UREA EFFECTS ON PROTEIN STABILITY - HYDROGEN-BONDING AND THE HYDROPHOBIC EFFECT

The effects of urea on protein stability have been studied using a mod el system in which we have determined the energetics of dissolution of a homologous series of cyclic dipeptides into aqueous urea solutions of varying concentration at 25 degrees C using calorimetry. The data s upport a model in which urea denatures proteins by decreasing the hydr ophobic effect and by directly binding to the amide units via hydrogen bonds. The data indicate also that the enthalpy of amide hydrogen bon d formation in water is considerably higher than previously estimated. Previous estimates included the contribution of hydrophobic transfer of the ct-carbon resulting in an overestimate of the binding between u rea and the amide unit of the backbone and an underestimate of the bin ding enthalpy. (C) 1998 Wiley-Liss,Inc.