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submitted on 15.10.2019 and posted on 21.10.2019by Jayesh Arun Bafna, Sushil Pangeni, Mathias Winterhalter, M. Alphan Aksoyoglu
We report that the dynamics of antibiotic capture and transport across a
voltage-biased OmpF nanopore is dominated by the electroosmotic flow rather than the
electrophoretic force. By reconstituting an OmpF porin in an artificial lipid bilayer and
applying an electric field across, we are able to elucidate the permeation of
molecules, and their mechanism of transport. This field gives rise to an
electrophoretic force acting directly on a charged substrate, but also
indirectly via coupling to all other mobile ions causing an electroosmotic
flow. The directionality and magnitude of this flow depends on the selectivity
of the channel. Modifying the charge state of three different substrates
(Norfloxacin, Ciprofloxacin, and Enoxacin) by varying the pH between 6 and 9,
while the charge and selectivity of OmpF is conserved, allows us to work under
conditions where EOF and electrophoretic forces add or oppose. This
configuration allows us to identify and distinguish the contributions of the
electroosmotic flow and the electrophoretic force on translocation. Statistical analysis of the resolvable dwell-times
reveals rich kinetic details regarding the direction and the stochastic
movement of antibiotics inside the nanopore. We quantitatively describe the
electroosmotic velocity component experienced by the substrates, and their
diffusion coefficients inside the porin with an estimate of the energy barrier experienced
by the molecules, caused by the interaction with the channel wall, slowing down
the permeation by several orders of magnitude.