Thermodynamics of denaturation of alpha-chymotrypsinogen A in aqueous urea and alkylurea solutions

The effects of pH, urea, and alkylureas on the thermal stability of alpha-chymotrypsinogen A (alpha-ctg A) have been investigated by differential scanning calorimetry (DSC) and UV spectroscopy. Heat capacity changes and enthalpies of transition of alpha-ctg A in the presence of urea and alkylureas were measured at the transition temperature. Using these data, the corresponding Gibbs free energies, enthalpies, and entropies of denaturation at 25 degrees C were calculated. Comparison of these values shows that at 25 degrees C denaturation with urea is characterized by a significantly smaller enthalpy and entropy of denaturation. At all denaturant concentrations the enthalpy term slightly dominates the entropy term in the Gibbs free energy function. The most obvious effect of alkylureas was lowering of the temperature of transition, which was increasing with alkylurea concentration and the size of alkyl chain. Destabilization of the folded protein in the presence of alkylureas appears to be primarily the result of the weakening of hydrophobic interactions due to diminished solvent ordering around the protein molecules. At pH lower than 2.0, alpha-ctg A still exists in a very stable form, probably the acid-denatured form (A-form).