Laura Baldini

Calixarenes: from biomimetic receptors to multivalent ligands for biomolecular recognition

Calixarenes are versatile platforms for the design and synthesis of molecular receptors and multivalent ligands that are able to mimic or affect specific biological functions. This review illustrates examples spanning the recognition of small peptides and carbohydrates to ion transport through membranes, biomimetic catalysis, DNA condensation and cell transfection, protein binding, sensing and inhibition, and gives perspectives for using calixarene macrocycles in bio-supramolecular chemistry.

Inhibiting angiogenesis and tumorigenesis by a synthetic molecule that blocks binding of both VEGF and PDGF to their receptors

Angiogenesis depends on vascular endothelial growth factor (VEGF) for initiation and platelet-derived growth factor (PDGF) for maintenance of blood vessels. We have designed a targeted library of compounds from which we identified a novel molecule, GFB-204, that binds PDGF and VEGF, blocks binding of PDGF and VEGF to their receptors (200–500 nM) and subsequently inhibits PDGFR and Flk-1 tyrosine phosphorylation and stimulation of the protein kinases Erk1, Erk2 and Akt and the signal transducer and activator of transcription STAT3. GFB-204 is selective for PDGF and VEGF and does not inhibit EGF, IGF-1 and FGF stimulation of Erk1/2, Akt and STAT3. GFB-204 inhibits endothelial cell migration and capillary network formation in vitro. Finally, treatment of mice with GFB-204 suppresses human tumor growth and angiogenesis. Thus, inhibition of VEGF and PDGF receptor binding with a synthetic molecule results in potent inhibition of angiogenesis and tumorigenesis.

Macrocyclic Nonviral Vectors: High Cell Transfection Efficiency and Low Toxicity in a Lower Rim Guanidinium Calix[4]arene

New multivalent cationic lipids, one of them showing high efficiency and low toxicity in cell transfection, have been obtained by attaching guanidinium groups at the lower rim of calix[4]arenes.

Biomimetic macrocyclic receptors for carboxylate anion recognition based on C-linked peptidocalix(4)arenes

Two neutral macrobicyclic anion receptors 4 and 6, containing a calix[4]arene in the cone conformation, two l-alanine units, and a 2,6-diacylpyridine or a phthaloyl bridge, are described. The x-ray crystal structure of the acetone complexes of the pyridine containing macrocycle 6 shows the four amide NH groups to be in close proximity to the chiral pocket delimited by the pyridine and one aromatic nucleus of the calix[4]arene. This conformation is also the most stable in acetone-d6 solution, as proven by one- and two-dimensional NMR spectral measurements. Electrospray ionization–MS and 1H NMR experiments reveal that the two ligands strongly bind carboxylate anions in acetone solution. H-bonding interactions between the carboxylate anions and the amide NH groups, together with π/π stacking, are invoked to explain the efficiency and the selectivity of these anion receptors.

Upper Rim Guanidinocalix[4]arenes as Artificial Phosphodiesterases

Calix[4]arene derivatives, blocked in the cone conformation and functionalized with two to four guanidinium units at the upper rim were synthesized and investigated as catalysts in the cleavage of the RNA model compound 2-hydroxypropyl p-nitrophenyl phosphate. When compared with the behavior of a monofunctional model compound, the catalytic superiority of the calix[4]arene derivatives points to a high level of cooperation between catalytic groups. Combination of acidity measurements with the pH dependence of catalytic rates unequivocally shows that a necessary requisite for effective catalysis is the simultaneous presence, on the same molecular framework, of a neutral guanidine acting as a general base and a protonated guanidine acting as an electrophilic activator. The additional guanidinium (guanidine) group in the diprotonated (monoprotonated) trifunctional calix[4]arene acts as a more or less innocent spectator. This is not the case with the tetrasubstituted calix[4]arene, whose mono-, di-, and triprotonated forms are slightly less effective than the corresponding di- and triguanidinocalix[4]arene derivatives, most likely on account of a steric interference with HPNP caused by overcrowding.

Conformationally Mobile Glucosylthioureidocalix[6]- and Calix[8]arenes: Synthesis, Aggregation and Lectin Binding

Two new glucoclusters 2a and 2b, in which the sugar units are connected to the upper rim of methoxycalix[6]- and calix[8]arene derivatives via thiourea linkages, were synthesised and their aggregation properties in water studied by 1H NMR, atomic force microscopy and dynamic light scattering. Small size aggregates (2–10 nm diameter) are formed by both macrocycles, which become much larger (200–300 nm) in the presence of a phosphate buffer, whereas other anions have no effect. The glycoclusters 2a and 2b interact with plasmid DNA but do not condense it, while in the presence of a glucose-specific lectin such as Concanavalin A (ConA) agglutination occurs. The data obtained give useful insights into the mode of binding of calixarene-based glycoclusters with lectins. The self-aggregation properties in water solution of glucosylthioureidocalix[6]- and [8]arenes were studied by a combination of different techniques. These glycocalixarenes showed a specific interaction with phosphate ion and ConA evidenced by an increase of the aggregate sizes from 3 to 10 nm up to 200–300 nm diameter.

CO2 Capture by Multivalent Amino-Functionalized Calix[4]arenes: Self-Assembly, Absorption, and QCM Detection Studies

The reactivity of CO(2) with polyamino substrates based on calix[4]arenes and on a difunctional, noncyclic model has been studied. All the compounds react with CO(2) in chloroform to form ammonium carbamate salts. However, the number, topology, and conformational features of the amino-functionalized arms present on the multivalent scaffold have a remarkable influence on the reaction efficiency and on the product composition. Tetraaminocalix[4]arenes 1-3 rapidly and efficiently react with 2 equiv of CO(2), yielding highly stable hydrogen-bonded dimers formed by the self-assembly of two bis-ammonium bis-carbamate intramolecular salts. 1,3-Diaminocalix[4]arene 4 absorbs 1 mol of CO(2), affording less stable zwitterionic ammonium carbamates. Gemini compound 5 reacts with CO(2) in a 1:1 stoichiometry, forming hydrogen-bonded dimers of ammonium carbamate derivatives of moderate stability. For upper rim 1,3-diaminocalix[4]arene 6, in addition to the labile intramolecular salt, the presence of a self-assembled polymer was also detected. These systems were fully characterized in solution by (1)H and (13)C NMR spectroscopy, whereas the corresponding gas-solid reactions were further investigated by QCM measurements. Interestingly, the high affinity and reversibility of CO(2) uptake shown by 1,3-diamino calix[4]arene 4 enabled us to attain a promising QCM device for carbon dioxide sensing.

Catalysis of Acyl Group Transfer by a Double-Displacement Mechanism: The Cleavage of Aryl Esters Catalyzed by Calixcrown-Ba2+ Complexes

The scope of the barium salt of p-tert-butylcalix[4]arene-crown-5 as a transacylation catalyst has been defined by evaluating its efficiency in the methanolysis of a series of aryl acetates at 25.0 degrees C in MeCN/MeOH 9:1 (v/v) under slightly basic conditions. In this system a phenolic hydroxyl is the acyl-receiving and -releasing unit in a double-displacement mechanism. The complexed barium ion acts both as a nucleophile carrier and a built-in Lewis acid in providing electrophilic assistance to the ester carbonyl both in the acylation and deacylation step (nucleophilic-electrophilic catalysis). Turnover capability is ensured by the acylated intermediate reacting with the solvent more rapidly than the original ester, but a serious drawback derives from the incursion of back-acylation of the liberated phenol. A gradual shift from rate-determining deacylation (p-nitrophenyl acetate) to rate-determining acylation (phenyl acetate) is observed along the investigated series. It is shown that the scope of the catalyst is restricted to acetate esters whose reactivity lies in the range approximately defined by the phenyl acetate-p-nitrophenyl acetate pair, with a maximum efficiency for p-chlorophenyl acetate. Moreover, the catalyst effectively promotes ester interchange between phenols, showing that its activity is not limited to solvolysis reactions. The very high sensitivity of the rate of acylation of the catalyst to leaving group basicity has been interpreted as due to rate-determining decomposition of the tetrahedral intermediate, which is believed to arise from the presumably low basicity of the metal ion stabilized nucleophile. The turnover frequency was in the range of 3.8 x 10(-4) min(-1) for phenyl acetate to 7.4 x 10(-3) min(-1) for p-nitrophenyl acetate ([ArOAc]0=4.0 mM]). A first attempt to enhance the rate of acylation of the catalyst through intramolecular general acid catalysis is also described.

One-shot preparation of an inherently chiral trifunctional calix[4]arene from an easily available cone-triformylcalix[4]arene

Via selective 1,3-distal intramolecular Cannizzaro disproportionation of an easily available cone-triformylcalix[4]arene, an inherently chiral trifunctional cone-calix[4]arene derivative has been prepared. The presence of three different functional groups (-CH2OH, -CHO and -COOH) at the upper rim of the calixarene scaffold makes this compound a versatile intermediate for the development of multifunctional devices. Interesting chiral discrimination of serine derivatives has been observed, presumably thanks to a multipoint-interaction involving the reversible imine bond formation and the hydrogen bonding of the hydroxyl group of the amino acid side-chain with the upper rim functional groups. Consistently, chiral discrimination was not observed with alanine and valine derivatives, lacking hydrogen bonding groups on the side-chain.

Peptidocalix[4]arene self-assembled nanotubes

Abstract The upper rim, C-linked and Cbz-protected cone calix[4]arene bis- l -alanyl derivative 2 forms self-assembled nanotubes in the solid state through a two-dimensional network of hydrogen bonds between the amide chains of adjacent conformers. The crystal lattice is quite different from that of the analogous N-linked derivative 1 which shows calix[4]arene macrocycles piled through van der Waals interactions.