β-Lactoglobulin adsorption layers at the water/air surface: 1. Adsorption kinetics and surface pressure isotherm: Effect of pH and ionic strength

Abstract

The dynamic surface pressure Π of air bubbles aging in buffered β-lactoglobulin (BLG) solutions containing various protein concentrations CBLG (10−9–10−4M), pH (3–7) and buffer concentrations Cbuff (1, 10 and 100mM) was measured as a function of time t by bubble profile analysis tensiometry. Adsorption kinetics was studied by recording Π(t) data for 80000s and the final Π-values were used to construct the surface pressure isotherm Π(CBLG) for the pH values of 3, 5, 7 (Cbuff=10mM) and 6.3 (in pure water). On the basis of obtained kinetic and concentration dependencies the effect of pH on the protein surface activity was qualitatively analyzed. At constant Cbuff=10mM and relatively low protein concentrations (CBLG<10−8M), BLG exhibits the shortest induction time τind and highest Π-values at pH 7 (negative net charge), in comparison to pH 5 and 3, whereas at sufficiently high protein concentrations (ca. CBLG>10−6M) BLG is the most surface active at pH 5 (negligible net charge in the vicinity of the isoelectric point, pI ≈5.1). At constant Cbuff, BLG solutions with pH 3 show the lowest surface activity. The influence of the ionic strength (buffer concentration) of the solution on the dynamic Π was studied at a selected protein concentration of CBLG=10−5M. The kinetics of adsorption is weakly affected by Cbuff for solutions with pH 5 and significantly enhanced by increasing Cbuff for solutions with pH≠pI. The effect of the chemical nature of the electrolyte was obtained for non-buffered aqueous BLG solutions (natural pH ∼6.3) at different concentrations of NaCl or CaCl2. It is demonstrated that the adsorption kinetics and the surface pressure isotherm of BLG at the water/air surface depend strongly on the protein effective charge, which is dictated by the solution pH and ionic strength.