Physicochemical basis for cosolvent modulation of β-lactoglobulin functionality: Interfacial tension study

The impact of neutral cosolvents on the thermal stability of globular proteins in aqueous solutions depends on the nature of the cosolvent, e.g., sorbitol causes a pronounced increase in the thermal denaturation temperature (T_m) of β-lactoglobulin (β-lg), whereas glycerol does not. When a protein unfolds there is a change in the exposed surface area and in the type of molecular interactions that occur at the protein–solvent–cosolvent interface. These changes contribute to the free energy change associated with protein denaturation and depend on cosolvent type. In this study we measured the equilibrium interfacial tensions of aqueous glycerol (0–70% w/w) and sorbitol (0–55% w/w) solutions as a function of temperature to provide insights into the role of the interfacial energy on the thermal stability of β-lg. There was a slight increase in interfacial tension with increasing sorbitol concentration, indicating its preferential exclusion from the oil–water interface. On the other hand, there was an appreciable decrease in interfacial tension with increasing glycerol concentration, indicating its preferential accumulation at the oil–water interface. These changes were largely independent of the measurement temperature (30–80 °C). Our results suggest that sorbitol increases the T_m of β-lg mainly through a steric exclusion effect, whereas glycerol has little effect on T_m because the steric exclusion effect is counter-balanced by a differential interaction effect.