Theoretical study of double ion occupancy in a gramicidin-like channel

The potential energy profiles for Cs⁺-Cs⁺, Na⁺-Na⁺, and Cs⁺-Cl^- double occupancy in a gramicidin-like channel are calculated by using the polarizable electropole model. The major part of the interaction energy is the summation of two single occupancy energies plus the Coulombic interaction energy between the two ions. The cooperative effect is small. The local solvation structure near each ion in the doubly occupied channel is very similar to that for single occupancy. Structural comparisons suggest that double occupancy is less probable with Na⁺ than with Cs⁺. A favorable cation-cation separation may lower the energy barrier for cation transport. Cation-anion double occupancy may account for possible anion contributions to conductance.