The hydration structure of the Na+ and K+ ions and the selectivity of their ionic channels

Some ions, mainly Na+ and K+, play a fundamental role in the excitability of the nerves and muscles because they are transported through the ionic channels of their cells. The selectivity of these channels is related to the hydration structures of the ions that were determined for Na+ and K+ by means of Monte Carlo simulations of systems constituted by water molecules and the ions at infinite dilution. The Na+ hydration shell, which is formed by three water molecules, is quite stable while the K+ ion can only perturb the water structure in its immediate neighborhood so that it is unable to attract the water molecules. As a result the diameter of the hydrated Na+ is about 0.5 nm while the K+ diameter is equal to its ionic diameter. Taking into account the steric hindrance and the energy profiles, K+ can pass through 0.3 nm diameter channels while the dimensions of the Na+ channel must be at least 0.5 x 0.3 nm(2) so that the ion maintains part of its solvation shell. Grand canonical Monte Carlo simulations were also conducted on water confined in cylindrical channels. In a 0.3-nm diameter rigid channel, the mean water concentration is close to 90% of the bulk one and the mean configurational energy is equal to 50% of the bulk energy. In a 0.5-nm diameter rigid channel, these fractions are 60 and 67% respectively. The results suggest that water molecules can be substituted by a K+ ion in its channel without large energetic cost and that the exchange between the molecules of the medium and the hydration molecules can occur at a low energetic cost in the Na+ channel. These results are in good agreement with experimental data on the selective transport of these ions.