Feike de Jong

Complete asymmetric chirality in a hydrogen-bonded assembly

Chirality at the supramolecular level involves the non-symmetric arrangement of molecular components in a non-covalent assembly1,2. Supramolecular chirality is abundant in biology, for example in the DNA double helix3, the triple helix of collagen4 and the α-helical coiled coil of myosin5. These structures are stabilized by inter-strand hydrogen bonds, and their handedness is determined by the configuration of chiral centres in the nucleotide or peptide backbone. Synthetic hydrogen-bonded assemblies have been reported that display supramolecular chirality in solution6,7,8 or in the solid state9,10,11,12. Complete asymmetric induction of supramolecular chirality—the formation of assemblies of a single handedness—has been widely studied in polymeric superstructures13,14. It has so far been achieved in inorganic metal-coordinated systems15,16,17, but not in organic hydrogen-bonded assemblies18,19,20. Here we describe the diastereoselective assembly of enantio-pure calix[4]arene dimelamines and 5,5-diethylbarbituric acid (DEB) into chiral hydrogen-bonded structures of one handedness. The system displays complete enantioselective self-resolution: the mixing of homomeric assemblies (composed of homochiral units) with opposite handedness does not lead to the formation of heteromeric assemblies. The non-covalent character of the chiral assemblies, the structural simplicity of the constituent building blocks and the ability to control the assembly process by means of peripheral chiral centres makes this system promising for the development of a wide range of homochiral supramolecular materials or enantioselective catalysts.Chirality at the supramolecular level involves the non-symmetric arrangement of molecular components in a non-covalent assembly,. Supramolecular chirality is abundant in biology, for example in the DNA double helix, the triple helix of collagen and the α-helical coiled coil of myosin. These structures are stabilized by inter-strand hydrogen bonds, and their handedness is determined by the configuration of chiral centres in the nucleotide or peptide backbone. Synthetic hydrogen-bonded assemblies have been reported that display supramolecular chirality in solution or in the solid state. Complete asymmetric induction of supramolecular chirality—the formation of assemblies of a single handedness—has been widely studied in polymeric superstructures,. It has so far been achieved in inorganic metal-coordinated systems, but not in organic hydrogen-bonded assemblies. Here we describe the diastereoselective assembly of enantio-pure calix[4]arene dimelamines and 5,5-diethylbarbituric acid (DEB) into chiral hydrogen-bonded structures of one handedness. The system displays complete enantioselective self-resolution: the mixing of homomeric assemblies (composed of homochiral units) with opposite handedness does not lead to the formation of heteromeric assemblies. The non-covalent character of the chiral assemblies, the structural simplicity of the constituent building blocks and the ability to control the assembly process by means of peripheral chiral centres makes this system promising for the development of a wide range of homochiral supramolecular materials or enantioselective catalysts.

An enantiomerically pure hydrogen-bonded assembly

Chiral molecules have asymmetric arrangements of atoms, forming structures that are non-superposable mirror images of each other. Specific mirror images (‘enantiomers’) may be obtained either from enantiomerically pure precursor compounds, through enantioselective synthesis, or by resolution of so-called racemic mixtures of opposite enantiomers, provided that racemization (the spontaneous interconversion of enantiomers) is sufficiently slow. Non-covalent assemblies can similarly adopt chiral supramolecular structures, and if they are held together by relatively strong interactions, such as metal coordination, methods analogous to those used to obtain chiral molecules yield enantiomerically pure non-covalent products. But the resolution of assemblies formed through weak interactions, such as hydrogen-bonding, remains challenging, reflecting their lower stability and significantly higher susceptibility to racemization. Here we report the design of supramolecular structures from achiral calix[4]arene dimelamines and cyanurates, which form multiple cooperative hydrogen bonds that together provide sufficient stability to allow the isolation of enantiomerically pure assemblies. Our design strategy is based on a non-covalent ‘chiral memory’ concept, whereby we first use chiral barbiturates to induce the supramolecular chirality in a hydrogen-bonded assembly, and then substitute them by achiral cyanurates. The stability of the resultant chiral assemblies in benzene, a non-polar solvent not competing for hydrogen bonds, is manifested by a half-life to racemization of more than four days at room temperature.

Facilitated transport of salts by neutral anion carriers

Partitioning of ions from water to the membrane solvent (NPOE) can be quantified by Gibbs free energies of transfer, deltaG(tr,NPOE)(ion). These were derived from transport studies of lipophilic salts through supported liquid membranes (SLMs) in the absence of the carrier. Partition coefficients Kp for various salts can now be calculated. The neutral anion receptors uranyl sal(oph)enes 1-5 transport Cl- and H2PO4- as tetrapropylammonium salts. The transport is diffusion-limited and can be described by two transport parameters Dm and K(ex). From the extraction constants K(ex) and the partition coefficients Kp of the transported salts, the association constants Ka of the anion receptors for Cl- and H2PO4- in NPOE were determined. Competitive transport with carriers 3 and 4 of NPr4H2PO4 and NPr4Cl demonstrated highly selective transport of H2PO4- even in the presence of excess of Cl-.

Kinetics of carrier-mediated alkali cation transport through supported liquid membranes: Effect of membrane solvent, co-transported anion, and support

The rate-limiting step in the transport of alkali cations through supported liquid membranes mediated by calix [4] arene carriers can be the diffusion of the carrier cation complex through the membrane and/or the kinetics of cation release from the complex. The effects of membrane solvent, co-transported anion, and support on the diffusion constant Dm, the extraction constant Kex and the rate constant k have been studied. These were determined from flux measurements as a function of source phase salt activity (Dm and Kex) and membrane thickness (Dm and k). Additional information about the transport resistances was obtained from variation of the operating temperature and from lag time measurements. The diffusion constants (Dm) for 1/NaClO4 and 2/KClO4 linearly increased with reciprocal solvent viscosity. On increasing the solvent polarity, the extraction constant increased, while the rate of cation release decreased. Both log Kex and log k correlate linearly with the Kirkwood function, (?r ? 1)/(2?r + 1). The co-transported anion (ClO4? vs. SCN?) affects the kinetics of release but not the diffusion constant. The normalized k and Dm values for 2/KClO4 in NPOE/Accurel® 1E-PP and NPOE/Celgard® 2500 are nearly the same. This means that the transport regime (diffusion or kinetic control) depends only on the tortuosity (?) and thickness of the support, irrespective of its morphology.

Stable heterotopic noncovalent resorcin[4]arene assemblies

Resorcin[4]arene tetracarboxylic acids 5,6 (A) and resorcin[4]arene tetrapyridines 2,3 (P) self-assemble in chloroform solution to form stable heterotopic AP dimers. Data from NMR titration and dilution experiments, as well as from vapor-pressure osmometry (VPO), indicate that the AP dimer is formed with an association constant greater than 107 M-1. Solid-solution extraction experiments are indicative of the formation of a 2:1 trimer (A2P), while self-associated homotopic species (A2 and A3) can be detected by NMR and VPO. Analysis of the heterotopic noncovalent assembly process over a range of compositions shows that these other species are much less stable than the AP heterodimer, which is the exclusive species at an A/P concentration ratio of 1:1 (> 99.7% of the total at 10 mM).

Mechanistic study of trivalent europium transport through supported liquid membranes (SLMs) and a novel immobilized phase solvent extraction (IPSE) system

As a new method of membrane formation, we have investigated microcellular foaming of thin (100 μm) polysulfone films containing varying trace concentrations of tetrahydrofuran using carbon dioxide as a physical blowing agent. Membrane morphologies were obtained by first saturating the polymer with carbon dioxide at 5 MPa, and subsequently heating the sample above the glass transition temperature (Tg) of the polymer/gas mixture at atmospheric pressure. The presence of tetrahydrofuran in the polymer at concentrations above 0.04 wt.% led to a transition from a closed cellular structure into novel open-cellular morphologies. The open structure manifests itself by small spot-like openings (diameters between 10 and 100 nm) in the cell walls. The mass transport resistances of the porous films were quantified using gas permeation measurements, and a Knudsen-type separation mechanism was observed. Detailed investigation showed that the transport resistance can mainly be controlled by two variables: (1) the concentration of the residual solvent in the polymer film, and (2) the foaming temperature. At optimal foaming temperatures, thin cell walls are obtained, which break up when fluctuations in the wall thickness are amplified by plasticizing solvent molecules.

Urea transport through supported liquid membranes using synthetic carriers

Urea can be transported through a supported liquid membrane (Accurel/NPOE) by carriers such as metallomacrocycles and polyaza (cleft-type) receptors. The urea flux is increased by a factor 4?8 using polyaza receptors and by a factor 10?15 using metallomacrocycles containing a salophene unit in which a uranyl cation is incorporated. These carriers have a high hydrophobicity and do not significantly leak from the membrane phase into the aqueous phases. The structure of the receptors and the type and number of binding sites have a pronounced influence on the transport rate. The lower urea fluxes found for the polyaza (cleft-type) carriers are most likely caused by a weaker complexation (only H-bond interactions). No transport is observed for carriers which form intramolecular H-bonds. Although lower fluxes are obtained than with a commercial haemodialysis membrane (Cuprophan), the selectivity of transport may be much higher using carrier-mediated transport.

Highly stable cage-like complexes by self-assembly of tetracationic Zn(II) porphyrinates and tetrasulfonatocalix[4]arenes in polar solvents

Tetracationic Zn(II) porphyrinates and tetraanionic calix[4]arenes can be assembled in polar solvents to obtain cage-like complexes in an entropy driven process. These structures are remarkably stable even in the presence of water or competing salts.

Adsorption of alkyltriphenylphosphonium amphiphiles on nafion membranes. X-ray photoelectron spectroscopy and static secondary ion mass spectrometry analysis.

Conductivity, UV, and attenuated total reflectance IR measurements show that n-alkyltriphenylphosphonium amphiphiles adsorb on a Ndion 117 membrane. Approximately 20% of the Ndion protons are exchanged for a cationic amphiphile (n-hexadecyltriphenylphoephonium). Diffusion of amphiphile through the membrane was not observed. Once adsorbed, the amphiphiles did not leach from the membrane. Surface-sensitive techniques (x-ray photoelectrion spectroscopy, static secondary ion mass spectrometry) were used to investigate the presence, concentration, and distribution of the amphiphile in the Ndion membrane. Our experiments point to an incomplete monolayer coverage of the membrane, the molar ratio of amphiphile to sulfonate groups being only slightly less than 1 in the uppermost 2-5 nm. The amphiphile is bonded to the membrane, most likely via an ionic bond with the sulfonate groups. X-ray fluorescence measurements show that the amphiphile is also present in the bulk of the membrane, at least in the uppermost micrometer. However, in the bulk the concentration of amphiphile is significantly lower than the sulfonate groups. These results show that thin and stable amphiphilic layers can be made on a solid support material using adsorption of an amphiphile and coupling via an ionic bond.

Carrier mediated transport through supported liquid membranes; determination of transport parameters from a single transport experiment

This paper describes a time-dependent transport model for carrier assisted cation transport through supported liquid membranes. The model describes the flux of salt as a function of time and two parameters viz. the diffusion coefficient of the cation complex (D), and the extraction constant (Kex). Simulations of transport experiments indicate that high extraction constants (Kex) are responsible for the decrease in the transport rate in time.A simulation program (CURSIM) was designed that is capable of determining the essential transport parameters D and Kex from only one time-dependent transport experiment. The non-linear curve-fitting Down-hill Simplex technique, gives directly the best-fit values of D and Kex.The CURSIM method was tested by evaluation of the transport of KClO4 by different calix[4]arene-crown-5 carriers and by valinomycin. Comparison of the results with those obtained previously by the initial flux approach, which requires a series of different experiments, shows a good agreement of both methods.