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

Applications of metal–organic
frameworks (MOFs) functionalized
with biomolecules have primarily focused on the use of these frameworks
for bioimaging, catalysis, chiral separation, nanomotors, and drug
delivery. However, their use in the design of artificial water channels
(AWCs) has yet to be explored. In this work, we computationally explore
the performance of a zwitterionic alanine-functionalized Ni-CPO-27
MOF as an AWC. Using density functional theory (DFT) calculations
and equilibrium/nonequilibrium molecular dynamics (MD) simulations,
the stability, water permeability, and ion selectivity of the proposed
AWC are studied. The DFT calculations predict that zwitterionic alanine
binds to the coordinatively unsaturated Ni sites almost twofold stronger
compared to water. Using the quantum theory of atoms in molecules,
it is also found that the zwitterionic alanine molecules are further
stabilized through hydrogen bonding between their carboxylate (COO–) and amino (NH3+) groups. Nonequilibrium
MD simulations show that the proposed AWC possesses a high osmotic
water permeability of 2.2 ± 0.3 × 10–15 cm3/s/channel, which lies between that observed in aquaporin-0
and aquaporin-1 proteins, while completely excluding Na+ and Cl– ions from the channel. The free energies
associated with the water and ion transport show that fast water transport
may be attributed to the relatively low free energy barriers for water
in the channel, whereas the ion exclusion is due to large free energy
barriers that the ions cannot overcome even under 100 MPa of applied
pressure. By using a crystalline material, the proposed design of
an amino acid-functionalized MOF-based AWC represents a departure
from previously developed AWCs, which rely on the self-assembly of
curated molecules in lipid bilayers or polymer matrices and are susceptible
to long-term stability issues.