van't Hoff and calorimetric enthalpies from isothermal titration calorimetry: Are there significant discrepancies?

The enthalpy of a reaction is most often determined through one of two means; it can be determined directly using calorimetry or indirectly by measuring the temperature dependence of the equilibrium constant (i.e., the van't Hoff method). Recently, discrepancies have been noted between the enthalpy measured by calorimetry, DeltaH degrees (cal), and the enthalpy determined by the van't Hoff method, DeltaH degrees (nuH). This has been suggested to indicate that the binding reaction is more complex than the simple one-to-one binding model used to describe the data. To better understand possible discrepancies between DeltaH degrees (cal), and DeltaH degrees (nuH), we have undertaken both experimental studies using isothermal titration calorimetry to measure the binding energetics of Ba2+ binding 18-crown-6 ether and 2'-CMP binding RNase A, along with a simulation of a system involving a molecule in conformational equilibrium coupled with binding. We find that when experimental setup and analysis are correctly performed, no statistically significant discrepancies between DeltaH degrees (cal), and DeltaH degrees (nuH) exist even for the linked system.