ON HEAT COAGULATION CF PROTEINS. 283 



The mean logarithmic difference in velocity per 1° C. was extra- 

 ordinarily high, viz., 01 13 for haemoglobin and (J'28 for egg albumen, 

 or, in other words, an increase in rate of 1*3 and 1*9 times respectively 

 per degree Centigrade rise in temperature. 



Effect of acidity on rate of dena titration. 



The effect of varying the acidity of the solution upon reaction rate 

 was studied with egg albumen. Direct measurements of the hydrogen- 

 ion concentration had to be made, as the protein combines with the 

 acid to form salts. The effect of very small modifications in acidity 

 is great and is not consistent; for example, on altering the H+ con- 

 centration from 25 to 50 x 10 -7 normal, the reaction rate was doubled, 

 whereas a change from 125 to 250xl0~ 7 normal trebled the velocity. 

 This accelerating effect of acid accounts for the old observation that 

 addition of acid lowers the coagulation temperature. 



Progressive diminution of acidity during the denaturation <>f egg 



albumen. 



A solution of egg albumen was maintained at a constant temperature 

 and samples withdrawn from time to time. The determination of pro- 

 tein content and hydrogen-ion concentration of the samples showed 

 that the acidity progressively diminished as the albumen was precipi- 

 tated. An idea of the extent of this diminution may be formed from 

 the following instance : — 



A 1 per cent, solution of egg albumen had an original acidity of 

 135 x 10~ 7 normal before heating. In a sample withdrawn during the 

 course of the experiment which contained 0*3 per cent, residual albumen 

 the acidity was reduced to one-seventh (19'6 x 10 ~ 7 normal). 



As this fall of concentration of free acid is associated with a corre- 

 sponding diminution in the velocity with which denaturation occurs it is 

 clear why this was a disturbing factor in our earlier attempts to deter- 

 mine the relation of rate of reaction to concentration of protein. 



The effect of acid on the reaction rate suggests that protein-acid- 

 salts are denaturated more readily than protein itself and that the more 

 acid the protein is combined with, the quicker the reaction. 



We have ascertained that the acid is actually removed from the 

 solution with the protein precipitate. 



If this view of the effect of acid be accepted, a simple interpretation 

 of the progressive diminution of acidity during denaturation is forth- 

 coming. In a solution of protein to which a small amount of, say, 

 hydrochloric acid has been added, the acidity represents the hydrolysis 

 of the protein-HCl plus the electrolytic dissociation of the H of the 

 carboxyl groups of the protein. 



Disregarding for the present the latter, we have — 



Protein HClt^ Protein + HC1 !r~ Protein + H<- + Cl~. 



If protein-HCl is removed a readjustment of equilibrium must occur. 

 This is brought about by combination of some of the free protein and 



