Electroosmosis through α-Hemolysin That Depends on Alkali Cation Type

We demonstrate experimentally the existence of an electroosmotic flow (EOF) through the wild-type nanopore of alpha-hemolysin in a large range of applied voltages and salt concentrations for two different salts, LiCl and KCl. EOF controls the entry frequency and residence time of small neutral molecules (beta-cyclodextrins, beta CD) in the nanopore. The strength of EOF depends on the applied voltage, on the salt concentration, and, interestingly, on the nature of the cations in solution. In particular, EOF is stronger in the presence of LiCl than KCl. We interpret our results with a simple theoretical model that takes into account the pore selectivity and the solvation of ions. A stronger EOF in the presence of LiCl is found to originate essentially in a stronger anionic selectivity of the pore. Our work provides a new and easy way to control EOF in protein nanopores, without resorting to chemical modifications of the pore.