Sodium and potassium ions are among the most abundant alkali metal ions in aqueous solution and the details of the interactions of these ions with water are crucial for understanding diverse chemical and biological processes. Of note is the selective permeability of cation channels, which is thought to rely on subtle structural differences between the hydration shells of Na+ and K+ ions. During the past decade, atomic-level molecular dynamics (MD) simulations have been used extensively to study K+-selective ion channels. As a result, a picture of the molecular mechanism became available suggesting that the selectivity filters (SF) of ion channels are finely tailored to mimic the structure of the first solvation shell of the permeant cation. The recent structure determinations of Na+-selective channels provide further support to the notion that the first solvation shell of cations plays a key role in selectivity. However, atomistic MD studies show that, in contrast to K+ channels, Na+ binds to the SF of Na+ channels in an almost fully solvated form, seemingly disproving the hypothesis that hydration-shell mimicry plays a major role. Multi-dimensional free energy calculations unveil a more complex role of solvation in ion conduction: while in K+ channels permeation occurs through a concerted displacement of waters and ions across four distinct sites, in Na+ channels diffusion of ions through the SF is less constrained and is related to a dynamic remodeling of the solvation shell of a pair of weakly coupled Na+ ions.

Solvation of cations in biological channels

248th National Meeting (2014)

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