Displacement of Adsorbed Insulin by Tween 80 Monitored Using Total Internal Reflection Fluorescence and Ellipsometry

Abstract

This study was conducted to investigate the mechanism of action in the displacement of adsorbed insulin from a hydrophobic surface by Tween 80 and of the competitive adsorption of the two species. Total internal reflection fluorescence (TIRF) and ellipsometry were used as in situ methods to examine the processes taking place at hydrophobic model surfaces in the presence of insulin and Tween 80. TIRF studies showed that the displacement of insulin by Tween 80 could be fitted to a sigmoidal function, indicating a nucleation-dependent process. Furthermore, a linear dependence between the apparent rate constant and the logarithm of the Tween 80 concentration was found. Competitive adsorption from solution mixtures of insulin and Tween 80 indicated that insulin was adsorbed first, but subsequently displaced by the surfactant. This displacement proved also to be dependent on the concentration of Tween 80 in the mixture. The results indicate that Tween 80 at concentrations above critical micelle concentration can be used to protect insulin against surface adsorption. The presence of a lag phase in the displacement at low surfactant concentration indicates that the mechanism of action for Tween 80 to reduce adsorption of insulin may be by competing for sites at the surface.