96 THE CHEMICAL COMPOSITION OF THE BODY 



identification. Dilute some white of egg with ten volumes of water, filter 

 off the precipitated globulin, and use the egg albumin in the following tests: 



1. Color Reactions of Proteids. a. Xanthoproteic. Add concentrated 

 nitric acid to 2 c.c. of the egg albumin in a test tube, a lemon-yellow color 

 appears on gently heating. Add excess of ammonia, the color deepens to 

 orange, or with potassium hydrate to reddish brown. Egg albumin is also 

 precipitated by the acid, but peptone gives only the color change. This re- 

 action depends upon the presence of the tyrosin nucleus, or that of indol, 

 in the proteid molecule. 



b. Milton's reaction. Millon's reagent (mercuric and mercurous nitrate 

 in weak nitric-acid solution) added to albumin solution gives a white co- 

 agulum in the cold which turns purple-red on heating to 70 C. or more. 

 The reaction is due to the tyrosin grouping. 



c. The biuret reaction. Excess of sodium or potassium hydrate with a few 

 drops of 2 per cent copper sulphate in albumin solution when heated gives a 

 violet color. Albumoses give a pinkish violet, and peptones a pink color in 

 this reaction, but care must be taken not to use an excess of copper sulphate. 

 The reaction seems to depend on the presence of the polypeptid groups. 



d. Adamkiewicz reaction. If dilute glyoxylic acid be added to proteid 

 solution, and concentrated sulphuric acid run under the mixture, a ring of 

 colors is produced at the junction of the layers when gentle heat is applied; 

 red at the bottom, then green and violet. When shaken the whole becomes 

 violet. The reaction depends upon the tryptophane group. 



2. Precipitations, a. Acid precipitation. Proteids form insoluble salts 

 with tannic acid, phospho-tungstic acid, hydroferrocyanic acid, picric acid, 

 etc. The proteid is changed in the reaction and cannot be recovered by 

 breaking up the salt. Strong mineral acids, hydrochloric acid, nitric acid, 

 etc., precipitate proteids, but the peptones are soluble in excess. 



b. Heavy metal precipitation. Proteids form insoluble compounds with 

 mercuric chloride, lead acetate, copper sulphate, silver nitrate, etc. 



c. Alcohol. Proteids are precipitated and coagulated by an excess of 

 alcohol. Peptone alone is recoverable from alcoholic precipitation. 



d. Heat coagulations. Make the egg-albumin very faintly acid with 

 2 per cent acetic and heat to boiling, a white cloudy coagulum appears. 

 Albumoses and peptones are not heat-coagulated. 



e. Precipitation by neutral salts. Add crystals of ammonium sulphate 

 to egg albumin solution to saturation, a white flocculent precipitate forms. 

 The precipitate can be recovered as unchanged albumin by removing the 

 excess of salt by dialysis. 



Reactions Characteristic of Individual Proteids. The proteid 

 groups most often met by the student are the albumins, globulins, albumi- 

 nates, albumoses, peptones, enzyme-coagulated proteid, and heat-coagulated 

 proteid. Each has certain characteristics. 



