| Transmembrane water channels play fundamental roles in organisms as well as in industrial applications. The dynamics of water across nanochannels has significant implications for both the understanding of biological activities and designing of new filtration materials novel nanofluidic machines. Water molecules confined in such nanoscale channels usually exhibit the behaviors different from those in bulk systems. In this thesis, a series of regularized cyclic D,L-peptides in aβ-sheet folded nanotube embedded in the POPE lipid bilayer were employed, to approach the structures, energical profiles, and transport behaviors of water confined in nanochannels.The distribution profile of water molecules along the transmembrane PNTs 8×(WL)n=3,4,5/POPE axis propose a wavelike pattern of water-chain in the nanotube, arraying in a form of 1-2-1-2 file in contrast to the single file in other nanochannels studied widely. A novel H-bond network is present in the nanotube. The primary reason of forming the water-chain pattern is the steric constraints from the nanotube wall. Strong orientations of the water molecular dipoles near the two opening-ends pointing to the opposite directions are found. The D-defect was found to explain the reversing of water dipole orientations in the nanotube.Three transmembrane PNTs, i.e., 8×(WL)n=3,4,5/POPE (with uniform lengths but various radii) were designed to investigate the radial dependences of the water-chain structures, diffusions and transportation properties. The diffusions of individual water molecules and collective coordinates of all the channel-water in the three systems are certified as unbiased Brownian motions. From the very well linear relationships between MSDs and time intervals, the diffusion coefficients (D and Dn) and transportation permeabilities (pf) have been deduced efficiently. Under the hydrostatic pressure differences across the membrane, a net unidirectional water flows rose up, and the osmotic permeabilities were determined, which are consisted with the results obtained from the analysis of collective coordinates of all the channel-water. The ratios of the osmotic and diffusion permeabilities (pf / pd) were examined for all the three channels.Five transmembrane PNTs (k×(WL)n=4/POPE, k=6,7,8,9,10) with same radii but different length were designed to study to effects of the channel length and membrane thickness on the water flow through the transmembrane PNTs. pf is found to decay with the channel length (L) along the axis (~ L-2.0), rather than linear or independent relationship. Energetic analysis shows that a series of water binding sites exist in the PNTs, with the barriers of ~ 3 kBT, which elucidates the length dependence of pf well. pd exhibits a relationship of ~ L-1.8, which is resulted from the novel 1-2-1-2 file in the confined nano-lumen. In the range of simulation accuracy, the two permeabilities are both inversely squared with L, and the ratio of them is approximately a constant. Simulations also reveal that the water permeabilities are independent of the thickness of a octane membrane, confirmed by the weak and nearly same interactions between channel-water and membrane with variant thickness. The critical factor affecting water permeation is the type of membrane rather than its thickness.Two sets of parameters used widely can describe the water permeation through a nanochannel. One is permeation rate and its diffusion, and the other is flow and net flux. The relationship between the two sets for a single-file nanochannel is firstly established and verified with molecular dynamics simulations in a transmembrane carbon nanotube. Simulation results are in excellent agreement with our prediction. In addition, how the flow and flux depend on the temperature and pressure difference between the two ends of a channel is also deduced.Our finding may be helpful for providing insights of permeation mechanism of biological water channels and designing filtration materials and artificial nanochannels. |
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