image: (a) Side (top panel) and top views (bottom panel) of the selectivity filter of the biological KcsA channel. Four binding sites (S1, S2, S3 and S4) are aligned in series along the selectivity filter. (b) Side (top panel) and top views (bottom panel) of the biomimetic nanochannel. Twenty carbonyl oxygens (shown as spheres) were modified inside the nanotube to mimic the structure of the KcsA selectivity filter. All atoms are colored based on their charges. (c) The simulation system. The modified nanotube channel was embedded between two graphene sheets, separating the solution into two chambers. The red spheres represent the carbonyl oxygens, and the yellow, brown, and blue spheres represent Na+, K+, and Cl- ions, respectively. An electric field was applied along the positive direction of the z-axis. (d) Cumulative ion fluxes of K+ (blue line) and Na+ (red line) through the biomimetic nanochannel under 0.2 V/nm.
Credit: ©Science China Press
Over the past decade, developing nanochannels with valence selectivity for cations have attracted increasing attention due to their potential role in a wide range of technological applications such as water desalination, energy conversion and biosensing. Among all pairs of ions, K+ and Na+ are probably the most difficult to differentiate from each other, because they are both monovalent and similar in molecular size.
To our surprise, this challenge can be readily addressed by biological potassium channels that can separate K+ from Na+ with a selectivity ratio exceeding 1000 while allowing for K+ conduction at a rate close to the diffusion limit. They accomplish this task with their selectivity filter, which contains a precisely arranged array of carbonyl oxygen atoms forming closely spaced carbonyl binding sites to coordinate ions. Inspired by the structure of biological potassium channels, many different types of biomimetic artificial potassium channels have been proposed, but the obtained selectivity remains well below that of biological potassium channels.
Recently, inspired by the selectivity filter structure of the biological potassium channel KcsA (a), for the first time, the research team led by Professor Wanlin Guo and Hu Qiu from Nanjing University of Aeronautics and Astronautics (NUAA) proposed a novel design of artificial potassium nanochannels, based on single-walled nanotubes modified with regularly arranged carbonyl atoms (b). The major structural feature of the present channels is that they fully mimic the arrangement of carbonyl groups in the KcsA selectivity filter and thus have a same number of binding sites and almost identical diameter. The biomimetic nanochannel exhibits a high K+ permeation rate and meanwhile a high K+/Na+ selectivity ratio (c-d). The free energy for ions passing through the nanochannel suggest that Na+ must overcome a higher energy barrier than K+ when entering the binding sites from the bulk. K+ and Na+ have distinct coordination features in the nanochannel: K+ is on-center coordinated at the cage sites and Na+ is off-center coordinated. Furthermore, when reducing the number of ion binding sites from four to one, the permeation rates of K+ and Na+ both increase but the K+/Na+ selectivity ratio decreases.
This study not only enhance our understanding of ion transport in biological channels but also are useful for designing nanochannels to sieve monovalent ions. Moreover, these biomimetic potassium nanochannels are ideal for developing highly sensitive and selective biosensors, which can be used in medical diagnostics, environmental monitoring and power generation.
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See the article:
A highly-selective biomimetic potassium channel
https://doi.org/10.1093/nsr/nwae242
Journal
National Science Review