Recent synthetic ion channels and pores

Synthetic ion channels and pores (2004-2005)

This critical review covers synthetic ion channels and pores created between January 2004 and December 2005 comprehensively. The discussion of a rich collection of structural motifs may particularly appeal to organic, biological, supramolecular and polymer chemists. Functions addressed include ion selectivity and molecular recognition, as well as responsiveness to light, heat, voltage and membrane composition. The practical applications involved concern certain topics in medicinal chemistry (antibiotics, drug delivery), catalysis and sensing. An introduction to principles and methods is provided for the non-specialist; some new sources of inspiration from fields beyond chemistry are highlighted.

The "triamino-analogue" of methyl allocholate; a rigid, functionalised scaffold for supramolecular chemistry

Cholic acid has been converted into triamine with the all-trans polycyclic allocholanoyl skeleton and co-directed, axial amino groups; the potential of this system as a scaffold is illustrated by conversion to a preorganised anion receptor.

Substrate-Independent Transduction of Chromophore-Free Organic and Biomolecular Transformations into Color

The concept of synthetic multifunctional pores as substrate-independent optical signal transducers of chemical reactions is introduced with emphasis on the combination with substrate-specific signal generation in biomolecular transformations. Comparison with the general electrochemical transduction, known from conventional biosensors, and the general optical transduction of analyte-specific biomolecular recognition (rather than transformation), known from immunosensing, reveals the fundamental nature of the concept as well as an attractive complementarity to existing methods. Examples with transferases, hydrolases, lyases, and even an isomerase demonstrate that optical transduction with synthetic multifunctional pores is general far beyond the substrate-specific signal generators of electrochemical transduction, that is, the oxidoreductases, and absolutely unproblematic. In part very recent breakthroughs are used to highlight the remarkable promise of synthetic multifunctional pores as optical transducers of biomolecular transformation with regard to practical sensing and screening applications.

A unimolecular G-quadruplex that functions as a synthetic transmembrane Na+ transporter

We describe the covalent post-modification of a hydrogen-bonded assembly with the subsequent formation of a potent transmembrane Na+ ion transporter. Olefin metathesis is used to cross-link all 16 guanosine subunits in a lipophilic G-quadruplex. The resulting unimolecular G-quadruplex folds in the environment of a phospholipid membrane and functions as a Na+ ion transporter as judged by fluorescence and 23Na NMR transport assays.

Dendritic Folate Rosettes as Ion Channels in Lipid Bilayers

The self-assembly of folate dendrimers into pi-stacked supramolecular rosettes is shown to produce ion channels in planar and spherical lipid bilayer membranes. The found ion channels are small, quite homogeneous, long-lived, ohmic, cation selective (Eisenman I), and blockable by the permeant cation.

Characterization of β-turn and Asx-turns mimicry in a model peptide: Stabilization via CH · · · O interaction

The chemical synthesis and single-crystal X-ray diffraction analysis of a model peptide, Boc-Thr-Thr-NH2 (1) comprised of proteinogenic residues bearing an amphiphilic Cbeta -stereogenic center, has been described. Interestingly, the analysis of its molecular structure revealed the existence of a distinct conformation that mimics a typical beta-turn and Asx-turns, i.e., the two Thr residues occupy the left- and right-corner positions. The main-chain torsion angles of the N- and C-terminal residues i.e., semiextended: phi = -68.9 degrees , psi = 128.6 degrees ; semifolded: phi = -138.1 degrees , psi = 2.5 degrees conformations, respectively, in conjunction with a gauche- disposition of the obligatory C-terminus Thr CgammaH3 group, characterize the occurrence of the newly described beta-turn- and Asx-turns-like topology. The preferred molecular structure is suggested to be stabilized by an effective nonconventional main-chain to side-chain Ci=O . . . H--Cgamma(i+2)-type intraturn hydrogen bond. Noteworthy, the observed topology of the resulting 10-membered hydrogen-bonded ring is essentially similar to the one perceived for a classical beta-turn and the Asx-turns, stabilized by a conventional intraturn hydrogen bond. Considering the signs as well as magnitudes of the backbone torsion angles and the orientation of the central peptide bond, the overall mimicked topology resembles the type II beta-turn or type II Asx-turns. An analysis of Xaa-Thr sequences in high-resolution X-ray elucidated protein structures revealed the novel topology prevalence in functional proteins (unpublished). In view of indubitable structural as well as functional importance of nonconventional interactions in bioorganic and biomacromolecules, we intend to highlight the participation of Thr CgammaH in the creation of a short-range C=O . . . H--Cgamma -type interaction in peptides and proteins.

Synthetic functional π-stack architecture in lipid bilayers

Neglected until recently, pi-stack architecture is rapidly emerging as a powerful strategy to create function in lipid bilayer membranes. Recent reports describe supramolecular rosettes acting as hosts of intercalating guests, to assemble in bilayer membranes and, in the case of stacked guanosine and folate quartets, to form ion channels. The introduction of rigid-rod pi-stack architecture allowed us to address one of the great challenges in the field, i.e. ligand gating. Inspiring pi-stack chemistry from related fields, covering rainbow coloration, conductivity, as well as the critical dependence of charge mobilities on the precision of supramolecular organization is summarized to zoom in on arguably the most promising application of functional pi-stack architecture in lipid bilayers, that is the creation of multifunctional photosystems.

Synthetic multifunctional pores that open and close in response to chemical stimulation

Studies on synthetic multifunctional pores with external and internal active sites for ligand gating and noncompetitive blockage are presented, with emphasis on the contribution of external ligands to the characteristics of pore. A comparison between different synthetic multifunctional pores reveals that the location of functional groups in rigid-rod beta-barrel pores is precisely reflected in the function: molecular recognition at the outer barrel surface results in pore opening, while molecular recognition at the inner barrel surface results in pore closing. Negligible nonspecific leakage, disappearance of pH gating, inhibition of intervesicular pore transfer, and maybe also the flickering of currents of single open pores characterize external ligands as adhesive cushions that liberate the pore from lateral pressure exerted by the surrounding membrane. Refined molecular models show good agreement with pore design and experimental facts with regard to function.

Ligand-Gated Synthetic Ion Channels

Supramolecular pi-stack architecture is fundamental in DNA chemistry but absent in biological and synthetic ion channels and pores. Here, a novel rigid-rod pi-stack architecture is introduced to create synthetic ion channels with characteristics that are at the forefront of rational design, that is, ligand gating by a conformational change of the functional supramolecule. Namely, the intercalation of electron-rich aromatics is designed to untwist inactive electron-poor helical pi-stacks without internal space into open barrel-stave ion channels. Conductance experiments in planar lipid bilayers corroborate results from spherical bilayers and molecular modeling: Highly cooperative and highly selective ligand gating produces small, long-lived, weakly anion selective, ohmic ion channels. Structural studies conducted under conditions relevant for function provide experimental support for helix-barrel transition as origin of ligand gating. Control experiments demonstrate that minor structural changes leading to internal decrowding suffice to cleanly annihilate chiral self-organization and function.

Receptor for Anionic Pyrene Derivatives Provides the Basis for New Biomembrane Assays

This study describes a new receptor cyclen 1 capable of strong selective binding of pyrene-based anionic dyes under near-physiological conditions. This receptor comprises four naphthylthiourea groups tethered to a cyclen core via an ester linkage. The complexation behavior of cyclen 1 receptor is characterized by a series of (1)H NMR, microcalorimetry, UV-vis, and fluorometry experiments. The relevance of structural features of this receptor to its recognition function is assessed using control compounds that lack some of the groups found in cyclen 1. The specificity of cyclen 1 toward pyrene-based dyes is assessed through experiments using dyes with different molecular organization. The most important finding was the ability of cyclen 1 to bind efficiently to a pH-sensitive dye pyranine, a dye that is commonly used in various biomembrane assays. The high affinity of cyclen 1 to pyranine, its impermeability to the lipid bilayer membrane, fast kinetics of binding, and ability to quench the pyranine's fluorescence were used as a basis for a new membrane leakage assay. This membrane leakage assay is fully compatible with the commonly applied pH-stat transport assay, and therefore it allows for differentiation of the ion transport and nonselective leakage mechanisms within a single set of experiments. The ability of cyclen 1 to quench the fluorescence of pyranine also finds limited applicability to the detection of endovesiculation.

Contributions of Lipid Bilayer Hosts to Structure and Activity of Multifunctional Supramolecular Guests

The question of whether or not the surrounding lipid bilayer host contributes to structure and activity of included functional guests is a general topic of current scientific concern. We report that synthetic multifunctional pores are of use to address this elusive question, because the detection of their catalytic activity is membrane independent. According to their salt-rate profiles, unstable multifunctional supramolecules with permanent internal charges show highest membrane sensitivity, and the dependence of membrane sensitivity on the acidity of internal cations exceeds that on supramolecule stability. These results can, with all appropriate caution, be interpreted as indications for the existence of long-range EMP-ICR interactions (EMP: external membrane pressure, ICR: internal charge repulsion) between membrane hosts and functional guests that can, for instance, prevent the 'explosion' and promote the 'implosion' of over- and undercharged transmembrane barrel-stave supramolecules, respectively.

Synthetic Ion Channels with Rigid-Rod π-Stack Architecture that Open in Response to Charge-Transfer Complex Formation

We describe the design, synthesis, and evaluation of synthetic ion channels with a new rigid-rod pi-stack architecture that open in response to guest binding by aromatic electron donor-acceptor interactions. Highly cooperative and highly selective ligand gating is shown to yield anion selective, small ion channels that have the characteristic plum color of the charge-transfer complexes formed between the dialkoxynaphthalene ligands and the stacked naphthalenediimide acceptors of the channel.

Arginine magic with new counterions up the sleeve

The elusive questions how arginine-rich sequences allow peptides and proteins to penetrate cells or to form voltage-gated ion channels are controversial topics of current scientific concern. The possible contributions of exchangeable counterions to these puzzling processes remain underexplored. The objective of this report is to clarify scope and limitations of certain counteranions to modulate cellular uptake and anion carrier activity of oligo/polyarginines. The key finding is that the efficiency of counteranion activators depends significantly on many parameters such as activator-membrane and activator-carrier interactions. This finding is important because it suggests that counteranions can be used to modulate not only efficiency but also selectivity. Specifically, activator efficiencies are found to increase with increasing aromatic surface of the activator, decreasing size of the transported anion, increasing carrier concentration as well as increasing membrane fluidity. Efficiency sequences depend on membrane composition with coronene > pyrene >>fullerene > calix[4]arene carboxylates in fluid and crystalline DPPC contrasting to fullerene > calix[4]arene approximately coronene > pyrene carboxylates in EYPC with or without cholesterol or ergosterol. In HeLa cells, the efficiency of planar activators (pyrene) exceeds that of spherical activators (fullerenes, calixarenes). Polyarginine complexes with pyrene and coronene activators exhibit exceptional excimer emission. Decreasing excimer emission with increasing ionic strength reveals dominant hydrophobic interactions with the most efficient carboxylate activators. Dominance of ion pairing with the inefficient high-affinity sulfate activators is corroborated by the reversed dependence on ionic strength. These findings on activator-carrier and activator-membrane interactions are discussed as supportive of arene-templated guanidinium-carboxylate pairing and interface-directed translocation as possible origins of the superb performance of higher arene carboxylates as activators.

Rigid-rod molecules in biomembrane models: from hydrogen-bonded chains to synthetic multifunctional pores

Synthetic ion channels and pores formed by rigid-rod molecules are summarized. This includes work on hydrogen-bonded chains installed along membrane-spanning rigid-rod scaffolds to transport protons. As a second topic, programmed assembly of p-septiphenyls with terminal iminodiacetate-copper complexes for potassium transport by cation-pi interactions is described. The third topic concerns rigid push-pull rods as fluorescent alpha-helix mimics to probe the importance of dipole-potential interactions for voltage gating, both on the functional and the structural level. Topic number four deals with p-octiphenyl staves as key scaffolds for the synthesis of rigid-rod beta-barrel pores. The description of internal and external design strategies for these rigid-rod beta-barrels covers a rich collection of pH-, pM-, voltage-, ligand-, and enzyme-gated synthetic multifunctional pores that can act as hosts, sensors, and catalysts. As far as practical applications are concerned, the possibility to detect chemical reactions with synthetic multifunctional pores appears most attractive. Recent molecular mechanics simulations are presented as a valuable approach to insights on the elusive suprastructures of multifunctional pores made from rigid rods.

A fluorescent assay for chloride transport; identification of a synthetic anionophore with improved activity

A fluorescent assay based on the chloride-sensitive probe, lucigenin, is developed for monitoring chloride transport into vesicles, and used to compare the effectiveness of three steroid-derived transporters.

The depth of molecular recognition: voltage-sensitive blockage of synthetic multifunctional pores with refined architecture

Voltage-sensitive blockage by ADP, ATP and phytate (IP6) demonstrates that active-site contraction toward the middle of newly synthesized rigid-rod beta-barrels provides a general strategy to rationally create and modulate the voltage sensitivity (and to increase the efficiency) of molecular recognition by synthetic multifunctional pores.

Use of the exciton chirality method in the investigation of ligand-gated synthetic ion channels

The objective of this brief highlight is to point out the central role of the exciton chirality method to gain insights on the structural basis of the recently achieved ligand gating of synthetic ion channels. This unprecedented ligand gating was achieved with an equally unprecedented transmembrane rigid-rod pi-stack architecture that is designed to adopt a closed conformation with helically stacked naphthalenediimide (NDI) acceptors. The intercalation of the complementary electron-rich dialkoxynaphthalene ligands then stimulates the untwisting of the closed pi-helices into hollow barrel-stave supramolecules. During this helix-barrel transition, the angle between the transition moments of the exciton-coupled NDI chromophores decreases toward zero. The corresponding disappearance of the split CD provides, according to the exciton chirality method, the otherwise elusive experimental support that ligand-gated ion channel formation really occurs by this rationally designed helix-barrel transition.

Artificial Ion Channel Formed by Cucurbit[n]uril Derivatives with a Carbonyl Group Fringed Portal Reminiscent of the Selectivity Filter of K+Channels

Novel artificial ion channels (1 and 2) based on CB[n] (n = 6 and 5, respectively) synthetic receptors with carbonyl-fringed portals (diameter 3.9 and 2.4 A, respectively) can transport proton and alkali metal ions across a lipid membrane with ion selectivity. Fluorometric experiments using large unilamellar vesicles showed that 1 mediates proton transport across the membranes, which can be blocked by a neurotransmitter, acetylcholine, reminiscent of the blocking of the K+ channels by polyamines. The alkali metal ion transport activity of 1 follows the order of Li+ > Cs+ approximately Rb+ > K+ > Na+, which is opposite to the binding affinity of CB[6] toward alkali metal ions. On the other hand, the transport activity of 2 follows the order of Li+ > Na+, which is also opposite to the binding affinity of 2 toward these metal ions, but virtually no transport was observed for K+, Rb+, and Cs+. It is presumably because the carbonyl-fringed portal size of 2 (diameter 2.4 A) is smaller than the diameters of these alkali metal ions. To determine the transport mechanism, voltage-clamp experiments on planar bilayer lipid membranes were carried out. The experiments showed that a single-channel current of 1 for Cs+ transport is approximately 5 pA, which corresponds to an ion flux of approximately 3 x 107 ions/s. These results are consistent with an ion channel mechanism. Not only the structural resemblance to the selectivity filter of K+ channels but also the remarkable ion selectivity makes this model system unique.

Synthetic multifunctional pores with external and internal active sites for ligand gating and noncompetitive blockage

Design, synthesis, and multifunctionality of p-octiphenyl beta-barrel pores with external LRL triads and internal HH dyads are described. Molecular recognition of anionic fullerenes > calixarenes > pyrenes by guanidinium arrays at the outer pore surface is shown to result in pore opening, whereas alpha-helix recognition within the topologically matching internal space is shown to result in noncompetitive pore blockage. This experimental evidence for multifunctionality is supported by comparison with pertinent control pores and blockers, by structural studies using FRET from p-octiphenyl donors in the pore to BODIPY acceptors in the bilayer, and by molecular mechanics simulations. Practical usefulness of ligand-gated synthetic multifunctional pores is exemplified with the continuous detection of chemical processes.