Electrostatic Control of Polymer Translocation Speed through α‑Hemolysin Protein Pore.

The electrostatic origins behind the speed of translocation of a uniformly charged flexible macromolecule through α-hemolysin (αHL) protein pores under a voltage are investigated using variations in pH and electrolyte concentration. We have measured durations of successful threading of poly(styrenesulfonate) through αHL at two different pH conditions, pH 4.5 and pH 7.5, under various salt concentration conditions. Salt concentrations in the donor (cis) and the recipient (trans) compartments influence the polymer translocation dynamics differently, depending on pH. At both pH 4.5 and pH 7.5, decreasing the cis salt concentration, cs,cis , results in faster polymer translocations. On the other hand, a decrease in trans salt concentration, cs,trans , retards the polymer transport process at pH 4.5, while at pH 7.5 the translocation time is observed to be independent of cs,trans . We present a theoretical model to calculate the translocation times from the free energy of the polymer along the translocation process to describe our experimental results. We show that the charge density of the polymer inside the nanopore is significantly affected by cs,cis , explaining the cis salt effect on the speed of polymer translocation. The trans salt effects are attributed to the electrostatic interaction between the polymer and the exit portion of the αHL pore, which is determined by the pH of the trans compartment. At low pH where the net charge of the end of the αHL is positive, the attractive electrostatic interaction in trans becomes stronger, as cs,trans decreases, resulting in delays in translocation process.