CII. XXVIII.] PROPERTIES OF PROTEINS 425 



a little acetic or sulphuric acid is added (Hopkins). Serum albumin 

 (from some animals) has also been similarly crystallised (Giirber). 



Action on Polarised Light. All proteins are Isevo-rotatory, the 

 amount of rotation varying with individual proteins. Several of the 

 conjugated proteins, e.g., haemoglobin and nucleo-proteins, are dextro- 

 rotatory, though their protein components are Isevo-rotatory (Gamgee). 



Colour Reactions. The principal colour reactions by which 

 proteins are recognised are the following: 



(1) The xantho-proteic reaction; if nitric acid is added to a 

 solution of a protein such as white of egg, the result is a white 

 precipitate; this and the surrounding liquid become yellow on 

 boiling and are turned orange by ammonia. The preliminary 

 white precipitate is not given by certain proteins such as peptones ; 

 but the colours are the same. The colour is due to the formation 

 of nitro-derivatives from the aromatic portion of the protein 

 molecule. 



(2) Millon's reaction. Millon's reagent is a mixture of mercuric 

 and mercurous nitrate with excess of nitric acid. This gives a 

 white precipitate which is turned brick -red on boiling. This reaction 

 depends on the presence of the tyrosine radical. 



(3) Copper sulphate (Hose's or Piotrowski's) test. A trace of copper 

 sulphate and excess of strong caustic potash give with most proteins 

 a violet solution. Proteoses and peptones, however, give a rose-red 

 colour instead; this same colour is given by the substance called 

 "biuret ; hence the test is generally called the "biuret reaction. This 

 name does not imply that biuret is present in protein; but both 

 protein and biuret give the reaction because they possess a common 

 radical, namely, two CONH 2 groups linked to a carbon or nitrogen 

 atom, or to one another. The native proteins give a violet colour, 

 because the red tint of the copper compound with the biuret group 

 is mixed with another copper compound with a blue colour. 



Biuret is formed by heating solid urea ; ammonia passes off and leaves biuret, 

 thus : 



2CON 2 H 4 = C 2 2 N 3 H 5 + NH 3 . 



[Urea.] [Biuret.] [Ammonia.] 



(4) Adamkiewicz reaction* When a solution of protein is mixed 

 with a dilute solution of formaldehyde, and then excess of commercial 

 sulphuric acid is added, an intense violet colour is obtained. This 

 is due to the tryptophane radical. 



Precipitants of Proteins. Solutions of most proteins are pre- 

 cipitated by: 



* In the original test, glacial acetic acid was used, but it is really an impurity 

 in this acid that gives the reaction. Rosenheim was the first to show that this 

 impurity is formaldehyde. The presence of impurities (oxidising agents) in the 

 sulphuric acid is also necessary. 



