PH-DEPENDENCE OF THE KINETICS OF INTERFACIAL-TENSION CHANGES DURING PROTEIN ADSORPTION FROM SESSILE DROPLETS ON FEP-TEFLON

Interfacial tension changes during protein adsorption at both the soli d-liquid and the liquid-vapor interface were measured simultaneously b y ADSA-P from sessile droplets of protein solutions on fluoroethylenep ropylene-Teflon. Four globular proteins of similar size, viz. lysozyme , ribonuclease, alpha-lactalbumin and Ca2+-free alpha-lactalbumin, and one larger protein, serum albumin, were adsorbed from phosphate solut ions at varying pH values (pH 3-12). The kinetics of the interfacial t ension changes were described using a model accounting for diffusion-c ontrolled adsorption of protein molecules and conformational changes o f already adsorbed molecules. The contribution of conformational chang es to the equilibrium interfacial-pressure was shown to be relatively small and constant with respect to pH when compared to the contributio n of adsorption of the protein molecules. The model also yields the di ffusion relaxation time and the rate constant for the conformational c hanges at the interface. Around the isoelectric point of a protein the calculated diffusion relaxation time was minimal, which is ascribed t o the absence of an energy barrier to adsorption. Energy barriers to a dsorption become larger at pH values away from the isoelectric point a nd can therefore become rate-limiting for the adsorption process. The rate constants for conformational changes at the liquid-vapor interfac e were maximal around the isoelectric point of a protein, suggesting a smaller structural stability of the adsorbed protein. At the solid-li quid interface the rate constants were smaller and independent of pH, indicating that conformational changes more readily occur at the liqui d-vapor than at the solid-liquid interface.