Ion transport through mechanically constrained lipid electropores

VERNIER, P. THOMAS; RISK, MARCELO RAÚL; FERNÁNDEZ ML

Norfolk, Virginia

Conferencia; 2 nd World Congress on Electroporation and Pulsed Electric Fields in Biology, Medicine and Food & Environmental Technologies; 2017

In this work we extend that method to eliminate the dependence of the pore radius on the applied electric field, a limitation of theprevious method. We do this by imposing a mechanical constraint on the phosphorus atoms in the phospholipid head groups after a pore has been field-stabilized to the desired radius. Different external electric fields are then applied to these constrained pores, with no effect on the radius, to evaluate ion transport through pores of different radii in the absence of any interaction between the applied field and the pore radius. Methods: The simulations were performed using GROMACS version 4.6.6 for 128 POPC bilayer systems with 8962 water molecules and 22 K+ and 22 Cl- for KCl systems and 40 Na+ and 40 Cl- for NaCl systems. A first simulation was performed at high electric field in order to obtain an initial configuration of an open pore. This configuration was usedto initiate simulations in the presence of three different sustaining electric fields (100, 75 and 50 MV/m) for 10 ns to obtain large, medium, and small radius pores. The final configuration of the large, medium, and small radius pore simulations was the initial configuration for the next steps. Position constraints were imposed on the phosphate groups. In these constraint conditions the pores remain open and stable for the whole simulation regardless of the applied electric field. Results and Discussion: Different sustaining electric fields (Es) generate pores with  correspondingly different radii when they are applied to unconstrained lipid bilayers. Position constraints affecting only the phosphorus atoms keep the pores in a fixed but flexible geometry, preventing the expansion or contraction of the pore. Under these conditions the geometry of the electropores becomes independent of the applied electric field. The range of the pore radius for NaCl systems is [0.04?0.08 nm] for simulations when Es is 50 MV/m, [0.49?0.57 nm] when Es is 75 MV/m, and [1.15?1.20 nm] when Es is 100 MV/m. For KCl systems the ranges are [0.30?0.32 nm], [0.74?0.81 nm] and [1.67?1.73 nm] for Es 50, 75, and 100 MV/m, respectively. With these mechanically constrained pores, ion transport can be measured through a pore of any desired radius with any desired applied voltage. Ion current was measured in systems containing NaCl and KCl. Both systems were exposed to different Es to obtain stabilized pores, and then these pores were constrained as described above. In a third step an external electric field was applied to the constrained geometries, E: 50, 75 and 100 MV/m. The current for the cations, and for the anions for the three applied electric fields was calculated. With this new mechanical constraint procedure, a high electric field can be applied to a small pore or a low electric field to a large pore, separating the size of the radius of the pore from the intensity of the applied electric field.