Figure 6. Binding of Cl^– and/or OH^– to H₃O⁺-bound mutant SFs.

B3LYP/6-31+G(3d,p) fully optimized structures of H₃O⁺–SF, Cl^––SF and OH^––SF complexes, and free energies (in kcal/mol) for (a) [SF(Ala-Arg⁺)-H₃O⁺] + Cl^– → [SF(Ala-Arg⁺)-Cl^–] + H₃O⁺, (b) [SF(Ala-Arg⁺)-H₃O⁺] + OH^– → [SF(Ala-Arg⁺)-OH^–] + H₃O⁺, and (c) [SF(His-Arg⁺)-H₃O⁺] + Cl^– → [SF(His-Arg⁺)-Cl^–] + H₃O⁺. ΔG¹ is the ion exchange free energy in the gas phase, whereas ΔG⁴ and ΔG³⁰ are the corresponding free energies in the SF characterized by an effective dielectric constant of 4 and 30, respectively. If the resulting free energy is negative, the pore is Cl^– or OH^–-selective, but if it is positive, the pore is proton-selective.