van't Hoff and calorimetric enthalpies II: effects of linked equilibria.

The complexity of binding reactions, including the linkage with other equilibria, is becoming increasingly apparent in biological processes such as signal transduction. Understanding these interactions requires obtaining thermodynamic profiles for each of the equilibria that occur in a binding event. Concern has been raised as to whether linked equilibria contribute differently to thermodynamics, such as DeltaH degrees and DeltaC(p), obtained from calorimetric and van't Hoff methods. We have previously shown that linked equilibria do not contribute differently to the van't Hoff and calorimetrically determined DeltaH degrees for processes such as linked folding or hydration. Here, examples of proton and ion linkage are examined. We show that there is no reason to expect the calorimetric and van't Hoff DeltaH degrees to be different, even without prior knowledge of the presence or absence of linked equilibria, as long as the system is permitted to equilibrate. However, it is possible to create experimental scenarios that result in and discrepancies. Furthermore, it is found that the presence of linked equilibria in all cases can result in "nonconventional" DeltaH degrees and DeltaC(p) profiles, making data analysis nontrivial.