PROTEINS 89 



the ease 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 

 sulphate, magnesium sulphate and sodium chloride. High and 

 varying concentrations of these salts are necessary to throw 

 down the different proteins. By the use of suitable amounts 

 of these salts some of the protein groups may be separated from 

 others. The process is known as "salting out." The proteins 

 are not coagulated, and may be redissolved on removal of the 

 salt. 



Alkaloidal Reagents. Many compounds known as alkaloidal 

 reagents will precipitate proteins. Among these are several 

 acids such as tannic, picric, phosphotungstic, phosphomolybdic, 

 ferrocyanic, chromic, and dichromic. The precipitates undoubt- 

 edly are compounds of protein with the negative-ion of the pre- 

 cipitating reagent. The tests are thus carried out to best ad- 

 vantage in weakly acid solution. Exceptions to this statement 

 depend upon conditions similar to those discussed under pre- 

 cipitation by the addition of salts. 



Alcohol. Alcohol in sufficient concentration will precipi- 

 tate many of the proteins. Some few are soluble in alcohol, 

 however. If the precipitate is allowed to stand in the alco- 

 hol, it will become coagulated, and cannot be redissolved on re- 

 moval of the alcohol. 



Structure of the Protein Molecule 



Since proteins make up so large a part of living tissue, and 

 are indispensable to life it would be of great interest to find out, 



