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 solution droplets on FEP-Teflon. Two large prote ins (albumin and immunoglobulin G), and four smaller proteins of simil ar size (lysozyme, ribonuclease, alpha-lactalbumin, and calcium deplet ed alpha-lactalbumin) were used at varying concentrations. The kinetic s of the interfacial tension changes were described using a model acco unting for diffusion-controlled adsorption of protein molecules and fo r conformational changes of already adsorbed molecules, Apart from the interfacial tension changes due to these two subprocesses, the model yields the diffusion relaxation time and the rate constant of the conf ormational changes. At low concentrations, adsorption of proteins did not always affect the interfacial tension, but its contribution to the decrease in interfacial tension increased with higher bulk concentrat ions. The decrease due to conformational changes remained a constant v alue for all proteins. The diffusion relaxation time could not be rela ted to the diffusion coefficient of the protein, probably because of n eglect of a reaction component in the model applied. Rate constants fo r conformational changes were generally lower at the solid-liquid inte rface, indicating that proteins are more apt to conformational changes at the liquid-vapor interface than at the solid-liquid interface. The least rigid protein, alpha LA(-Ca2+), had the largest rate constant f or the conformational change at the two interfaces. (C) 1996 Academic Press, Inc.