Columnar Self-Assembled Ureido Crown Ethers:  An Example of Ion-Channel Organization in Lipid Bilayers

Abstract

The self-assembly of ureido crown-ether derivatives has been examined in homogeneous solution, in the solid state, and in planar bilayer membranes. The self-assembly is driven by head-to-tail hydrogen bonding between the urea functional groups. Dimers and higher oligomers are formed in CDCl3 solution as assessed by the change in the ureido NH chemical shift as a function of concentration. Single-crystal X-ray diffraction shows that an antiparallel association of the ureas produces columnar channels composed of face-to-face crown ethers. Powder X-ray diffraction studies also show the presence of a minor phase based upon a parallel urea association leading to an alternative columnar arrangement of the crown ethers. In bilayer membranes at low concentration of ureido crown ether added, membrane disruption is observed together with rare single-channel openings, but at higher concentration, a rich array of interconverting channel conductance states is observed. The channel results are interpreted as arising from discreet stacks of ureido crown ethers where the transport of cations would occur via the macrocycles, admixed with larger pores formed by association of the crown ether headgroups around a central large pore.

Dynamic hybrid materials for constitutional self-instructed membranes

“…1 A) or in solid hybrid membranes (Fig. 1B) (27)(28)(29). Our efforts involve now the noncovalent binding of hexylureidobenzo-15-crown-5 (1) and hexylureidobenzo-18-crown-6 (2) macrocyclic receptors,confined in a lipophilic mesoporous silica scaffolding matrix (Fig.…”

“…We have previously shown that in the solid state a dominant antiparallel packing of 2 via urea ribbons leads to extended stacks of crown ethers (27)(28)(29). Comparing the diffractograms generated from the single-crystal structure solution (Fig.…”

“…The primary goal of our work was to form oriented membrane ion-channel superstructures confined in a hydrophobic silica scaffolding nanospace taking advantage of the dynamic features of reversible connections between the formers' macrocyclic components. For this reason we have chosen to use heteroditopic receptors 1 and 2 with macrocyclic-cation and ureaanion binding sites, which can simultaneously bind ion pairs (27)(28)(29). The transport experiments, according to the solution-diffusion mechanism (Fick's law) were evaluated by using passive competitive conditions with both Na ϩ and K ϩ present in the feed phase and deionized water in the receiving phase, assuming that the chemical potential gradient across the membrane is only caused by a concentration gradient (30)(31)(32)(33)(34)(35).…”

“…1). Recently, we proved the possibility to create ion-conduction pathways by self-assembly (27)(28)(29), and the urea ribbons and a silica scaffolding matrix can be used to orient the directional transporting superstructures in hybrid membrane materials. Our previous studies on receptors covalently attached to the silica matrix focused on their potential ability to recognize ions or molecules (30)(31)(32) or to control the ''fixed-site jumping'' diffusion along the directional nanometric pathways (33)(34)(35).…”

Self-assembling subnanometer pores with unusual mass-transport properties

“…Biological nanotubular complexes, such as the non-selective pore formed by α-hemolysin 2 , and the highly selective channels of ions 3 and water 4 , provide some of the most spectacular examples of supramolecular assemblies with extraordinary structural and functional sophistication. Taking lessons from biological assemblies, the recent integration of molecular design and synthesis with noncovalent forces such as hydrogenbonding, electrostatic, hydrophobic, π − π and ion-π interactions, along with metal-organic architectures and covalent capture of supramolecular assemblies, has made a significant inroad towards producing various synthetic channels and pores, either from organic building blocks or from cyclic peptides [5][6][7][8][9][10][11][12][13] . Many of these synthetic molecular and supramolecular structures exhibit interesting masstransporting properties.…”