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PubMed1999

Sulphonic acid derivatives as probes of pore properties of volume-regulated anion channels in endothelial cells.

1. We have used the whole-cell patch-clamp technique to study the effects of 4-sulphonic-calixarenes and some other poly-sulphonic acid agents, such as suramin and basilen blue, on volume-regulated anion channel (VRAC) currents in cultured endothelial cells (CPAE cells). 2. The 4-sulphonic-calixarenes induced a fast inhibition at positive potentials but were ineffective at negative potentials. At small positive potentials, 4-sulphonic-calix[4]arene was a more effective inhibitor than 4-sulphonic-calix[6]arene and -calix[8]arene, which became more effective at more positive potentials. 3. Also suramin and basilen blue induced a voltage dependent current inhibition, reaching a maximum around +40 mV and declining at more positive potentials. 4. The voltage dependence of inhibition was modelled by assuming that these negatively charged molecules bind to a site inside VRAC that senses a fraction delta of the applied electrical field, ranging beween 0.16 to 0.32. 4-Sulphonic-calix[4]arene, suramin and basilen blue bind and occlude VRAC at moderate potentials, but permeate the channel at more positive potentials. 4-Sulphonic-calix[6]arene and -calix[8]arene however do not permeate the channel. From the structural information of the calixarenes, we estimate a lower and upper limit of 11*12 and 17*12 A2 respectively for the cross-sectional area of the pore.

Animals↗
PubMed2000

Interactions at the outside faces of calix

Attractive pi-pi interactions between two of the four outside cavity faces of 1,3-bis-pyridylmethylcalix[4]arene (1) and both faces of 1,4-diiodotetrafluorobenzene (2a) form infinite one-dimensional non-covalent ribbons where the two modules alternate. These ribbons are cross-linked by electron donor-acceptor interactions between picolyl nitrogen atoms of calixarene 1 in one chain and iodine atoms of perfluoroarene 2a in another chain and the two-dimensional supramolecular network 3a is formed. A similar behaviour is also shown by 1,4-dibromotetrafluorobenzene (2b). The halogen bonding and the attractive pi-pi interactions occur in directions which are nearly orthogonal each other. Diiodotetrafluorobenzene, being involved in both these interactions, appears to be a particularly interesting tecton. The ability of electron-poor arenes to elicit the exo-receptor potential of calixarene module by connecting their outside faces through pi-pi interactions may be developed as a new and general binding protocol in calixarene self-assembly processes.

Journal Article↗
PubMed2000

Hydrogen-bonding effects in calix

The synthesis and spectroscopic characterization of self-assembling calix[4]arene based capsules 1a.1a and 1b.1b are described. These compounds feature four urea substituents at the upper rims and four secondary amide fragments at the lower rims that can participate in inter- and intramolecular hydrogen bonding in apolar solution. Communication between the calixarene rims in 1a, b influences the self-assembled cavity's size and shape. Specifically. dimerization results in a perfect cone conformation of the calixarene skeleton in 1a, b and stabilizes a seam of intramolecular amide C=O...H-N hydrogen bonds at the lower rim. This seam is cycloenantiomeric, with either clockwise or counterclockwise arrangements of the head-to-tail amides. Complexation of Na+-cation breaks hydrogen bonds at the lower rim but maintains the capsular assembly. Encapsulation properties of 1a.1a and 1b.1b were studied in nonpolar solvents and their binary mixtures as well as through heterodimerization experiments. The presence of amide groups at the lower rim causes notable differences in the capsule's binding affinities when compared to the corresponding tetraester capsules 1c.1c and 1d.1d. In the monomeric state calixarenes 1a, b are in a pinched cone conformation. The solid state X-ray crystallographic studies with monomeric 1a reveal only two intramolecular C=O...H-N hydrogen bonds between the adjacent amides at the lower rim, and an extensive network of intermolecular hydrogen bonds between urea groups at the upper rim.

Journal Article↗
PubMed2006

[Towards new antibacterial drugs. Interest of para-guanidinoethylcalix[4]arene].

We present here the results concerning the antibacterial properties evaluation of para-guanidinoethylcalix[4]arene, compared with its constitutive monomer, the para-guanidinoethylphenol, and hexamidine (Hexomédine), an antiseptic from the diamidine family widely used in therapeutic, chosen as a reference in this study for its resemblance in terms of functional groups. Antibacterial activities of those three compounds were evaluated by microdilution methods, in Mueller Hinton broth, onto 5 bacterial strains: Escherichia coli ATCC 25922, Pseudomonas aeruginosa ATCC 27853, Staphylococcus aureus ATCC 25923 & ATCC 29213 and Enterococcus faecalis ATCC 29212, according to CA-SFM and CLSI (formerly NCCLS) approved standards. In parallel, the effects of these three compounds on MRC-5 eukaryotic cell viability were evaluated with MTT assay. The results obtained here confirm a lack of activity for the monomer compound (MIC> or =512 mg/l) and a real antibacterial activity for the calixarene, comparable to hexamidine. This activity is expressed, both on Gram+and Gram- bacteria (MIC=4 mg/l for E. coli, 8 mg/l on both S. aureus strains) and at a lesser degree on E. faecalis and P. aeruginosa (MIC=32 mg/l). Similarly, both compounds, monomer and calixarene, slightly induce any modification on MRC-5 cells viability, and this until 168 h of treatment for concentrations reaching 10(-4) mol/L while hexamidine demonstrates a significant and increasing effect during the time of experiment and this for 100 to 1000 times lower concentrations. Thus, this study tends to confirm the significance of the organization of the para-guanidinoethylphenol monomer into its cyclic calixarenic tetramer for the gain of an antibacterial activity, similar to a widely used antiseptic one.

Anti-Bacterial Agents↗
PubMed2006

Control of the optical properties of quantum dots by surface coating with calix[n]arene carboxylic acids.

A new method for the control of the optical properties of quantum dots (QDs) has been developed using calix[n]arene carboxylic acids (1-3) as surface coating agents for QDs. The calixarene coating of CdSe/ZnS QDs was easily performed in tetrahydrofuran at room temperature. Deprotonation of the carboxyl groups of the calixarene derivatives surrounding the QDs resulted in highly fluorescent water-soluble QDs. The emission peak of the calixarene-coated QDs shifted to longer wavelengths depending on the oligomer size of the calix[n]arene derivative used for the surface coating. Although the red shift of the emission peak decreases with the increase in the particle size of QDs, this surface coating method is useful for the preparation of multi-colored water-soluble QDs from a single-colored hydrophobic QD.

Journal Article↗
PubMed2002

Functionalization of the methylene groups of p-tert-butylcalix[4]arene: S-C, N-C, and C-C bond formation.

Reaction of the calixarene derivative 7 with two exocyclic double bonds with carbon-, nitrogen-, oxygen-, or sulfur-containing nucleophiles afforded bis(spirodienone) derivatives substituted at two opposite methylene groups in a trans fashion. LiAlH(4) reduction of the bis(spirodienone) derivatives with two methylenes functionalized by thiomethoxy, diethyl malonate, or anilino substituents yielded trans methylene-substituted calix[4]arenes. Upon standing in solution, the calixarene derivative incorporating SMe groups on the bridges underwent trans right harpoon over left harpoon cis isomerization. An equilibration study performed on this calixarene derivative (tetrachloroethane-d(2), 430 K) indicated that the cis isomer is the form of lower free energy.

Journal Article↗
PubMed2005

Vibrational spectra, co-operative intramolecular hydrogen bonding and conformations of calix[4]arene and thiacalix[4]arene molecules and their para-tert-butyl derivatives.

The IR and Raman spectra and conformations of calix[4]arene, thiacalix[4]arene and their p-tert-butyl derivatives have been analysed within the framework of scaled quantum mechanics (SQM). It is shown that the introduction of four p-tert-Bu groups into the calixarene molecules influences the relative energies of their conformers and the enthalpy of the cooperative intramolecular H-bonding (DeltaH(intra)) almost negligibly. DeltaH(intra), evaluated from Iogansen's rule, amounts to approximately 26-28 kcal mol(-1) for the calixarenes and approximately 20-21 kcal mol(-1) for the thiacalixarenes, which essentially exceeds the enthalpies of non-cooperative H-bonds formed by related phenols. As a result of this strong bonding, bands of stretching, bending and torsion vibrations of an eight-membered cyclic system (OH...)4 arise, e.g., two delta(OH)4 bands are observed in the IR spectra of the most highly symmetric C4 cone conformations of calix[4]arene and thiacalix[4]arene. The "duplication" of the number of OH infrared bands is a good new indicator of cooperativity of intramolecular H-bonding of the calixarenes.

Journal Article↗
PubMed2004

Aqueous solubilization of furosemide by supramolecular complexation with 4-sulphonic calix[n]arenes.

The solubilization of the practically water insoluble drug furosemide by guest:host inclusion complexation with 4-sulphonic calix[n]arenes has been reported. The 4-sulphonic calix[n]arenes are water-soluble phenolic cyclooligomers that form inclusion complexes with neutral molecules. The solubility of furosemide in acidic (pH < 4) aqueous solutions containing increasing concentrations of the calixarenes was determined at 30 degrees C and the concentration of furosemide in solution was determined by HPLC. Results showed that the molecular size of the 4-sulphonic calix[n]arenes and the concentration of the calix[n]arenes significantly influenced the increase in the solubility of furosemide. 4-Sulphonic calix[6]arene improved the solubility of furosemide the most (+/-104%) followed by 4-sulphonic calix[8]arene (+/-84-102%), while 4-sulphonic calix[4]arene increased the solubility of furosemide the least (+/-73-81%). The increase in furosemide solubility afforded by the calixarenes was most probably the result of the incorporation of the non-polar portions of the furosemide molecule into the non-polar cavities of the calixarenes similar to furosemide:cyclodextrin complexes. The driving force for this interaction was the reduction in the non-polar-water interfacial surface area when the furosemide (guest) molecules were inserted into the 4-sulphonic calix[n]arenes (host).

Benzenesulfonates↗
PubMed2001

Molecular motions within self-assembled dimeric capsules with tetraethylammonium cations as guest.

Hydrogen-bonded, dimeric capsules of calix[4]arenes substituted at the wide rim by four urea functions show unprecedented dynamic features when a tetraethylammonium cation is included as a guest. The seam of hydrogen bonds C=O...(HN)2C=O in the equatorial region which holds the two calixarene counterparts together changes its directionality fast (at 25 degrees C), while the dimer itself is kinetically stable on the NMR time scale. An energy barrier of deltaG++ = 49.9 kJmol(-1) (Tc 276 K) was estimated for this reorientation from variable-temperature (VT) NMR measurements. Lowering the temperature to about -50 degrees C restricts also the rotation of the encapsulated tetraethylammonium cation around a pseudo C2-symmetry axis in the equatorial plane of the capsule, while its rotation around the C4 axis is still fast. As a result of this restriction two ethyl groups of the tetraethylammonium cation point towards the "poles" of the capsule, while the other pair lies in the "equator" region. Variable-temperature 1H NMR experiments led to a barrier of deltaG++=54.8kJ mol(-1) (Tc 306 K) for the exchange of these different ethyl groups. Studies with the ternary complex formed by a C2v-symmetrical tetraurea proved that both processes, reorientation of the hydrogen bonds and rotation of the guest, take place independently. Molecular dynamics simulations suggest that the capsule is strongly expanded by the larger tetraethylammonium cation in comparison with benzene as guest. Thus, on average only one N-H...O hydrogen bond is formed per urea function and the interaction energy between the two tetraurea calixarenes is less favorable by about 15 kcal mol(-1). This is overcompensated by an energy gain of about 36 kcal mol(-1) due to cation-pi interactions. These results provide a rationale to understand the high stability of the complex inspite of the mobility of the hydrogen-bonded belt.

Calixarenes↗
PubMed2005

High operationally stable sol-gel diglycidyloxycalix[4]arene fiber for solid-phase microextraction of propranolol in human urine.

A simple, sensitive, and accurate method for the determination of propranolol in human urine has been developed based on solid-phase microextraction (SPME) followed by GC-flame ionization detection (FID). The sol-gel 5,11,17,23-tetra-tert-butyl-25,27-dihydroxy-26,28-diglycidyloxycalix[4]arene/hydroxy-terminated silicone oil (diglycidyloxy-C[4]/OH-TSO) fiber was prepared to accommodate to the harsh extraction conditions. It possesses excellent alkali-proof ability and retains its extraction characteristics intact even after treatment with highly alkaline (4 mol/L) NaOH solution. Direct chemical bonding of the coating to the fiber surface provides it with excellent solvent resistance and the introduction of calixarene enhances its thermal stability. The newly developed sol-gel calixarene coating was effectively used for the extraction of propranolol in human urine. No interference with the determination of propranolol was observed from the urine components. Standard curves were linear in the range 50-5000 microg/L for headspace-SPME (HS-SPME) and 25-25000 microg/L for direct-SPME (Dir-SPME) with correlation coefficients better than 0.9999. The detection limit was 0.275 microg/L for HS-SPME and 0.193 microg/L for Dir-SPME. The method was validated using standard addition methodology and recovery values were between 91.4 and 117% for both the sampling modes with the RSDs less than 6% at different concentration levels in the linear ranges. The results obtained by both the sampling modes were feasible, and no significant differences between them regarding accuracy, precision, and detection limits were seen.

Calixarenes↗
PubMed2004

Reorientational dynamics of p-sulfonatocalix[4]arene and of its La(III) complex in water.

The reorientational dynamics of p-sulfonatocalix[4]arene and of its La(III) complex in deuterated water were studied by 1H NMR longitudinal relaxation rates. It is shown that the relaxation is purely dipolar in the non-extreme narrowing regime. The distance between the geminal protons could be determined from the NMR data, giving good agreement with the values generally used in correlation time calculations. The correlation times show an Arrhenius behaviour in good agreement with previously reported data from 13C measurements for a similar uncomplexed calixarene. The Arrhenius energies of activation are identical for the uncomplexed and the complexed calixarenes, suggesting a reorientational motion strongly dependent on the structure of the water cage around the complex. This is also in agreement with a complexation of the La(III) cation in the second sphere of solvation of the sulfonate groups, as shown by molecular dynamics simulations.

Calixarenes↗
PubMed2005

Selective transport of ions and molecules across layer-by-layer assembled membranes of polyelectrolytes, p-sulfonato-calix[n]arenes and Prussian Blue-type complex salts.

Our recent studies in the field of ultrathin membranes prepared upon layer-by-layer assembly of various polyionic compounds such as polyelectrolytes, calixarenes and polyelectrolytes, and metal hexacyanoferrate salts such as Prussian Blue are reviewed. It is demonstrated that polyelectrolyte multilayers can be used (a) as nanofiltration and reverse osmosis membranes suitable for water softening and seawater desalination and (b) as molecular sieves and ion sieves for size-selective separation of neutral and charged aromatic compounds. Furthermore, hybrid membranes of p-sulfonato-calixarenes and cationic polyelectrolytes showing specific host-guest interactions with permeating ions are described. The membranes exhibit high selectivities for distinct metal ions. Finally, it is demonstrated that purely inorganic membranes of Prussian Blue (PB) and analogues can be prepared upon multiple sequential adsorption of transition metal cations and hexacyanoferrate anions. Due to the porous lattice of PB, the membranes are useful as ion filters able to separate cesium from sodium ions, for example.

Calixarenes↗
PubMed2006

Supramolecular assemblies with calix[6]arenes and copper ions: from dinuclear to trinuclear linear arrangements of hydroxo-Cu(II) complexes.

Complexation of copper(II) by calix[6]arene-based ligands bearing either two or three N-benzylimidazole coordinating arms under basic conditions has been studied. Whereas the tris(imidazole) derivative stabilizes dicationic 5-coordinate aqua complexes in a mononuclear state with an intracavity bound guest, in the presence of hydroxide ions, the latter undergo dimerization. An X-ray structure revealed decoordination of one imidazole arm and formation of a bis(hydroxo) bridged Cu(II) core with a square-planar geometry for both metal centers sandwiched by two empty calixarene cavities. Upon methanolysis, the dinuclear complex underwent an unexpected rearrangement leading to the clean formation of a trinuclear complex. X-ray diffraction analyses of this novel species revealed a trinuclear core constructed around a central Cu(II) ion that is doubly bridged through either methoxide or hydroxide anions to two Cu(II) ions hold by two calixarene units. The same complex could be directly synthesized by reacting the ligand with copper(II) perchlorate in a 2:3 ratio in the presence of base. In solution, the tetrahydroxo Cu(3) complex was characterized by UV-vis and (1)H NMR spectroscopies and displayed an electron paramagnetic resonance (EPR) signal only below 100 K that accounts for a S = 1/2 fundamental state. Formation of the same di- and trinuclear species was observed with a calix[6]arene-based bis(imidazole) ligand, which demonstrates the generality of the reaction schemes. All these results emphasize the versatility of the calix[6]arene scaffold for the stabilization of metal complexes with various nuclearities.

Calixarenes↗
PubMed2003

Self-sorting: the exception or the rule?

In this paper, we pose the question of whether self-sorting in designed systems is exceptional behavior or whether it is likely to become a more general phenomenon governing molecular recognition and self-assembly. To address this question we prepared a mixture comprising two of Davis' self-assembled ionophores, Rebek's tennis ball and calixarene tetraurea capsule, Meijer's ureidopyrimidinone, Reinhoudt's calixarene bis(rosette), and two molecular clips in CDCl(3) solution and observed the behavior of this ensemble by (1)H NMR. As hypothesized, high-fidelity self-sorting behavior was observed. The influence of several key variables-temperature, concentration, equilibrium constants, and the presence of competitors-on the fidelity of self-sorting is described. These results show that self-sorting is neither the exception nor the rule. They suggest, however, that the subset of known molecular aggregates that exceed the criteria required for thermodynamic self-sorting is larger than previously appreciated and potentially quite broad.

Binding, Competitive↗
PubMed2004

Synthesis of a calix[4]arene derivative for isolation of a stable cation radical salt for use as a colorimetric sensor of nitric oxide.

We have designed and synthesized a modified calixarene derivative (1) that allows, for the first time, the isolation of a stable cation radical salt that binds a single molecule of nitric oxide deep within its cavity with remarkable efficiency (KNO >108 M-1), as demonstrated by isolation of a crystalline complex [1, NO]+ and its characterization by X-ray crystallography as well as by optical spectroscopy. Furthermore, the ready accessibility of the calixarene cation radical will allow the exploration of its use for developing efficient sensing devices for nitric oxide based on the accompanied color changes.

Biosensing Techniques↗
PubMed2005

Nanomolar protein sensing with embedded receptor molecules.

A new concept of protein sensing at the air-water interface is introduced, based on amphiphilic receptor molecules embedded in a lipid monolayer. The process begins with incorporation of a small amount (0.13 equiv) of one or two different calix[4]arenes, adorned with charged functional groups at their upper rims, into a stearic acid monolayer. These doped monolayers are subsequently shown to attract peptides and proteins from the aqueous subphase. Depending on the host structure, the monolayers can be made selective for basic or acidic proteins. A working model is proposed, which explains the large observed p/A shifts with reincorporation of excess receptor molecules into the lipid monolayer after complex formation with the oppositely charged protein. This requires a self-assembly of multiple calixarene units over the protein surface, which bind the protein in a cooperative fashion. Oppositely charged calixarene derivatives do not form molecular capsules inside the monolayer, but rather remain separate inside the lipid layer, adopting a perpendicular orientation. They combine their hydrogen bond donor and acceptor capacities, and thus markedly enhance the sensitivity of the sensor system toward proteins, pushing the detection limits to 10 pM concentrations. The response pattern obtained from various receptor units inside the monolayer toward the same protein creates a fingerprint for this protein, which can hence be selectively detected at nanomolar concentrations (pattern recognition).

Amino Acids↗
PubMed2005

Biomimetic zinc funnel complexes based on calix[6]N3ArO ligands: an acid-base switch for guest binding.

The coordination chemistry of Zn in an N(3)ArOH environment has been explored. The ligands are based on calix[6]arenes that present two imidazole arms and an amino phenol moiety at the narrow rim. Three different types of complexes have been characterized. One is dicationic with Zn(2+) coordinated to the three nitrogen atoms and to the oxygen of the phenol group of the calix[6]ligand. This complex is very sensitive to exogenous coordinating molecules and exists as a 5-coordinate species due to the endo-complexation of a guest. The second species is a monocationic complex for which the phenol group has been deprotonated. The resulting N(3)ArOZn complex can also bind a guest ligand albeit with a lower affinity than the dicationic complex. The third species is neutral. It can be obtained upon reaction with a base to yield a hydroxo complex or with an anion such as a chloride that coordinates the metal center from the outside of the calixarene cavity. The simultaneous binding of two anionic donors decreases the Zn Lewis acidity, allowing an impressive conformational reorganization of the system. One imidazole arm is released by the metal center. The other one undergoes self-inclusion into the pi-basic calixarene cavity because the low affinity of the metal center for neutral ligand does not allow the endo-coordination of an exogenous guest. Hence, the calix[6]N(3)ArOH-based Zn complexes act as an acid-base switch for guest binding. Several aspects of this system appear reminiscent of Zn-peptidases of the astacin and serralisin families.

Acid-Base Equilibrium↗
PubMed2001

Synthesis and conformational property of tannin-like p-tert-butylcalix[4]arene 1,3-diesters stabilized by intramolecular hydrogen bonds.

Tannin-like p-tert-butylcalix[4]arene 1,3-digallate was synthesized, and its conformational property was investigated by dynamic (1)H NMR and X-ray crystallography. It was found that the 3-OH (or 5-OH) group of the galloyl group in p-tert-butylcalix[4]arene 1,3-digallate is placed at the position where an unusual nonbonded close contact is observed between the OH group and the aromatic ring of the galloyl group facing each other. The calixarene 1,3-diesters of various hydroxybenzoic acids were also prepared, and the conformational properties of those calixarenes were compared with that of p-tert-butylcalix[4]arene 1,3-digallate. A significant contribution of the 3- and 5-OH groups in pendant groups toward the close contact was found. It was suggested that the conformation of p-tert-butylcalix[4]arene 1,3-digallate was stabilized by intramolecular hydrogen bonds including OH.O and OH-pi interactions.

Calixarenes↗