Overcharging below the nanoscale: multivalent cations reverse the ion selectivity of a biological channel.
We report charge inversion within a nanoscopic biological protein ion channel in salts of multivalent ions. The presence of positive divalent and trivalent counterions reverses the cationic selectivity of the OmpF channel, a general diffusion porin located in the outer membrane of E. coli. We discuss the conditions under which charge inversion can be inferred from the change in sign of the measured quantity, the channel zero current potential. By comparing experimental results in protein channels whose charge has been modified after site-directed mutagenesis, the predictions of current theories of charge inversion are critically examined. It is emphasized that charge inversion does not necessarily increase with the bare surface charge density of the interface and that even this concept of surface charge density may become meaningless in some biological ion channels. Thus, any theory based on electrostatic correlations or chemical binding should explicitly take into account the particular structure of the charged interface.
García-Giménez
Elena
E
Department of Physics, Laboratory of Molecular Biophysics, University Jaume I, Avenida Sos Baynat, s/n 12080 Castellón, Spain.
Alcaraz
Antonio
A
Aguilella
Vicente M
VM
D019943:Amino Acid Substitution; D002412:Cations / Q000378:metabolism*; D029968:Escherichia coli Proteins / Q000737:chemistry / Q000235:genetics / Q000378:metabolism; D015640:Ion Channel Gating; D008954:Models, Biological; D009154:Mutation; D036103:Nanotechnology; D018272:Porins / Q000737:chemistry* / Q000235:genetics / Q000378:metabolism*; D012492:Salts / Q000737:chemistry / Q000378:metabolism; D013379:Substrate Specificity
D002412:Cations; D029968:Escherichia coli Proteins; C109237:OmpF protein; D018272:Porins; D012492:Salts