Activation Parameters for Dynamics of Lysozyme and α -Lactalbumin Adsorption onto Silica Surface in Aqueous Medium

Abstract

The adsorption densities of lysozyme and α -Lactalbumin from aqueous solutions onto silica surface have been studied as a function of time and temperature. Kinetic measurements were carried out to determine the rate of adsorption of the proteins on the surface at various temperatures. The rates of adsorption fitted into first-order kinetic equation with two kinetic rate constants, k1 and k2, indicating that two kinetic steps are involved in the adsorption process. k1 represents the initial binding or anchorage of the protein molecules at the active sites of the solid surface and k2 represents the denaturation and reorganization of the bound proteins at the surface. The activation energies ∆E1 * and ∆E2 * for the proteins adsorption were obtained from Arrhenius equation and are given (in kJ mol -1 ) as 45.637, 41.430 and 17.942, 15.971 respectively for the adsorption of α -lactalbumin and lysozyme onto the silica surface. 1 H   and 2 H   correspond to the enthalpy changes for the two kinetic steps with values (in kJ mol -1 ) of 43.121, 38.836 and 15.427, 11.379 for adsorption of α-lactalbumin and lysozyme onto the silica surface respectively. The change in entropy of activation (S1* and S2*) for the two kinetic steps with values (in Jmol -1 K -1 ) of -114.422, -128.095 and -217.081, -229.183 were obtained for adsorption onto the silica surface following the initial order. The negative S* values thus signify that refolding as well as unfolding of the proteins at the interface is controlled mostly by the order-disorder parameter, S*, occurring in the activated state. Results further show that for k1 * step; H1*>TS1* and for k2 * step, TS2* > H2*, thus buttressing the fact that the second step is mostly entropy-controlled. The results generally show that α-lactalbumin was slightly better adsorbed onto the silica surface as compared to the lysozyme.