Synthesis of blockwise alkylated tetrasaccharide–organic quantum dot complexes and their utilization for live cell labeling with low cytotoxicity

Bioimaging is a key to understanding immune responses, cell differentiation, and development. Quantum dots (QDs) conjugated with monoclonal antibodies and other biomolecules are currently utilized for flow cytometry and immunohistochemistry, but monoclonal antibody–QD complexes are of limited use when cell surface markers are not available. In this study, we synthesized novel amphiphilic blockwise alkylated tetrasaccharides and developed a simple method for labeling a wide variety of live cells with organic QDs encapsulated with these carbohydrates. The novel amphiphilic blockwise alkylated tetrasaccharides were as follows: methyl β-d-glucopyranosyl-(1 → 4)-β-d-glucopyranosyl-(1 → 4)-2,3,6-tri-O-methyl-β-d-glucopyranosyl-(1 → 4)-2,3,6-tri-O-methyl-d-glucopyranoside (1), methyl β-d-galactopyranosyl-(1 → 4)-β-d-glucopyranosyl-(1 → 4)-2,3,6-tri-O-methyl-β-d-glucopyranosyl-(1 → 4)-2,3,6-tri-O-methyl-d-glucopyranoside (2), ethyl β-d-glucopyranosyl-(1 → 4)-β-d-glucopyranosyl-(1 → 4)-2,3,6-tri-O-ethyl-β-d-glucopyranosyl-(1 → 4)-2,3,6-tri-O-ethyl-d-glucopyranoside, (3), and ethyl β-d-galactopyranosyl-(1 → 4)-β-d-glucopyranosyl-(1 → 4)-2,3,6-tri-O-ethyl-β-d-glucopyranosyl-(1 → 4)-2,3,6-tri-O-ethyl-d-glucopyranoside (4). The newly synthesized blockwise alkylated tetrasaccharides spontaneously assembled into micelle-like particles, in which the hydrophobic moiety of the blockwise alkylated tetrasaccharides played an important role. They were less toxic to human cells than octyl β-d-glucopyranoside, a commonly used amphiphilic glucoside. Flow cytometry and confocal laser scanning microscopy revealed that the blockwise alkylated tetrasaccharide–organic QD complexes were stably attached to live cells. The affinity of compounds 1 and 2 to the live cell surface was slightly higher than that of compounds 3 and 4. Because the preparation of these carbohydrate–QD complexes is simple and does not require sophisticated equipment, and because the complexes can be autonomously attached to a wide spectrum of cell lines, they can be used as cell labeling reagents in biomedical studies.