Figure 5.

The free energies, ΔG^x (in kcal/mol), for Na⁺ to displace K⁺ (numbers in black) and Ca²⁺ (numbers in blue) bound to (a) 3 –CONHCH₃ ligating groups (representing backbone peptide groups denoted by B) in the BBB filter and (b) 3 OH-ligating groups (representing Ser side chains) in the SSS filter (eq 1). ΔG¹refers to the metal exchange free energy in the gas phase, whereas ΔG¹⁰ and ΔG³⁰ refer to the metal exchange free energies in an environment characterized by an effective dielectric constant of 10 and 30, respectively. The free energies for metal exchange in a rigid Na⁺-optimized SSS and BBB filters prohibited from relaxing upon K⁺ or Ca²⁺ binding are in parentheses. Shown are B3-LYP/6-31+G(3d,p) fully optimized structures of Na⁺ bound to the model SFs.