Controlling Water Flow through a Synthetic Nanopore with Permeable Cations

Abstract

There is presently intense interest in the development of synthetic nanopores that recapitulate the functional properties of biological water channels for a wide range of applications. To date, all known synthetic water channels have a hydrophobic lumen, and while many exhibit a comparable rate of water transport as biological water channels, there is presently no rationally designed system with the ability to regulate water transport, a critical property of many natural water channels. Here, we describe a self-assembling nanopore consisting of stacked macrocyclic molecules with a hybrid hydrophilic/hydrophobic lumen exhibiting water transport that can be regulated by alkali metal ions. Stopped-flow kinetic assays reveal a non-monotonic-dependence of transport on cation size as well as a strikingly broad range of water flow, from essentially none in the presence of the sodium ion to as high a flow as that of the biological water channel, aquaporin 1, in the absence of the cations. All-atom molecular dynamics simulations show that the mechanism underlying the observed sensitivity is the binding of cations to defined sites within this hybrid pore, which perturbs water flow through the channel. Thus, beyond revealing insights into factors that can modulate a high-flux water transport through sub-nm pores, the obtained results provide a proof-of-concept for the rational design of next-generation, controllable synthetic water channels.

Figure 1

Vesicle-based stopped-flow kinetic assay to determine water permeability through nanopores. (A) Hybrid macrocycle 1, with a backbone consisting of a hydrogen-bond-rigidified aromatic oligoamide segment and a diethynylbenzene segment. (B) Schematic diagram of the assay. (C) Real-time traces showing the water permeability through pure lipid vesicles (no nanopores) under different magnitudes of osmotic pressure induced by PEG 1000. (D) Real-time traces showing the differences in water permeability through the membrane with or without nanopores under osmotic pressure.

Figure 2

Molecular dynamics simulations of water transport through hybrid nanopores. (A) Energetically minimized structure of the hybrid macrocycle determined from density functional calculations. (B) The nanopore is constructed by attaching amide/acyl chains to the energy-minimized ring and stacking together 10 macrocycles. (C) Following equilibration, the channel (shown in line representation) is found to be filled with water molecules (shown as van der Waals spheres). The inset shows the typical orientation of pore waters. In this depiction, the peripheral carbonyls of the channel lumen are on the left side. (D) Water flow through the nanopore in simulations without any ions in solution or in the presence of 100 mM NaCl. The red and black regions correspond to times of higher and lower flux, respectively. (E) Snapshots of the pore waters at 0.2 ns intervals corresponding to the regions labeled in (B). In the left of each panel, the pore waters were colored orange, and their movement at two subsequent times is shown.

Figure 3

Molecular dynamics simulations of water transport through the hybrid nanopores in the presence of cations. (A) Snapshot of the Na⁺ and water organization within the pore during the equilibration simulations. The ions are found to localize either at the distal sites (left) or within the axial sites. (B) Two views showing the binding sites in more detail. The top panel shows the ion in the distal site (right) between two macrocycles, whereas the ion in the central site is not as precisely positioned along the central axis and can be found within a single macrocycle as shown. The lower panel is a top-down view showing the location of the ions with respect to the central axis of the pore. (C) The axial and radial positions of the Na⁺ ions that were in the pore during the simulation, including one (Sodium #4) that entered the pore after about 40 ns. The ions in the distal binding site are at approximately 2 Å in the radial direction, while those in the axial binding site are near 0 Å. (D) Snapshot during the equilibration simulations showing the number and locations of Li⁺, K⁺, or Cs⁺, together with the water molecules in the pore. (E) Number of water molecules transported through the nanopore after 70 ns of simulations in the presence of 100 mM LiCl, NaCl, KCl, or CsCl or in the absence of any salt in the solution. (F) Axial and radial positions of all Li⁺, K⁺, or Cs⁺ in the pore during the simulations.

Figure 4

Experimental measurement of the cation-dependence of water flow through the nanopores. (A) Stopped-flow measurements of water permeability through the nanopores in the presence or absence of different salt solutions, each 100 mM of Cl-based salts, together with the control measurement without any nanopores. (B) Measured water permeability through the nanopores, p_nano, as determined from the fitting of the stopped-flow traces to eqs 1 and 2 (see Methods in the Supporting Information).