Ion channel models based on self-assembling cyclic peptide nanotubes

Abstract

The lipid bilayer membranes are Nature's dynamic structural motifs that individualize cells and keep ions, proteins, biopolymers and metabolites confined in the appropriate location. The compartmentalization and isolation of these molecules from the external media facilitate the sophisticated functions and connections between the different biological processes accomplished by living organisms. However, cells require assistance from minimal energy shortcuts for the transport of molecules across membranes so that they can interact with the exterior and regulate their internal environments. Ion channels and pores stand out from all other possible transport mechanisms due to their high selectivity and efficiency in discriminating and transporting ions or molecules across membrane barriers. Nevertheless, the complexity of these smart "membrane holes" has driven researchers to develop simpler artificial structures with comparable performance to the natural systems. As a broad range of supramolecular interactions have emerged as efficient tools for the rational design and preparation of stable 3D superstructures, these results have stimulated the creativity of chemists to design synthetic mimics of natural active macromolecules and even to develop artificial structures with functions and properties. In this Account, we highlight results from our laboratories on the construction of artificial ion channel models that exploit the self-assembly of conformationally flat cyclic peptides (CPs) into supramolecular nanotubes. Because of the straightforward synthesis of the cyclic peptide monomers and the complete control over the internal diameter and external surface properties of the resulting hollow tubular suprastructure, CPs are the optimal candidates for the fabrication of ion channels. The ion channel activity and selective transport of small molecules by these structures are examples of the great potential that cyclic peptide nanotubes show for the construction of functional artificial transmembrane transporters. Our experience to date suggests that the next steps for achieving conceptual devices with better performance and selectivity will derive from the topological control over cyclic peptide assembly and the functionalization of the lumen.

Figure 1

CP1 and CP4. a) Self-assembled nanotube structure of CP1 (most of the side chains are omitted for clarity). b) Electron microscopy low-magnification image of nanotube suspension adsorbed on a carbon support film (scale bar 1 µm); inset shows low-dose image enhancement of a single crystal and reveals the longitudinal striations that correspond to each nanotube. c) Cylindrical dimer of CP4. d) Crystal structure of D4 emphasizing the internalized water binding sites (blue rod).

Figure 2

d,l-α-SCPNs as functional ion channels or pores. a) Structures of CP2a–e and CP5 and b) model of the transmembrane channels in the lipid bilayer (most of the side chains are omitted for clarity), which were able to transport ions (green), glucose (blue) or glutamic acid (red). c) 20-s continuous K⁺ (1M KCl) single channel conductance recorded at +100 mV for CP2 a–e. d) Enzymatic assay to motorize glucose efflux from liposomes by NADPH detection (CP5). e) Current/voltage (I/V) plot for gramicidin A (●), CP2a (□) and CP5 (○) in planar BLMs separating NaCl and NaGlu solutions (300 mM). The inset shows the higher reversal potential required for CP5. Figure 2e is reproduced by permission of John Wiley and Sons.

Figure 3

a) Different conducting states recorded at +100 mV, showing at least four working channels formed by CP2b in l-α-lecithin bilayers in 1M KCl. b) Probable mechanisms of gating and of variable conductance events.

Figure 4

a) Structures of CP2b, CP6-10. b) Mode of interaction of the CP caps (CP69) with the transporting ensemble of CP2b to afford heteromeric transmembrane channels. c) SCPN2b embedded in the gold SAMs for the construction of selective ion sensors. d) Proposed model of parallel nanotube orientation for amphiphilic antibacterial d,l-α-cyclic peptides (CP10).

Figure 5

Structure of β³-CPs, CP11-13 and the corresponding membrane β³-SCPN. The alignment of amide and carbonyl groups generates a net dipole moment.

Figure 6

a) Structure of α,γ-CPs CP14a–c. b) Different hydrogen-bonding pattern in the tubular arrangement of α,γ-SCPNs. c) Crystal structure of dimeric assembly of CP14c (R = Bn), in which one chloroform molecule occupies their cavity: (left) top view; (right) side view.

Figure 7

a) Structure of α,γ-CPs CP1517. b) Single-channel recordings of Na⁺ current through CP16 channels (+100 mV and −100 mV) and 3.0 s detail of the trace showing two different current levels (1.8 and 3.1 pA). c) Snapshots of simulated SCPNs in 0.5 M electrolyte solutions after 25 ns of simulation (lipids and external water and ions have been removed for clarity) and Z coordinate for the ions inside the nanotube along the trajectory. [38] – Part of this figure is reproduced by permission of The Royal Society of Chemistry

Figure 8

a) Structure of CP18 and model for nanotube formation showing the hydroxyl groups (blue color) projected towards the cavity.