Electrostatic gating of a nanometer water channel

Abstract

Water permeation across a single-walled carbon nanotube (SWNT) under the influence of a mobile external charge has been studied with molecular dynamics simulations. This designed nanopore shows an excellent on-off gating behavior by a single external charge (of value ؉1.0e): it is both sensitive to the available charge signal when it is close (less than a critical distance of 0.85 Å or about half the size of a water molecule) and effectively resistant to charge noise, i.e., the effect on the flow and net flux across the channel is found to be negligible when the charge is >0.85 Å away from the wall of the nanopore. This critical distance can be estimated from the interaction balance for the water molecule in the SWNT closest to the imposed charge with its neighboring water molecules and with the charge. The flow and net flux decay exponentially with respect to the difference between these two interaction energies when the charge gets closer to the wall of the SWNT and reaches a very small value once the charge crosses the wall, suggesting a dominating effect on the permeation properties from local water molecules near the external charge. These findings might have biological implications because membrane water channels share a similar single-file water chain inside these nanoscale channels.carbon nanotube ͉ molecular switch ͉ nanogate T he transportation of water molecules across nanometer water channels in membranes plays a key role in biological activities (1-8). It has been recognized that the existence of the charged residues in these water channels greatly reduces the permeation of protons across the channel but maintains quite stable water flows (4, 5, 9, 10). Moreover, because charges are indispensable in both membrane proteins and physiological solutions inside and outside the cells, it is also important to understand how external charges influence the water permeation.By using molecular dynamics simulations, the importance of the charged residues in channel proteins such as aquaporin (AQP) and Glpf on the behavior of water molecules inside the channel has been studied recently (9-11). It has been found that the charged groups in the conserved NPA and ar/R regions that dominate the energetics of water permeation in these regions can interrupt the hydrogen bond along the water chain and generate the electrostatic field to exclude proton transfer (9-13). Furthermore, very recently, the electrostatic environment of the water channel has been found to be able to regulate water permeability, using mutational analysis (14).However, the complex structure of biological channels and protein-water interactions often make further investigations of the mechanism of biological water channels very complicated. It has been well recognized that simple nanochannels can be used as model systems to exploit some of the primary behavior of the biological water channels. In 2001, Hummer et al. proposed that single-walled carbon nanotubes (SWNTs) can be designed as molecular channels for water (15)(16)(17)(18). They showed tha...

Pore waters regulate ion permeation in a calcium release-activated calcium channel

“…Another example comes from nanotube-based channels, in which the orientation and occupancy of water were found to be affected by the external electric field (37). It was further reported that a designed carbon nanopore, which mimics the biological water channel, shows an on/off gating behavior controlled by a single external charge (38).…”

Entropy of single-file water in (6,6) carbon nanotubes

“…Despite their apparent nonpolar character, carbon nanotubes tend to fill with water at ambient conditions, as shown by simulations [1][2][3][4][5][6][7][8][9] and observed in a broad range of experiments. [10][11][12][13][14][15] In narrow tubes with subnanometer diameters, water forms a single-file arrangement, in which each water molecule donates one hydrogen bond, and accepts one from another water molecule.…”

Fast water channeling across carbon nanotubes in far infrared terahertz electric fields

“…In the past, the water transport properties have been studied extensively in singlewalled carbon nanotubes (SWCNTs) [8][9][10][11][12] . Novel studies on water transports in nanochannels have continued to appear in recent years with various designs concerning temperature gradients 13 , charge modification 14 , coulomb dragging 15 , collective mode oscillations 16 , and phononinduced frictions 17 . Besides the biological implications in these studies, the stream of these novel progresses may potentially become integrated into the application pool for the design of innovative nanofluidic devices, such as flow sensors 18 , desalination of seawater 19 , molecular sieves 20 , and so on.…”