Self-assembled monolayer of sugar-carrying polymer chain: Sugar balls from 2-methacryloyloxyethyl D-glucopyranoside

A sugar-carrying polymer chain with a disulfide group (DTPA-PMEGlc) was prepared by the coupling of poly(2-methacryloyloxyethyl D-glucopyranoside) (PMEGlc), which carried an amino group at its end, with 3,3'-dithiodipropionic acid di-p-nitrophenyl ester. The polymer obtained was incubated with colloidal silver, and a self-assembled monolayer (SAM) of the polymer chain (PMEGlc) was formed on the surface of the colloid as evidenced by surface-enhanced Raman spectroscopy, cyclic voltammetry, dynamic light scattering, and ellipsometry. The silver colloid was largely stabilized by the modification with PMEGlc chains due to the formation of a thick diffuse layer on the surface. The critical flocculation concentration (CFC) of the modified colloid was not detectable, which is a contrast to the presence of CFC for the starting silver colloid dispersion (0.1 M NaCl at 25 degrees C). The polymer-coated colloids obtained ("sugar balls") were aggregated when a solution of lectin (concanavalin A (Con A) from Canavalia ensiformis) was added to the dispersion, due to a specific binding of D-glucopyranoside residues on the colloid particles by a tetrameric lectin molecule. The association constant (K-asn) for glucose residues on the colloid with Con A (7.1 x 10(5) M-1) was much larger than those for the complexation of Con a with small molecular weight sugars such as alpha-methyl D-glucopyranoside (4.9 x 10(3) M-1) due to the so-called "cluster effect". Thermodynamic parameters for the binding of Con A to sugar residues in the SAM of PMEGlc clearly showed that the binding is governed by entropy change (Delta S degrees = 108 J/K.mol). The usability of polymeric SAM in the biomedical field was strongly suggested.