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K+ channels exhibit strong selectivity for K+ ions over Na+ ions based on electrophysiology experiments that mea-sure ions competing for passage through the channel. During this conduction process, multiple ions interact within the region of the channel called the selectivity filter. Ion selectivity may arise from an equilibrium prefer-ence for K+ ions within the selectivity filter or from a kinetic mechanism whereby Na+ ions are precluded from entering the selectivity filter. Here, we measure the equilibrium affinity and selectivity of K+ and Na+ ions binding to two different K+ channels, KcsA and MthK, using isothermal titration calorimetry. Both channels exhibit a large preference for K+ over Na+ ions at equilibrium, in line with electrophysiology recordings of reversal potentials and Ba 2+ block experiments used to measure the selectivity of the external-most ion-binding sites. These results suggest that the high selectivity observed during ion conduction can originate from a strong equilibrium preference for K+ ions in the selectivity filter, and that K+ selectivity is an intrinsic property of the filter. We hypothesize that the equilibrium preference for K+ ions originates in part through the optimal spacing between sites to accommodate multiple K+ ions within the selectivity filter. [ABSTRACT FROM AUTHOR]