Nonstatistical binding of a protein to clustered carbohydrates

Carbohydrate-derivatized self-assembled monolayers (SAMs) are used as a mod el system to address issues involving cell-surface carbohydrate-protein int eractions. Here we examine the influence of carbohydrate surface density on protein-binding avidity. We show that the binding selectivity of Bauhinia purpurea lectin switches from one carbohydrate ligand to another as the sur face density of the carbohydrate ligands increases from values of chi(sugar ) approximate to 0.1-1.0. Polyvalent binding is possible at all surface den sities investigated; hence, the switch in selectivity is not due simply to the achievement of a critical density that permits polyvalent contacts. Ins tead, secondary interactions at high surface densities promote a switch in carbohydrate-binding selectivity. These findings may have implications for how changes in the composition and the density of cell-surface carbohydrate s influence biological recognition processes and regulatory pathways.