Monte Carlo Simulations of the Mechanism for Channel Selectivity:  The Competition between Volume Exclusion and Charge Neutrality

A mechanism for selectivity of calcium over sodium in a biological channel is studied by means of a Monte Carlo simulation. This model channel is an infinite cylinder containing negatively charged glutamates, each modeled as a pair of half charged oxygens, and an electrolyte. The fluid in the channel is in equilibrium with a bulk electrolyte. The proportion of Ca2+ ions in the channel is greatly in excess of that in the bulk because, in the restricted space in the channel, the doubly charged Ca2+ ions require less volume and are more effective at neutralizing the charge of glutamates than twice the number of singly charged Na+ ions with the same amount of charge.