104 THE CHEMICAL CONSTITUTION OF THE PROTEINS 



three-fourths of the arginine nitrogen (found previously) represents two- 

 thirds of the histidine nitrogen. Hence 



Histidine N = f (D - f Arg.) 



= i -5 D - 1-125 Arg. 1 



Calculation of Lysine Value. 



Knowing the other values, that of lysine is found by difference : 

 Lysine N = Total N - (Arg. N + Cyst. N + Hist. N). 



1 1. Determination of the Total Nitrogen of the Mono-amino Acids. 

 To the combined filtrate and washings from the phosphotungstate 

 precipitate 50 per cent, caustic soda is carefully added, until the solu- 

 tion becomes turbid by precipitation of lime and is just alkaline. 

 Excess must be carefully avoided as the precipitate may then not 

 completely dissolve in acetic acid, which is added to clear the solution 

 when just alkaline. The acid solution is concentrated in vacuo until 

 salt commences to crystallise out. The solution is then washed into 

 a 150 c.c. measuring flask and diluted up to the mark. 



Total nitrogen is estimated in 25 c.c. portions by Kjeldahl's 

 method, using 35 c.c. of sulphuric acid, 15 grams potassium sulphate 

 and 0*25 grams of copper sulphate. The acid must be added care- 

 fully on account of the evolution of hydrochloric acid. The digestion 

 must be continued for three hours after the solution has become clear. 



12. Determination of the Amino Nitrogen of the Mono-amino Acids. 

 10 c.c. portions of the solution are used for this purpose and the time 

 for the reaction with nitrous acid is six to ten minutes. Smaller por- 

 tions are used with the micro apparatus. 



The volume of nitrogen evolved by a given amount of amino 

 nitrogen is 2*5 times the volume of decinormal acid neutralised, if the 

 same amount is determined by the Kjeldahl method. Therefore, the 

 portions of 25 c.c. and 10 c.c. used in these last estimations give 

 results of similar accuracy. 



13. Determination of the Nitrogen in the Mono-aminodicarboxylic 

 Acids. In estimating the nitrogen of the mono-aminodicarboxylic 

 acids, ammonia, di-amino acids, glucosamine and organic acids must 

 be absent from the solution ; further, calcium salts must not be 

 present. 



Andersen and Roed-Muller [1915] therefore adopt a procedure 

 differing from that of Van Slyke : 



VIn the original paper the formula 1-667 D - 1*125 Arg. was given [Van Slyke, 

 , 3]- 



