Synthesis and Binding of Mannose-Specific Synthetic Carbohydrate Receptors.

Synthetic carbohydrate receptors (SCRs) that selectively recognize cell-surface glycans could be used for detection, drug delivery, or as therapeutics. Here we report the synthesis of seven new C2h symmetric tetrapodal SCRs. The structures of these SCRs possess a conserved biaryl core, and they vary in the four heterocyclic binding groups that are linked to the biaryl core via secondary amines. Supramolecular association between these SCRs and five biologically relevant C1 -O-octyloxy glycans, α/ß-glucoside (α/ß-Glc), α/ß-mannoside (α/ß-Man), and ß-galactoside (ß-Gal), was studied by mass spectrometry, 1 H NMR titrations, and molecular modeling. These studies revealed that selectivity can be achieved in these tetrapodal SCRs by varying the heterocyclic binding group. We found that SCR017 (3-pyrrole), SCR021 (3-pyridine), and SCR022 (2-phenol) bind only to ß-Glc. SCR019 (3-indole) binds only to ß-Man. SCR020 (2-pyridine) binds ß-Man and α-Man with a preference to the latter. SCR018 (2-indole) binds α-Man and ß-Gal with a preference to the former. The glycan guests bound within their SCR hosts in one of three supramolecular geometries: center-parallel, center-perpendicular, and off-center. Many host-guest combinations formed higher stoichiometry complexes, 2:1 glycan⋅SCR or 1:2 glycan⋅SCR, where the former are driven by positive allosteric cooperativity induced by glycan-glycan contacts.

Anti-Zika Activity of a Library of Synthetic Carbohydrate Receptors.

Zika virus (ZIKV), a mosquito-borne flavivirus, is a global health concern because of its association with severe neurological disorders. Currently, there are no antiviral therapies that have been specifically approved to treat ZIKV, and there is an urgent need to develop effective anti-ZIKV agents. Here, we report anti-ZIKV activity of 16 synthetic carbohydrate receptors (SCRs) that inhibit ZIKV infection in Vero and HeLa cells. Using a ZIKV reporter virus particle-based infection assay, our data demonstrates these SCRs are highly potent with IC50s as low as 0.16 µM and negligible toxicity at several-fold higher concentrations. Time-of-addition studies showed that these SCRs inhibit the early stages of the virus infection, which is consistent with the proposed mode of action, where the SCRs likely inhibit binding between the virus and cell-surface glycans, thereby preventing viral entry into the cells and, as such, this study demonstrates a potential new strategy against ZIKV.

Binding Studies on a Library of Induced-Fit Synthetic Carbohydrate Receptors with Mannoside Selectivity.

Synthetic carbohydrate receptors could serve as agents for disease detection, drug delivery, or even therapeutics, however, they are rarely used for these applications because they bind weakly and with a preference towards the all-equatorial glucosides that are not prevalent on the cell surface. Herein the binding of 8 receptors with 5 distinct octyloxy pyranosides, which was measured by mass spectrometry and by 1 H NMR titrations in CD2 Cl2 at 298 K, is reported, providing binding affinities that vary from ≈101 -104 m-1 . Although the receptors are promiscuous, 1 shows selectivity for ß-Man at a ratio of 103:1 ß-Man:ß-Gal, receptors 2-4 and 6 have preference for α-Man, 5 is selective for ß-Gal, and 10 prefers α-Glc (Man=mannose; Gal=galactose, Glc=glucose). A variety of 1D and 2D NMR, and computational techniques were used to determine the thermodynamic binding parameters (ΔHo and ΔSo ) and the structure of the host-guest complex, revealing that dimeric receptor 10 binds ß-Man with increased enthalpy, but a larger entropic penalty than 1. The first-principles modelling suggests that 10⋅ß-Man forms an inclusion-type complex where the glycan engages both monomeric subunits of 10 through H-bonding and C-H⋅⋅⋅π interactions. Like natural glycan-binding proteins, these receptors bind pyranosides by accessing multivalent and cooperative interactions, and these studies suggest a new approach towards biomimetic synthetic carbohydrate receptors, where conformational flexibility and promiscuity are incorporated into design.