INFLUENCE OF ELECTROSTATIC INTERACTIONS ON BETA-CASEIN LAYERS ADSORBED ON POLYSTYRENE LATICES

Electrophoretic mobility, dynamic light scattering and solution-deplet ion measurements have been combined to study the influence of electros tatic interactions on the thickness and structure of adsorbed layers o f beta-casein at the solid/liquid interface (polystyrene latex particl es). 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 o bservations on hydrodynamic thickness of the bound protein layer are i nterpreted in terms of a previously proposed model where most of the p rotein molecule lies close to the surface, leaving the highly charged N-terminal portion forming a loop extending into solution. The extensi on of this loop into solution is dominated by electrostatic interactio ns. Moderating these by diminishing protein charge via dephosphorylati on or calcium-binding or by reducing the range of repulsion by increas ing ionic strength, causes the loop to relax and decreases the effecti ve thickness of the protein layer.