Resolution of the Ambiguity of van’t Hoff Plots by the Effect of Pressure on the Equilibrium

Abstract

The change in the Gibbs free energy function, ΔG, of chemical reaction is determined by the difference between the heats respectively released to and absorbed from the environment, and separation of the enthalpy and entropy changes that these changes represent cannot be achieved without specific hypotheses as to their relations. The determination of the enthalpy of reaction by the plot of ΔG/T against 1/T (van’t Hoff plot) implicitly assumes that the enthalpy ΔH and entropy ΔS are temperature independent, and this assumption leads to very large errors when this is not the case and ΔH « TΔS. It is therefore inapplicable to the reactions of molecules, such as proteins, that have thermally activated local motions. The concepts offered previously by the author to relate the entropy and enthalpy changes in protein associations are reviewed briefly and applied to account for the temperature dependence of ΔH and ΔS. It is shown that two different values of the enthalpy computed in that manner correspond to each value of the apparent van’t Hoff enthalpy, but that the choice between the two is easily made by reference to the volume change on reaction. The enthalpies of association of subunit pairs of seven oligomers are all found to be positive and much more uniformly related to the size of the intersubunit surface than those previously assigned by use of the classical van’t Hoff plot.