A Water Molecule Triggers Guest Exchange at a Mono-Zinc Centre Confined in a Biomimetic Calixarene Pocket: A Model for Understanding Ligand Stability in Zn Proteins.

Invited for the cover of this issue are Kristin Bartik, Olivia Reinaud and co-workers at the Université libre de Bruxelles and Université Paris Descartes. The image depicts a Zn protein and highlights the role that a single water molecule can play in catalysing ligand exchange. Read the full text of the article at 10.

Closing a Calix[6]arene-Based Funnel Zn Complex at Its Large Rim Entrance: Consequences on Metal Ion Affinity and Host-Guest Properties.

A molecular capsule based on a calix[6]arene core closed at the small rim by a three-point coordinated metal ion and at the large rim by a three-point covalent capping is described. It is derived from a trisimidazole funnel complex capped by a trenamide unit that prevents in/out exchange of guest molecules through the large rim. A detailed comparative study with three different calixarenes provides a unique opportunity for (i) comparing the binding ability of two different coordination sites in well three-dimensional (3D)-structured macrocyclic receptors and (ii) evaluating the impact of a covalent closing of one rim of a funnel receptor while the other rim is closed by weaker coordination bonds.

A Water Molecule Triggers Guest Exchange at a Mono-Zinc Centre Confined in a Biomimetic Calixarene Pocket: a Model for Understanding Ligand Stability in Zn Proteins.

In this study, the ligand exchange mechanism at a biomimetic Zn centre, embedded in a pocket mimicking the possible constrains induced by a proteic structure, is explored. The residence time of different guest ligands (dimethylformamide, acetonitrile and ethanol) inside the cavity of a calix[6]arene-based tris(imidazole) tetrahedral zinc complex was probed using 1D EXchange SpectroscopY NMR experiments. A strong dependence of residence time on water content was observed with no exchange occurring under anhydrous conditions, even in the presence of a large excess of guest ligand.

Antiproliferative activity of a new xanthone derivative from leaves of Engl.

A new xanthone, mboudiexanthone (), together with five known compounds, euxanthone (), isogarcinol (), garcinol (), betulinic acid () and zeorin () were isolated from the leaves of Engl. The structures were determined by 1D and 2D NMR techniques and X-ray diffraction for . The antiproliferative properties of isolated compounds were evaluated against the human breast cancer cell line MCF-7.

Tuning the Fluorescence Through Reorientation of the Axle in Calix[6]arene-Based Pseudorotaxanes.

This work describes a calix[6]arene-based wheel that binds, in non-polar media, a stilbazolium salt to yield a mixture of pseudorotaxane orientational isomers. The isomer's abundance ratio evolves with time and can be reversibly tuned by adjusting the temperature. The spectroscopic properties, and notably the emission spectrum, of the bound guest depend on its orientation inside the non-palindromic wheel, suggesting such a system as a switch with spectroscopic readout.

H NMR study of the interaction of trans-resveratrol with soybean phosphatidylcholine liposomes.

Resveratrol (RSV) is a well-known natural derivative with a wide range of biological and pharmacological activities. Despite of these demonstrated properties, it exhibits low both aqueous solubility and chemical stability and therefore low bioavailability. Consequently, the major concern of the technological research is to exploit delivery systems able to overcome bioavailability problems.

Synthesis and Binding Properties of a Tren-Capped Hexahomotrioxacalix[3]arene.

The straightforward synthesis of a new hexahomotrioxacalix[3]arene-based ligand capped by a tren subunit was developed and the binding properties of the corresponding zinc complex were explored by NMR spectroscopy. Similarly to the closely related calix[6]tren-based systems, the homooxacalixarene core ensures the mononuclearity of the zinc complex and the metal center displays a labile coordination site for exogenous guests. However, very different host-guest properties were observed: i) in CDCl , the zinc complex strongly binds a water molecule and is reluctant to recognize other neutral guests, ii) in CD CN, the exo-coordination of anions prevails.

Selective EPR Detection of Primary Amines in Water with a Calix[6]azacryptand-Based Copper(II) Funnel Complex.

A water-soluble calix[6]arene-based azacryptand was synthesized. The corresponding tren [tris(2-aminoethyl)amine] cap grafted at the small rim coordinates strongly a copper(II) ion over a wide range of pH. The host-guest properties of the complex were explored by EPR spectroscopy.

Photo-CIDNP Reveals Different Protonation Sites Depending on the Primary Step of the Photoinduced Electron-/Proton-Transfer Process with Ru(II) Polyazaaromatic Complexes.

The excited-state quenching of [Ru(TAP)(HAT)] (TAP = 1,4,5,8-tetraazaphenanthrene, HAT= 1,4,5,8,9,12-hexaazatriphenylene) by hydroquinone (HQ), N-acetyl-tyrosine (N-Ac-Tyr) or guanosine-5'-monophosphate (GMP) was investigated at various pH values. The quenching occurs via electron/proton transfer, as evidenced by transient absorption spectroscopy and confirmed by H photochemically induced dynamic nuclear polarization (photo-CIDNP). Reductive quenching also occurs in strongly acidic solution despite a much shorter lifetime of the protonated excited-state complex.

NMR Study of the Reductive Decomposition of [BMIm][NTf ] at Gold Electrodes and Indirect Electrochemical Conversion of CO.

Potential controlled electrolyses of [BMIm][NTf ] ionic liquid were performed at a gold cathode under nitrogen atmosphere. The structures of the major conversion products of the BMIm cation were elucidated on the basis of 1D and 2D nuclear magnetic resonance (NMR) analyses and gas chromatography (GC) analysis of the volatile compounds. Recombination of the imidazol-2-yl radicals, generated at the electrode by single electron transfer, leads to neutral diastereomeric dimers in equal proportions, with a faradaic efficiency of 80 %, while disproportionation of these radicals and/or reaction with hydrogen atoms adsorbed at the electrode generates a neutral monomer with 20 % faradaic efficiency.

Encapsulation and solid state sequestration of gases by calix[6]arene-based molecular containers.

Two calix[6]arene-based molecular containers were synthesized in high yields. These containers can encapsulate small guests through a unique "rotating door" complexation process. The sequestration of greenhouse gases is clearly demonstrated.

pH Dependence of Photoinduced Electron Transfer with [Ru(TAP)].

The quenching of the excited state of [Ru(TAP)] (TAP = 1,4,5,8-tetraazaphenanthrene) by guanosine-5'-monophosphate (GMP), N-acetyltyrosine (N-Ac-Tyr), and hydroquinone (HQ) has been studied in aqueous solution over a wide range of pH values including, for the first time, strongly acidic media. This quenching by electron transfer was examined by steady-state H photochemically induced dynamic nuclear polarization (photo-CIDNP) as well as by more conventional techniques, among which are pulsed laser-induced transient absorption and emission experiments. A deeper knowledge of the photochemical behavior of [Ru(TAP)] has been gained thanks to the combined use of these two approaches, photo-CIDNP and electronic spectroscopies, highlighting their complementarity.

A nano-sized container for specific encapsulation of isolated water molecules.

A calix[4]arene-based molecular box was synthesized. Its properties were characterized through XRD and extensive NMR studies. This receptor is able to encapsulate specifically two isolated water molecules in both non-protic and protic solvents.

Kinetic and Thermodynamic Stabilization of Metal Complexes by Introverted Coordination in a Calix[6]azacryptand.

The Huisgen thermal reaction between an organic azide and an acetylene was employed for the selective monofunctionalization of a X6 -azacryptand ligand bearing a tren coordinating unit [X6 stands for calix[6]arene and tren for tris(2-aminoethyl)amine]. Supramolecular assistance, originating from the formation of a host-guest inclusion complex between the reactants, greatly accelerates the reaction while self-inhibition affords a remarkable selectivity. The new ligand possesses a single amino-leg appended at the large rim of the calixarene core and the corresponding Zn(2+) complex was characterized both in solution and in the solid state.

Triflate-functionalized calix[6]arenes as versatile building-blocks: application to the synthesis of an inherently chiral Zn(ii) complex.

Cavity-based metal complexes can find many applications notably in the fields of catalysis and biomimicry. In this context, it was shown that metal complexes of calix[6]arenes bearing three aza-coordinating arms at the small rim provide excellent structural models of the poly-imidazole sites found in the active site of many metallo-enzymes. All these N-donor ligands were synthesized from the 1,3,5-tris-methoxy-p-tBu-calix[6]arene platform, which presents some limitations in terms of functionalization.

Selective recognition of neutral guests in an aqueous medium by a biomimetic calix[6]cryptamide receptor.

The design of artificial receptors that can efficiently work in water is a challenging research area. A possible biomimetic approach for the elaboration of such receptors consists of associating a hydrophobic cavity with a polar polyfunctional binding site. On this basis, a hydrophilic calix[6]cryptamide decorated with oligo(ethylene glycol) units (i.

Synthesis of (Homooxa)calixarene-Monoquinones through the "All-but-One" Methodology.

The iteroselective "all-but-one" carbamatation methodology has been successfully extended to homooxacalixarenes and used for the selective and controlled synthesis of homooxacalixarene-monoquinones and calixarene-monoquinones. These moquinone derivatives constitute interesting molecular platforms that, until now, were inaccessible through any efficient means.

Insights into the structure-activity relationships of chiral 1,2-diaminophenylalkane platinum(II) anticancer derivatives.

The structure-activity relationships of chiral 1,2-diaminophenylalkane platinum(II) anticancer derivatives are studied, including interactions with telomeric- and genomic-like DNA sequences, the pKa of their diaqua species, structural properties obtained from DFT calculations and resonant X-ray emission spectroscopy. The binding modes of the compounds to telomeric sequences were elucidated, showing no major differences with conventional cis-platinum(II) complexes like cisplatin, supporting that the cis-square planar geometry governs the binding of small Pt(II) complexes to G4 structures. Double-stranded DNA platination kinetics and acid-base constants of the diaqua species of the compounds were measured and compared, highlighting a strong steric dependence of the DNA-binding kinetics, but independent to stereoisomerism.

17O NMR study of diamagnetic and paramagnetic lanthanide(III)-DOTA complexes in aqueous solution.

The complexes between the polyaminocarboxylate DOTA ligand and the whole series of stable lanthanide(III) metal ions, except Gd(3+), were studied in aqueous solution by (17)O NMR. For all of the paramagnetic systems, the (17)O NMR signals of both the nonchelating (O1) and chelating (O2) oxygen atoms could be detected, and for some of them, the signals of both the SAP and TSAP (TSAP') conformational isomers were also observed. Line width data analysis reveals that signal broadening is not dominated by paramagnetic relaxation enhancement, as it was believed to be.

Tailored functionalization of polyphenol-based molecular platforms.

General and efficient methods for selective modification of macrocyclic oligomers are rare, mostly because restricting a reaction to a defined number of identical functional groups is difficult to achieve. This work describes a unique, general, and rational methodology for the iteroselective functionalization of polyphenolic platforms by N-tert-butylaminocarbonyl (Bac) groups. The methodology consists of reacting the oligomeric platform with t-BuNCO and an inorganic base in an apolar solvent.

Effect of magnetic field and iron content on NMR proton relaxation of liver, spleen and brain tissues.

Iron accumulation is observed in liver and spleen during hemochromatosis and important neurodegenerative diseases involve iron overload in brain. Storage of iron is ensured by ferritin, which contains a magnetic core. It causes a darkening on T2 -weighted MR images.

An induced-fit process through mechanical pivoting of aromatic walls in host-guest chemistry of calix[6]arene aza-cryptands.

The per-ipso-nitration of a TMPA-capped calix[6]arene has been achieved. The substitution of the six bulky tBu substituents for nitro groups has a strong impact on the behavior of the ligand during guest recognition. The complexation of the aza cap (by H(+) or Cu(+)) associated with the encapsulation of a guest triggers an induced-fit process leading to the loss of the cone conformation of the host in favor of alternate conformations.

Supramolecular assistance for the selective monofunctionalization of a calix[6]arene tris-carboxylic acid-based receptor.

The selective functionalization of macrocyclic receptors remains extremely challenging because it generally requires the transformation of one and only one functional group among several identical groups. Recently, some of us described that the host-guest properties of a calix[6]arene-based Zn complex could be exploited for its selective monofunctionalization. Herein, we report on the extension of this synthetic strategy to a calix[6]arene-based receptor displaying a different recognition pattern with its guest.

An oxygen-17 dynamic NMR study of the Pr-DOTA complex.

The complex between (17)O-enriched DOTA (tetraazacyclododecanetetraacetic acid) and praseodymium(III) (Pr(3+)) was studied in aqueous solution by variable-temperature (17)O NMR at 14.1 T. pH effects as well as the influence of metal ions free in solution were investigated.

Acid-base-controlled stereoselective metalation of overhanging carboxylic acid porphyrins: consequences for the formation of heterobimetallic complexes.

Overhanging carboxylic acid porphyrins have revealed promising ditopic ligands offering a new entry in the field of supramolecular coordination chemistry of porphyrinoids. Notably, the adjunction of a so-called hanging-atop (HAT) Pb(II) cation to regular Pb(II) porphyrin complexes allowed a stereoselective incorporation of the N-core bound cation, and an allosterically controlled Newton's cradle-like motion of the two Pb(II) ions also emerged from such bimetallic complexes. In this contribution, we have extended this work to other ligands and metal ions, aiming at understanding the parameters that control the HAT Pb(II) coordination.