CHEMICAL COMPOSITION OF PROTEIN MOLECULE 19 



(3) Extraction of the Esters of the Amino Acids. 



The solution is concentrated in vacuo at 40 C. to a syrup. The 

 esters are then liberated from their hydrochlorides by one of the follow- 

 ing methods : 



(a) About one-third to one-half the volume of water is added to 

 dissolve the syrup, and, if I kilo of protein has been used, the solution 

 is divided into two or four portions for convenience and to ensure the 

 subsequent thorough cooling ; to each portion two or three volumes of 

 ether are added, and the mixture is thoroughly cooled in a freezing 

 mixture of ice and salt ; strong caustic soda (33 per cent.) is now 

 added till the free hydrochloric acid is neutralised ; this can be tested 

 by adding a small quantity of a saturated solution of potassium 

 carbonate. The feebly basic esters of aspartic and glutamic acids, 

 which are very sensitive to free alkali, are thus liberated and are dis- 

 solved by the ether, which is quickly poured off and replenished by a 

 fresh quantity. Caustic soda and solid potassium carbonate added in 

 small portions at a time set 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 com- 

 bine with the whole of the hydrochloric acid, and as much potassium 

 carbonate as is necessary to form finally a pasty mass in order that 

 the 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, which is continued until the ether separates in a colour- 

 less state, 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 each dried by shaking for about 

 five 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 small quantities at 

 a time and in vacuo at the ordinary temperature ; 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. A brown oil, 

 consisting of the esters of the amino acids, results ; this is fractionally 

 distilled in vacuo. 



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