22 BIOLOGICAL CHEMISTRY 



orange. Compare this test with the yellow colour of picric 

 acid, an aromatic nitro compound, which becomes orange 

 on rendering alkaline. 



Tyrosine Reaction. On heating the protein solution with 

 a few drops of Millon's reagent the precipitate formed by the 

 addition of the reagent turns brick red in colour. This 

 reaction is due to the phenol group in tyrosine.* 



Tryptophane Reaction. A drop or two of glyoxalic acid 

 solution is added to the protein solution and the mixture is 

 floated on strong sulphuric acid. A purple ring at the junc- 

 tion of the watery mixture with the strong sulphuric acid 

 indicates the presence of the amino acid, tryptophane.f 



Sulphur Test. To a solution of protein some lead sub- 

 acetate is added and then just sufficient alkali to redissolve 

 the precipitate that forms at first. If the solution becomes 

 brown or black on heating the test indicates the presence 

 of loosely combined sulphur which is removed by the alkali 

 in the form of sulphide and forms black lead sulphide. 



The reaction is probably due to cystine in the molecule. 



Molisch reaction (p. 8) shows the presence of a carbo- 

 hydrate group in protein. 



Other reactions of proteins are not so strictly chemical in 

 nature. They depend mainly on the ease of precipitation 

 of the proteins from their solutions. We can distinguish 

 two separate processes, reversible precipitation and irrevers- 

 ible precipitation or coagulation. In the case of the former 

 there is a precipitate which will redissolve on removal of 

 the precipitant and in the case of the latter there is a preci- 

 pitate which must be chemically altered before it can be made 

 to redissolve. 



Many of the substances which cause precipitation will cause 

 coagulation if their action is allowed to continue ; thus the 

 distinction between a precipitant and coagulant is partly a 

 matter of the length of time that the reagents are allowed to 

 act on the protein. 



Protein precipitants and coagulants. 

 Strong mineral acids. 

 Alcohol. 



Alkaloidal reagents (hydroferrocyanic acid, picric acid, 

 potassio-mercuric iodide, trichloracetic acid, tannic 

 acid, phosphotungstic acid and bromine water). 

 Excess of neutral salts of alkalies and alkaline earths. 



* E. Salkowski, Zeit. f. physiol. Chem., 1888, vol. 12, p. 215. 



f F. G. Hopkins and S. W. Cole, Journ. Physiol., 1901, vol. 27, p. 418. 



