INTRODUCTION 3 



the proteins are built up by the condensation of several amino-acids 

 according to the scheme 



Ri Rii Riii 



NH,.CH CO.IOH HNH.CH CO!OH. H. NH 2 CH COOH HNH a . 



Rv 

 . . . CO OH HNH 2 .CH.COOH 



thereby forming a class of products which have been designated 

 the polypeptides by Fischer. Such polypeptides are held to form the 

 essential part of the structure of the protein molecule ; the latter may, 

 however, contain other groups, such as phosphoric acid, and possibly 

 also carbohydrates ; as to whether such groups form an essential part 

 of the actual protein molecule, or whether they are held in loose 

 combination in the form of what Hoppe-Seyler described as 

 " prosthetic groups," need not be discussed in detail here. There is, 

 however, a certain amount of evidence that phosphoric acid, in the case 

 of caseinogen, for example, forms an essential part of the molecule. 

 Whatever view may be held as to the other groups, there is little doubt 

 that the polypeptide group forms the essential part of the protein 

 molecule, and that it is formed by the conjugation of different amino 

 acids in the different individual members of the protein class. 



THE RELATIVE VALUE OF THE VARIOUS PHYSICAL AND 

 CHEMICAL CHARACTERS FOR FIXING THE IDENTITY OF A 

 PROTEIN CONSIDERED IN THE LIGHT OF PRESENT 

 KNOWLEDGE. 



As already mentioned, the majority of the proteins differ from one 

 another but slightly in their empirical chemical constitution ; the 

 numbers obtained by the ultimate chemical analysis are therefore, as 

 a rule, of but little value for the characterisation of proteins. The 

 most obvious method for differentiation of the proteins is that of the 

 quantitative estimation of hydrolysis products. This process, however, 

 yields reliable results only when relatively large quantities of material 

 are available for examination, and it requires, furthermore, considerable 

 expenditure of time. In actual practice, such as in physiological and 

 pathological research, or in the investigation of foodstuffs, it is, as a 

 rule, quite inapplicable ; it affords, moreover, no certain criterion as to 

 the homogeneity or heterogeneity of the substance under investigation. 



On the assumption, then, that the protein is a polypeptide, it 

 remains to be considered what chemical and physical properties may 

 most fittingly serve for its characterisation. 



The Acid and Basic Functions of the Protein Molecule and their 

 Relations to the Physical Properties. 



The discussion of the above question will be facilitated by the 

 consideration of the properties of a typical polypeptide, e.g., 

 the tetrapeptide : 



