Supramolecular Channels Assembled within Intercellular Gaps.

Tubular structures exist broadly in biological systems and exhibit important functions including mediating cellular communications. The construction of artificial analogues in living cells would provide a new strategy for chemotherapy. In this report, a kind of supramolecular channel has been constructed within intercellular gaps by mimicking the assembly process and structure of natural gap junctional channels, which consist of hydrophobic tubular modules located in the adjacent cell membranes and hydrophilic modules within the extracellular space. The assembly of the channels was driven by electrostatic interactions. The channels could inhibit tumor cell invasion by preventing cell migration.

Methylene-bridge tryptophan fatty acylation regulates PI3K-AKT signaling and glucose uptake.

Protein fatty acylation regulates numerous cell signaling pathways. Polyunsaturated fatty acids (PUFAs) exert a plethora of physiological effects, including cell signaling regulation, with underlying mechanisms to be fully understood. Herein, we report that docosahexaenoic acid (DHA) and eicosapentaenoic acid (EPA) regulate PI3K-AKT signaling by modifying PDK1 and AKT2. DHA-administered mice exhibit altered phosphorylation of proteins in signaling pathways. Methylene bridge-containing DHA/EPA acylate δ1 carbon of tryptophan 448/543 in PDK1 and tryptophan 414 in AKT2 via free radical pathway, recruit both the proteins to the cytoplasmic membrane, and activate PI3K signaling and glucose uptake in a tryptophan acylation-dependent but insulin-independent manner in cultured cells and in mice. DHA/EPA deplete cytosolic PDK1 and AKT2 and induce insulin resistance. Akt2 knockout in mice abrogates DHA/EPA-induced PI3K-AKT signaling. Our results identify PUFA's methylene bridge tryptophan acylation, a protein fatty acylation that regulates cell signaling and may underlie multifaceted effects of methylene-bridge-containing PUFAs.

Gramicidin A-based unimolecular channel: cancer cell-targeting behavior and ion transport-induced apoptosis.

A series of glycoside-peptide conjugates were prepared by engineering at the N-terminus of the natural peptide gramicidin A. The conjugate containing galactose moiety formed a unimolecular transmembrane channel and mediated ion transport to induce apoptosis of cancer cells. More importantly, it exhibited liver cancer cell-targeting behavior due to the galactose-asialoglycoprotein receptor recognition.

Artificial Aquaporin That Restores Wound Healing of Impaired Cells.

Artificial aquaporins are synthetic molecules that mimic the structure and function of natural aquaporins (AQPs) in cell membranes. The development of artificial aquaporins would provide an alternative strategy for treatment of AQP-related diseases. In this report, an artificial aquaporin has been constructed from an amino-terminated tubular molecule, which operates in a unimolecular mechanism. The artificial channel can work in cell membranes with high water permeability and selectivity rivaling those of AQPs. Importantly, the channel can restore wound healing of the cells that contain function-lost AQPs.

Artificial water channels enable fast and selective water permeation through water-wire networks.

Artificial water channels are synthetic molecules that aim to mimic the structural and functional features of biological water channels (aquaporins). Here we report on a cluster-forming organic nanoarchitecture, peptide-appended hybrid[4]arene (PAH[4]), as a new class of artificial water channels. Fluorescence experiments and simulations demonstrated that PAH[4]s can form, through lateral diffusion, clusters in lipid membranes that provide synergistic membrane-spanning paths for a rapid and selective water permeation through water-wire networks. Quantitative transport studies revealed that PAH[4]s can transport >109 water molecules per second per molecule, which is comparable to aquaporin water channels. The performance of these channels exceeds the upper bound limit of current desalination membranes by a factor of ~104, as illustrated by the water/NaCl permeability-selectivity trade-off curve. PAH[4]'s unique properties of a high water/solute permselectivity via cooperative water-wire formation could usher in an alternative design paradigm for permeable membrane materials in separations, energy production and barrier applications.

Targeting the Cell Membrane by Charge-Reversal Amphiphilic Pillar[5]arene for the Selective Killing of Cancer Cells.

A charge-reversal amphiphilic pillar[5]arene, P5NH-DCA, bearing 10 charge-reversal headgroups is reported. It targets the cell membrane of cancer cells and selectively destroys the cancer cells by disrupting the membrane. In the acidic tumor microenvironment, the headgroup charge of P5NH-DCA reversed from negative to positive owing to hydrolysis of the acid-labile amide group. The hydrolyzed product bearing multiple positive charges can bind to the cell membrane and then disrupt the membrane of cancer cells with high efficiency. However, under the neutral microenvironment of healthy cells, the negatively charged P5NH-DCA remains stable and the cytotoxicity is considerably reduced. The strategy killing the cancer cells by membrane disruption may represent a new route of cancer chemotherapy.

A synthetic channel that efficiently inserts into mammalian cell membranes and destroys cancer cells.

Despite the fact that a large number of synthetic channels have been developed in the last three decades, few of them can function in mammalian cell membranes because of their weak membrane insertion abilities. This study describes a tubular molecule with terminal positively charged amino groups that displays a strong ability to insert into lipid bilayers composed of phosphatidylcholine and consequently forming unimolecular transmembrane channels. It has been demonstrated that the insertion of the channel into the phosphatidylcholine bilayers was driven by the electrostatic interaction between the positively charged amino groups of the channel molecules and the negatively charged phosphate groups of the lipid molecules. The high affinity of the channels for lipid bilayers led to efficient mammalian cell membrane insertion. The channels showed high effective activity against HepG2 cancer cells at concentrations above 5.1 µM.

Reversible photo-gated transmembrane channel assembled from an acylhydrazone-containing crown ether triad.

We have prepared a crown ether triad containing acylhydrazone units. In solution, the triad can self-assemble linearly to form an organogel. UV light-induced E/Z isomerization of the C[double bond, length as m-dash]N bond of the acylhydrazone unit endows the assembly with photo-sensitivity. The triad was able to insert into the lipid bilayer to form a supramolecular transmembrane channel which showed transport selectivity for NH4+ over K+. The channel exhibited photo-gating properties under microscopic and macroscopic conditions. The transport of the channel could be reversibly switched off and on by irradiation with alternating 320 and 365 nm UV light as supported by the conductance measurements.

Synthetic Channel Specifically Inserts into the Lipid Bilayer of Gram-Positive Bacteria but not that of Mammalian Erythrocytes.

A series of tubular molecules with different lengths have been synthesized by attaching Trp-incorporated peptides to the pillar[5]arene backbone. The tubular molecules are able to insert into the lipid bilayer to form unimolecular transmembrane channels. One of the channels has been revealed to specifically insert into the bilayer of the Gram-positive bacteria. In contrast, this channel cannot insert into the membranes of the mammalian rat erythrocytes even at the high concentration of 100 µm. It was further demonstrated that, as a result of this high membrane selectivity, the channel exhibits efficient antimicrobial activity for the Gram-positive bacteria and very low hemolytic toxicity for mammalian erythrocytes.

Highly permeable artificial water channels that can self-assemble into two-dimensional arrays.

Bioinspired artificial water channels aim to combine the high permeability and selectivity of biological aquaporin (AQP) water channels with chemical stability. Here, we carefully characterized a class of artificial water channels, peptide-appended pillar[5]arenes (PAPs). The average single-channel osmotic water permeability for PAPs is 1.0(± 0.3) × 10(-14) cm(3)/s or 3.5(± 1.0) × 10(8) water molecules per s, which is in the range of AQPs (3.4 ∼ 40.3 × 10(8) water molecules per s) and their current synthetic analogs, carbon nanotubes (CNTs, 9.0 × 10(8) water molecules per s). This permeability is an order of magnitude higher than first-generation artificial water channels (20 to ∼ 10(7) water molecules per s). Furthermore, within lipid bilayers, PAP channels can self-assemble into 2D arrays. Relevant to permeable membrane design, the pore density of PAP channel arrays (∼ 2.6 × 10(5) pores per µm(2)) is two orders of magnitude higher than that of CNT membranes (0.1 ∼ 2.5 × 10(3) pores per µm(2)). PAP channels thus combine the advantages of biological channels and CNTs and improve upon them through their relatively simple synthesis, chemical stability, and propensity to form arrays.

Chiral selective transmembrane transport of amino acids through artificial channels.

Peptide-appended pillar[n]arene (n = 5, 6) derivatives have been synthesized. (1)H NMR and IR studies revealed that the molecules adopt a tubular conformation in solution and lipid bilayer membranes. Kinetic measurements using the fluorescent labeling method with lipid vesicles revealed that these molecules can efficiently mediate the transport of amino acids across lipid membranes at a very low channel-to-lipid ratio (EC(50) = 0.002 mol %). In several cases, chiral selectivity for amino acid enantiomers was achieved, which is one of the key functions of natural amino acid channels.

Peptide mimics by linear arylamides: a structural and functional diversity test.

Hydrogen-bonded oligoamide foldamers represent a large family of peptide mimics. Pioneered by Gellman and Seebach (Appella , J. Am. Chem. Soc. 1996, 118, 13071- 13072; Seebach , Helv. Chim. Acta 1996, 79, 913- 941), aliphatic amino acid-based mimic structures have been extensively studied. Results of these studies have found many useful applications in areas including chemical biology and drug design. This Account describes our efforts in creating arylamide-based foldamers whose compact conformations are stabilized by hydrogen bonding. The aim of our study was to test whether this class of mimic structures is sufficiently rigid to lead to new interesting functions. It was envisioned that, if our approach was workable, it might be developed into a new family of useful soft frameworks for studies toward molecular recognition, self-assembly, and materials science. Three classes of mimic structures, that is, folded or helical, zigzag, and straight oligomers, have been constructed by simply changing the positions of the substituents at the benzene rings in the backbones. Both amide and hydrazide units have been employed to construct the frameworks. In most cases, O...H-N hydrogen bonding was chosen to stabilize the compact conformations. Notably, for the first time the F...H-N hydrogen-bonding pattern has been used to tune the size of the cavity. To test their usefulness, these frameworks have been extensively modified and functionalized. (1)H NMR, UV-vis, fluorescence, circular dichroism, and X-ray diffraction techniques have all been employed to establish the compact structures and their interactions with guest molecules. The properties or functions of the mimic structures have been studied in seven aspects. (1) Acyclic molecular receptors: The amide foldamers can bind amine cations, while the hydrazide foldamers can complex saccharides. (2) Acceleration of anisole hydrolysis: Several folded oligomers are able to bind alkali metal cations and consequently promote the hydrolysis of the nitro-substituted anisole by alkali hydroxides. (3) Facilitation of macrocyclization: The straight and zigzag backbones can be readily functionalized, from which two classes of macrocycles have been prepared. (4) Homoduplex assembly: Zigzag oligomers that are appended with amide units at one side can form stable homoduplexes through the cooperative self-binding of the amide units. (5) Assembly of molecular tweezers: Discrete binding moieties are introduced at the ends of the oligomers, which can bind structurally matched guests. (6) Assembly of nano networks: F...H-N hydrogen-bonded foldamers can stack with fullerenes; thus a mixture of fullerenes with a trifoldamer generates honeycomb-styled nanoarchitectures. (7) Assembly of dynamic [2]catenanes: A preorganized porphyrin tweezer has been synthesized, from which dynamic three-component [2]catenanes have been assembled in high yields. Our results demonstrate that hydrogen-bonding-driven arylamide oligomers are a class of structurally unique mimic structures. The folded oligomers themselves can be used as synthetic receptors for binding different guest molecules, while incorporation of different segments into one system can produce many desired shapes. In addition, all of the rigid frameworks can be readily functionalized at specific sites. We believe that our results have helped to open the door for some new chemistry in molecular recognition, self-assembly, and other related areas.

Solvophobically-driven oligo(ethylene glycol) helical foldamers. Synthesis, characterization, and complexation with ethane-1,2-diaminium.

Oligo(ethylene glycols) 1a-h, which are incorporated with one to eight 2,3-naphthylene units, respectively, have been synthesized and characterized. The conformational changes of the new oligomers have been investigated in chloroform-acetonitrile binary solvents by the UV-vis, (1)H NMR, and fluorescent spectroscopy. It has been revealed that the naphthalene units in hexamer 1f, heptamer 1g, and octamer 1h are driven by solvophobic interaction to stack in polar solvents. As a result, compact helical conformations are formed that give rise to a cavity similar to that of 18-crown-6. Shorter oligomers 1b-e exhibit weaker folding tendency. (1)H NMR studies reveal that 1f-h are able to complex ammonium or ethane-1,2-diaminium 19, but not secondary ammonium compounds. The association constants of complexes 1f.19, 1g.19, and 1h.19 in acetonitrile are determined to be 3.5(+/-0.4) x 10(3), 1.0(+/-0.12) x 10(4), and 2.5(+/-0.4) x 10(4) M(-1), respectively, with the (1)H NMR titration method. For comparison, hexamer 22, which incorporates six 1,5-naphthylene units, is also prepared. The UV-vis and fluorescent investigations show that 22 is also able to fold in polar solvents, but no helical structure can be produced due to mismatch of the stacking naphthalene units and consequently there is no obvious complexation between 22 with ethane-1,2-diaminium ion. The structures of the longest foldamer 1h and its complex with 19 have been studied with molecular mechanics calculations. This work represents a new approach to building folding conformations from flexible linear molecules.