A Highly Active Anion-Selective Aminocyclodextrin Ion Channel

Self-Assembly Vesicles Made from a Cyclodextrin Supramolecular Complex

Self-assembly vesicles have been made from a cyclodextrin (CD) supramolecular complex, which is cooperatively formed with natural beta-CD, 1-naphthylammonium chloride (NA), and sodium bis(2-ethyl-1-hexyl)sulfosuccinate (AOT) by weak noncovalent interactions. In the complex structure, a NA molecule is included inside a beta-CD molecule while it is coupled with an AOT molecule on one side. The supramolecular structure and morphology of the vesicles were characterized by transmission electron microscopy (TEM) and dynamic light scattering (DLS), respectively. The mechanism of vesicle formation and transition is discussed along with the data obtained from induced circular dichroism (ICD) and UV/visible spectroscopy, polarized optical microscopy (POM), and (1)H NMR spectroscopy. Both the fabrication and the transition of vesicles are controlled by the inclusion equilibria and the cooperative binding of noncovalent interactions, which include the "key-lock" principle, electrostatic interactions, pi-pi stacking, and amphiphilic hydrophobic association.

Increasing pH Causes Faster Anion- and Cation-Transport Rates through a Synthetic Ion Channel

Ion-channel mimics are able to transmit electrical signals across phospholipid membranes, and can be envisioned as nanoswitches for molecular electronics. Here, we reported the use of pH to alter ion-transport rates through a synthetic aminocyclodextrin ion channel. Both cation- and anion-transport rates were found to increase with an increase in pH due to the unique electrostatics of the multiple ammonium groups that line the channel pore. Such pH regulation of ion transport rates is unique and can be exploited for sensing applications.

Sodium ion internalized within phospholipid membranes

Seven phospholipids, modified with ester groups in their hydrophobic chains, were synthesized and examined for their ability to promote sodium ion flux across vesicular membranes. It was found by 23Na NMR that only the phospholipids having short chain segments beyond their terminal ester groups catalyze sodium ion transfer by up to 2 orders of magnitude relative to a conventional phospholipid, POPC. The rates increase with the concentration of the ester-phospholipid admixed with POPC in the bilayer. More surprisingly, the rates increase with the time allowed for the vesicles to age. This was attributed to ester-phospholipid migrating in the bilayers to form domains that solubilize the sodium ion within the hydrocarbon interior of the membrane. Such membrane domains explain why shift reagent-modified NMR spectra display three 23Na signals representing sodium outside the vesicles, sodium within the vesicular water pools, and sodium within the membranes themselves.

Recent Advances in Synthetic Membrane Transporters

It is 25 years since the first report of a synthetic ion channel transporter. Today, dozens of molecular and supramolecular designs have been developed to facilitate ion and small molecule transport across a bilayer membrane. Presented here is a concise summary of the advances made over the past four years. The transporters are grouped into three mechanistic classes: mobile carrier, monomeric channel, and self-assembled pore. Common building blocks are crown ethers, steroids, cyclodextrins, peptides, curcubiturils, and calixarenes. The eventual goal is to produce functional supramolecular devices such as sensors, enzyme assays, and lead candidates for pharmaceutical development.

Biologically active, synthetic ion transporters

The compelling chemical goal of modeling protein channel behavior has led to synthetic compounds that are true ion channels. Although they largely lack the selectivity and sophistication of highly evolved proteins, they successfully perform a variety of biological functions. This tutorial review describes these novel structures and their activity in living systems. Different channel structures show antibacterial to anticancer activity when tested against a variety of cell types.

Ion channels and pores, made from scratch

We elaborate on the structural diversity well beyond the biological limitations that becomes accessible with synthetic ion channels and pores, and on the importance of advanced nanoarchitecture to create significant function.

Rigid-rod anion–π slides for multiion hopping across lipid bilayers

Shape-persistent oligo-p-phenylene-N,N-naphthalenediimide (O-NDI) rods are introduced as anion-pi slides for chloride-selective multiion hopping across lipid bilayers. Results from end-group engineering and covalent capture as O-NDI hairpins suggested that self-assembly into transmembrane O-NDI bundles is essential for activity. A halide topology VI (Cl > F > Br approximately I, Cl/Br approximately Cl/I > 7) implied strong anion binding along the anion-pi slides with relatively weak contributions from size exclusion (F >or= OAc). Anomalous mole fraction effects (AMFE) supported the occurrence of multiion hopping along the pi-acidic O-NDI rods. The existence of anion-pi interactions was corroborated by high-level ab initio and DFT calculations. The latter revealed positive NDI quadrupole moments far beyond the hexafluorobenzene standard. Computational studies further suggested that anion binding occurs at the confined, pi-acidic edges of the sticky NDI surface and is influenced by the nature of the phenyl spacer between two NDIs. With regard to methods development, a detailed analysis of the detection of ion selectivity with the HPTS assay including AMFE in vesicles is provided.

Development of synthetic membrane transporters for anions

The development of low molecular weight anion transporters is an emerging topic in supramolecular chemistry. The major focus of this tutorial review is on synthetic chloride transport systems that operate in vesicle and cell membranes. The transporters alter transmembrane concentration gradients, and thus they have applications as reagents for cell biology research and as potential chemotherapeutic agents. The molecular designs include monomolecular channels, self-assembled channels and mobile carriers. Also discussed are the experimental assays that measure transport rates across model bilayer membranes.

Planar bilayer studies reveal multiple conductance states for synthetic anion transporters

Compounds of the general type R(1)(2)NCOCH(2)OCH(2)CO-(Gly)(3)-Pro-(Gly)(3)-OCH(2)Ph insert in phospholipid bilayers and conduct ions. Different levels of activity were observed when R(1) was either decyl or octadecyl, as judged either by Cl(-) release, detected by ion selective electrodes, or carboxyfluorescein dequenching, detected by fluorescence. Either method reports average behavior for all ionophores over all liposomes. These methods also show that at least two ionophores are involved in the formation of each pore. Planar bilayer experiments reported here confirm pore formation by these compounds but identify more than one conductance state for each. The pseudo-dimer, in which two molecules of the type shown above are covalently linked, shows only two conductance states, of which one is dominant. This state has been characterized by use of a current-voltage plot.

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.