Degreve L., Vechi S.M., Quintale C.J. The hydration structure of the Na + and K + ions and the selectivity of their ionic channels

Biochimica et Biophysica Acta, V. 1274, 1996, 149-156 pp.
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 nm² so that the ion maintains part of its solvation shell. Grand canonical Monte Carlo imulations were also conducted on water confined in cylindrical channels.