Rectified Proton Grotthuss Conduction Across a Long Water-Wire in the Test Nanotube of the Polytheonamide B Channel.
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A hydrogen-bonded water-chain in a nano-tube is highly proton conductive, and examining the proton flux under electric fields is crucial to understanding the one-dimensional Grotthuss conduction. Here, we exploited a nano-tube-forming natural product, the peptide polytheonamide B (pTB), to examine proton conduction mechanisms at a single-molecule level. The pTB nano-tube has a length of ~40 Å that spans the membrane and a uniform inner diameter of 4 Å that holds a single-file water-chain. Single-channel proton currents were measured using planar lipid bilayers in various proton concentrations and membrane potentials (±400 mV). We found, surprisingly, that the current–voltage curves were asymmetric with symmetric proton concentrations in both solu-tions across the membrane (rectification). The proton flux from the C-terminal to the N-terminal end was 1.6 times higher than the opposite. At lower proton concentrations, the degree of rectification was attenuated, but by adding a pH-buffer (dichloroacetate) that supplies protons near the entrance, the rectification emerged. These results indicate that the permeation processes inside the pore generate the rectification, which is masked at low concentrations by the diffusion-limited access of protons to the pore entrance. The permeation processes were characterized by a discrete-state Markov model, in which hops of a proton followed by water-chain turn-overs were implemented. The optimized model revealed that the water-chain turnover exhibited unusual voltage dependence, and the distinct voltage-dependencies of the forward and backward transition rates yielded the rectification. The pTB nano-tube serves as a rectified proton conductor, and the design principles can be exploited for proton-conducting materials.
- Journal of the American Chemical Society
Journal of the American Chemical Society 138(12), 4168-4177, 2016-03-09
American Chemical Society