The energetics of ionic selectivity in the neuronal sodium channels is
studied. A simple model constructed for the selectivity filter of the channel
is used. The selectivity filter of this channel type contains aspartate (D),
glutamate (E), lysine (K), and alanine (A) residues (the DEKA locus). We use
Grand Canonical Monte Carlo simulations to compute equilibrium binding
selectivity in the selectivity filter and to obtain various terms of the excess
chemical potential from a particle insertion procedure based on Widom's method.
We show that K+ ions in competition with Na+ are efficiently excluded
from the selectivity filter due to entropic hard sphere exclusion. The
dielectric constant of protein has no effect on this selectivity. Ca2+
ions, on the other hand, are excluded from the filter due to a free energetic
penalty which is enhanced by the low dielectric constant of protein.Comment: 14 pages, 7 figure