Enthalpy and entropy of hippuraldehyde hydration and binding to papain

In aqueous solution hippuraldehyde, PhCONHCH2CHO, is approx. 10-times more extensively hydrated than simple aliphatic aldehydes. Studies of the temperature dependence of the NMR spectrum of hippuraldehyde and its hydrate in 2H2O indicate that the dehydration is endothermic (delta H = 3.9 +/- 0.9 kcal/mol), but entropically favored (delta S = 7.9 +/- 1.9 cal/mol per deg). For simple aliphatic aldehydes the corresponding dehydration is slightly more endothermic, but somewhat more favorable entropically. Fluorescence titration studies show that Kd for the hemithioacetal adduct of hippuraldehyde with papain-SH (1.2 microM at 298 K) is approx. 3000-times smaller than that estimated for simple thiols. Dissociation of hippuraldehyde from papain is endothermic by 18.1 kcal/mol, and this is only partly offset by the favorable entropy change of 33.7 cal/mol per deg. Thus, at least in thermodynamic terms, the binding of this putative 'transition-state analog' to papain is not unlike the binding of other small ligands to the active site of chymotrypsin.