44 THE CHEMICAL CONSTITUTION OF THE PROTEINS 



of the protein is considerably larger than the figures given in the tables 

 (pp. 111-130). Abderhalden and Kautzsch [1912] consider that the 

 yields of both forms of proline should be always separately given. 

 The yield if it be based on the yield of copper salts is not a proper 

 value. 



The real proline content can be accurately determined, as Van 

 Slyke [1911, 3] has shown by his nitrous acid method (p. 89), by a 

 determination of the total and amino-nitrogen of the product soluble 

 in absolute alcohol. The alcoholic solution is made up to a definite 

 volume and aliquot portions are taken for these estimations. The 

 difference gives the amount of nitrogen present as proline from which 

 the amount of proline can be calculated. In the case of caseinogen 

 the proline content was found to be 6*7 per cent., a figure which is 

 twice that found by Abderhalden. It agrees with that found by 

 Engeland by his method of methylation (p. 69). Abderhalden and 

 Kautzsch [1912] believe that the proline content determined by the 

 Van Slyke method is not accurate, as the solution may contain other 

 nitrogenous products which do not evolve nitrogen on treatment with 

 nitrous acid. 



(b) Glycine, Alanine, Valine, Leucine, and Isoleucine. 



These five amino acids are present together in varying proportions 

 in the residues which are insoluble in absolute alcohol. Their 

 separation is only effected with great difficulty, and the procedure 

 depends very largely upon which amino acids are present in the 

 several fractions. 



In each case the residue is dissolved in water and, if necessary, the 

 solution is decolorised by boiling with charcoal. The aqueous solu- 

 tions are concentrated and fractionally crystallised ; the final mother 

 liquor is evaporated to dryness. Each fraction is dried and weighed. 

 Indications of the constituents of each fraction may be obtained : 



1. By elementary analysis of the carbon, hydrogen and nitrogen 

 content. 



2. By determining the melting-point ; the substance must be 

 rapidly heated. Glycine melts at 240, alanine about 297, leucine 

 about 300, and valine about 315. 



3. By the taste. Glycine and alanine have a sweet taste ; valine 

 is less sweet and it leaves a bitter after-taste ; leucine is insipid and 

 slightly bitter. 



The following fractions may be obtained by the fractional crys- 



