174 CHEMISTRY OF PLANT LIFE 



better, glyoxylic acid) has previously been added, a violet color is 

 produced. This color will appear as a ring at the juncture of the 

 two liquids, if the sulfuric acid is poured carefully down the sides 

 of the tube, or throughout the mixture if it is shaken up. It 

 depends upon the interaction of the glyoxylic acid (which is gen- 

 erally present as an impurity in acetic acid) upon the tryptophane 

 group, and is therefore given by all proteins which contain tryp- 

 tophane. 



(e) Molisch's reaction for furfural will be shown by those pro- 

 teins which contain a carbohydrate group. In applying this test, 

 the solution to be tested is first treated with a few drops of an alco- 

 holic solution of a-naphthol, and then concentrated sulfuric acid 

 is poured carefully down the sides of the test-tube. If carbo- 

 hydrates are present, either free or as a part of a protein molecule, 

 a red-violet ring forms at the juncture of the two liquids. 



(/) Sulfur Test. If a drop of a solution of lead acetate be 

 added to a solution containing a protein, followed by sufficient 

 sodium hydroxide solution to dissolve the precipitate which forms, 

 and the mixture is heated to boiling, a black or brown coloration 

 will be produced if the protein contains cystine, the sulfur-con- 

 taining amino-acid. 



THE CLASSIFICATION OF THE PROTEINS 



Formerly, the classification of proteins was based almost 

 wholly upon their solubility and coagulation reactions. More 

 recently, since their products of hydrolysis have been extensively 

 studied, their classification has been modified, in attempts to make 

 it correspond as closely as possible to their chemical constitution 

 and physical properties. As knowledge of these matters progresses, 

 the schemes of classification change. On that account, no one 

 definite scheme is universally used. For example, the English 

 system varies considerably from the one commonly used by 

 American biochemists, which is the one presented below. 



The proteins are divided into three main classes, as follows : 



(1) Simple proteins, which yield only amino-acids when 

 hydrolyzed. 



(2) Conjugated proteins, compounds of proteins with some 

 other non-protein group. 



