Structural characterization and surface activity of spread and adsorbed soy globulin films at equilibrium

Abstract

In this paper we studied the β-conglycinin (fraction 7S) and glycinin (fraction 11S) structural characterization, hydrophobicity, and solubility, and the surface activity at equilibrium of adsorbed and spread films at the air–water interface. These properties were analyzed as a function of the aqueous phase pH and the protein concentration in the bulk phase, including the chemical reduction of glycinin with dithiothreitol (DTT). The surface activity of 7S, 11S, and 11S+10 mM DTT in aqueous solutions has been investigated over a range of protein concentrations (5–1×10 −6% wt/wt) at pH 2.0, 5.0, and 8.0. The temperature and the ionic strength were maintained constant at 20 °C and 0.05 M, respectively. The surface activity at equilibrium was quantified by the surface pressure, which was determined by the Wilhelmy plate method. The fractions 7S and 11S would be under native conditions at pH 8.0, and they would be progressively unfolded when the pH diminishes towards the acidic region. These fractions are found to be partially denatured at pH 5.0 near the isoelectric point. The conformational change at the highest acidic pH takes place together with a greater exposure of the hydrophilic residues in a more polar region. Reduction of glycinin resulted in molecular conformational changes, which enhanced molecular hydrophobicity and increase molecule flexibility. Surface pressure data at low protein concentration indicated a low surface activity that rises to a plateau as the monolayer is saturated at higher protein concentrations. The protein concentration and the surface pressure at the plateau depend on the pH, but not on the type of protein in the aqueous phase and on the chemical reduction of glycinin with DTT neither. Poor surface activity was observed for pH 5.0. However, the equilibrium spreading pressure depends on the protein and the aqueous phase pH. Some differences were observed between surface pressure at the plateau for adsorbed protein and equilibrium spreading pressure for 7S, 11S, and 11S+10 mM DTT.