Anion Transport by Bambusuril-Bile Acid Conjugates: Drastic Effect of the Cholesterol Content

Abstract

Artificial anion transporters offer a potential way to treat deficiencies in cellular anion transport of genetic origins. In contrast to the large variety of mobile anion carriers and self-assembled anion channels reported, unimolecular anion channels are less investigated. Herein, we present a unique example of a unimolecular anion channel based on a bambusuril (BU) macrocycle, a well-established anion receptor. The BU structure was expanded by appending various bile acid residues allowing a single molecule to span the membrane. Chloride transport mediated by BUs through lipid bilayers was investigated in liposomes and these studies revealed a surprisingly high dependence of the anion transport activity on the cholesterol content in the liposomal membrane.

Keywords: Anion Channels; Anion transport; Bambusurils; Macrocycles; Supramolecular Chemistry.

Figure 1

a) Schematic representation of compounds with different linkers; b) 3D model of L3 as complex with Br⁻, which has a calculated length of 5.5 nm.

Scheme 1

Cycloaddition reactions of propargyl BU (BU1) and azido‐functionalized bile acids resulting in the target BUs. BUs connected to the bile acid residues via a longer linker are L1–L3, while the ones with a shorter linker are S1–S3. BUs conjugated with lithocholic acid are L1 and S1; with chenodeoxycholic acid L2 and S2; with cholic acid L3 and S3.

Figure 2

a) Chloride transport by BUs (pre‐incorporated at 0.2 mol% transporter to lipid ratio) monitored by the SPBA assay in 225 mM NaHCO₃, upon addition of 25 mM NaCl to the LUVs (0.4 mM lipids) made of POPC; b) Schematic representation of the compounds with different numbers of hydroxy groups, leading to different transport activities.

Figure 3

a) Chloride transport by L3 and S3 (pre‐incorporated at 0.2 mol% transporter:lipid ratio) monitored by the SPBA assay in 225 mM NaHCO₃, upon addition of 25 mM NaCl to the LUVs (0.4 mM lipids) made of DPPC at 25 °C; b) PLB recordings obtained for L3 in DOPC membranes at +80 mV, showing channel‐like currents of ~1 pA.

Figure 4

a) Chloride transport by S2 (pre‐incorporated at 1:500 transporter to lipid ratio) monitored by the SPBA assay in 225 mM NaHCO₃, upon addition of 25 mM NaCl to the LUVs (0.4 mM lipids) at various mol% of cholesterol in the POPC membrane of the LUVs; b) Chloride transport rate constants (k) obtained from fitting transport data of L3, S3, L2, and S2 at various cholesterol contents with a single exponential function; c) Schematic representation of the impact of cholesterol on BU‐bile acid in the membrane.