COAGULATION-REACTIONS 123 



acid introduced at the bottom of the tube by means of a pipette. A 

 reddish- violet ring is formed at the junction of the two liquids. 



4. Acree's Reaction. To the solution is added an equal volume 

 of a 0.02 per cent, solution of formaldehyde containing a trace of 

 ferric chloride. Concentrated sulphuric acid is then introduced below 

 the mixture and at the junction of the two fluids a violet ring is formed. 



5. The Biuret-reaction. The protein solution is rendered strongly 

 alkaline with concentrated sodium or potassium hydroxide, and j, 

 dilute solution of cupria^ulphate ls_added, one^drop at a time. A 

 reddish or bluish violet results in solutions of proteins, and a pink 

 color in solutions of their digestion-products, the peptones. Excep- 

 tions are afforded by the protamiiie group of proteins, which yield a 

 pink biuret-reaction without preliminary hydrolysis. 



6. The Ninhydrin Reaction. One-tenth of a gram of Triketohydrin- 



denehydrate ("Ninhydrin"): 



CO 



is dissolved in from thirty to forty c.c. of water, one or tw.o drops 

 of this solution are added to one c.c. of the protein solution, and the 

 mixture is heated for a short time to boiling. On cooling, an intense 

 blue or bluish violet color develops. This reaction is given not only 

 by proteins, but also by their cleavage-products, proteoses, peptones 

 and even amino-acids, with the exception of proline and oxyproline. 

 This reaction is exceedingly delicate and is given by substances con- 

 taining at least one free carboxyl-group, and one amino-group; it will 

 detect glycine (amino-acetic acid) in solutions containing only one part 

 in ten thousand. This extreme delicacy, in fact, renders the reaction 

 rather unserviceable as a practical test for proteins or their decom- 

 position-products, since such extraordinary precautions have to be 

 taken to ensure that a positive test may not have been attributable to 

 accidental contamination of reagents or apparatus with traces of the 

 many substances that will yield a positive reaction. 



All of these color reactions, with the exception of the biuret reaction 

 and the ninhydrin reaction, depend" upon specific atomic groupings 

 which are usually, but not invariably present in the protein molecule. 

 Thus, Millon's reaction is attributable to a hydroxy-benzene radical 

 which is usually present in proteins in the form of the amino-acid 

 Tyrosine, but is absent from certain proteins, for example gelatin. The 

 xanthoproteic reaction is attributable to aromatic groups which are 

 provided by the Tyrosine, Phenylalanine and Tryptophane radicals in the 

 protein molecule. The xanthoproteic reaction is therefore not given 

 by proteins such as the members of the protamine group in which 



