Tools to Quantify Molecular Transport: Electroosmosis Dominates Electrophoresis of Fluoroquinolones Across the Outer Membrane Porin F (OmpF)

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.

Tools to Quantify Molecular Transport: Electroosmosis Dominates Electrophoresis of Fluoroquinolones Across the Outer Membrane Porin F (OmpF)

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.

Electroosmotic Flow in Hydrophobic Microchannels of General Cross Section

Adopting the Navier slip conditions, we analyze the fully developed electroosmotic flow in hydrophobic microducts of general cross section under the Debye–Hückel approximation. The method of analysis includes series solutions which their coefficients are obtained by applying the wall boundary conditions using the least-squares matching method. Although the procedure is general enough to be applied to almost any arbitrary cross section, eight microgeometries including trapezoidal, double-trapezoidal, isosceles triangular, rhombic, elliptical, semi-elliptical, rectangular, and isotropically etched profiles are selected for presentation. We find that the flow rate is a linear increasing function of the slip length with thinner electric double layers (EDLs) providing higher slip effects. We also discover that, unlike the no-slip conditions, there is not a limit for the electroosmotic velocity when EDL extent is reduced. In fact, utilizing an analysis valid for very thin EDLs, it is shown that the maximum electroosmotic velocity in the presence of surface hydrophobicity is by a factor of slip length to Debye length higher than the Helmholtz–Smoluchowski velocity. This approximate procedure also provides an expression for the flow rate which is almost exact when the ratio of the channel hydraulic diameter to the Debye length is equal to or higher than 50.