Statistical physics of ion channels. No life without entropy!

by Prof. Boris I. Shklovskii (Department of Physics, University of Minnesota)

Tuesday 8 Sept 2009, 16:00 → 18:00 Europe/Rome

Aula Conversi (Dipartimento di Fisica - Ed. G.Marconi)

I consider ion transport of a protein ion channel in a lipid cell membrane. It is known that due to the large ratio of dielectric constants of water filling the channel (81) and of the surrounding lipid (2), an ion placed inside the channel has its electric lines confined in the channel. This should lead to a large electrostatic self-energy barrier and to exponentially large Ohmic resistance of the channel. In other words, one can say that in the channel a pair of positive and negative salt ions is bound by a string with potential energy growing linearly with the distance between them, and, therefore, a large energy is required to break the pair. In this sense, the ion pairs should resemble mesons of two confined quarks. Nevertheless biological channels are well transparent for ions. In order to address this paradox, we study reduction of the electrostatic barrier by a finite concentration of salt in water and/or by immobile charges on the internal channel walls. We show that both types of charges lead to the insulator-metal crossover (elimination of the barrier) with their increasing concentrations. The first one resembles the Mott insulator-metal transition in an exciton gas with increasing density of excitons. The second one resembles the Mott transition in a doped semiconductor with growing concentration of impurities. What happens in ion channel is actually very similar to quark de-confinement at large density of mesons in Brookhaven experiments with heavy ion collisions.