Permeation selectivity of alkali metal ions through crown ether based ion channels

Synthetic ion channels mimic the natural ion channels for various biological activities such as drug delivery, signal transduction, as biosensors and as antibacterial agents. Among the synthetic ion channels, crown ether-based ion channels show greater selectivity for K+ and Na+, compared to other metal ions. These synthetic ion channels can therefore be utilized for the selective transport of K+ and Na+ which is important for various biological processes. In this work, theoretical insight for the permeability and selectivity of crown ether-based ion channels is provided. Permeability is studied by passing the alkali metal ion directly through the crown ether macrocyclic cavity, as well as through the empty spaces present in the packings. For this purpose, 18-crown-6, 15-crown-5 ether and their substituted derivatives have been studied. The results show that 18-crown-6-ethers are more selective for K+, while 15-crown-5 ethers are selective for Li+. Moreover, the results show that the ion transport can also occur through the empty spaces present in crown ether packings. The factors responsible for the greater selectivity of ions through a certain system are the sizes of the ion and cavity. Furthermore, substituents on the crown ether cycle help the system to form columnar packing which is found to decrease the permeation barrier for passing ions. Additionally, we find that the permeability of ions is sensitive to the hydration of ions.