Crown ethers: sensors for ions and molecular scaffolds for materials and biological models.

Supramolecular chemistry has expanded dramatically in recent years both in terms of potential applications and in its relevance to analogous biological systems. The formation and function of supramolecular complexes occur through a multiplicity of often difficult to differentiate noncovalent forces. The aim of this Review is to describe the crucial interaction mechanisms in context, and thus classify the entire subject. In most cases, organic host-guest complexes have been selected as examples, but biologically relevant problems are also considered. An understanding and quantification of intermolecular interactions is of importance both for the rational planning of new supramolecular systems, including intelligent materials, as well as for developing new biologically active agents.

Binding Mechanisms in Supramolecular Complexes

The challenge of cyclic and acyclic schiff bases and related derivatives

1. Host−Guest Supramolecular Chemistry A 1.

Biomedical Applications of Supramolecular Systems Based on Host-Guest Interactions.

Structural and functional evolution of metallacrowns.

Introductory chapter.- Calixdendrimers.- Molecular tubes derived of calixarenes.- Catenated calixarenes.- Capsules and catenands.- Calixarenes as Cluster Keepers.- Calixarenes and C60.- Calixarene-Based Nanoporous Supramolecular Architectures.- Calixarenes on molecular printboards: multivalent binding, surface patterning, and capsule formation.- Calixarene-based Supramolecular Materials for Sensing and Fixation of Gases.- Enzyme models.- Supramolecular and Biomimetic Properties of Peptido- and Glycocalixarenes.- Fluorescent calixarene-based sensors.- Nanoprotein sensing.- Pharmaceutical applications of calixarenes.

Calixarenes in the nanoworld

The water-soluble tetra- and hexasulfonated calix[4] and calix[6]arenes bind very strongly the neurotransmitter acetylcholine and other quaternary ammonium cations with association constants Ks up to 4×105 M−1 for NEt4 +. The high affinities for choline and acetylcholine (Ks=5×104 to 8×104 M−1) are comparable to those of the biological recognition sites. The crystal structure of the [choline-tetrasulfonated calix[4]arene] complex was determined. Two complexes A and B were observed in the asymmetric unit. In each one, the choline inserts its N-terminal inside the cavity of the receptor. The calix[4]arene adopts a single cone conformation but the choline substrate is in an extended conformation in complex A and adopts two other folded conformations in complex B (75 % –25 %). The very high association constants and the crystal structure provide important information about the nature of the binding in this biologically most relevant complex.

Binding of acetylcholine and other quaternary ammonium cations by sulfonated calixarenes. Crystal structure of a [choline-tetrasulfonated calix[4]arene] complex

The synthesis of 1,2- and 1,3-calix[4]-bis-crowns, double calix[4]arenes and double calixcrowns have been shown to depend on the reaction conditions (nature of the base, structure of the ditosylates, and the stoichiometry of the reactants). The 1,3-alternate conformation of the 1,3-calix[4]-bis-crowns was shown to be favourable to the selective complexation of cesium cation. The observed Na+/Cs+ selectivity was exploited in separation processes using them as carriers in transport through supported liquid membranes (SLMs). The best Na+/Cs+ selectivity (1/45 000) was observed for the naphthyl derivative 7. Calix(aza) crowns and 1,3-calix[4]-bis-(aza)-crowns were also produced through the preliminary formation of the Schiff base-calixarenes, which were further hydrogenated. The syntheses consisted of the 1,3-selective alkylation of calixarenes followed by cyclization into a 1,3-bridged calixarene or by the direct 1,3-capping of the calixarene with appropriate ditosylates. Soft metal complexation by these ligands is also presented.

Calixcrowns and related molecules

A series of novel N-chromogenic calix[4]arene azacrown ethers were synthesized as selective extractants of potassium ion. 1,3-Alternate calix[4]arene azacrown ethers were prepared by reacting 25,27-dipropyloxy-26,28-bis(5-chloro-3-oxapentyloxy) calix[4]arenes with p-toluenesulfonamide in the presence of potassium carbonate. The coupling reaction of calix[4]arene azacrown ether with 2-hydroxy-5-nitrobenzyl bromide in the presence of triethylamine in THF gave the chromogenic calix[4]arene azacrown ether in moderate yield. These compounds show high potassium selectivity over other metal ions as shown by two-phase extraction, bulk liquid membrane, and 1H NMR studies on a ligand−metal complex. It is assumed that the OH of the chromogenic group attached on nitrogen can assist the complexation by encapsulation of the metal.

Synthesis and Metal Ion Complexation Studies of Proton-Ionizable Calix[4]azacrown Ethers in the 1,3-Alternate Conformation

The synthesis of 1,2- and 1,3-calix[4]-bis-crowns, double calix[4]arenes and double calixcrowns have been shown to depend on the reaction conditions (nature of the base, structure of the ditosylates, and the stoichiometry of the reactants). The 1,3-alternate conformation of the 1,3-calix[4]-bis-crowns was shown to be favourable to the selective complexation of cesium cation. The observed Na+/Cs+ selectivity was exploited in separation processes using them as carriers in transport through supported liquid membranes (SLMs). The best Na+/Cs+ selectivity (1/45 000) was observed for the naphthyl derivative 7. Calix(aza)crowns and 1,3-calix[4]-bis-(aza)-crowns were also produced through the preliminary formation of the Schiff base-calixarenes, which were further hydrogenated. The syntheses consisted of the 1,3-selective alkylation of calixarenes followed by cyclization into a 1,3-bridged calixarene or by the direct 1,3-capping of the calixarene with appropriate ditosylates. Soft metal complexation by these ligands is also presented.

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Jacques Vicens

Papers

Calixarenes in the nanoworld

Binding of acetylcholine and other quaternary ammonium cations by sulfonated calixarenes. Crystal structure of a [choline-tetrasulfonated calix[4]arene] complex

Calixcrowns and related molecules

Synthesis and Metal Ion Complexation Studies of Proton-Ionizable Calix[4]azacrown Ethers in the 1,3-Alternate Conformation

Calixcrowns and related molecules