Carbohydrate-Functionalized Anthracene Carboximides as Multivalent Ligands and Bio-Imaging Agents

Graphical Abstract

Carbohydrate-conjugated anthracene carboximides (ACI glycosides) are synthesized which displayed excellent optical properties such as strong fluorescence, large Stokes shift in aq. solution, along with an increased aq. solubility. ACI glycosides show minimal cytotoxicity and are cell permeable as evidenced by the intracellular fluorescence. The glycosides self-assemble in aq. solutions, forming large vesicular structures and act as a multivalent glycocluster, showing specific lectin binding.

Abstract

Anthracene carboximides (ACIs) conjugated with gluco-, galacto- and mannopyranosides are synthesized, by glycosylation of N-hydroxyethylanthracene carboximide acceptor with glycosyl donors. Glycoconjugation of anthracene carboximide increases the aq. solubility by more than 3-fold. The glycoconjugates display red-shifted absorption and emission, as compared to anthracene. Large Stokes shift (λ_abs/λ_em=445/525 nm) and high fluorescence quantum yields (Φ) of 0.86 and 0.5 occur in THF and water, respectively. The ACI-glycosides undergo facile photodimerization in aqueous solutions, leading to the formation of the head-to-tail dimer, as a mixture of syn and anti-isomers. Solution phase and solid-state characterizations by dynamic light scattering (DLS), microscopic imaging by atomic force (AFM) and transmission electron (TEM) microscopies reveal self-assembled vesicle structures of ACI glycosides. These self-assembled structures act as multivalent glycoclusters for ligand-specific lectin binding, as evidenced by the binding of Man-ACI to Con A, by fluorescence and turbidity assays. The conjugates do not show cellular cytotoxicity (IC₅₀) till concentrations of 50 μM with HeLa and HepG2 cell lines and are cell-permeable, showing strong fluorescence inside the cells. These properties enable the glycoconjugates to be used in cell imaging. The non-selective cellular uptake of the glycoconjugates suggests a passive diffusion through the membrane.