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 °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 × 105 M-1) was much larger than those for the complexation of Con A with small molecular weight sugars such as α-methyl d-glucopyranoside (4.9 × 103 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 (ΔS° = 108 J/K·mol). The usability of polymeric SAM in the biomedical field was strongly suggested.