1.D.213.  The Self-Assembled Peptide Anion Channel (SAP-ACh) Family 

Wanting to mimic a protein α-helical bundle, Ren et al. 2018 developed a modular synthesis of amino acids containing alkylamides at N- and C-termini, exemplified by 60 (see the figure below. X-ray crystal structures of related compounds had shown that the 3 different side chains (R1, R2, and R3) stacked on themselves to give 1D helices, held together by intermolecular CO⋯HN hydrogen bonds (Davis et al. 2020). The former authors hypothesized that such assemblies might pack, using sidechain–sidechain interactions, to give membrane-spanning pores.

Using a combinatorial approach, the authors synthesised and screened 100 monopeptides in an HPTS assay in EYPC liposomes. Over 25 of 100 monopeptides had Y_max values for Cl⁻ efflux ≥50% at 5 μM. Further screening at lower ligand concentrations identified two peptides 60 and the parent monopeptide bearing a n-hexyl chain as the best chloride transporters, and both formed ion channels in planar bilayers with specific conductance values of ≈550 fS. In EYPC liposomes the EC₅₀ values for reaching 50% for chloride transport with concomitant pH gradient discharge were quite low: 0.32 μM for 60 and 0.27 μM for the parent monopeptide bearing an n-hexyl chain. A Hill coefficient of n = 3.3 for 60 was consistent with the membrane-active species arising via self-assembly. Finally, both compounds were nitrate-selective transporters, showing a qualitative trend of NO₃⁻ > ClO₄⁻ > Br⁻ > I⁻ > Cl⁻.

Molecular models based on crystal structures of analogues indicated that a column of 6–8 molecules of 60 is needed to span a EYPC membrane of 28 Å. Molecular dynamics simulations showed that (1) R1, R2 and R3 side chains, which were superimposable in the starting structure, became less organised (2) all 8 molecules of 60 remained H-bonded to one another and (3) trimeric bundles of (60)₈ columns were stable. The authors suggested that these persistent H-bonded columns can self-associate using sidechain–sidechain interactions to generate a membrane-active channel.

The liposome and planar bilayer experiments and MD calculations established that low molecular weight compounds 60 and other monopeptide analogues are potent anion transporters that form self-assembled ion channels in the membrane. The combinatorial synthesis and screening of such “easy-to-make” monopeptides is a promising strategy to rapidly identify anion transporters.

Halogen bonds are noncovalent interactions wherein Lewis acidic halogens use σ* orbitals to pair with Lewis bases. Jentzsch and Matile 2015 introduced halogen bonds in synthetic ion channels, using rigid-rods with iodine atoms located along the rod to facilitate anion transport. Ren et al. 2018 extended this concept with a self-assembly approach, which benefits from combinatorial synthesis and screening of channel candidates (see the figure below). As often occurs, the authors were guided by evaluating crystal structures. Zeng's group had found that lipophilic monopeptides with N-terminal FMOC groups form H-bonded chains in the solid state. The three subunits always stacked with their own identical subunits to give supramolecular structures that featured columns of co-linear FMOC groups. The authors reasoned that replacing FMOC with a tetrafluoroiodobenzyl group would enable hydrogen-bonded assembly in the lipid membrane of 6–7 monomers to give a path lined with electron-deficient iodines. As depicted below, these evenly spaced iodines would enable anions to cross the bilayer by hopping along halogen-bonding sites.