main chain atoms. The precise orientation
of individual carbonyl oxygens cannot be
discerned at the resolution of this x-ray
analysis, but we propose that they are
directed inward to account for K1 ion
coordination (Fig. 8B). A single water
molecule (the only one modeled in the
structure) located between the two K1
ions in the selectivity filter was justified
by the presence of a strong electron density
peak in the experimental map, which
was never associated with an ion peak in
the difference Fourier maps (19).