Fig. 3: V625 backbone carbonyls can flip when K⁺ ions are present in the filter.

A Structures of high-K (left) and low-K (right) selectivity filters determined with C4 symmetry. The labels S0–S4 highlight canonical K⁺ ion binding sites. B Distribution of backbone carbonyl angles, θ (0° indicates pointing towards center of the pore) from MD simulations with ions held in the configurations shown. The proportion of time all 4 backbone carbonyls were pointing inwards (defined as <70° deviation from 0°) is shown at the bottom. Backbone carbonyl angle distributions for single ion configurations are shown in Supplementary Fig. S8. C Free energy profile derived from Umbrella Sampling of rotation of the Ψ angle of the V625-Gly626 linkage when ions are held in S0/S2/S4 (blue), or S1/S3/Cav (cavity) (orange). The energy minima for V625 backbone carbonyls pointing towards and away from the central axis are indicated at Ψ = −50° and +70°, respectively. D Potential mean force for ion binding in the filter from REST2 simulations for 0 (red), 1 (green), 2 (blue), 3 (cyan), or 4 (black) flipped V625 backbone carbonyls. Barriers for the 4 flipped conformation that are not well sampled are shown as black dashed lines. Data shown as mean ± SD obtained from MBAR³³.