Hydration Mimicry by Membrane Ion Channels

Abstract

Ions transiting biomembranes might pass readily from water through ion-specific membrane proteins if those protein channels provide environments similar to the aqueous solution hydration environment. Indeed, bulk aqueous solution is an important reference condition for the ion permeation process. Assessment of this hydration mimicry view depends on understanding the hydration structure and free energies of metal ions in water to provide a comparison for the membrane channel environment. To refine these considerations, we review local hydration structures of ions in bulk water, and the molecular quasi-chemical theory that provides hydration free energies. In that process, we note some current views of ion-binding to membrane channels, and suggest new physical chemical calculations and experiments that might further clarify the hydration mimicry view.

Absolute ion hydration free energy scale and the surface potential of water via quantum simulation

“…We employ quasi-chemical theory (QCT) (6,10,11) to compute the hydration free energy of the Na + ion. QCT provides a means to partition the free energy spatially into three physical parts.…”

“…single-ion thermodynamics | hydration free energy | DFT | ab initio quantum molecular dynamics | surface potential I on solvation phenomena are central to diverse condensedphase physical, chemical, biological, materials, and energyrelated systems (1)(2)(3)(4)(5)(6). The physical behaviors of solutions that include ions of the same charge but with different size, shape, and/or chemical makeup can vary dramatically.…”

Free Energies of Hydrated Halide Anions: High throughput Computations on Clusters to Treat Rough Energy-Landscapes

“…Important aspects of those mecha-23 nisms in aqueous solution involves the local ion hydration properties, and exchange of 24 hydrating water molecules for molecular ligating groups [3,5,[10][11][12]. As a step toward 25 enhancing our understanding of ion-specific effects, this paper studies the structures 26 and free energies of hydration clusters of the anions F − , Cl − , Br − , and I − in the dilute 27 gas-phase, with the longer term goal of addressing the comparative behavior of this se-28 ries of ions in liquid water on the basis of quasi-chemical theory (QCT) [12][13][14][15]. Accurate 29 characterization of gas-phase clusters can indeed helpfully inform QCT applications, 30 and has been successful for cations [12].…”

“…As a step toward 25 enhancing our understanding of ion-specific effects, this paper studies the structures 26 and free energies of hydration clusters of the anions F − , Cl − , Br − , and I − in the dilute 27 gas-phase, with the longer term goal of addressing the comparative behavior of this se-28 ries of ions in liquid water on the basis of quasi-chemical theory (QCT) [12][13][14][15]. Accurate 29 characterization of gas-phase clusters can indeed helpfully inform QCT applications, 30 and has been successful for cations [12]. Because our analyses here will be limited to 31 gas phase systems, we restrict our work to traditional physical cluster theories [16][17][18][19][20][21][22], 32 progenitors of molecular QCT [23,24].…”

“…They as n varies, the energy combination of Eq. ( 5) is not affected by the electrostatic potential 197 of the phase [12].…”