Effect of electroosmotic flow on the transport of α-cyclodextrin through the channel CymA

Abstract

CymA, an outer membrane channel of Klebsiella oxytoca, allows the passive diffusion of the bulky molecule α-cyclodextrin (α-CD, M.W. 972.8 Da) to the periplasm of the bacterium [1]. In single channel electrophysiology experiments, the flow of the uncharged α-CD was found asymmetric with respect to the applied voltage and ionic salts used. The net water current associated with the ion movement, i.e., the so-called electroosmotic flow (EOF), is induced by the ionic selectivity of the pore. This effects has been found to be a major factor behind the modified interactions of the α-CD molecule with the channel [2]. To get atomistic insight into the EOF on α-CD permeation, we have performed ∼40 μs free energy calculations in presence of three different ionic conditions, i.e., in the absence of ions, in the presence of 1 M KCl and of 1 M MgCl2, using well-tempered matadynamics simulations [3] applying an external field of 0 V, +1 V and −1 V. No major changes in the free energy landscapes were observed in the absence of ions at both polarities of the voltage. This finding indicates the absence of an electrophoretic effect on the neutral α-CD molecule and of an EOF mediated effect due to the absence ions. However, using an electric field together 1 M KCl salt, we observed significant free energy changes in the transport of the α-CD consistent with net EOF at both voltage polarities. Moreover, using 1 M MgCl2 salt, we demonstrate an alteration of the pore selectivity from cationic to anionic. Thereby, the direction of the resulting EOF at a particular voltage polarity and its effect on the α-CD permeation is inverted. These results highlight the role of the EOF in the transport of α-CD through the nanometer-sized CymA channel.