12 THE CHEMICAL CONSTITUTION OF THE PROTEINS 



five to six times the quantity of absolute alcohol, which dissolves the 

 proline, leaving the alanine, which is then purified by crystallisation 

 from water. It may be characterised by its melting point, its rotation 

 in hydrochloric acid solution, by elementary analysis, or by conversion 

 into its benzoyl derivative, which may, however, have a rather low 

 melting point on account of its partial racemisation. It is estimated 

 by its yield. 



Valine. This amino acid is contained mixed with leucine in the 

 fractions of the esters which boil between 60 and 90 C. Its isolation 

 and separation from leucine is of extreme difficulty, since these com- 

 pounds, as well as their copper salts into which they are converted by 

 boiling with freshly precipitated cupric oxide, tend to form mixed 

 crystals. Its isolation was only effected by these means in certain 

 cases, and its amount is really much more than the figures represent 

 from its yield. It is best characterised by conversion into its phenylhy- 

 dantoine derivative by treatment with phenylisocyanate in alkaline 

 solution. The phenylureido acid is first formed, and this loses a 

 molecule of water, as shown by Mouneyrat, and is changed into its 

 anhydride or phenylhydantoine by treatment with hydrochloric acid. 

 The following reactions occur :- 



CH 3 v CH 3 x 



)CH 2 CH COOH >CH 2 CH COOH 



CH 3 / | + C 6 H 5 NCO = CH 3 / 



NH 2 NH.CO.NH.C 6 H 5 



CH 3 x CH 3 \ 



)CH 2 CH COOH /CH 2 CH CO\ 



CH 3 / | = H 2 0+CH 3 / | )N.C 6 H 5 



NH.CO.NH.C 6 H 5 NH CO/ 



Leucine. The greater part of the leucine is contained in the ester 

 fractions, which boil between 70 and 90 C. It generally occurs in 

 considerable amounts in the protein, and is obtained by crystallisation 

 from water, in which it is less soluble than the other amino acids which 

 may be present. It is seldom present in its pure, optically active form, as 

 this is easily racemised, and the various crops of crystals most probably 

 also contain isoleucine. It is more easily isolated by completely racemis- 

 ing the mixture of amino acids contained in this fraction by heating in 

 an autoclave with baryta to 160-180 C., and then, after removal of the 

 baryta, separating it by crystallisation. The difficulty of separating it 

 from the other amino acids, especially valine and isoleucine, makes an 

 exact quantitative estimation almost impossible. The values which 

 have been found are therefore minimal ones, and they will also include 

 in many cases the yield of isoleucine. 



Isoleucine. The separation of leucine and isoleucine has been 



