70 PROPERTIES OF CERTAIN AMINO-ACIDS. [CH. IV. 



R.CH.NH 2 R.CH.N = CH 2 



| + H.CHO= | + H 2 0. 



COOH COOH. 



This reaction is the basis of Sorensen's method of estimating 

 the production of amino-acids during tryptic digestion. 



4. They form insoluble crystalline compounds with 

 #-naphthalene-sulphochloride. 



SO 2 C1 H 2 N.CH.R qo 



+ = bU 



COOH 



5. They form moderately soluble copper salts. With 

 the exception of tryptophane (see p. 95) these can be readily 

 crystallised from boiling water. The tryptophane copper 

 salt is characteristically insoluble, even in boiling water. 

 In the presence of even traces of other amino-acids it be- 

 comes soluble in the solution of their copper salts. 



6. They are acted upon by nitrous acid, yielding 

 nitrogen gas (see Ex. 81). 



R.CH.NH 2 HNO 2 = R.CH.OH 



| + + N 2 +H 2 0. 



COOH COOH 



This important reaction is the basis of Van Slyke's gaso- 

 metric method for the estimation of amino-acids in blood 

 and tissues. 



7. They are optically active with the exception of 

 glycine, which does not contain an asymmetric carbon 

 atom (see p. 147). 



Methods of separation. The proteins can be hydro- 

 lysed by boiling acids, boiling alkalies, or by the action of 

 certain enzymes, e.g. trypsin. The products are then 

 separated by : 



I. Fractional crystallisation of the amino-acids, or of 

 their hydrochlorides. In this way tyrosine, 

 leucine, cystine, and glutaminic acid hydrochlor- 

 ide are obtained. 



