8 THE CHEMICAL CONSTITUTION OF THE PROTEINS 



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 ; we 

 now know the chemical constitution of every clearly defined unit 

 in the protein molecule. Except glycine, all the amino acids con- 

 tained in proteins are optically active and this property must also 

 be considered. 



The final problem in the chemical constitution of the proteins 

 the synthesis remains. This problem is still in its infancy. After 

 numerous attempts by the earlier investigators its foundation was 

 laid by Emil Fischer, who has synthesised a compound which, if 

 it had been found in nature, would have been described as a protein. 

 The difficulties in this part of the subject are very considerable. Not 

 only is the amino acid required, but also its natural optical isomer is 

 required. Many of the amino acids can only be readily obtained by 

 decomposition of the protein, and, even if they be prepared by synthe- 

 sis, much time and expense is involved. The proper conjunction of 

 the amino acids is then necessary ; the results of analysis give no 

 clue as to whether the arrangement is a, b, c, d, e or b, c, d> a, e, or 

 d, e, b, a, c> etc. Some idea of the arrangement of the units in the 

 protein molecule has been obtained by the isolation of several poly- 

 peptides, as the combinations together of the amino acids are termed, 

 from proteins. Their isolation is so difficult that there must be many 

 years of incessant labour before a real natural protein will be actually 

 produced in the laboratory. 



The study of the Chemical Constitution of the Proteins can there- 

 fore be divided into three sections : 



I. The Chemical Composition of the Protein Molecule. Analysis. 



II. The Chemical Constitution of the Units. The Amino Acids. 



III. The Synthesis of the Proteins. The Polypeptides. 



