Selective Complexation of K+ and Na+ in Simple Polarizable Ion-Ligating Systems

An influx of experimental and theoretical studies of ion transport protein structure has inspired efforts to understand underlying determinants of ionic selectivity. Design principles for selective ion binding can be effectively isolated and interrogated using simplified models composed of a single ion surrounded by a set of ion-ligating molecular species. While quantum mechanical treatments of such systems naturally incorporate electronic degrees of freedom, their computational overhead typically prohibits thorough dynamic sampling of configurational space and, thus, requires approximations when determining ion-selective free energy. As an alternative, we employ dynamical simulations with a polarizable force field to probe the structure and K+/Na+ selectivity in simple models composed of one central K+/Na+ ion surrounded by 0-8 identical model compounds: N-methylacetamide, formamide, or water. In the absence of external restraints, these models represent gas-phase clusters displaying relaxed coordination structures with low coordination number. Such systems display Na+ selectivity when composed of more than similar to 3 organic carbonyl-containing compounds and always display K+ selectivity when composed of water molecules. Upon imposing restraints that solely enforce specific coordination numbers, we find all models are K+-selective when similar to 7-8-fold ion coordination is achieved. However, when models composed of the organic compounds provide similar to 4-6-fold coordination, they retain their Na+ selectivity. From these trends, design principles emerge that are of basic importance in the behavior of K+ channel selectivity filters and suggest a basis not only for K+ selectivity but also for modulation of block and closure by smaller ions.