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At least 253 records · Page 14Linked to original sources
PubMed2003

Thiourea-linked upper rim calix[4]arene neoglycoconjugates: synthesis, conformations and binding properties.

The thiourea group has been exploited to link two or four carbohydrate units at the upper rim of tetrapropoxycalix[4]arene derivatives in the cone conformation. Two synthetic methodologies were used, the first one consisting of the condensation of di- and tetraminocalix[4]arenes with the isothiocyanate of monosaccharides in dry CH2Cl2 at room temperature and the second one exploiting the condensation of an aminolactoside with a calixarene isothiocyanate. The first method allows the glycoconjugates to be obtained in 75-80% overall yields. The disfunctionalized derivatives exist in a closed flattened cone conformation in CDCl3 and CD3OD due to the formation of intramolecular hydrogen bonds involving the thiourea groups which are broken in DMSO-d6 to give an open flattened cone conformation. The thiourea groups act not only as linkers but also as binding units for anionic substrates as evidenced by solution 1H NMR and ESI-MS experiments. Turbidimetric analysis indicates that the tetraglucoside and tetragalactoside clusters give specific interactions with Concanavalin A (Con A) and peanut lectin (PNA), respectively. Both features show that the neoglycoconjugates could also be used as site specific molecular delivery systems.

Calixarenes↗
PubMed2003

Compensation of steric demand by cation-pi interactions, cobaltocenium cations as guests in tetraurea calix[4]arene dimers.

The affinities of ferrocene (2) and the cobaltocenium cation (3+), which have roughly the same size and differ in their charge, towards the inner cavity of the dimeric capsule formed by tetraurea calix[4]arene (1) were studied in C2D4Cl2 solutions. While 3+, which occupies more than 75% of the internal volume of the dimer, is readily encapsulated this is not the case for 2. This is probably due to cation-pi interactions, which operate only between 3+ and the aromatic rings of the calix[4]arene dimer. We found that the affinity of the cobaltocenium cation is higher than that of the tropylium cation (4+) and is only 2-3 times less than that of the tetraethylammoniun cation (5+). From the variable temperature 1H NMR spectra of this capsule, the free energy of activation at 298 K (deltaGdouble dagger(298K)) for the reorientation of the hydrogen bonded belt between the two parts of the dimer could be determined by total line shape analysis for the aromatic protons of the calixarene. The value of 14.3 +/- 0.2 kcal mol(-1) for the dimeric capsules of 3+ PF6- is very similar to the free activation energy found for dimeric capsules of 1 with 4+ PF6- and 5+ PF6- in C2D4Cl2. It becomes significantly lower, if PF6- is replaced by BF4-. We also found that ten times more DMSO is needed to disrupt the capsule 1 x 3+ x 1 than the corresponding 1 x 1 dimer containing benzene as guest. This demonstrates again the importance of the cation-pi interactions for the stability of such hydrogen-bonded dimeric capsules.

Calixarenes↗
PubMed2004

Recognition of alpha-amino acid derivatives by N,N'-dibenzylated S,S-(+)-tetrandrine.

Complexation of free and N-acetylated alpha-amino acid anions (Gly, Ala, Phe) and some structurally related guests by a dicationic cyclophane-type N,N'-dibenzylated chiral derivative of a bisisoquinoline macrocyclic alkaloid S,S-(+)-tetrandrine (DBT) has been studied by (1)H-NMR titrations in D(2)O. In contrast to other macrocyclic hosts like cyclodextrins and calixarenes, DBT shows highest affinity and large enantioselectivity (K(S)/K(R) >/=10) toward smaller N-acetylalanine and binds larger phenylalanine derivatives more weakly and non-selectively. With 1,2,3,4-tetrahydroisoquinoline-3-carboxylate, a rigid analog of phenylalanine, binding again becomes enantioselective with K(S)/K(R)= 3.8. The binding specificity of DBT is rationalized on the basis of molecular mechanics calculations.

Acetylation↗
PubMed2004

Synthesis and binding properties of calix[4]arenes with [2 + 2'] mixed ligating functional groups.

A series of mixed [2 + 2'] p-tert-butylcalix[4]arene have been synthesised by selective 1,3-dialkylation of phenolic groups using various alkylating agents such as benzyl bromide, methyl iodide, ethyl bromoacetate, and 2-methoxyethyl tosylate. The extraction and complexation properties of the synthesized calixarenes towards alkali and alkaline earth metal cations have been investigated in acetonitrile by means of UV spectrophotometry and 1H NMR spectroscopy. The results show the formation of ML and/or ML2 species depending on the ligand and the cation. The enthalpies and entropies of complexation of alkali metal cations by a tetraglycol, diglycol-dibenzyl and diglycol-diester derivatives have been obtained from calorimetric measurements. The results revealed that the formation of ML species is controlled by enthalpy while the formation of ML2 from ML is entropy driven.

Calixarenes↗
PubMed2004

Calix[4]arene methylenebisphosphonic acids as calf intestine alkaline phosphatase inhibitors.

Calix[4]arenes bearing one or two methylenebisphosphonic acid fragments were prepared via addition of diethylphosphite to the parent calix[4]arene aldehydes. The resulting compounds displayed stronger inhibition of calf intestine alkaline phosphatase than simple methylenebisphosphonic or 4-hydroxyphenyl methylenebisphosphonic acids. The action of these phosphorylated calix[4]arenes is concordant with partial mixed-type inhibition. The inhibition constants Ki and Ki' for the calix[4]arene bis(methylenebisphosphonic) acid in Tris-HCl buffer at pH 9 are 0.38 microM and 2.8 microM respectively. The replacement of the phosphoric acid moieties on the macrocycle with diethylphosphonates results in a sharp decrease of its inhibitory action. Preorganizing phosphonic acid fragments using a calixarene platform therefore provides a promising approach for the design of efficient alkaline phosphatase inhibitors.

Alkaline Phosphatase↗
PubMed2005

Playing with podands based on cone-shaped cavities. How can a cavity influence the properties of an appended metal centre?

The potential of molecules that combine the properties of a conical cavity with those of a covalently-linked transition-metal centre is highlighted through the assessment of cyclodextrin- and calixarene-derived podands ("cavitand" ligands) in coordination chemistry and catalysis. Metallocavitands with coordination sites directed towards the interior of the generic cavity provide interesting systems for studying host-guest complexation processes, their enhanced strength of metal-ion binding allowing for regioselective catalysis in a confined environment, and stabilisation of coordination complexes of unusual forms. Where cavitands have exo-oriented podand arms, the intrinsic dynamics of the cavity can dramatically modify metal chelation behaviour and the catalytic properties of the complexes. Such functionalised cavities are also useful as metal-ion transporters.

Calixarenes↗
PubMed2006

Orientational preferences of aromatic guests in dimeric capsules of tetraurea calix[4]arenes--MD and NMR studies.

Molecular dynamics (MD) simulations have been performed for complexes of a dimeric capsule of a tetraurea calixarene with a series of twelve aromatic guests. A distinct orientational preference and a restriction of the internal mobility was found which depend on the size and electronic properties of the guests. The results are in agreement with the CIS values obtained from (1)H NMR spectroscopic measurements and with complexation selectivities obtained by competition experiments.

Benzene Derivatives↗
PubMed2006

Fluoride anion binding by cyclic boronic esters: influence of backbone chelate on receptor integrity.

A systematic investigation of fluoride anion binding properties as a function of chelate backbone has been carried out for ferrocene functionalised boronic esters of the types FcB(OR)2 and fc[B(OR)2]2 [Fc = ferrocenyl = (eta5-C5H5)Fe(eta5-C5H4); fc = ferrocendiyl = Fe(eta5-C5H4)2]. Cyclic boronic esters containing a saturated five- or six-membered chelate ring are readily synthesized from ferrocene, and selectively bind fluoride via Lewis acid/base chemistry in chloroform solution. The resulting complexes are characterized by relatively weak fluoride binding (e.g.K = 35.8 +/- 9.8 M(-1) for FcBO2C2H2Ph2-S,S), and by cathodic shifts in the ferrocene oxidation potential that form the basis for electrochemical or colorimetric fluoride detection. The fluoride selectivity of these systems is attributed to relatively weak Lewis acidity, resulting in weak F- binding, and essentially no binding of potentially competitive anions. By contrast, more elaborate Lewis acid frameworks based on calix[4]arene (calixH4), such as (FcB)2calix or fcB2calix, do not survive intact exposure to standard fluoride sources (e.g. [nBu4N]F.xH2O solutions in chloroform or acetonitrile). Instead B-O bond cleavage occurs yielding the parent calixarene; the differences between alkoxo- and aryloxo-functionalised derivatives can be rationalised, at least in part, by consideration of the differences in electron donating capabilities of RO- (R = alkyl, aryl).

Boronic Acids↗
PubMed2006

Sulfonatocalixarenes: molecular capsule and 'Russian doll' arrays to structures mimicking viral geometry.

p-Sulfonatocalix[4,5,6,8]arenes are versatile building blocks, able to assemble into 'molecular capsule' arrays based on two calixarenes, as well as a variety of other structural motifs, with the extended structures dominated by the formation of bilayers. For p-sulfonatocalix[4]arene, assembly into nanometre scale spheroids (of either icosahedral or cuboctahedral geometries) as well as nanotubules (all of which take on structural features akin to those of viruses) is possible, depending on the guest molecules and lanthanides present in solution.

Calixarenes↗
PubMed1995

Synthesis and assembly of self-complementary calix[4]arenes.

A calix[4]arene was designed to reversibly dimerize and form an egg-shaped enclosure. Adhesive interactions in the assembly were provided by four self-associating ureas, which form a cyclic array containing 16 hydrogen bonds. The synthesis was completed in four steps from the previously described O,O',O",O"'-tetrabenzylcalix[4]arene. Evidence for dimerization of the calixarene tetraurea was provided by H NMR, mass spectrometry, and the observation of encapsulated molecules. The resulting cavity was of sufficient size to capture guests such as ethyl benzene and p-xylene.

Benzene↗
PubMed2006

Design of nonpeptidic topomimetics of antiangiogenic proteins with antitumor activities.

The inhibition of angiogenesis is a promising avenue for cancer treatment. Although some angiostatic compounds are in the process of development and testing, these often prove ineffective in vivo or have unwanted side effects. We have designed, synthesized, and evaluated a small library of nonpeptidic, calixarene-based protein surface topomimetics that display chemical substituents to approximate the molecular dimensions and amphipathic features (hydrophobic and positively charged residues) of the antiangiogenic peptide anginex, which, like many antiangiogenic proteins, consists primarily of an antiparallel beta-sheet structure as the functional unit. Two of the topomimetics (0118 and 1097) were potent angiogenesis inhibitors in vitro, as determined by endothelial cell proliferation, migration, and chick embryo chorioallantoic membrane assays. Moreover, both compounds were highly effective at inhibiting tumor angiogenesis and growth in two mouse models (MA148 human ovarian carcinoma and B16 murine melanoma). Our results demonstrate the feasibility of designing nonpeptidic protein surface topomimetics as novel pharmaceutical agents for clinical intervention against cancer through angiostatic or other mechanisms.

Angiogenesis Inhibitors↗
PubMed2006

Unusual conformations of 1,3-dialkoxythiacalix[4]arenes in the solid state.

The structures of three syn-1,3-dialkoxythiacalix[4]arenes with unusual conformations in the solid state are reported. The pinched cone conformation of syn-2(2),4(2)-dihydroxy-1(2),3(2)-bis(prop-2-enyloxy)thiacalix[4]arene, C30H24O4S4, (3a), is stabilized by two intramolecular hydrogen bonds, remarkably formed from both OH groups to the same ether O atom. In syn-2(2),4(2)-dihydroxy-1(5),2(5),3(5),4(5)-tetranitro-1(2),3(2)-bis(prop-2-enyloxy)thiacalix[4]arene acetone disolvate, C30H20N4O12S4.2C3H6O, (3b1), the molecule is found in the 1,3-alternate conformation. The crystallographic C2 symmetry is due to a twofold rotation axis running through the centre of the calixarene ring. The hydroxy groups cannot form intramolecular hydrogen bonds as in (3a) and both are bonded to an acetone solvent molecule. The molecule of the pseudo-polymorph of (3b1) in which the same compound crystallized without any solvent, viz. (3b2), is located on a crystallographic mirror plane. Only one of the two hydroxy groups forms a hydrogen bond, and this is with a nitro group of a neighbouring molecule as acceptor. Molecular mechanics calculations for syn-1,3-diethers suggest a preference of the 1,3-alternate over the usual cone conformation for thiacalix[4]arene versus calix[4]arene and for para-nitro versus para-H derivatives.

Calixarenes↗
PubMed1994

Both CFTR and outwardly rectifying chloride channels contribute to cAMP-stimulated whole cell chloride currents.

From whole cell patch-clamp recordings at 35 degrees C utilizing either nystatin perforation or conventional methods with 5 mM MgATP in the pipette solution, it was demonstrated that both cystic fibrosis transmembrane conductance regulator (CFTR) chloride (Cl-) channels and outwardly rectifying Cl- channels (ORCC) contribute to adenosine 3',5'-cyclic monophosphate (cAMP)-activated whole cell Cl- currents in cultured human airway epithelial cells. These results were similar whether recordings were performed on two normal human cell lines or on two cystic fibrosis (CF) cell lines stably complemented with wild-type CF gene. These results were obtained by exploiting dissimilar biophysical properties of CFTR and ORCC currents such as the degree of rectification of the current-voltage relationship, the difference in sensitivity to Cl- channel-blocking drugs such as 4,4'-diisothiocyanostilbene-2,2'-disulfonic acid (DIDS), calixarenes, and diphenylamine carboxylic acid (DPC), and the opposing Cl- relative to I- permeabilities of the two channels. In normal cells or complemented CF cells, 8-(4-chlorophenylthio)adenosine 3',5'-cyclic monophosphate stimulated outwardly rectifying whole cell Cl- currents. Addition of DIDS in the presence of cAMP inhibited the outwardly rectifying portion of the cAMP-activated Cl- current. The remaining cAMP-activated, DIDS-insensitive, linear CFTR Cl- current was inhibited completely by DPC. Additional results showed that not only do ORCC and CFTR Cl- channels contribute to cAMP-activated Cl- currents in airway epithelial cells where wild-type CFTR is expressed but that both channels fail to respond to cAMP in delta F508-CFTR-containing CF airway cells. We conclude that CFTR not only functions as a cAMP-regulated Cl- channel in airway epithelial cells but also controls the regulation of ORCC.

4,4'-Diisothiocyanostilbene-2,2'-Disulfonic Acid↗
PubMed2000

Optical resolution of phosphorus containing calix[4]arenes by additive RP HPLC method.

The RP HPLC method (LiChrosorb RP18 or Separon SGX C18 column, acetonitrile-water 86:14 v/v mobile phase) was applied for enantiomeric separation of topologically chiral 5-bromo-25,26-bis(diethoxyphosphoryl)calix[4]arene 9 or 25-ethyl-26-dihydroxyphosphoryl-calix[4]arene 5. Separation of calixarene 9 was achieved by the addition of D-(-)-tartaric acid which formed hydrogen bonded diastereomeric associates with 9 to the mobile phase. Calixarenephosphoric acid 5 was transformed into diastereomeric salt 6 by addition of L-(-)-alpha-phenylethylamine before RP HPLC separation.

Calixarenes↗
PubMed2004

[Characteristics and applications of amide bridged calix [ 4 ] arene solid-phase microextraction fiber].

A novel solid-phase microextraction (SPME) fiber coated with 25,27-dihydroxy-26, 28-(1', 10'-dioxa-4', 7'-diaza-3', 8'-dioxooctylene ) -p-tert-butylcalix [ 4 ] arene/hydroxy-terminated silicone oil (amide bridged -C[4]/OH-TSO) was prepared with sol-gel technology and applied to the analysis of aliphatic amines and aromatic amines without derivatization. It demonstrated high thermal stability of up to 380 degrees C and stability in organic and inorganic solvents, a prolonged lifetime of over 200 times for headspace SPME, great fiber-to-fiber reproducibility with a relative standard deviation (RSD) of no more than 6.3% for aromatic amines, etc. Due to the introduction of polar amide bridge in calixarene molecules, polarity of the coating increases and thus the fiber shows better extraction efficiency in most of the investigated aliphatic amines and aromatic amines compared to the extraction efficiency of commercial fibers, indicating specific selectivity and sensitivity to amines. The detection limits (LODs) were determined to be 0.19 - 39.51 microg/L, the linear ranges were 3 orders of magnitude with a precision better than 5.1% for aliphatic amines, and they were 1.21 - 40.73 ng/L, 4 - 6 orders of magnitude with a precision better than 6.0% for aromatic amines. The application of this SPME technique coupled with gas chromatography has been shown to be feasible for the rapid determination of fish freshness.

Amines↗
PubMed2000

Biomimetic copper(I)--CO complexes: a structural and dynamic study of a cali.

Four novel calix[6]arene-based cuprous complexes are described. They present a biomimetic tris(imidazole) coordination core associated with a hydrophobic cavity that wraps the apical binding site. Each differs from the other by the methyl or ethyl substituents present on the phenoxyl groups (OR1) and on the imidazole arms (NR2) of the calix[6]arene structure. In solution, stable CO complexes were obtained. We have investigated their geometrical and dynamic properties with respect to the steric demand. IR and NMR studies revealed that, in solution, these complexes adopted two distinct conformations. The preferred conformation was dictated only by the size of the OR1 group. When R1 was an ethyl group, the complex preferentially adopted a flattened C3-symmetrical structure. The corresponding helical enantiomers were in conformational equilibrium, which, however, was slow on the 1H NMR time scale at -80 degrees C. When R1 was a methyl group, the low-temperature NMR spectra revealed the partial inclusion of one tBu group. The complex wobbled between three dissymmetric but equivalent conformations. Hence, small differences in the steric demand of the calixarene's skeleton changed the geometry and dynamics of the system. Indeed, this supramolecular control was promoted by the strong conformational coupling between the metal center and the host structure. Interestingly, this was not only the result of a covalent preorganization, but also stemmed from weak interactions within the hydrophobic pocket. The vibrational spectra of the bound CO were revealed to be a sensitive gauge of this supramolecular behavior, similar to copper proteins in which allosteric effects are common.

Carbon Monoxide↗