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, and the enthalpy determined by the van't Hoff method,. 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 and, we have undertaken both experimental studies using isothermal titration calorimetry to measure the binding energetics of Ba(2+) 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 and exist even for the linked system.

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Pressure perturbation differential scanning calorimetry was used to determine thermal expansion coefficients and thus temperature-induced volume changes of DNA duplexes differing in their GC/AT content. It was shown that the temperature-induced unfolding of the DNA duplexes proceeds with a significant increase of the thermal expansion coefficient and the partial volume of the DNA. Unusually, large temperature-induced changes in the partial volume were observed for an AT-rich dodecamer, a finding consistent with previous crystallographic studies showing the presence of highly ordered water molecules hydrating the minor groove of such duplexes. The data show that the density of this ordered water is substantially higher than that of the bulk water. This ordered water cannot, therefore, be equated to ice at normal pressures but it thermodynamically resembles ice formed at high pressures.

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