Synthetic ion channels have been known for nearly three decades, but it is only in the past decade that analysis of the currents these ionic conductors carry has become a standard technique. A broad range of structural types have been explored and these reports have produced a very diverse collection of ion channel conductance behaviours. In this critical review we describe a notational method to extract salient information from reported ion channel experiments. We use an activity grid to represent quantitative information on conductance and opening duration with a five-level colour code to represent qualitative information on the nature of the conductance-time profile. Analysis of the cumulative dataset suggests that the reported conductance data can reflect the structural features of the compounds prepared, but does also reflect the energetic landscape of the bilayer membrane in which synthetic ion channels function (143 references).

Abstract:Cumulative formation constants for the interaction of phenylboronic acids with 1,2-diols and structurally related α-hydroxy carboxylic acids were determined by potentiometric titration in aqueous solution. Although there is a significant electronic effect on the acidity of phenylboronic acid (ρ = 2.1), there is no marked electronic effect on the stability of the complexes. Rather, the complexes are significantly destabilized by adjacent anionic groups, by steric interactions across the face of the cyclic boronate ester, and by angle strain within the boronate ester ring. Binding that is nearly independent of pH is observed for some favorably constituted α-hydroxy acid complexes as a result of the relatively high acidity of the acids, which in turn allows tetrahedral boronate complexes to persist in acidic solution (pH < 3).

Cyclodextrin-based ion channels are readily prepared via a "click" reaction and exhibit a range of membrane activities consistent with the formation of ion channels in planar bilayer membranes. The durations of irregular and transient conductance events appear to follow a power-law scaling.

SummaryVoltage-clamp experiments of eight oligoester bolaamphiphiles in two subclasses are described. Syntheses of three new terephthalate-based compounds were achieved in three linear steps. Together with five previously described, related compounds, the ion transport activity was assessed by means of the voltage-clamp technique. All of the compounds show multiple types of conductance behavior in planar bilayers, a subset of which was exponentially voltage-dependent. The varied and irregular activities were summarized with the aid of a recently developed “activity-grid” method.

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Seventeen derivatives of α- and β-cyclodextrins were prepared from the cyclodextrin per-6-azide by "click" cyclization with terminal alkynes. Sixteen of these "half-channel" compounds showed significant activity as ion channels in planar bilayer members as assessed by the voltage-clamp technique. Activity ranged from persistent square-top openings to highly erratic conductance; mixed behaviours were evident in virtually all data recorded. Some of the erratic behaviours were shown to follow an apparent power-law distribution of open duration times. The activities observed for the suite were summarized using a model-free activity grid method which displays conductance, duration, and opening behaviour. The overall activity shows the clustering of conductance-duration indicating that activity arises from system properties rather that solely as a property of the compound. The activity grids also support an analysis of structure-activity relationships as they apply to the global behaviour of the compounds and reveal the complexity of a single structure change in controlling the distribution of concurrent conductance behaviours. Transient blockage of channel activity by the hydrophobic guest of the cyclodextrin (1-adamantyl carboxylate) is consistent with the formation of an end-to-end dimer channel among several other competing and interconverting structures.

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Shaft torque and corresponding stresses in a motor driven - direct coupled - reciprocating compressor system, are significantly influenced by the system’s mass-elastic properties. Uncertainty in the system’s mass-elastic properties will therefore translate into uncertainty in the calculated system stresses. Three case studies provide the reader with an appreciation for the importance of defining uncertainty bands in the mass-elastic properties of a torsional system. The case studies are followed by a theoretical discussion section. The reader is introduced to the concept of torsional resonance, uncertainty in calculated system torsional natural frequencies is defined, and the relative influence that specific system mass-elastic properties have on the overall system is discussed. A full compliment of system mass-elastic uncertainties is presented.