INTRODUCTION 7 



containing nitrogen is contained in the glucoproteins in their prosthetic 

 group, but it is doubtful if it be present in the protein part of the mole- 

 cule, although a carbohydrate has been obtained from carefully purified 

 proteins containing no prosthetic group, such as crystallised egg-albu- 

 min and serum-albumin. The fact that the yield of carbohydrate from 

 such a protein becomes smaller the more often it is recrystallised, 

 suggests that the presumably pure protein still contained an impurity ; 

 this impurity would be a glucoprotein, which is found in both egg 

 white and serum from which the crystallised proteins are separated, 

 and this would give rise to the carbohydrate. Glucosamine is therefore 

 excluded from the above list. 



The composition of the protein molecule has been determined by 

 the method of hydrolysis. As the result of hydrolysis a complex 

 mixture of all, or nearly all, the previously mentioned units is ob- 

 tained. These have been isolated by various methods based upon 

 the fractional crystallisation of the compounds themselves, or of their 

 copper, silver and other salts. Only when one or more of the amino 

 acids occurred in somewhat large amounts was their isolation and 

 characterisation effected ; their amount seldom reached a value higher 

 than 20 per cent, of the total quantity, and the remainder was 

 represented by uncrystallisable syrups of unknown nature. The 

 products, termed leuceines, acids of the series C M H 2W -iNO2, and 

 glucoproteines, acids of the series C w H 2 nN 2 O 4 , in gelatin, glucopro- 

 teines-a and tyroleucine in albumin, described by Schiitzenberger 

 [1879] and by Lepierre [1903] have been shown by Hugounenq and 

 Morel [1906, 1907] and Galimard, Lacomme and Morel [1906] to be 

 mixtures of now definitely known substances. 



A great advance was made when Drechsel discovered that the 

 protein molecule contained di-amino acids as well as mono-amino 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 and separating the mono-amino 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 



