THE CHESIISTRY OF THE PROTEINS 113 
with more than one asymmetric carbon atom are represented 
as follows: 
d-alanyl-d-leucine d-alany]-l-leucine I 
]-alanyl-l-leucine l-alanyl-d-leucine 
but as their configuration must be first established, Fischer has 
simply labelled them A and B, the A denoting the more insoluble. 
The synthesis of the optically active forms and their combination 
together would definitely decide the constitution, but a simpler 
method has been found by studying the action of trypsin upon 
them. The A combination was hydrolysed; d-alanyl-l-leucine 
contains the natural amino acids, and hence the form II. most 
probably corresponds to the more insoluble racemic dipeptide. 
The configuration of the diketopiperazines is far more 
complex and need not be entered into here. 
Although the amino acids are in all probability joined 
together in the manner of acid amides, it is possible that other 
couplings may also be present in the protein molecule. Ethers 
of the oxyamino acids, their esters with other amino acids are 
conceivable ; and polyoxyamino acids, if present, suggest a still 
more complex combination. The presence of diketopiperazine 
rings is also likely, as they are so easily converted by alkali into 
the dipeptides; a change of this kind may take place in the 
formation of the so-called albuminates and on coagulation. 
The hydrolysis of the polypeptides by the proteolytic enzymes, 
and their giving of the red biuret reaction like peptone, point 
most strongly to the acid amide manner of combination of the 
amino acids in the protein molecule. 
IV. Tue AcTion or ENZYMES UPON THE POLYPEPTIDES 
We have seen that, by synthesis, a very large number of 
polypeptides have been prepared from the amino acids; their 
number is almost infinite, even if only the optically active forms, 
as they occur in nature, be coupled together. Some light might 
be shed upon what combinations actually occur in the protein 
molecule by the study of the products of hydrolysis, when this 
is not carried to the ultimate stage. Such intermediate products 
are already known and designated proteoses and peptones. 
They consist of mixtures of complex polypeptides, and at 
present our methods are not sufficiently perfect for separating 
polypeptides from one another and from amino acids. They 
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