Rapid and selective ion transport is essential for the generation and regulation of electrical signaling pathways in living organisms. In this study, we use molecular dynamics simulations and free energy calculations to investigate how the bacterial sodium channel Na(v)Ab (Arcobacter butzleri) differentiates between Na+ and Ca2+ ions. Multiple nanosecond molecular dynamics simulations revealed distinct binding patterns for these two cations in the selectivity filter and suggested a high affinity calcium binding site formed by backbone atoms of residues Leu-176 and Thr-175 (S-CEN) in the sodium channel selectivity filter. (C) 2012 Elsevier Inc. All rights reserved.