Effect of Electrostatic Interaction on the Adsorption of Globular Proteins on Octacalcium Phosphate Crystal Film

Abstract

The electrostatic effect on the adsorption of globular proteins, such as bovine serum albumin (BSA), hen egg white lysozyme (LZM), and β-lactoglobulin (β-Lg), on octacalcium phosphate (OCP)-like crystal thin films was investigated. A poorly crystalline thin film was synthesized on a tissue culture polystyrene (TCP) surface and used as a model surface in this study. The solution pH clearly affected the electrostatic properties of both proteins and surface. The adsorbed amounts obtained at quasi-steady state were readily related to the solution pH for each protein. The adsorption rate is fast during the initial period and levels off gradually. The maximum adsorbed mass occurred at pH 7 for BSA and at pH 9 for LZM. β-Lg adsorbed similar amounts at pHs lower than 9, but the adsorbed mass decreased at pHs higher than 9 where electrostatic repulsion exists. The pH values where the maximum adsorbed mass occurred may be considered as the conditions where electrostatic attraction is most favorable. The adsorbed mass of β-Lg was the greatest among the proteins of interest while BSA adsorbed the least despite its greater molecular mass. LZM falls into the intermediate region. According to these observations, BSA has undergone conformational changes that prevent further adsorption to a greater extent than the others. A simple relationship between the adsorption rate and the electrostatic properties was not established. However, the order of magnitude of the adsorption rate at the initial period tends to be the same as that of maximum adsorbed mass for each protein.