DSC study of cold and heat denaturation processes of β-lactoglobulin A with guanidine hydrochloride

The cold and heat denaturations of bovine β-lactoglobuhn A (β-lg A) has been studied in solutions of guanidine hydrochloride (GuHCl) by differential scanning calorimelry (DSC) The experimental results are presented and discussed.It is shown that the number of protons bound by the monomeric molecules of β-lg A was unchanged before and after its heat denaturation below pH 3,and that the activation energy of the heat denaturation was depressed owing to the presence of GuHCl.In the solutions with 2.50 and 3.06 mol/L of GuHCl,both the cold and heat denat-urations of β-lg A were observed.In comparison with the heat denaturation,the activation energy of cold denaturation was far lower and the number of GuHCl molecules bound by the unfolded polypeptide chains after cold denaturation increased a lot.The absolute value of the enthalpy of cold denaturation was larger than that of heat denaturation It was found by the analysis that the contribution to the total denaturational enthalpy of conformational change i