DSC STUDY OF DENATURATION OF BETA-LACTOGLOBULIN-B

The denaturation of bovine beta-lactoglobulin B (beta-Lg B) has been studied in phosphate solutions with various concentrations of GuHCl with differential scanning calorimetry. The experiments demonstrated that the presence of GuHCl made the beta-Lg B undergo both cold denaturation and heat denaturation under the condition of a high concentration of the protein. The enthalpy changes of both kinds of denaturation exhibit opposite signs. Both the cold denaturation and the renaturation of the protein are reproducible, but its heat denaturation is irreversible. The cooperation among monomer molecules of the protein is involved in its heat denaturation. The heat denaturation of the protein can be represented by the thermodynamic model N-c reversible arrow D-->F. The activation energy of heat denaturation is 285 kJ/mol, which implies that the depression of temperature and enthalpy of heat denaturation of the beta-Lg B does not result from decreasing considerably the activation energy by GuHCl. As for the cold denaturation of the protein, especially in the solvent with 3.10 mol/L of GuHCl,;it can be described by the two-state model N reversible arrow D.