Confinement of water by pore geometry to a one-dimensional file of molecules interacting with the pore alters the diffusion coefficient D(W). Here we report an exponential dependence of D(W) on the number of water positions in the pore. The result is based on measurements of single channel water permeabilities of structurally similar peptidic nanopores of different length. The inconsistency with predictions from continuum or kinetic models indicates that pore occupancy is reduced in single file transport. In longer pores (e.g., in aquaporins) the presence of charged residues increases D(W).

Progress in the synthesis of membrane-bound ion channels is reviewed. Important results from several groups are highlighted and contributions from the authors' group are summarized. Topics covered include synthetic approaches towards oligo-THF amino acids, oligopyrrolidines and gramicidin hybrid channels. Points of interest are the importance of self-assembly systems, the functional analysis (ion selectivity, gating, asymmetric insertion and asymmetric function) and the structural characterization of the channel.

Predictable ion selectivity is a major goal for the use of synthetic channels in biological systems. Gramicidin A (gA) is a suitable lead structure for the design of such channels. The crown ether–gA hybrid structure shown forms ion channels with a higher ion selectivity for K+ than for Cs+.

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