308 PHYSICAL PROPERTIES 



then it would follow, from van't Hoff's "Principle of Mobile 

 Equilibrium" that the hydrolysis of protein (hydration) is ac- 

 companied by an evolution of heat, which conclusion is in com- 

 plete accord with experimental observation.* The apparent 

 discrepancy between the conclusion reached by Chick and Martin 

 and those which have been formulated by other investigators 

 in this field may therefore be due to the fact that dry protein 

 has been deprived of the elements of water in its end — NH2 

 and — COOH groups, and that these, consequently, cannot react 

 to form polymers of the protein. f It is evident that in the process 

 of dehydrating proteins by heat not only the reaction : 



H2N.R.COH.N.R.COOH = HN.R.COH.N.R.CO + H2O 



occurs, but also reactions of the type 



2 H2N.R.COH.N.R.COOH 



= H2N.R.COH.N.R.COH.N.R.COH.N.R.COOH -1- H2O, 



and it is the formation of these polymers which leads to the 

 apparently irreversible character of the process; apparently and 

 not actually irreversible because as Corin and Ansiaux (22) 

 have shown, if a solution of protein be cooled and vigorously 



* Cf. 1 and the results of Wiedemann and Liideking cited in this chap- 

 ter. The heat of reaction of protein hydrolysis is extremely small; observers 

 using the usual indirect methods of determination either failed to detect 

 any change in the heat-content of the system or else have observed a very 

 slight disengagement of heat (101), (49), (36). Henderson and Ryder, however, 

 using the beautiful and excessively sensitive isothermal method of calorimetry 

 of T. W. Richards, have demonstrated that the tryptic hydrolysis of casein 

 is accomphshed by a progressive evolution of heat (37). It should be clearly 

 borne in mind in this connection that the effect of raising the temperature upon 

 a chemical reaction is always twofold. It shifts the station of equilibrium in 

 one sense or in the opposite and, always, it accelerates the reaction in either 

 sense to a greater or less degree (i.e., it magnifies both velocity constants, but 

 unequally). The action of heat upon proteins must always be to shift the 

 station of equilibrium in the direction of polymerization (i.e., condensation), 

 since the reaction of hydrolysis is exothermic. But the fact that fairly com- 

 plete hydrolysis will occur at the temperature of steam shows that the shift in 

 equilibrium between the lower protein complexes and the amino-acids which 

 are the products of their hydrolysis is not so great as to extinguish the reaction 

 of hydrolysis. 



t It should also be recollected, in this connection, that reactions occurring 

 in solid systems are notoriously extremely slow, owing to the high internal 

 friction of the system and the consequent hampering of molecular motion. 



