Size selectivity by cation-pi interactions: Solvation of K+ and Na+ by benzene and water

Size-specific interaction of alkali metal ions with aromatic side chains ha s been proposed as a mechanism for selectivity in some K+ channel proteins. Experiments on gas-phase cluster ions of the form M+(C6H6)(n) (H2O)(m), wi th M = Na or K, have demonstrated that the interaction between benzene and K+ is sufficiently strong to result in partial dehydration of the ion, i.e. , benzene will displace some water molecules from direct contact with the i on. In sharp contrast, there is no evidence that benzene can displace water from the first hydration shell of Na+. The resistance of Na+ (H2O)(4) towa rds dehydration in an aromatic environment suggests a molecular-level mecha nism for the low permeability of Na+ through the pore region of K+ channel proteins: the hydrated Na+ ion is too large to pass, while K+ can shed enou gh of its hydration shell to fit through the pore. These results also sugge st that it may be possible to design a new class of ionophores that take ad vantage of the cation-pi interaction to confer ion selectivity. This is the first experimental evidence that K+ selectively interacts with an aromatic complex in an aqueous environment, while Na+ does not. A remarkable sideli ght from this study was the discovery of a self-assembled cluster ion, Na(C6H6)(8)(H2O)(4), with a single structure: an inner shell of four water mo lecules and an outer layer of eight benzene molecules, each of the latter f ixed by a pi-hydrogen bond to one of the eight interior O-H groups. (C) 199 9 American Institute of Physics. [S0021-9606(99)00817-X].