42 THE CHEMICAL CONSTITUTION OF THE PROTEINS 



when the water is removed by evaporation in vacua. Phelps and 

 Tillotson's method seems to be preferable. 



5. Separation of Glycine Ester Hydrochloride. The glycine is never 

 completely isolated as ester hydrochloride ; the remainder is obtained 

 as ester. 



6. Extraction of Esters. Loss always occurs in this part of the pro- 

 cess, but is covered when the process is repeated. The loss is largely 

 mechanical and cannot be avoided. A small quantity of organic 

 matter is retained by the sodium sulphate used for drying the esters. 



7. Distillation of Esters. (a) In distilling off the ether, especially 



if its volume be large, a considerable quantity of esters distils 

 at the same time. Two receivers should be used and the 

 esters extracted from the distillate. 



(&) Decomposition of the esters occurs during the distillation, and 

 a more or less large residue represents the loss. The products 

 can be recovered, if necessary. Abderhalden has found that 

 in the process up to this stage 25 per cent, of glutamic acid 

 and 40 per cent, of aspartic acid are lost ; some of the glutamic 

 acid is lost by conversion into pyrrolidone carboxylic acid. 

 The loss is greatly diminished if the distillation of fraction IV. 

 be omitted. Osborne and Jones find that the separation of 

 the esters in this fraction is not more troublesome than when 

 they are distilled. 



8. Separation of the Individual Amino Acids. The greatest loss 

 occurs here as none of the methods of isolating the compounds are 

 perfect. 



(a] Proline. The quantity extracted by alcohol is much greater 

 than that obtained in a pure crystalline state and reckoned as 

 proline. Estimation by Van Slyke's method gives the actual 

 amount. 



(&} V aline, Leucine, Isoleucine. The figures given represent the 

 quantity of substance isolated in a pure state. The leucine 

 figures are really those for leucine + isoleucine. 



(c) Glycine and Alanine. Nearly all the glycine can be obtained, 



but the actual amount of alanine is much greater. 



(d) Glutamic Acid. Probably the figures given for this amino acid 



most nearly approach the real content of the protein in this 

 constituent. 



(e) Aspartic Acid and Serine. The figures are much too 1 low, as the 



method of separation is extremely unsatisfactory. 



