The Heat-Coagulation of Proteins

IT has been long pointed out that the heat-coagulation of proteins is not so simple a phenomenon as it at first may seem to be.Schmidt and his pupils, Aronstein [1874], Heynsius [1874], Kieseritzky [1882] and Rosenberg [1883], showed that albumin solutions from which the electrolytes had been removed by dialysis did not coagulate on heating; heat-coagulation could only be produced after the addition of electrolytes to such solutions; it took place also if a small amount of any salt was added to the cold albumin solutions which had been preliminarily heated.These authors therefore sug- gested the hypothesis that heating may transform the albumin into an unknown body, the solutions of which coagulate on the addition of a very small quantity of salts [Rosenberg, 1883, p. 34].Hardy [1899, 1900, 1906] showed later that in solutions of protein altered by heating ultramicrons appear and that the protein can now be precipitated by adjusting the reaction so as to render the particles isoelectric with the solution.When the protein is made negative by adding alkali the valency of cations is of importance for the precipitation; on the contrary, the cations have no part in the precipitation if protein is made positive.Then Michaelis came to the conclusion [Michaelis] that the properties of albumin solutions may be changed by heating from those of an "emulsoid" to those of a "suspensoid," whereas the high viscosity and the low surface- tension of solutions of albumin altered by heating point rather to their hydrophilic character.These solutions probably do not fall into either category of the colloidal state but form an intermediate group with some of the properties of both.Chick and Martin [1913, 1] who investigated the heat-coagulation of pro- teins and the influence of various factors upon its course confirm the conclusion that this phenomenon consists of two distinct processes: (1) "denaturation" and (2) "agglutination" of the denaturated protein by electrolytes.They regard the former as a reaction between protein and hot water without determining this reaction more precisely.