Water Nanoconfined in a Hydrophobic Pore: MD Simulations and Water Models

Abstract

Water molecules within biological ion channels are in a nano-confined environment and therefore exhibit novel behaviours which differ from that of bulk water. Here, we investigate the phenomenon of hydrophobic gating, the process by which a nanopore may spontaneously de-wet to form a vapour lock if the pore is sufficiently hydrophobic and/or narrow. Notably, this occurs without steric occlusion of the pore. Using molecular dynamics simulations with both additive and polarisable (AMOEBA) force fields, we investigate this wetting/de-wetting behaviour in the TMEM175 ion channel. We examine how a range of rigid fixed-charge (i.e. additive) and polarisable water models affect wetting/de-wetting in both the wild-type structure and in mutants chosen to cover a range of nanopore radii and pore-lining hydrophobicities. Crucially, we find that the rigid fixed-charge water models lead to similar wetting/de-wetting behaviours, but that the polarisable water model resulted in an increased wettability of the hydrophobic gating region of the pore. This has significant implications for molecular simulations of nano-confined water, as it implies that polarisability may need to be included if we are to gain detailed mechanistic insights into wetting/de-wetting processes. These findings are of importance for the design of functionalised biomimetic nanopores (for e.g. sensing or desalination), as well as for furthering our understanding of the mechanistic processes underlying biological ion channel function.