Artificial ion channels of amphiphilic ion pairs composed of oligoether-ammonium and hydrophobic carboxylate or phosphates

Photo-activation of Tolane-based Synthetic Ion Channel for Transmembrane Chloride Transport

While natural channels respond to external stimuli to regulate ion concentration across cell membranes, creating a synthetic version remains challenging. Here, we present a photo-responsive uncaging technique within an artificial ion channel system, which activates the ion transport process from a transport-inactive o-nitrobenzyl-based caged system. From the comparative ion transport screening, 1 b emerged as the most active transporter. Interestingly, its bis(o-nitrobenzyl) derivative, i.e., protransporter 1 b' was inefficient in transporting ions. Detailed transport studies indicated that compound 1 b is an anion selective transporter with a prominent selectivity towards chloride ions by following the antiport mechanism. Compound 1 b' did not form an ion channel, but after the o-nitrobenzyl groups were photocleaved, it released 1 b, forming a transmembrane ion channel. The channel exhibited an average diameter of 6.5±0.2 Å and a permeability ratio ofP C l - / P K + = 7 . 3 ± 1 . 5 ${{P}_{{Cl}^{-}}/{P}_{{K}^{+}}=7.3\pm 1.5}$ . The geometry-optimization of protransporter 1 b' indicated significant non-planarity, corroborating its inefficient self-assembly. In contrast, the crystal structure of 1 b demonstrates strong self-assembly via the formation of an intermolecular H-bond. Geometry optimization studies revealed the plausible self-assembled channel model and the interactions between the channel and chloride ion.

Photocontrolled activation of doubly o-nitrobenzyl-protected small molecule benzimidazoles leads to cancer cell death

Artificial biomimetic chloride anionophores have shown promising applications as anticancer scaffolds. Importantly, stimuli-responsive chloride transporters that can be selectively activated inside the cancer cells to avoid undesired toxicity to normal, healthy cells are very rare. Particularly, light-responsive systems promise better applicability for photodynamic therapy because of their spatiotemporal controllability, low toxicity, and high tunability. Here, in this work, we report o-nitrobenzyl-linked, benzimidazole-based singly and doubly protected photocaged protransporters 2a, 2b, 3a, and 3b, respectively, and benzimidazole-2-amine-based active transporters 1a-1d. Among the active compounds, trifluoromethyl-based anionophore 1a showed efficient ion transport activity (EC50 = 1.2 ± 0.2 μM). Detailed mechanistic studies revealed Cl-/NO3- antiport as the main ion transport process. Interestingly, double protection with photocages was found to be necessary to achieve the complete "OFF-state" that could be activated by external light. The procarriers were eventually activated inside the MCF-7 cancer cells to induce phototoxic cell death.

Stimuli-Responsive Anion Transport through Acylhydrazone-Based Synthetic Anionophores

Photoswitchable acylhydrazone-based synthetic anionophores are reported. Single-crystal X-ray structure and ¹H NMR titration studies confirmed the chloride binding in solid and solution states. The ion transport activity of 1a was greatly attenuated through a phototriggered E to Z photoisomerization process, and the photoisomerized deactivated state showed high kinetic stability due to an intramolecular hydrogen bond. Switchable "OFF-ON" transport activity was achieved by the application of light and acid-catalyzed reactivation process.

A Sandwich Azobenzene-Diamide Dimer for Photoregulated Chloride Transport

There has been a tremendous evolution for artificial ion transport systems, especially gated synthetic systems, which closely mimic their natural congeners. Herein, we demonstrate a trans-azobenzene-based photoregulatory anionophoric system that transports chloride by forming a sandwich dimeric complex. Further studies confirmed a carrier-mediated chloride-anion antiport mechanism, and the supramolecular interactions involved in chloride recognition within the sandwich complex were revealed from theoretical studies. Reversible trans-cis photoisomerization of the azobenzene was achieved without any significant contribution from the thermal cis→trans isomerization at room temperature. Photoregulatory transport activity across the lipid bilayer membrane inferred an outstanding off-on response of the azobenzene photoswitch.

Synthetic, biologically active amphiphilic peptides

Amphiphilic peptides typically consist of a peptide portion that may be 5-25 (or more) amino acids in length. The hydrophobic portion may be a single fatty acid residue, but can also be more elaborate. The main focus of this article lies on the family of synthetic anion binders (SATs) of the general structure (R(1))(2)N-COCH(2)OCH(2)CO-(Aaa)(n)-OR(3). The most-common R(1) group is the octadecyl (C(18)H(37)) group. The most studied peptide sequence in this family is (Gly)(3)-Pro-(Gly)(3), although different sequences (and longer and shorter peptides) have been prepared as well. The C-terminal ester residue providing the most effective anion release from liposomes is heptyl (C(7)H(15)), although many others have been examined. The compound (C(18)H(37))(2)N-COCH(2)OCH(2)CO-(Gly)(3)-Pro-(Gly)(3)-OBn (Bn=benzyl) was found to mediate Cl(-) transport in mouse epithelial cells.

Transmembrane ion channels constructed of cholic acid derivatives

A new class of supramolecular transmembrane ion channels was prepared by linking two amphiphilic cholic acid methyl ethers through biscarbamate bonds to afford bis(7,12-dimethyl-24-carboxy-3-cholanyl)-N,N'-xylylene dicarbamate 2 and bis[7,12-dimethyl-24-(N,N,N-trimethylethanaminium-2-carboxylate)-3-cholanyl]-N,N'-xylylene dicarbamate dichloride 3. When incorporated into a planar bilayer membrane, both compounds showed stable (lasting 10 ms to 10 s) single ion channel currents. Only limited numbers of relatively small conductances were characterized for these channels (5-20 pS for 2 and 5-10 pS for 3, 10 and 17 pS for 2, and 9 pS for 3 in particular). Both channels were cation selective, and permeability ratios of potassium cation to chloride anion were 17 and 7.9 for 2 and 3, respectively, reflecting the difference in ionic species of the headgroup. Both channels 2 and 3 showed significant potassium selectivity over sodium by a factor of 3.1 and 3.2, respectively. No Li(+) currents were observed for 2, showing sharp discrimination between Na(+) or K(+).

Conformational and orientation studies of artificial ion channels incorporated into lipid bilayers

The conformational and orientation studies in lipid bilayers of 21 amino acid peptides bearing six crown ethers are reported. The compounds were designed to form artificial ion channels by stacking the crown rings, and were shown to be functional in bilayer membranes. We used Fourier transform infrared spectroscopy and CD spectropolarimetry to study the conformation of the peptides in solution and in lipid bilayers. These studies revealed that hexacrown peptides retain their alpha-helical conformation when incorporated in a lipid bilayer environment. Attenuated total reflectance spectroscopy was used to investigate the orientation of the peptides in a lipid bilayer. Results demonstrated that the peptides are not oriented at a fixed angle in membrane, but rather are in incorporation equilibrium between an active state parallel to the lipid chain and an inactive state adsorbed at the surface of the bilayer. From these results, we propose a model for the channel activity and the gating mechanism of these hexacrown peptides in bilayer membranes.

Molecular design and synthesis of artificial ion channels based on cyclic peptides containing unnatural amino acids

A series of novel cyclic peptides composed of 3 to 5 dipeptide units with alternating natural-unnatural amino acid units, have been designed and synthesized, employing 5-(N-alkanoylamino)-3-aminobenzoic acid with a long alkanoyl chain as the unnatural amino acid. All cyclic peptides with systematically varying pore size, shape, and lipophilicity are found to form ion channels with a conductance of ca. 9 pS in aqueous KCl (500 mM) upon examination by the voltage clamp method. These peptide channels are cation selective with the permeability ratio P(Cl(-))/P(K(+)) of around 0.17. The ion channels formed by the neutral, cationic, and anionic cyclic peptides containing L-alanine, L-lysine, and L-aspartate, respectively, show the monovalent cation selectivity with the permeability ratio P(Na(+))/P(K(+)) of ca. 0.39. On the basis of structural information provided by voltage-dependent blockade of the single channel current of all the tested peptides by Ca(2+), we inferred that each channel is formed from a dimer of the peptide with its peptide ring constructing the channel entrance and its alkanoyl chains lining across the membrane to build up the channel pore. The experimental results are consistent with an idea that the rate of ion conduction is determined by the nature of the hydrophobic alkanoyl chain region, which is common to all the channels.

Hydraphile channels: models for transmembrane, cation-conducting transporters

A completely synthetic, non-peptide channel has been prepared and shown to conduct cations across a phospholipid bilayer membrane. Studies have been undertaken to assess the compound's location within the bilayer and to better understand its function. These studies are described along with background on the design and concept of the channel.