Science Topics – 105

In bulk water, water molecules are hydrogen bonded each other, and proton moves freely and randomly through this network by the Grotthuss mechanism. How does proton permeate in one-dimensional water chain (single-file water chain)? The mechanism underlying this issue is important not only for the proton-related chemical reaction but for understanding permeation mechanism of proton channels. Several types of proton channels exist in biomembranes, and they have a narrow pore, in which water molecules are aligned in single-file and proton jumps through one-dimensional hydrogen-bonded chain. We applied a peptide channel, polytheonamide B, extracted from marine sponge, as a test nanotube to examine single-channel proton conduction through the single-file water chain. We found surprisingly that the flow rate of proton in one direction overwhelmed the other. This is called a rectification, which is the first observation of the rectified proton permeation. The mechanism underlying the rectification was related to the water turnover. These results help to understand mechanism in proton channels and served as a template for designing proton permeable membranes for the fuel cell.

Figure 1. Proton jumps in the three-dimensional water network and one-dimensional water wire.
A. Water molecules in bulk solution form a three-dimensional hydrogen-bonded network, and proton in water (H3O +) jumps freely through the network. B. Water molecules in the narrow pore of channels are aligned in single file, and mechanism of proton transfer differs substantially. C. Proton jumps along the hydrogen bonded-chain (hop). After a proton exits the pore, all of the water molecules in the pore are reversely oriented. For permeating another proton, water molecules must reverse their orientation (turn). We revealed that the flow rate of proton from the C-terminal side of the polytheonamide B channel to the N-terminal side (red arrow) overwhelmed the other (blue arrow)(rectification). The analysis suggested that the turn process is responsible for the rectification.

Y. Matsuki, M. Iwamoto, K. Mita, K. Shigemi, S. Matsunaga, S. Oiki: Rectified proton grotthuss conduction across a long water-wire in the test nanotube of the polytheonamide B channel. J. Am. Chem. Soc. 136, 4168-4177, 2016

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1. Departments of Anesthesiology and Reanimatology, University of Fukui, Faculty of Medicine Sciences, Fukui, Japan
2. Departments of Molecular Physiology and Biophysics, Faculty of Medicine Sciences, The University of Fukui, Japan