GENERAL CHEMICAL CHARACTERS OF PROTEINS 39 



that in the proteins they investigated (edestin and egg-albumin) 

 scission of tyrosine by tryptic digestion is a very rapid process 

 (whereas there is no scission by peptic digestion) ; the whole of the 

 tyrosine from edestin and egg-proteins was liberated in from one to 

 three hours. It is conceivable that the determination of tyrosine in 

 a protein by this method, after a short tryptic digestion, may afford 

 another valuable factor for the characterisation of proteins. 



SECTION XVL SALT FORMATION BY PROTEINS. COMBINA- 

 TION WITH ACIDS AND BASES. 



One class of the proteins, vis., the protamines, react as strong 

 bases, giving alkaline solutions and characteristic salts with acids ; 

 others, of which caseinogen may be taken as an example, react as 

 acids, giving salts on treatment with bases. The majority of the 

 proteins, however, possess an amphoteric reaction, acting as bases 

 towards acids and as acids towards bases. 



For reasons already mentioned in the introduction, the deter- 

 mination of the acidity or basicity of proteins offers several diffi- 

 culties. The chief of these is due to their high molecular weight ; 

 relatively large amounts of protein will require for neutralisation but 

 small amounts of acids and bases. Furthermore, as the proteins in 

 several instances act only as very weak acids or bases, the salts 

 readily undergo hydrolysis in aqueous solutions ; the ordinary methods 

 of titration, with the use of indicators, are not therefore available for 

 determining their combining weights with acids and bases. Con- 

 sequently it is necessary to employ indirect methods, of which the 

 following are the chief: 



A. Physical Methods. 



I. The measurement of electrolytic conductivity when acids or 

 bases are added to protein solutions. 



II. The measurement of the potential of concentration cells, and 

 the changes produced when proteins are added to either electrode. 



III. The determination of the influence of the addition of pro- 

 teins on the depression of the freezing points of solutions of acids 

 and bases. 



IV. The determination of the influence of the addition of proteins 

 on the rate of hydrolysis of cane-sugar and esters by acids and 

 bases. 



B. Chemical Methods. 



I. Direct titration in presence of indicators. 



II. Determination of the solubility in acids and bases of proteins 

 which are insoluble in pure water. 



III. Determination of the acidity of the filtrate from protein 

 precipitates produced by neutral salts or alkaloidal reagents in 

 solutions containing an excess of acids. 



It is proposed to consider these general methods first, and after- 

 wards to refer to some of the more recent investigations on the salt 

 formation of individual proteins. 



4 



