanyl-d-alanine 



from d-alanylchloride and d-alanine ester. 



Two products again result when one of the components is optically 

 active and the other racemic. The various combinations of optically 

 active tyrosine and aspartic acid with racemic leucine, alanine, etc., come 

 into this category ; they are designated as, e.g. 



dl-alanyl-l-tyrosine 

 glycyl-dl-leucine 



These compounds are not optical antipodes, and can therefore be 

 separated by simple crystallisation. In the case of the leucyl-aspara- 

 gines, 



d-leucyl-l-asparagine 

 1-leucyl-l-asparagine 



this separation has been effected, but in the majority of cases no separa- 

 tion was carried out, since the similarity of the isomers was so great 

 that they formed apparent mixed crystals. Such a condition was 

 termed by Fischer in 1894 "partial racemism". It occurs almost 

 always when a racemic compound in combination with an active 

 residue cannot be separated into its two isomeric forms by simple 

 crystallisation. 



Cystine, as its constitution shows, 



COOH . CH(NH 2 ) . CH 2 . S . S . CH 2 . CH(NH 2 ) . COOH, 



resembles the tartaric acids in its stereochemistry ; it is composed of 

 two exactly similar halves, and it matters very little with which amino 

 group combination is effected . But if it be combined with two mole- 



