99 



of all proteins and is given due to the presence of an a-amino radical next 

 to a free carboxyl ( COOH). 



Precipitating Reactions of Proteins. All proteins are precipitated by 

 absolute alcohol. With dilute alcohol the precipitating point of the differ- 

 ent proteins is different, and C. Tebb, 1904, has worked out a means of dif- 

 ferentiating between different proteins. 



Various mineral acids, like nitric, metaphosphoric, and ferrocyanic 

 acid, as well as the alkaloidal reagents like phosphotungstic, phosphomo- 

 lybdic, tannic and picric acids, potassium mercuric iodids and potassium 

 bismuth iodids, have the power of precipitating the proteins. 



Practically all the salts of the heavy metals have the power of precipi- 

 tating the proteins. Those that are employed for that purpose most fre- 

 quently are ferric chlorid, ferric acetate, copper sulphate, mercuric chlorid, 

 basic or neutral lead acetate, zinc acetate and uranyl acetate. The strongly 

 basic proteins, like histones and protamins, also possess the power of pre- 

 cipitating the proteins. Most of the above precipitations are irreversible, 

 i. e., by removing the precipitating agent the proteins cannot be dissolved 

 in water. On adding an excess of some of the salts of the heavy metals 

 to precipitated proteins, the proteins may go into solution again. This 

 is accounted for by the fact that the proteins undergo a certain degree of 

 hydrolysis and break up into molecules which are smaller and soluble. 



The "Salting Out "of Proteins by Means of Electrolytes. It was al- 

 ready recognized by Denis (1856 (a)) and worked out in great detail by 

 Kuhne (1886), Hofmeister (1887 (6)), and T. B. Osborne and his collab- 

 orators, that a great many salts have the power of throwing the proteins 

 out of their solutions by precipitating them. Those precipitated pro- 

 teins, after removal of the salts, can be redissolved in distilled water, which 

 makes the reaction a reversible one. It was further found that different 

 proteins will be precipitated out by the different salts at definite points of 

 salt concentration. This, therefore, enabled the above workers to frac- 

 tionate the proteins and to obtain them in fairly pure state. 



Kauder, working in Hofmeister's laboratory (1886), found that when 

 small quantities of ammonium sulphate was added to blood serum, the 

 precipitation of globulins commenced when the salt concentration reached 

 13 per cent of complete saturation, and ended when it reached 24.11 per 

 cent. After the globulins were filtered off and fresh ammonium sulphate 

 was added, no precipitation took place until the concentration of the 

 ammonium sulphate reached 33.55 per cent, when the albumin fraction 

 began to be precipitated. The latter precipitation was completed when 

 the concentration reached 47.18 per cent. 



Hofmeister further studied the relative influence of anions and the 

 cations on the power of precipitating proteins. His results are summarized 

 in the following table. 



