104 PHYSIOLOGICAL CHEMISTRY 



stance is said to be coagulated. Proteins may be coagulated 

 also in other ways. 



Mineral Acids. Proteins are precipitated by the addition 

 of small amounts of the strong mineral acids, hydrochloric, 

 sulphuric and nitric. The precipitate dissolves in excess of the 

 acid, particularly if the solution is heated. Glacial acetic acid 

 does not precipitate proteins. There has been much discussion 

 of the nature of this precipitation. Probably the proteins are 

 thrown down in the form of salts. Precipitation with concen- 

 trated nitric acid is often used as a test for proteins. To the 

 solution to be tested, concentrated nitric acid is added carefully 

 so that it will form a layer at the bottom of the test tube. In 

 the presence of protein a cloudy ring appears at the juncture of 

 the two liquids. 



Salts. Proteins are precipitated by salts. The salts of 

 heavy metals, such as copper, iron, mercury, lead, etc., will 

 throw down the proteins from their solutions. The precipi- 

 tates formed are in many cases true salts of the protein and 

 the metal. Where this is the case, the precipitation takes place 

 best usually in a weakly alkaline solution, for in this condition 

 the proteins are negatively charged, and will combine with the 

 positive metal ions. Some proteins, such as protamines and his- 

 tones, which have large amounts of diamino acids, form alkaline 

 solutions and the protein carries positive charges. More alkali 

 must be added to give these proteins negative charges than in 

 the case of albumins. Casein, which contains much dicarboxylic 

 acid (glutamic) carries negative charges (since it gives off 

 hydrogen-ions to the water). Casein may thus be precipitated 

 by metals even in slightly acid solution. 



The conditions governing precipitation of proteins by metals 

 are somewhat complicated by the fact that some metals such 

 as mercury, gold, copper, and others (these metals have a lower 

 solution tension than hydrogen) combine with the amino group 

 also, so that salts of these metals will precipitate proteins in 

 weakly acid as well as in weakly alkaline solution. 



Three salts much used to precipitate proteins are ammonium 



