106 SCIENCE PROGRESS 
Ill. THe SyNTHESIS OF THE PROTEINS. THE POLYPEPTIDES 
Many theories have been advanced concerning the constitu- 
tion of the proteins as to how the amino acids, the key-stones, 
are combined together. The majority of the views regard the 
amino acids as being bound together in an anhydride-like 
manner in the form of acid amides. This view has been very 
strongly supported by Hofmeister, and several experimenters 
had already attempted to combine the amino acids together in 
this manner. Thus, Schaal made the anhydride of aspartic 
acid, which was further condensed by Grimaux and Schiff. 
Schutzenberger condensed amino acids with urea, Lilienfeld 
employed a mixture of amino acid esters, and Balbiano and 
Frasciatta obtained a horny mass, an anhydride of glycine. 
Not, however, until Fischer commenced his systematic 
researches upon the combination of the amino acids with one 
another in 1901 was any real progress made. Since then the 
enormous number and variety of products which have been 
prepared by Fischer and his pupils have given us an actual 
knowledge as to how the proteins are built up, and they show 
us how much more complicated the proteins are than the fats 
and carbohydrates, the other natural constituents of animal 
and vegetable cells. The combinations of the amino acids 
have been called by Fischer polypeptides, those containing 
two, three, four, etc., acids being di-, tri-, tetra-peptides, etc. 
They remind us in their nomenclature of monosaccharides, 
dissacharides, polysaccharides, and of peptone, which Fischer 
regards as a mixture of polypeptides. 
It was found by Curtius and Goebel many years ago that 
the ethyl ester of glycine, when dissolved in water, lost two 
molecules of alcohol, and was converted into diketopiperazine, 
or, as it is now better called, glycine anhydride, 
NH.CH,.CO 
| | 
CO.CH,.NH 
This substance formed the starting-point of E. Fischer’s investi- 
gations ; by boiling it with concentrated hydrochloric acid the 
piperazine ring was split, and it gave the hydrochloride of 
the base C,H,N.O;, thus 
NH.CH,.CO 
| | +H,O = H.N.CH,.CO.NH.CH,. COOH 
CO.CH,.NH 
