The Entry of Ions into a Molecular Synthetic Channel in a Membrane

A molecular dynamics simulation is conducted to describe the behaviour of sodium and chloride ions as they enter a synthetic ion channel (mounted in a bilayer membrane) from aqueous NaCl solutions on either side of the membrane. The channel consists of an α-helical peptide chain scaffold with six aligned crown ether (CE) rings (18-CE-6) as side groups, forming a molecular chamber between each neighbouring pair. Responding to the channel’s axial −2 to −1 V electric potential, the Na+ ions, but not Cl−, enter the channel spontaneously but they do not proceed beyond the first chamber formed between CE rings 1 and 2. The application of an axial electric field promotes the entry of a Na+ ion and its migration over the internal length of the channel. The forces that drive the migration phenomena are predominantly coulombic. Although the same electric field simultaneously allows a Cl− ion initially to access the channel the ion is subsequently expelled from the first chamber into the bilayer. Although a Na+ ion may make a facile or even spontaneous entry to the channel it requires an energy estimated from Coulomb forces as ~5 eV to pass subsequent CE rings, and considerably more to exit the channel. An important role is found for the vibrational activity of the ether rings’ C-O-C units in their facilitating contribution to the migration of Na+ in the channel.