CHEMICAL COMPOSITION OF PROTEIN MOLECULE 9 



amino acids is then concentrated to a syrup in vacuo at 40 C. and from 

 this syrup the free esters are extracted as follows : 



About an equal volume of water is added to dissolve the syrup, 

 and, if I kilo, of protein has been used, it is divided into four or six 

 portions for convenience and to ensure the subsequent thorough cooling ; 

 to each portion one to two volumes of ether are added, and the mixture 

 is thoroughly cooled in a freezing mixture of ice and salt ; strong caustic 

 soda is now added till the free hydrochloric acid is neutralised, and then 

 a considerable excess of finely granulated potassium carbonate. The 

 feebly basic esters of aspartic and glutamic acids, which are very 

 sensitive to free alkali, are thus liberated and are dissolved by the ether, 

 which is quickly poured off and replenished by a fresh quantity. The 

 addition of 33 per cent, caustic soda in small portions at a time and of 

 solid potassium carbonate sets free the other esters from their hydro- 

 chlorides ; these are dissolved by the ether, which is frequently renewed 

 throughout the process and thoroughly mixed with the mass of salt and 

 potassium carbonate ; sufficient caustic soda must be added to combine 

 with the whole of the hydrochloric acid, and as much potassium carbon- 

 ate as is necessary to form a pasty mass in order that the free esters, 

 which are very easily soluble in water, are salted out and dissolved by 

 the ether. A large amount of ether is required for this extraction, and 

 an essential condition is that, throughout the process of extraction, the 

 various portions be kept thoroughly cold by shaking in the freezing 

 mixture. 



The several ethereal extracts are then dried by shaking for about 

 fifteen minutes with potassium carbonate; they are then combined 

 together and allowed to stand for twelve hours with anhydrous sodium 

 sulphate. 



The ether is next evaporated off, preferably in vacuo at the ordinary 

 temperature, as in this way the lower boiling esters do not distil with 

 the ether and the danger of decomposing them by a higher temperature 

 is avoided, and a brown oil, consisting of the esters of the amino acids, 

 is obtained, which is fractionally distilled in vacuo. 



By this method of extracting the esters from their hydrochlorides, 

 neither that of tyrosine, which remains behind combined with alkali, nor 

 those of the diamino acids, which are soluble with difficulty in ether, are 

 obtained. This is advantageous for the subsequent process of separation, 

 but the method has the disadvantage that the whole quantity of esters 

 is not taken up by the ether on account of their destruction by the 

 alkali. In order to avoid their loss, the mass of carbonate is treated 

 with excess of hydrochloric acid and evaporated down, the potassium 



