GENERAL CHEMICAL CHARACTERS OF PROTEINS 31 



making alkaline, becomes orange yellow. This is probably due to 

 the presence of a radical containing the benzene ring. 



III. Milloris Reagent. This consists of a solution of mercury in 

 nitric acid, which contains nitrous acid. This reaction is due to the 

 presence of a tyrosine group. 



IV. Colour Reactions due to the Presence of the Tryptophane 

 Group : 



(a) The chief of these is the Hopkins and Cole modification of 



the so-called Adamkiewicz reaction. As originally pro- 

 posed by Adamkiewicz, the reaction consisted in treating 

 the protein solution with one volume of concentrated sul- 

 phuric acid and two volumes of glacial acetic acid ; a reddish 

 violet colour was thereby produced. Hopkins and Cole 

 have shown that this reaction is due to the presence in the 

 acetic acid of glyoxylic acid, produced by the oxidation of 

 the former body, a process which readily takes place on its 

 exposure to sunlight. They now use a solution of glyoxylic 

 acid itself, which is produced by the reducing action of 

 sodium amalgam on a solution of oxalic acid. 



(b) ReichPs Reaction. On mixing a protein with an alcoholic 



solution of benzaldehyde and adding dilute sulphuric acid 

 (one volume of acid to one volume of water) and ferric 

 sulphate a blue coloration is produced. 



(c) Rhode's Reaction. A weak solution of dimethylaminobenz- 



aldehyde is mixed with the protein solution, and concentrated 

 sulphuric acid is allowed to flow into the mixture. A reddish 

 violet colour is thereby produced which changes to dark violet. 



(d) Liebermanns Reaction. When a protein is boiled with 



alcohol, then treated with ether, and then heated with con- 

 centrated hydrochloric acid, a blue solution is produced. 

 This is, according to Cole, due to glyoxylic acid, contained 

 as an impurity in the ether. If this is the case, the Lieber- 

 mann reaction is identical with that of Hopkins and Cole. 



(e) According to Cole, proteins on treatment with furfurol and 



hydrochloric acid yield a purple-red colour, which is also due 

 to tryptophane. Some proteins containing a carbohydrate 

 group will yield the reaction directly (cf. Molisch-Udransky 

 reaction below). 

 This series of reactions is only given by those proteins which 



yield tryptophane on hydrolysis. Gelatin, for example, does not give 



these reactions. 



V. Diazobenzene sulphonic acid in the presence of potassium 

 hydroxide yields an orange to brownish red colour, which on treat- 

 ment with zinc dust changes, owing to reduction, to a fuchsin colour. 

 The same reaction is given with tyrosine and histidine. 



VI. Reactions due to the Presence of a Carbohydrate Group. 

 Certain proteins, which contain a carbohydrate grouping (glyco- 

 proteins, and possibly certain albumins, such as egg-albumin and 

 serum-albumin J ), yield reactions indicating the presence of sugars. 

 The chief of these are (a) the Molisch-Udransky reaction, and (b) 

 the orcin reaction. 



1 In these cases it is not definitely proved whether the carbohydrate group is 

 actually contained in the protein molecule, or whether the protein is contaminated with 

 a sugar or glyco-protein. 



