Assessing Biomolecular Hydration Mimicry for Ion Permeation through Membrane Channels

Membranes separate molecular components from liquid and gas mixtures, with current and urgent applications in water purification and carbon dioxide capture. Compared with cellular membranes in biological systems, traditional polymeric membranes lag behind in terms of selectivity and flux, thus limiting their utility. An alternative approach is to develop bio-inspired membranes that incorporate key structural features of cellular membranes to improve performance, but with those structures translated into a robust synthetic framework that can withstand industrial operating conditions. To facilitate membrane design, we apply molecular simulation and highly accurate free energy calculations to probe how atomic structure relates to function in cellular membranes.

An interesting idea relevant to designing ion-selective membranes is that ions pass readily through ion-specific membrane proteins when pore-lining structures provide an environment similar to the hydration environment of ions in aqueous solution. The local biomolecular structure may arrange an ion binding free energy approximately equal to the free energy of ions in bulk liquid water. Here, we evaluate this hydration mimicry idea by applying ab initio molecular dynamics, electronic structure calculations, and a statistical mechanical free energy theory to alkali metal ions (Li, Na, K, Rb), alkaline earth ions (Mg, Ca, Sr, Ba) and binding sites from channels selective for K, Mg, or Ca. Our results lead to several new conclusions about the mechanism of selective ion binding and permeation that highlight the surprising role of the distant environment, and provide an alternative mechanism in cases where hydration mimicry does not apply.