sumably, K1 ions) in a high contour native
electron density map (not shown). Thus,
the selectivity filter contains two K1 ions.
A third weaker peak is located below the
selectivity filter at the center of the large
cavity in the Rb1 difference map (Fig. 6a,
cavity ion) and in the Cs1 difference map
at a lower contour (not shown). Electron
density at the cavity center is prominent in
MIR maps, even prior to averaging (Fig. 6c,
lower diffuse peak). The difference electron
density maps show this to be related to the
presence of one or more poorly localized
cations situated at least 4 Å away from
the closest protein groups.