INTRODUCTION 7 



chapter has afforded to the subjects of " alcoholic fermentation " and 

 "ptomaines " or "natural amines". The amino acids have been 

 found to be the mother substances of the by-products (fusel oil) of 

 alcoholic fermentation and of those products of putrefaction the 

 amines which are of biological importance. A description of these 

 compounds is given in Harden's Alcoholic Fermentation and in Barger's 

 Simple Natural Bases. 



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 decomposi- 

 tion of the protein, and even if they be prepared by synthesis 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, a, d, e or d, e, c, a, b, etc. 



The proper arrangement of the units in the molecule is under in- 

 vestigation. Several polypeptides, as the combinations together of 

 the amino acids are termed, have now been prepared directly from the 

 proteins ; their isolation is so difficult that there must be many years 

 of incessant labour before a real natural protein will be actually pro- 

 duced in the laboratory. 



Like the complex proteins the polypeptides are hydrolysed into their 

 constituent amino acids by the proteoclastic enzymes' present in animals 

 and plants. Some are hydrolysed by trypsin, others are not hydrolysed 

 by this enzyme, but they may be hydrolysed by other enzymes. A 

 complex polypeptide may be attacked at different junctions by dif- 

 ferent enzymes. Such data serve in determining the arrangement of 

 the amino acids in the protein molecule, and they will no doubt help 

 towards the elucidation of the nature of the enzymes themselves. 



Use is also being made of the polypeptides in the studyiof disease. 



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

 fore be divided into four main sections : 



I. The Chemical Composition of i the Protein Molecule. 

 II. The Chemical Constitution of the Units. 



III. The Synthesis of the Proteins. 



IV. The Action of Enzymes on the Polypeptides. 



