Thermal Denaturation of Myoglobin in Water−Disaccharide Matrixes: Relation with the Glass Transition of the System

Proteins embedded in glassy saccharide systems are protected against adverse environmental conditions [Crowe et al. Annu. ReV. Physiol. 1998, 60, 73-103]. To further characterize this process, we studied the relationship between the glass transition temperature of the protein-containing saccharide system (Tg) and the temperature of thermal denaturation of the embedded protein (Tden). To this end, we studied by differential scanning calorimetry the thermal denaturation of ferric myoglobin in water/disaccharide mixtures containing nonreducing (trehalose, sucrose) or reducing (maltose, lactose) disaccharides. All the samples studied are, at room temperature, liquid systems whose viscosity varies from very low to very large values, depending on the water content. At a high water/saccharide mole ratio, homogeneous glass formation does not occur; regions of glass form, whose Tg does not vary by varying the saccharide content, and the disaccharide barely affects the myoglobin denaturation temperature. At a suitably low water/saccharide mole ratio, by lowering the temperature, the systems undergo transition to the glassy state whose Tg is determined by the water content; the Gordon-Taylor relationship between Tg and the water/disaccharide mole ratio is obeyed; and Tden increases by decreasing the hydration regardless of the disaccharide, such effect being entropy-driven. The presence of the protein was found to lower the Tg. Furthermore, for nonreducing disaccharides, plots of Tden vs Tg give linear correlations, whereas for reducing disaccharides, data exhibit an erratic behavior below a critical water/ disaccharide ratio. We ascribe this behavior to the likelihood that in the latter samples, proteins have undergone Maillard reaction before thermal denaturation.