42 THE CHEMICAL CONSTITUTION OF THE PROTEINS 



ing to Osborne and Jones [1910, 2] the distillation of the esters boil- 

 ing above 110 can be dispensed with. An equally good separation 

 of this residual mixture can be effected if it be carried out in the same 

 way as is described for the esters after they have been distilled. 



The following table shows the fractions which are usually collected, 

 and the amino acid esters which they may contain : 



Sometimes no fractionation of the esters boiling up to 100 at 0-5 

 mm. is made, the whole mixture being simply divided into esters 

 volatile below 100 at 0*5 mm. and a residue, which is not distilled. 



(6) The Isolation of the Individual Mono-Amino Acids. 



The separation and characterisation of each constituent contained 

 in an ester fraction has now to be carried out. A somewhat special 

 process has to be adopted for each individual product. 



Fractions /.,//., ///. 



The esters contained in these fractions are immediately reconverted 

 into the amino acids. This is effected by boiling the fractions with 

 5-10 volumes of water under a reflux condenser for six to seven hours, 

 until the alkaline reaction has disappeared. 



If leucine be present in considerable amount, as may be the case in 

 fraction III., the solution on cooling may deposit crystals of this sub- 

 stance. These are filtered off, washed with water, dried and weighed. 

 Identification of the product is carried out as described below. 



The three solutions are then evaporated to dryness in vacuo in 

 weighed flasks and the amount of residue ascertained for each fraction. 



Levene and Van Slyke [1908, 2] have found that leucine ester, 

 like phenylalanine ester, is readily dissolved by ether from water. The 

 ester fractions may therefore, before hydrolysis, be mixed with 3 

 volumes of water and extracted with ether, and the ether extract 

 washed three times with water. The aqueous solution is then 

 saponified by boiling and the extract, after removal of the ether, is 

 treated in the same way. 



