1 6 PROXIMATE CONSTITUENTS OF PLANTS 



Special Reactions : — 



^. Millon's Reaction. — Egg albumin and other pro- 

 teins containing tyrosine or oxyphenyl groups give 

 a red precipitate on boiling with Millon's reagent.^ 

 In the absence of tyrosine or other bodies containing 

 the oxyphenyl group, this reaction does not take place, 

 and is therefore not obtained in the case of gluten or 

 gelatine. 



e. Adamkiewicz' s Reaction. — Gluten, ^^^ albumin, 

 and other proteins containing tryptophane (indole-^- 

 amino-propionic acid), on dissolving in glacial acetic 

 acid, and pouring a small quantity of concentrated 

 H2SO4 down the side of the test-tube, give an intense 

 purple ring at the junction of the two liquids. Hopkins 

 and Cole have shown this reaction to be due to the 

 presence in the acetic acid of glyoxylic acid as an 

 impurity, and the presence of this latter compound 

 should be ensured by adding to the acetic acid, previous 

 to making the test, a small quantity of a solution of 

 oxalic acid that has been partially reduced to glyoxylic 

 acid by sodium amalgam. The glyoxylic acid con- 

 denses with the indole-amino-propionic acid formed by 

 the hydrolysis of the albumin, with production of a 

 purple body of complex composition. This reaction is 

 not given by gelatine and other proteins that do not 

 contain tryptophane. 



f. VogePs Lead Sulphide Reaction. — This reaction 

 is given only by cystine and proteins containing 

 sulphur, and therefore is not obtained with gelatine. 



The substance is boiled for a short time with strong 

 NaOH solution, whereby the molecule is broken up 



1 Millon's reagent should be freshly prepared by dissolving 

 mercury in an equal weight of concentrated nitric acid with 

 application of heat, and then diluting the solution of mercuric 

 nitrate with an equal volume of water. 



