Epimeric monosaccharide-quinone hybrids on gold electrodes toward the electrochemical probing of specific carbohydrate-protein recognitions.

Carbohydrates represent one of the most significant natural building blocks, which govern numerous critical biological and pathological processes through specific carbohydrate-receptor interactions on the cell surface. We present here a new class of electrochemical probes based on gold surface-coated epimeric monosaccharide-quinone hybrids toward the ingenious detection of specific epimeric carbohydrate-protein interactions. Glucose and galactose, which represent a pair of natural monosaccharide C4 epimers, were used to closely and solidly conjugate with the 1,4-dimethoxybenzene moiety via a single C-C glycosidic bond, followed by the introduction of a sulfhydryl anchor. The functionalized aryl C-glycosides were sequentially coated on the gold electrode via the self-assembled monolayer (SAM) technique. X-ray photoelectron spectroscopy (XPS) was used to confirm the SAM formation, by which different binding energies (BE) between the glucosyl and the galactosyl SAMs on the surface, probably rendered by their epimeric identity, were observed. The subsequent electrochemical deprotection process readily furnished the surface-confined quinone/hydroquinone redox couple, leading to the formation of electrochemically active epimeric monosaccharide-quinone SAMs on the gold electrode. Cyclic voltammetry (CV) and differential pulse voltammetry (DPV) used for the detection of specific sugar-lectin interactions indicated that the addition of specific lectin to the corresponding monosaccharide-quinone surface, i.e., concanavalin A (Con A) to the glucosyl SAM and peanut agglutinin (PNA) to the galactosyl SAM, resulted in an obvious decrease in peak current, whereas the addition of nonspecific lectins to the same SAMs gave very minor current variations. Such data suggested our uniquely constructed gold surface coated by sugar-quinone hybrids to be applicable as electrochemical probes for the detection of specific sugar-protein interactions, presumably leading to a new electrochemistry platform toward the study of carbohydrate-mediated intercellular recognitions.