Resonsive anion transport with a Hamilton receptor-based anionophore controlled by photo-activation and host-guest competitive inhibition

Abstract

Ion transport across biological membranes, facilitated by naturally occurring ion channels and pumps, is crucial for many biological functions. Many of these transport systems are gated, such that ion transport is regulated by a range of external stimuli, including light, small molecule ligand binding, and membrane potential. Synthetic ion transport systems, including those with similar gating mechanisms, have garnered significant attention due to their potential applications in targeted therapeutics as anticancer agents or to treat channelopathies. In this work, we report stimuli-responsive anion transporters based on dynamic hydrogen bonding interactions of hydroxyl-functionalised Hamilton-receptor-based anionophores. Caging of the hydroxyl groups with a light-responsive ortho-nitrobenzyl (ONB) moiety locks the amide protons through intramolecular hydrogen bonding, making them unavailable for anion binding and transport. Decaging with light reverses the hydrogen bonding pattern, rendering the amide protons available for anion binding and transport. Addition of a barbiturate ligand switches OFF the ion transport activity by blocking the anion binding cavity through competitive inhibition. OFF-ON-OFF reversible control over anion transport is therefore achieved using a combination of light and competitive small molecular ligand binding.

Fig. 1. (A) Schematic representation of stimuli-responsive anion transport. (B) Chemical structure of transporters 1–3, with clogP values calculated using MarvinSketch. (C) Caged pro-transporter 1a.

Fig. 2. (A) Synthesis of transporters 1–3. (B) Chemical structure of transporter 1 complexed to chloride. (C) ¹H NMR titration spectra for 1 (2.5 mM in THF-d₈) with stepwise addition of TBACl in THF-d₈. Protons assigned in panel B, and the equivalents of added TBACl are shown on the stacked spectra. (D) The plot of chemical shift (δ) of proton H₁vs. concentration of TBACl added for transporter 1, fitted to a 1 : 1 binding model using BindFit.

Fig. 3. (A) Ion transport activity comparison of 1–3 (1.34 mol%) across POPC-LUVs⊃HPTS. (B) Dependence of activity of 1 (0.193 mol%) on external anion NaX (where X^– = Cl⁻, Br⁻, I⁻, NO₃⁻, and ClO₄⁻). (C). Ion transport activity of 1 (0.33 mol%) across DPPC-based vesicles at 25 °C and 45 °C temperatures, respectively. (D) Ion transport activity of 1 across POPC-LUVs⊃lucigenin. (E) Schematic representation of chloride efflux with either extravesicular SO₄²⁻ and NO₃⁻ ions in the lucigenin assay. (F) Ion transport activity of 1 (3.0 mol%) in the presence of external SO₄²⁻ and NO₃⁻ ions.

Fig. 4. (A) Synthesis of pro-transporter 1a. (B) ¹H NMR titration spectra for 1a (2.5 mM in THF-d₈) with stepwise addition of TBACl in THF-d₈. The equivalents of added TBACl are shown on the stacked spectra. (C) Plot of chemical shift (δ) of H₁ proton vs. concentration of TBACl added for transporter 1, and fit to a 1 : 1 binding model.

Fig. 5. (A) Ion transport activities across POPC-LUVs⊃lucigenin after photo-irradiating 1a at 405 nm light using an LED (1 W) and (B) treating 1a with light for 5 minutes followed by the addition of barbiturate ligand B1.