INTRODUCTION 



5 



such as some of the globulins and caseinogen, although this character 

 may be ascribed in the latter substance to the presence of the 

 phosphoric acid group in the molecule. 



The majority of the proteins, however, have neither marked basic 

 nor acidic character, and in this respect resemble the typical 

 polypeptide of the typical formula with the radicals R 1 , R", etc., 

 containing only carbon and hydrogen atoms. 



The determination of the acidity and basicity of proteins of this 

 character has entailed many difficulties. This arises from the capacity 

 of proteins as colloids to adsorb simpler substances and from the 

 large molecular weights of this class of substances and relatively small 

 number of active carboxylic and amino groups ; the equivalent com- 

 bining weight is therefore large ; in the case of the crude egg- 

 albumins Sjoqvist has shown (p. 40) that between 800 and 900 grams 

 of protein combine with I gram mol. equivalent of hydrochloric acid 

 to form the hydrochloride salt. 



Now a solution which contains a relatively large percentage of a 

 solute of large molecular weight is technically very dilute ; on the 

 assumption that the protein of egg-white is a mono-acid base, its 

 molecular weight would be about 850, which, according to the 

 experiments of Sjoqvist, is the lowest possible ; a 5 per cent, solution 

 would be therefore only j^ normal. The salts of weak bases in 

 dilute solution readily undergo hydrolysis according to the equation 



B . HC1 + HOH = B . OH + HC1. 



The acidity or basicity of a protein cannot, as a rule, be determined, 

 therefore, by titration with the use of indicators, since hydrolysis of 

 salts can take place in solutions containing relatively large amounts 

 of protein. 



Furthermore, Hardy has pointed out in the case of the globulins 

 the possibility of the formation of basic salts. If serum-globulin be 

 submitted to dialysis (see p. 54) it can hydrolyse according to the 

 equation 



x GHAc + y HOH = (GHOH)j, (GHAc) x - y + y HAc. 



Where Ac represents an acid ion. As y increases, the protein 

 becomes more and more basic^ and the negative ion increases in size, 

 the change being indicated by the alteration in the appearance of the 

 solution, which becomes gradually more and more opalescent until it 

 is finally nearly opaque. It has still more or less the properties of a 

 true solution, the opacity being due to the formation of what Hardy 

 calls pseudo-ions, which can still take part in the transport of electricity, 

 and which, on increasing hydrolysis, become so large as to be capable 

 of diffracting light. A basic salt is finally obtained containing a very 

 small amount of acid. 



The above considerations have an important bearing upon the 

 choice of constants to be chosen for the characterisation of proteins, 

 for it will be obviously impossible to directly determine in most cases 

 whether a given solution contains a free protein or a salt of the same. 

 Indicators, as already stated, are useless for the purpose, and owing to 

 the possibility of the formation of acid or basic salts very minute 

 quantities of an acid or base will very often suffice to entirely alter 

 the character of a solution. There exists, therefore, very great 



