Effects of solute multivalence on the evaluation of binding constants by biosensor technology: studies with concanavalin A and interleukin-6 as partitioning proteins.

The interaction of concanavalin A with immobilized carboxylmethyldextran has been characterized by means of a biosensor based on surface plasmon resonance detection. Adsorption and desorption of this bivalent lectin to/from the biosensor surface are shown to deviate markedly from pseudo-first-order kinetics, an assumption inherent in the usual kinetic approach to the characterization of interactions by biosensor technology. Similar results for the interaction of a dimeric and hence bivalent form of human interleukin-6 with its receptor immobilized on the biosensor plate support the conclusion that this deviation from pseudo-first-order kinetics originates from multivalence of the partitioning protein. Use of the kinetic approach to characterize the binding of multivalent proteins to immobilized affinity sites on the biosensor chip is therefore precluded because of nonconformity with the model on which the quantitative analysis is based. Instead, an intrinsic binding constant of 2.5 x 10(5) M-1 for the interaction of concanavalin A with the carboxymethylated dextran layer coating the biosensor chip has been obtained by interpreting the equilibrium biosensor responses in terms of expressions developed in the context of quantitative affinity chromatography of multivalent partitioning solutes.