Transport Properties Of Simple Organic Molecules In A Transmembrane Cyclic Peptide Nanotube And Separations Of Alcohol/Water Mixtures With Various CPNTs

Multiple molecular dynanmics simulations have been performed to explore the transport properties of single methane, methanol, and ethanol molecules through a water-filled transmembrane cyclic peptide nanotube(CPNT) of 8?(WL)4/POPE, and the potential application of the CPNT in the separation of an alcohol/water mixture. In addition, the separations of methanol/water mixtures with various CPNTs have also been explored.Molecular size and hydrophilicity/hydrophobicity both significantly influence molecular diffusion behavior in a nanochannel. The vdW interaction energy between methane and the channel wall of 8?(WL)4/POPE is the greatest contributor to the whole interaction of methane with the surroundings. Methane drifts faster near an α-plane zone and displays more explicit distrbutions in midplane regions. For amphiphilic methanol with a moderate molecular size among three molecules, the electrostatic interation with water and the vdW interaction with the CPNT wall are almost evenly matched and jointly play critical roles in methanol’s non-bonded interations with the surroundings, which results in the methanol’s diffusing uniformly in the whole transmembrane region, and slightly preferentially occuring in α-plane zones. Compared with methanol, ethanol possesses a larger alkyl group—C2H5, resulting in the powerful vdW with the CPNT wall. Ethanol drifts fast near an α-plane zone and displays obviously preferential distributions in midplane regions. The PMF(potential of mean force) profiles disclose the different diffusion behaviors of three organic molecules through the CPNT.The dipole orientation of channel methanol is significantly affected by the bare carbonyl groups at the tube mouths, resulting in methanol taking positive or negative dipole orientation, and flips mainly occur in gap 4. The dipole orientation of ethanol is seriously blocked, almost keeping the initial molecular orientation when entering the CPNT. Amphiphic alcohols can form direct H-bonds with channel water and the CPNT wall. Both single- and double-water bridges with the CPNT exist in the methanol and ethanol systems. Compared with the methanol hydroxyl, the ethanol hydroxyl is closer to the CPNT wall, resulting in the higher probabilities of water-bridges formed between ethanol and the CPNT.The different adsorption behaviors of alcohols and water in the dehydrated 8?(WL)4/POPE may provide potential applications in the separations of alcohols and water. For a methanol/water mixture with 75% methanol MF(mole fraction), the CPNT exhibits a high separation efficiency as the methanol MF in the CPNT exceeds 99%. For an ethanol/water mixture with a higher ethanol MF than 75%, the CPNT demonstrates an absolute advantage in ethanol dehydration. Five CPNTs composed of various amino acid residues exhibit differences in the separations of methanol/water mixtuers. The CPNTs named 8?(QL)4 and 8?(AQ)4, which consist of hydrophilic Gln(Q) and hydrophobic Leu(L) or Ala(A), possess ambitious capacities of methanol enrichment from methanol/water mixtures with 10%~50% methanol MF. To a methanol/ water mixture with a methanol MF higher than 20%, the CPNT named 8?[(WL)3QL] also possesses desirable enrichment effectiveness to methanol component.