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

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.