Thermal unfolding of monomeric and dimeric beta-lactoglobulins

The thermal stabilities of dimeric bovine beta -lactoglobulin and monomeric equine beta -lactoglobulin were investigated at neutral pH by means of dif ferential scanning calorimetry, circular dichroism, tryptophan fluorescence , and by binding of an hydrophobic probe. Differential scanning calorimetry showed the presence of two structural domains with different thermal stabi lities in both proteins. Thermodynamic analysis of the calorimetric signal revealed that the two domains unfold independently according to a mechanism where an equilibrium step is followed by an irreversible transition. The s pectroscopic data supported this model and allowed recognition of the struc tural regions corresponding to the more thermally stable domain. The differ ences in thermal stability between the two proteins can be primarily ascrib ed to the properties of the less stable domain.