Accounting for Apparent Deviations between Calorimetric and van’t Hoff Enthalpies

Abstract

AbstractBackgroundIn theory, binding enthalpies directly obtained from calorimetry (such as ITC) and the temperature dependence of the binding free energy (van’t Hoff method) should agree. However, previous studies have often found them to be discrepant.MethodsExperimental binding enthalpies (both calorimetric and van’t Hoff) are obtained for two host-guest pairs using ITC, and the discrepancy between the two enthalpies is examined. Modeling of artificial ITC data is also used to examine how different sources of error propagate to both types of binding enthalpies.ResultsFor the host-guest pairs examined here, good agreement, to within about 0.4 kcal/mol, is obtained between the two enthalpies. Additionally, using artificial data, we find that different sources of error propagate to either enthalpy uniquely, with concentration error and heat error propagating primarily to calorimetric and van’t Hoff enthalpies, respectively.ConclusionsWith modern calorimeters, good agreement between van’t Hoff and calorimetric enthalpies should be achievable, barring issues due to non-ideality or unanticipated measurement pathologies. Indeed, disagreement between the two can serve as a flag for error-prone datasets. A review of the underlying theory supports the expectation that these two quantities should be in agreement.General SignificanceWe address and arguably resolve long-standing questions regarding the relationship between calorimetric and van’t Hoff enthalpies. In addition, we show that comparison of these two quantities can be used as an internal consistency check of a calorimetry study.HighlightsAgreement within ~0.4 kcal/mol between calorimetric and van’t Hoff enthalpies can be achieved for systems with typical heat and concentration errors, if solution non-ideality is not an issue.Concentration error chiefly affects calorimetric enthalpies, while error in measured heat chiefly affects van’t Hoff enthalpies.Large discrepancies between calorimetric and van’t Hoff enthalpies can be used to flag experimental error.There is no theoretical basis to expect discrepancies between these two methods of determining the binding enthalpy.