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Proton Transport Through Membrane-Embedded Carbon Nanotubes

Emad Tajkhorshid, University of Illinois at Urbana-Champaign

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John Stone, Emad Tajkhorshid, Tao Jiang, Shashank Pant, Sepehr Dehghanighahnaviyeh, Mariano Spivak

Carbon nanotubes embedded in biological membranes are shown to act as biomimetic channels that can transport protons at high rates. The hydrophobic walls of narrow nanotubes can confine water to one dimensional arrangements, thereby achieving proton diffusion constants an order of magnitude larger than in bulk water. The study of proton transport in this context has been focused on the mechanistic details inside the nanotube walls, where the motion of the excess charge is almost barrier-less. However, previous experimental work suggests the limiting factor of the mechanism be the transport of the excess charge from the bulk/solution to the water chain inside the carbon nanotube.

To gain an overall understanding of the entire proton transport process, in this work, we will apply hybrid quantum mechanical/molecular mechanics method to simulate proton transport between the bulk water on two sides of the membrane through membrane-embedded nanotubes. To characterize the effect of nanotube and membrane environment on proton transport, the bias exchange umbrella sampling method will be used to calculate the energetics of proton transport through nanotubes of different diameters, in the presence of different lipid compositions.

The calculation of free energies requires massive computing resources, which can only be realized on petascale platforms such as Blue Waters. Using the state-of-art computational techniques, the obtained knowledge from this project will help to unravel the mechanistic details underlying the proton transport mechanism through biomimetic channels, which provide insight into the broad application of man-made materials in transmembrane transport under physiological conditions.