Selective Water Transport in an Alanine-Functionalized Metal-Organic Framework: A Computational Study

Applications of metal-organicframeworks (MOFs) functionalizedwith biomolecules have primarily focused on the use of these frameworksfor bioimaging, catalysis, chiral separation, nanomotors, and drugdelivery. However, their use in the design of artificial water channels(AWCs) has yet to be explored. In this work, we computationally explorethe performance of a zwitterionic alanine-functionalized Ni-CPO-27MOF as an AWC. Using density functional theory (DFT) calculationsand equilibrium/nonequilibrium molecular dynamics (MD) simulations,the stability, water permeability, and ion selectivity of the proposedAWC are studied. The DFT calculations predict that zwitterionic alaninebinds to the coordinatively unsaturated Ni sites almost twofold strongercompared to water. Using the quantum theory of atoms in molecules,it is also found that the zwitterionic alanine molecules are furtherstabilized through hydrogen bonding between their carboxylate (COO-) and amino (NH3 (+)) groups. NonequilibriumMD simulations show that the proposed AWC possesses a high osmoticwater permeability of 2.2 & PLUSMN; 0.3 x 10(-15) cm(3)/s/channel, which lies between that observed in aquaporin-0and aquaporin-1 proteins, while completely excluding Na+ and Cl- ions from the channel. The free energiesassociated with the water and ion transport show that fast water transportmay be attributed to the relatively low free energy barriers for waterin the channel, whereas the ion exclusion is due to large free energybarriers that the ions cannot overcome even under 100 MPa of appliedpressure. By using a crystalline material, the proposed design ofan amino acid-functionalized MOF-based AWC represents a departurefrom previously developed AWCs, which rely on the self-assembly ofcurated molecules in lipid bilayers or polymer matrices and are susceptibleto long-term stability issues.