We have measured the ionic current blockages produced by single molecules of sodium poly(styrene sulfonate) passing through an α-hemolysin protein pore under an electric field. Most of the blockage events were composed of one or two blockage levels of ionic current. By analyzing the statistics of different event types for different polymer lengths, applied voltages, and pH conditions, we have identified the molecular mechanism behind the two-level blockages. Our analysis of the data shows that not all blockages are successful translocation events and the propensity of successful translocation can be tuned by pH gradients across the protein pore. We interpret our results as the change in protein-polymer interaction via protonation of charged amino acid residues of α-hemolysin pore. In addition, we have constructed a stochastic theory for polymer translocation through α-hemolysin pore with tunable polymer-pore interactions. The theoretical calculations capture many features observed in our experiments.
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