Selectivity filters of ion channels are commonly single-filing. This paper focuses on ion interactions in such narrow pores and their effects on the pores' electrical properties. Using semi-microscopic Monte Carlo modeling, we discuss the dielectric properties of channel water and aspects of ionic permeation energetics in idealized models of two well characterized systems: gramicidin and the KcsA K⁺-channel. We note that: 1) ionic occupancy dramatically modifies the permittivity of channel water; 2) in both channels carbonyl reorientation is crucial for ion stabilization; 3) in both channels differential selectivity may depend significantly on the binding cavity¹s rigidity; 4) due to selectivity filter design, multiple occupancy at small interionic separation is far more favorable energetically in the KcsA K⁺-channel than in gramicidin.

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