Biological Water Channels Research Articles

ChemInform Abstract: Artificial Water Channels — Incipient Innovative Developments

AbstractReview: [30 refs.

Salt-Excluding Artificial Water Channels Exhibiting Enhanced Dipolar Water and Proton Translocation.

Aquaporins (AQPs) are biological water channels known for fast water transport (∼10(8)-10(9) molecules/s/channel) with ion exclusion. Few synthetic channels have been designed to mimic this high water permeability, and none reject ions at a significant level. Selective water translocation has previously been shown to depend on water-wires spanning the AQP pore that reverse their orientation, combined with correlated channel motions. No quantitative correlation between the dipolar orientation of the water-wires and their effects on water and proton translocation has been reported. Here, we use complementary X-ray structural data, bilayer transport experiments, and molecular dynamics (MD) simulations to gain key insights and quantify transport. We report artificial imidazole-quartet water channels with 2.6 Å pores, similar to AQP channels, that encapsulate oriented dipolar water-wires in a confined chiral conduit. These channels are able to transport ∼10(6) water molecules/s, which is within 2 orders of magnitude of AQPs' rates, and reject all ions except protons. The proton conductance is high (∼5 H(+)/s/channel) and approximately half that of the M2 proton channel at neutral pH. Chirality is a key feature influencing channel efficiency.

Highly Permeable Artificial Water Channels that Self-Assemble into Two-Dimensional Arrays

Artificial nanoscale water channels are the future of cheap, low-power water filtration and desalinization. Biological water channels such as the aquaporin membrane protein selectively pass water across the cell membranes of living creatures, but are prohibitively difficult to incorporate into large-scale technological applications. In response to these challenges, many artificial channels have been designed to imitate the function of aquaporins, including a new single-molecule water channel we introduce here, peptide-appended pillar[5]arene (PAP). PAP is comprised of a single benzyl ring decorated by hydrophobic phenylalanine amino acid arms. Stopped-flow light scattering measurements combined with fluorescence correlation spectroscopy accurately characterized single channel PAP permeability as near to that of aquaporin itself, orders of magnitude better than first-generation artificial water channels. Molecular dynamics simulation supported this assessment and further predicted wetting-dewetting transitions to occur in individual PAP channels on the nanosecond timescale. Molecular dynamics simulation also predicted PAP self-assembly into two-dimentional arrays of lattice size 21 Å, driven by hydrogen bonds among the channel's peptide arms. Aggregation and lattice size were verified to within 1 Å by cryo-electron microscopy. The self-assembly of individual, highly permeable water channels into densely packed two-dimensional arrays can be potentially utilized to design a new generation of high-flux water filtration membranes. Furthermore, as the phenylalanine arms of PAP can be switched out for other amino acids, the individual channels and their self-assembly can be programmed to enable selective transmembrane transport.

Tunable, Strain-Controlled Nanoporous MoS₂ Filter for Water Desalination.

The deteriorating state of global fresh water resources represents one of the most serious challenges that scientists and policymakers currently face. Desalination technologies, which are designed to extract potable water from the planet's bountiful stores of seawater, could serve to alleviate much of the stress that presently plagues fresh water supplies. In recent decades, desalination methods have improved via water-filtering architectures based on nanoporous graphene filters and artificial membranes integrated with biological water channels. Here, we report the auspicious performance (in simulations) of an alternative nanoporous desalination filter constructed from a MoS2 nanosheet. In striking contrast to graphene-based filters, we find that the "open" and "closed" states of the MoS2 filter can be regulated by the introduction of mechanical strain, yielding a highly tunable nanopore interface. By applying lateral strain to the MoS2 filter in our simulations, we see that the transition point between "open" and "closed" states occurs under tension that induces about 6% cross-sectional expansion in the membrane (6% strain); the open state of the MoS2 filter demonstrates high water transparency and a strong salt filtering capability even under 12% strain. Our results thus demonstrate the promise of a controllable nanoporous MoS2 desalination filter, wherein the morphology and size of the central nanopore can be precisely regulated by tensile strain. These findings support the design and proliferation of tunable nanodevices for filtration and other applications.

Artificial water channels--incipient innovative developments.

Aquaporins (AQPs) are biological water channels known for fast water transport (∼10(8)-10(9) water molecules per s per channel), with complete proton/ion exclusion. Few synthetic channels have been designed to mimic this high water permeability and to reject ions at a significant level. This Feature Article will discuss the incipient developments of the first artificial water channel systems.

Thermodynamic insight into spontaneous hydration and rapid water permeation in aquaporins.

We report here a detailed thermodynamic description of water molecules inside a biological water channel. Taking advantage of high-resolution molecular dynamics trajectories calculated for an aquaporin (AQP) channel, we compute the spatial translational and rotational components of water diffusion and entropy in AQP. Our results reveal that the spontaneous filling and entry of water into the pore in AQPs are driven by an entropic gain. Specifically, water molecules exhibit an elevated degree of rotational motion inside the pore, while their translational motion is slow compared with bulk. The partial charges of the lining asparagine residues at the conserved signature Asn-Pro-Ala motifs play a key role in enhancing rotational diffusion and facilitating dipole flipping of water inside the pore. The frequencies of the translational and rotational motions in the power spectra overlap indicating a strong coupling of these motions in AQPs. A shooting mechanism with diffusive behavior is observed in the extracellular region which might be a key factor in the fast conduction of water in AQPs.

Electricity resonance-induced fast transport of water through nanochannels.

We performed molecular dynamics simulations to study water permeation through a single-walled carbon nanotube with electrical interference. It was found that the water net flux across the nanochannel is greatly affected by the external electrical interference, with the maximal net flux occurred at an electrical interference frequency of 16670 GHz being about nine times as high as the net flux at the low or high frequency range of (<1000 GHz or >80,000 GHz). The above phenomena can be attributed to the breakage of hydrogen bonds as the electrical interference frequency approaches to the inherent resonant frequency of hydrogen bonds. The new mechanism of regulating water flux across nanochannels revealed in this study provides an insight into the water transportation through biological water channels and has tremendous potential in the design of high-flux nanofluidic systems.

Dependences of Water Permeation through Cyclic Octa-peptide Nanotubes on Channel Length and Membrane Thickness

Effects of the channel length and membrane thickness on the water permeation through the transmembrane cyclic octa-peptide nanotubes (octa-PNTs) have been studied by molecular dynamics (MD) simulations. The water osmotic permeability (p(f)) through the PNTs of k × (WL)(4)/POPE (1-palmitoyl-2-oleoyl-glycerophosphoethanolamine; k = 6, 7, 8, 9, and 10) was found to decay with the channel length (L) along the axis (~L(-2.0)). Energetic analysis showed that a series of water binding sites exist in these transmembrane PNTs, with the barriers of ~3k(B)T, which elucidates the tendency of p(f) well. Water diffusion permeability (p(d)) exhibits a relationship of ~L(-1.8), which results from the novel 1-2-1-2 structure of water chain in such confined nanolumens. In the range of simulation accuracy, the ratio (p(f)/p(d)) of the water osmotic and diffusion permeability is approximately a constant. MD simulations of water permeation through the transmembrane PNTs of 8 × (WL)(4)/octane with the different octane membrane thickness revealed that the water osmotic and diffusion permeability (p(f) and p(d)) are both independent of the octane membrane thickness, confirmed by the weak and nearly same interactions between the channel water and octane membranes with the different thickness. The results may be helpful for revealing the permeation mechanisms of biological water channels and designing artificial nanochannels.

Mechanism of water permeation through modified carbon nanotubes as a model for peptide nanotube channels

It is of interest to explore transfer of fluid through nanopores because of widespread applications for such systems. Carbon Nanotubes (CNTs) with their exceptional properties are the best candidates as building blocks for nanostructures. Water transfer in lots of biological systems acts as an important role for keeping the tissue working properly. Peptide nanotube is one of the best biological channels which was proposed recently. While the mechanism of water permeation through channels is very complex, however, investigations such as effect of charge distributions and temperature on water permeation could shed light on the determinants of water and proton conduction rates in biological water channels. A neutral and two different charge-distributed CNTs have been used to investigate the effect of charge distribution on water permeation as well as investigation of the mechanism of water permeation through them. The charge distribution on CNTs is in accordance with peptide nanotube channels. We have investigated both the osmotic and diffusion permeations in two possible model systems. We have applied a pressure gradient (about 0.12-0.14 Mpa/13.4 A) on the systems for applying a chemical potential difference to explore the osmotic permeation. The diffusion permeation is obtained using no pressure difference. The amount of permeation for one model system was inconsistent with the continuous-time random-walk (CTRW) model. The charge distribution in this model prevents water molecules to flip around which shows that the main part of diffusion and permeation mechanism is flipping the water molecules according to water charge interaction. We have also investigated the effect of temperature on water permeation in the range of 280 K to 370 K. An interesting result is an abrupt increase in number of diffusion permeation around 340 K.

Enhancement of Water Permeation across a Nanochannel by the Structure outside the Channel

We used molecular dynamics simulation to study the effect of the external structure on water permeation across a single-walled nanochannel. In contrast with the macroscopic scenario, the outside structure greatly affects the water transport across the nanochannel. Remarkably, the ratio of maximal to minimal flux reached a value of about two for different outside structures. These findings are expected to be helpful in design of high-flux nanochannels and provide an insight into the contribution of the lipid membrane to water permeation across biological water channels.

Enhancement of water permeation across nanochannels by partial charges mimicked from biological channels

In biological water channel aquaporins (AQPs), it is believed that the bipolar orientation of the single-file water molecules inside the channel blocks proton permeation but not water transport. In this paper, the water permeation and particularly the water-selective behaviour across a single-walled carbon nanotube (SWNT) with two partial charges adjacent to the wall of the SWNT are studied by molecular dynamics simulations, in which the distance between the two partial charges is varied from 0.14 nm to 0.5 nm and the charges each have a quantity of 0.5 e. The two partial charges are used to mimic the charge distribution of the conserved non-pseudoautosomal (NPA) (asparagine/proline/alanine) regions in AQPs. Compared with across the nanochannel in a system with one +1 e charge, the water permeation across the nanochannel is greatly enhanced in a system with two +0.5 e charges when charges are close to the nanotube, i.e. the two partial charges permit more rapid water diffusion and maintain better bipolar order along the water file when the distance between the two charges and the wall of SWNT is smaller than about 0.05 nm. The bipolar orientation of the single-file water molecules is crucial for the exclusion of proton transfer. These findings may serve as guidelines for the future nanodevices by using charges to transport water and have biological implications because membrane water channels share a similar single-file water chain and positive charged region at centre and provide an insight into why two residues are necessitated in the central region of water channel protein.

The structure and dynamics of water inside armchair carbon nanotube

In this paper we present some simulation results about the behaviour of water molecules inside a single wall carbon nanotube (SWNT). We find that the confinement of water in an SWNT can induce a wave-like pattern distribution along the channel axis, similar phenomena are also observed in biological water channels. Carbon nanotubes(CNTs) can serve as simple nonpolar water channels. Molecular transport through narrow CNTs is highly collective because of tight hydrogen bonds in the protective environment of the pore. The hydrogen bond net is important for proton and other signal transports. The average dipoles of water molecules inside CNTs (7,7), (8,8) and (9,9) are discussed in detail. Simulation results indicate that the states of dipole are affected by the diameter of SWNT. The number of hydrogen bonds, the water–water interaction and water–CNT interaction are also studied in this paper.

Kinetics and Mechanism of Proton Transport across Membrane Nanopores

We use computer simulations to study the kinetics and mechanism of proton passage through a narrow-pore carbon-nanotube membrane separating reservoirs of liquid water. Free energy and rate constant calculations show that protons move across the membrane diffusively along single-file chains of hydrogen-bonded water molecules. Proton passage through the membrane is opposed by a high barrier in the effective potential, reflecting the large electrostatic penalty for desolvation and reminiscent of charge exclusion in biological water channels. At neutral pH, we estimate a translocation rate of about 1 proton per hour and tube.

Controllable Water Channel Gating of Nanometer Dimensions

The dynamics of water molecules in a single-walled carbon nanotube (SWNT) under continuous deformations was studied with molecular dynamics simulations. The flux and occupancy remain almost fixed within a deformation of 2.0 A but decrease sharply for a further deformation of 0.6 A. The nanopore is an excellent on-off gate that is both effectively resistant to deformation noises and sensitive to available signals. Biological water channels are expected to share this advantage due to similar wavelike water distributions. The minimal external force required for triggering an open-close transition falls within the working range of many available experimental facilities, which provides the possibility of developing SWNT-based nanoscale devices.

Water and Proton Conduction through Carbon Nanotubes as Models for Biological Channels

Carbon nanotubes, unmodified (pristine) and modified through charged atoms, were simulated in water, and their water conduction rates determined. The conducted water inside the nanotubes was found to exhibit a strong ordering of its dipole moments. In pristine nanotubes the water dipoles adopt a single orientation along the tube axis with a low flipping rate between the two possible alignments. Modification can induce in nanotubes a bipolar ordering as previously observed in biological water channels. Network thermodynamics was applied to investigate proton conduction through the nanotubes.

Synthesis and the Unique Crystal Structure of [(H2O)⊂(DB18C6)(μ2‐H2O)2/2][(H3O)⊂(DB18C6)(μ2‐H2O)2/2]I3 (DB18C6 = dibenzo‐18‐crown‐6)

AbstractIn the title compound 1, the macrocylic ligand DB18C6 arranges to build two types of channels in which either only water or water and H3O+ molecules are stacked to linear polymers. The counter ions, I3—, also form chains and fill in the spaces left between the parallel stacks of the crown ethers. Compound 1 should therefore possess interesting conducting properties and might as well serve as model for biological water channels.