Increasing pH Causes Faster Anion‐ and Cation‐Transport Rates through a Synthetic Ion Channel

Abstract

Ion-channel mimics are able to transmit electrical signals across phospholipid membranes, and can be envisioned as nanoswitches for molecular electronics. Here, we reported the use of pH to alter ion-transport rates through a synthetic aminocyclodextrin ion channel. Both cation- and anion-transport rates were found to increase with an increase in pH due to the unique electrostatics of the multiple ammonium groups that line the channel pore. Such pH regulation of ion transport rates is unique and can be exploited for sensing applications.

Artificial Anion Transporters in Bilayer Membranes

“…Bromide and, particularly, iodide are transported at a faster rate than Na + . Interestingly, both the rates of cation and anion trans- port were found to be pH-dependent and to increase with the pH values [61]. This was explained with a retardation effect induced by the protonated amines: in the case of sodium ion, electrostatic repulsion hampers the cation transport, in the case of halides, the strong interaction of the positively charged amines with the anions induces the blockage of the channel (Fig.…”

Macrocycle-Based Synthetic Ion Channels

“…At lower pH, protonation of amine groups leads to slower cation transport due to electrostatic repulsion, while high electrostatic attraction was mentioned to explain the slower anion transport rate at the lower pH [99]. After developing pH-gated ion channel, the author has successfully installed a photoswitchable azobenzene group at the upper rim of the cyclodextrin to develop a synthetic light-gated ion channel [100]. A perfect matching of trans-azobenzene within the macrocycle cavity of 76 allowed transport of smaller cations while hindering the larger anions.…”

Generalized microscopic theory of ion selectivity in voltage-gated ion channels

“…The macroscopic models for ion channels developed thus far do take the intrinsic conductance of an ion in a given ion channel into account, but it is treated as a constant [13,23,61,62]. Subsequently, this constant is adjusted (by means of some guessed functions or hypothesis) whenever one changes the type of ions and/or the type of ion channels [13,23,61,62] to fit the experimental conductance data. The reason why the intrinsic conductances or other intrinsic parameters are treated as constants are explained in the following four examples.…”