86 THE CHEMICAL CONSTITUTION OF THE PROTEINS 



neutral reaction whereas the monoaminodicarboxylic acids have an 

 acid reaction ; by neutralising the solution the dicarboxylic acids react 

 with i molecule of sodium hydroxide and on incineration leave an 

 equivalent quantity of sodium carbonate, which can be titrated (see 

 pp. 104-106). Only a few determinations have so far been made by 

 Andersen and Roed-Miiller, but they agree with the figure calculated 

 from the amounts of aspartic and glutamic acids isolated from the 

 products of hydrolysis : 



Per cent, of Total Calculated from data by 



Nitrogen. 



Gliadin . . . 24-3 24*0 Osborne and Guest 



Zein .... 17-9 i6'6 Osborne and Liddle 



Caseinogen. . . . 17-7 10-4 Osborne and Guest 



In the case of caseinogen the value is distinctly higher, but it may 

 represent the amount of dicarboxylic acid in this protein more closely 

 than the calculated figure, more especially as in this case the complete 

 isolation of glutamic acid is more difficult than in vegetable proteins. 



If we regard the data for tyrosine as almost accurate, we have still 

 one more value of service for the chemical differentiation of the pro- 

 teins. It is unfortunate that we cannot yet measure the tryptophan 

 content of a protein, especially as this unit is so readily detected by 

 means of its colour reactions. 



