PROTEINS 105 



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. 



Tungstic acid as a protein precipitant is of particular in- 

 terest, since it is used to precipitate the proteins of the blood in 

 the preparation of the protein-free blood filtrate used in the 

 Polin and Wu system of blood analysis. Sodium tungstate is 

 added to the diluted blood, and then sulphuric acid. Tungstic 

 acid is set free, and precipitates the blood proteins completely, 

 so that the filtrate is water-clear, and free from all protein. 



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, 

 how the protein molecule is constructed. This problem has been 

 studied by some of the foremost biochemists for a long time, and 

 although the formula for a protein is still unknown, much is 

 now known as to the manner in which the parts of a protein 



