Accounting for apparent deviations between calorimetric and van't Hoff enthalpies

Area of Science:

• Biophysical Chemistry
• Thermodynamics
• Analytical Chemistry

Background:

• Calorimetric and van't Hoff enthalpies are expected to agree in theory.
• Previous studies report discrepancies between these two enthalpy measurements.

Purpose of the Study:

• To investigate the agreement between experimental binding enthalpies from isothermal titration calorimetry (ITC) and the van't Hoff method.
• To analyze how different error sources affect calorimetric and van't Hoff enthalpies.
• To resolve questions about the relationship between these two enthalpy measurements.

Main Methods:

• Experimental ITC measurements for two host-guest pairs.
• Analysis of binding enthalpies derived from calorimetry and van't Hoff analysis.
• Modeling of artificial ITC data to simulate error propagation.

Main Results:

• Good agreement (within 0.4 kcal/mol) was observed between calorimetric and van't Hoff enthalpies for the studied systems.
• Concentration errors primarily impact calorimetric enthalpies, while heat errors affect van't Hoff enthalpies.
• Different error sources propagate uniquely to each enthalpy measurement.

Conclusions:

• Achieving agreement between van't Hoff and calorimetric enthalpies is feasible with modern calorimeters.
• Disagreement between these enthalpies can serve as an indicator of data quality issues.
• Comparing calorimetric and van't Hoff enthalpies provides an internal consistency check for calorimetry studies.