6 THE CHEMICAL CONSTITUTION OF THE PROTEINS 



A great advance was made when Drechsel discovered that the 

 protein molecule contained diamino acids as well as monoamino acids, 

 and to Kossel and Kutscher we owe our chief knowledge concerning 

 their isolation and estimation. Emil Fischer, in 1901, by his study 

 of the amino acids and their derivatives, introduced a new method of 

 isolating arid separating the monoamino acids, which depended upon 

 the fractional distillation in vacuo of their esters, and which is now 

 commonly known as the ester method. This method, though not yet 

 really quantitative, has enabled us to obtain a knowledge of some 70 

 per cent, of the total products resulting by hydrolysis, and it has shown 

 us that phenylalanine, serine, and alanine, which were only known to 

 occur in a few, are present in all proteins, and that phenylalanine 

 in its distribution is the principal aromatic constituent, for it often 

 exceeds in amount that of tyrosine and occurs when this latter is 

 absent. Further, it has demonstrated the presence of two new com- 

 pounds, proline and oxyproline. 



New units have thus been discovered with each improvement in, 

 and development of, the methods of analysis of the proteins. The 

 exact constitution of these units had also to be determined. This 

 portion of the subject has been attended with entire success ; with 

 the exception of oxyproline we now know definitely the chemical 

 constitution of every unit in the protein molecule mentioned in the 

 above list. Considerable labour has been expended by investigators 

 in proving the composition of the amino acids ; thus, for example, 

 tyrosine was discovered by Liebig in 1846 and its constitution 

 established by synthesis in 1883 by Erlenmeyer and Lipp ; serine 

 was first isolated by Cramer in 1865 and synthesised in 1902 by 

 Fischer and Leuchs ; cystine was known in 1810 (Wollaston), was 

 proved to be a constituent of proteins by Morner in 1899, and its com- 

 position was established in 1903 by Erlenmeyer, jun. The latest 

 addition to the list is histidine, which was first obtained by Kossel in 

 1896 and was synthesised by Pyman in 1911. 



A further detail in our exact knowledge is still required. With 

 the exception of glycine all the amino acids are optically active ; the 

 actual synthesis of the natural compound is only accomplished when 

 the synthetic form has been separated into its isomers. A brief 

 summary of the discovery and synthesis of the amino acids present 

 in the protein molecule is given in Table A (p. 8). Several blank spaces 

 still exist. 



Mention must be made of the help which this nearly complete 



