ADSORPTION OF GLOBULAR-PROTEINS ON LOCALLY PLANAR SURFACES - MODELS FOR THE EFFECT OF EXCLUDED SURFACE-AREA AND AGGREGATION OF ADSORBED PROTEIN ON ADSORPTION EQUILIBRIA

Equilibrium statistical-thermodynamic models are presented for the sur face adsorption of proteins modeled as regular convex hard particles. The adsorbed phase is treated as a two-dimensional fluid, and the chem ical potential of adsorbed protein is obtained from scaled particle th eory. Adsorption isotherms are calculated for nonassociating and self- associating adsorbing proteins. Area exclusion broadens adsorption iso therms relative to the Langmuir isotherm (negative cooperativity), whe reas self-association steepens them (positive cooperativity). The calc ulated isotherm for adsorption of hard spheres using scaled particle t heory for hard discs agrees well with that calculated from the hard di sc virial expansion. As the cross section of the adsorbing protein in the plane of the surface becomes less discoidal, the apparent negative cooperativity manifested in the isotherm becomes more pronounced. The model is extended to the case of simultaneous adsorption of a tracer protein at low saturation and a competitor protein with a different si ze and/or shape at arbitrary fractional saturation. Area exclusion by competitor for tracer (and vice versa) is shown to substantially enhan ce the displacement of tracer by competitor and to qualitatively inval idate the standard interpretation of ligand competition experiments, a ccording to which the fractional displacement of tracer by competitor is equal to the fractional saturation by competitor.