Controlled Release of Proteins Bound to Spherical Polyelectrolyte Brushes

We discuss the interaction of proteins dissolved in aqueous solution with spherical polyelectrolyte brushes (SPB). The SPB consist of a solid core particle of colloidal dimensions (ca. 100 nm in diameter) onto which long polyelectrolyte chains have been grafted. Immersed in aqueous solution of proteins these SPB will take up high amounts of protein if the ionic strength is low. At high ionic strength, however, virtually no protein will enter into the brush layer attached to the surface of the core particles. We show that bovine serum albumin (BSA) bound at low ionic strength will gradually be released upon raising the salt concentration in the solution in a well-controlled manner: For each raise of the ionic strength in solution there is a well-defined amount of protein that is released. We show that BSA adsorbed to a conventional carboxylated latex will not be released if treated in the same manner. All findings, namely the uptake of protein as well as the controlled release can be explained by the " counterion release force " : Patches of positive charge on the surface of the proteins which are immersed in the brush layer become multivalent counterions of the polyelectrolyte chains thus releasing a concomitant number of counter-and coions. Release of counterions as induced by the adsorption of proteins is hence the main driving force for the polyelectrolyte-mediated protein adsorption (PMPA).