Physics Colloquium

I will discuss ion transport of a protein ion channel in lipid membranes or water filled nanopores in silicon films. It is known that due to the large ratio of dielectric constants of water filling the channel and of the surrounding media, the electric field of an ion placed inside the channel is bundled inside the channel, so that the ion has a large electrostatic self-energy barrier. Two such ions are connected by linearly growing with distance interaction similarly to two quarks. This should lead to negligible conductance of the channel. Nevertheless ion channels function. I will talk about two mechanisms employed by Nature for reduction of the electrostatic barrier namely effects of salt ions dissolved in water and of immobile charges on the internal channel walls. Both type of charges lead to insulator-metal crossover. The first transition resembles Mott transition in the exciton gas with increasing density of excitons (or de-confinement of quarks in high density of matter); the second one resembles transition in a doped semiconductor with growing concentration of impurities. Of course, I will be talking about completely classical phenomenon (happening in water at room temperature), where the entropy plays the role of quantum mechanics.