Revised equilibrium thermodynamic parameters for thermal denaturation of β-lactoglobulin at pH 2.6

Thermodynamic parameters for thermal denaturation of β-lactoglobulin (β-lg) should account for the dissociation coupled unfolding (DCU) transitions, Dimer⇌Monomer⇌Unfolded state. Purified β-lg (0.4–4 mg ml−1 in 50 mM glycine–glycine–HCl buffer, pH 2.6) was heated and monitored by UV-difference spectrophotometry. The Monomer⇌Unfolded state transition occurred at 65–95°C with Tm equal to 82°C and a Gibbs free energy change (ΔGU0) of 51 kJ mol−1. Such results were combined with parameters for β-lg dissociation leading to the Gibbs free energy change for DCU (ΔGDCU0) of 128 (±8.3) kJ mol−1. The enthalpy and entropy change for DCU was (ΔHDCU0) equal to 373 kJ mol−1 and (ΔHDCU0) 824 J mol−1 K−1. Thus, the room temperature stability of β-lg is 76 kJ mol−1 higher than reported previously. The possible significance of such results for protein stability function relations (PSFR) is discussed.