Isothermal equilibrium adsorption of concanavalin A on dextran-modified poly(methyl methacrylate) latex particles

Isothermal equilibrium adsorption experiments were carried out to study the adsorption of concanavalin A (Con A) on dextran-modified poly(methyl methacrylate) (PMMA) latex particles. Three PMMA particles with various levels of dextran modification were selected for study: 0% (designated as DO), 1.24% (D20), and 2.45% (D75) based on total polymer weight. The Langmuir model is applicable to both DO and D20 systems, although the data for the D20 system are somewhat scattered. On the other hand, the amount of Con A adsorbed per gram polymer particles (q*) versus the Con A concentration in water (c*) curve for the D75 system cannot be described by the Langmuir model. The deviation is attributed to the formation of a crosslinked network structure, caused by specific binding of the dimeric Con A molecules onto two neighboring particles with grafted dextran. The ratio of the initial number of Con A molecules to the initial number of active binding sites on the dextran-modified particle surface plays an important role in determining the structure of flocs formed. The maximum amount of Con A adsorbed on the particle surface (q(max)) is of the order of 10(-1) mu mol per gram particles and q(max) in decreasing order is D75 > D20 > DO. The dissociation constant of the Con A-D2O (or Con A-D75) pair is of the order of 10(-1) mu mol dm(-3) which is 1 order of magnitude smaller than that of the Con A-DO pair. Thus, the electrostatic interaction between Con A and DO is much weaker than the affinity interaction between Con A and D20 (or D75). An empirical model is proposed to qualitatively explain the q* versus c* data.