COMPETITIVE ADSORPTION OF ALPHA-LACTALBUMIN IN THE MOLTEN GLOBULE STATE

The molten globule state of alpha-lactalbumin is a partially denatured form with native-like secondary structure and disordered tertiary structure. Using circular dichroism measurements, it was demonstrated that the molten globule state was produced by decreasing the pH to 2.0 at 25 degrees C or by removing bound Ca2+ by treatment with ethylenediamine-tetraacetic acid (EDTA) at pH 7.5 and 40 degrees C. Tension measurements showed that alpha-lactalbumin in the molten globule state is more easily unfolded at liquid interfaces than is the native protein. Results of competitive adsorption experiments involving alpha-lactalbumin and beta-lactoglobulin at the oil droplet surface in emulsions are consistent with preferential adsorption of alpha-lactalbumin during emulsification when it is in the molten globule state. In contrast to the difficulty of exchange between alpha-lactalbumin and beta-lactoglobulin at the oil-water interface in emulsions at 25 degrees C, it has been found that the two whey proteins are able partially to displace one another from the oil-water interface at 40 degrees C. While native alpha-lactalbumin was found to be readily displaced from the oil-water interface by beta-lactoglobulin at 40 degrees C, it was found that alpha-lactalbumin in the molten globule state in the presence of EDTA at 40 degrees C had itself the capacity for displacing beta-lactoglobulin from the interface.