Significant discrepancies between van't hoff and calorimetric enthalpies. II

Abstract

Isothermal calorimetric titration of 18-crown-6 ether with BaClz in pure aqueous solution over the temperature range 7-40 "C gives precise binding constants and enthalpy changes. Nonlinear least-squares fitting of the binding constants to the integrated van't Hoff equation, including a temperature-independent change in heat capacity, leads to van't Hoff enthalpies that differ significantly from the observed calorimetric enthalpies. This perplexing discrepancy appears at present to be very widely occurring. Keywords: calorimetric enthalpies; calorimetry; temperature variation of equilibrium constants; van't Hoff enthalpiesIn a recent paper (Naghibi et al., 1995), we pointed out the frequent occurrence of discrepancies between van't Hoff enthalpies derived from the temperature variation of equilibrium constants and enthalpies determined by direct calorimetric measurements. To date we have not found a single case where there is agreement, to within experimental uncertainty, between these two types of enthalpy values. All the systems but two considered in our earlier paper involved proteins, the two exceptions being the binding of cyclohexanol to a-and 0-cyclodextrins. Even in these cases involving relatively simple molecules, there were significant differences between van't Hoff and calorimetric enthalpies. Izatt et al. (1976) and Briggner and Wadso (1991) published calorimetric data on the binding of barium chloride to 18-crown-6 ether, the latter publication including equilibrium data at three temperatures. This system would appear to be an excellent system for further study of the problem concerning van't Hoff and calorimetric enthalpies because of its simplicity:18-crown-6 is a solid cyclic polyether of 264 Da molecular mass; both reactants are readily available in very pure form; and they can be weighed and dissolved in pure water for the reaction. Izatt et al. reported data at 25 "C only, and Briggner and Wadso limited their study to three temperatures. Because our method for analyzing such data requires measurements at four or more temperatures, we have undertaken further study of this system.

Complexes of Silver with Histidine and Imidazole Investigated by the Calorimetric and Potentiometric Methods

“…ions to 18-crown-6 ether were found slightly higher than the values reported in the literature, by approximately 0.5% [4] and 1.5% [5,6]. These differences lie within the limits of the measurement accuracy.…”

Carbonic Anhydrase as a Model for Biophysical and Physical-Organic Studies of Proteins and Protein−Ligand Binding

“…Such discrepancies have been reported previously in the literature for protein-ligand binding. [578][579][580] While a rigorous comparison of thermodynamic values obtained from the two techniques is not possible because so few directly comparable data are available in the literature, we believe that calorimetry is the best technique for measuring the dissected thermodynamics for protein-ligand association for reasons discussed in section 10.2.2.…”

Multivalency in Ligand Design

“…ITC can thus determine both the free energy and the enthalpy of binding; and the entropy of binding can be determined from these two values (TDS o = DH o -DG o ). Because ITC is carried out at one temperature (and so without concerns for changes in protein structure with temperature, and without assuming the change in heat capacity upon complexation is independent of temperature), it generates much more reliable thermodynamic information for biological systems than does van't Hoff analysis, which requires measurements at different temperatures [66][67][68][69]. For thermodynamic analysis of multivalent systems, the accuracy provided by ITC is essential in understanding both the entropic and enthalpic contributions to binding, in determining the thermodynamic advantage gained from multivalency (relative to monovalency), and in assessing the cooperativity of the system (Section 2.4.4).…”