The Role of Electrostatic Forces in Anomalous Adsorption Behavior of Phosvitin at the Air/Water Interface

Abstract

The adsorption of phosvitin at the air–water interface has been studied to elucidate the influence of electrostatic forces on protein adsorption at liquid interfaces. Because of its polyanionic character, adsorption of phosvitin at the air–water interface takes place only at low pH, but not near neutral pH. Phosvitin adsorbed at an initial pH of 2.0 is completely desorbed from the interface when the pH is increased to neutral pH. The saturated monolayer coverage for phosvitin is about 1.25 mg/m2at pH 2.0. However, in spite this significant amount of adsorption, no decrease in surface tension occurs. Instead, a consistent increase in surface tension of the solution occurs, which apparently violates the Gibbs adsorption equation. A model based on the configuration of phosvitin at the interface has been proposed to explain the thermodynamic reasons for this apparent violation of the Gibbs equation. It is shown that phosvitin is anchored to the interface only via a short C-terminus hydrophobic segment and the rest of the highly hydrophilic molecule is suspended in the subsurface. These suspended loops exert an electrostatic pull on surface water molecules, causing an increase in surface tension. However, the reduction in free energy resulting from removal of the hydrophobic segment from water to the interface is much greater than the increase in surface tension caused by charge–dipole interactions, so that there is actually a net reduction in free energy of thesystem.Thus, although adsorption of phosvitin apparently violates the Gibbs adsorption equation, it does not violate the basic thermodynamic principle. The results also show that proteins can adsorb to an interface against seemingly excessive electrostatic repulsive forces through attachment of only a small hydrophobic peptide segment.