Effect of salt concentrations on the displacement adsorption enthalpies of denatured protein folding at a moderately hydrophobic surface

The displacement adsorption enthalpies (ΔH) of the refolding of lysozyme (Lys) denatured by 1.8 mol L–1 guanidine hydrochloride (GuHCl) on a moderately hydrophobic surface at 298 K, pH 7.0 and various (NH4)2SO4 concentrations were determined by using a Micro DSC-III calorimeter. The study shows that the effect of salt concentrations on the three fractions of the enthalpy is that with increasing (NH4)2SO4 concentrations, the molecular conformation enthalpy of the adsorbed Lys has probably no distinct change at 1.8 mol L–1 GuHCl; the adsorption affinity enthalpy (exothermic) becomes more negative; and the dehydration enthalpy (endothermic) decreases. At lower salt concentrations, the dehydration, especially squeezing water molecules led by molecular conformation, which leads to an entropy-driving process, predominates over the adsorption affinity (also including the orderly orientation of molecular conformation), while at higher salt concentrations, the latter is prior to the former for contribution to ΔH and induces an enthalpy-driving process. Also, the optimal NH4)2SO4 concentration favoring refolding and renaturing of Lys denatured by 1.8 mol L–1 GuHCl was found.