Unraveling the mechanism of selective ion transport in hydrophobic subnanometer channels.
Recently reported synthetic organic nanopore (SONP) can mimic a key feature of natural ion channels, i.e., selective ion transport. However, the physical mechanism underlying the K(+)/Na(+) selectivity for the SONPs is dramatically different from that of natural ion channels. To achieve a better understanding of the selective ion transport in hydrophobic subnanometer channels in general and SONPs in particular, we perform a series of ab initio molecular dynamics simulations to investigate the diffusivity of aqua Na(+) and K(+) ions in two prototype hydrophobic nanochannels: (i) an SONP with radius of 3.2 Å, and (ii) single-walled carbon nanotubes (CNTs) with radii of 3-5 Å (these radii are comparable to those of the biological potassium K(+) channels). We find that the hydration shell of aqua Na(+) ion is smaller than that of aqua K(+) ion but notably more structured and less yielding. The aqua ions do not lower the diffusivity of water molecules in CNTs, but in SONP the diffusivity of aqua ions (Na(+) in particular) is strongly suppressed due to the rugged inner surface. Moreover, the aqua Na(+) ion requires higher formation energy than aqua K(+) ion in the hydrophobic nanochannels. As such, we find that the ion (K(+) vs. Na(+)) selectivity of the (8, 8) CNT is ∼20× higher than that of SONP. Hence, the (8, 8) CNT is likely the most efficient artificial K(+) channel due in part to its special interior environment in which Na(+) can be fully solvated, whereas K(+) cannot. This work provides deeper insights into the physical chemistry behind selective ion transport in nanochannels.
Beijing National Laboratory for Condensed Matter Physics, Chinese Academy of Sciences, Beijing 100190, China; Institute of Physics, Chinese Academy of Sciences, Beijing 100190, China;
Hui
H
Li
Department of Chemistry, University of Nebraska-Lincoln, Lincoln, NE 68588 jfrancisco3@unl.edu xzeng1@unl.edu.
Joseph S
JS
Francisco
Department of Chemistry, University of Nebraska-Lincoln, Lincoln, NE 68588 jfrancisco3@unl.edu xzeng1@unl.edu.
Xiao Cheng
XC
Zeng
D057927:Hydrophobic and Hydrophilic Interactions; D007473:Ion Channels / Q000378:metabolism*; D017136:Ion Transport
D007473:Ion Channels
ion; molecular-dynamics; nanotubes; selective; transport