Origin of current blockades in nanopore translocation experiments.

We present a detailed investigation of the ionic current in a cylindrical model nanopore in the absence and the presence of a double stranded DNA homopolymer. Our atomistic simulations are capable of reproducing almost exactly the experimental data obtained by Smeets et al., including notably the crossover salt concentration that yields equal current measurements in both situations. We can rule out that the observed current blockade is due to the steric exclusion of charge carriers from the DNA, since for all investigated salt concentrations the charge carrier density is higher when the DNA is present. Calculations using a mean-field electrokinetic model proposed by van Dorp et al. fail quantitatively in predicting this effect. We can relate the shortcomings of the mean-field model to a surface related molecular drag that the ions feel in the presence of the DNA. This drag is independent of the salt concentration and originates from electrostatic, hydrodynamic, and excluded volume interactions.