A comparative study on the electrokinetic behavior of bovine serum albumin molecules adsorbed onto different polymer colloids

Abstract

The adsorption of protein onto polymer surfaces is mainly due to the dehydration of hydrophobic patches on the polymers. Hence the degree of hydrophobicity of the polymer surfaces plays an important role in the adsorption of bovine serum albumin (BSA) molecules onto polystyrene beads. The main purpose of the present work is to study the electrokinetic behavior of BSA molecules adsorbed onto polystyrene beads of different superficial hydrophobicities. Four different samples of polystyrene (PS) beads were used in this study. Two samples showed a marked hydrophobic character ([PS(−)] and [PS (+)]), and were prepared by the conventional emulsion polymerization of styrene, using potassium persulfate and ADMBA as initiators. Both latex samples, although charged differently (−5.17 μC cm −2 and +11.0 μC cm −2 respectively), showed a lower occupancy area per adsorbed surfactant molecule at saturation ( A m), indicating a strong hydrophobic character. However, the other latex samples prepared by copolymerization of styrene and 2-hydroxyethyl methacrylate (HEMA) ([PS—H]) and acrylic acid ([PS—A]) showed a high A m, indicating a pronounced hydrophilic character. Adsorption isotherms at low ionic strength and different pH were obtained. In all cases the adsorption isotherms showed well-defined plateaus. The maximum adsorbed amount on the hydrophobic latices occurred at around the isoelectric point of the dissolved BSA (i.e. pH 5), whereas it occurred at around pH 4 on the hydrophilic latices. The dependence of electrophoretic mobility on the adsorbed amount seems to indicate that ions of small molecular weight are adsorbed on the protein—polymer interface. This ionic adsorption effect depends mainly on the surface charge density of the latex particles.