Binding of aroma compounds with legumin. II. Effect of hexyl acetate on thermodynamic properties of 11S globulin in aqueous medium

This paper presents a study of the effect of aroma compound (hexyl acetate, HxAc) on thermodynamic properties of legumin (11S globulin from broad beans) in an aqueous medium (protein conformational stability and protein–protein interactions) by a combination of gas–liquid chromatography, differential scanning microcalorimetry, static light scattering and sedimentation velocity analysis. The aqueous phase at the ionic strength of 0.05 mol dm−3 was buffered at pH 7.2 and 3.0. The conformational stability of the native 11S globulin (pH 7.2) was maximum at the point of saturation of the protein molecules with the aroma ligand, which was determined from the binding isotherm. At pH 3.0, the protein conformational stability did not change over the same concentration range of HxAc, i.e. where the protein binding capacity for HxAc was studied. As a result of the binding of HxAc with 11S globulin the thermodynamic affinity between protein molecules in an aqueous medium increased in the case of the native protein, remained unchanged for the acid-denatured protein and reduced in the case of the heat-denatured protein at the same concentration of HxAc in the system. An excess of concentration of HxAc in the solutions above the saturation level for the protein, according to the binding isotherm, led to protein aggregation in an aqueous medium owing to spontaneous protein unfolding.