Influence of electrostatic interactions on β-casein layers adsorbed on polystyrene latices

Abstract

Electrophoretic mobility, dynamic light scattering and solution-depletion measurements have been combined to study the influence of electrostatic interactions on the thickness and structure of adsorbed layers of β-casein at the solid/liquid interface (polystyrene latex particles). Charge effects are investigated by removing phosphate groups from the protein, by varying the ionic strength and by including divalent calcium ions which are known to bind to the protein in solution. The observations on hydrodynamic thickness of the bound protein layer are interpreted in terms of a previously proposed model where most of the protein molecule lies close to the surface, leaving the highly charged N-terminal portion forming a loop extending into solution. The extension of this loop into solution is dominated by electrostatic interactions. Moderating these by diminishing protein charge via dephosphorylation or calcium-binding or by reducing the range of repulsion by increasing ionic strength, causes the loop to relax and decreases the effective thickness of the protein layer.