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First proof of hydrogen-bonded water wires could lead to better desalination

Posted March 28, 2018

Approximately 70 percent of the Earth’s surface is covered by water, but only 2.5 percent of that is fresh water, and less than half of that is easily accessible. Efficient desalination, therefore, is crucial for sustaining life now and in years to come.

But a challenge in developing bioinspired synthetic purification systems lies in deciphering the complexity of natural systems like aquaporins (AQPs), nature’s most efficient water transport system. These proteins form pores in the membranes of biological cells, facilitating highly efficient water transport.

An illustration of the first direct observation of a dipolar oriented chiral water wire inside a transmembrane channel. Credit: Cornell University

An international research partnership – co-led by Poul Petersen, assistant professor in the Department of Chemistry and Chemical Biology – has made the first direct observation of the structure of water inside artificial water channels. Understanding the “shape of water,” so to speak, under those conditions will help inform biomimetic purification systems of the future.

Petersen is co-senior author of “Oriented Chiral Water Wires in Artificial Transmembrane Channels,” which published in Science Advances. The paper’s other senior authors are Mihail Barboiu of the Université de Montpellier, France; and Georges Belfort of Rensselaer Polytechnic Institute.

Water wires – hydrogen-bonded chains of associated water molecules, which have properties that are very different from bulk water – have been confirmed in theory for years but never actually seen. Petersen and his collaborators, through nonlinear sum-frequency vibrational spectroscopy, claim the first experimental observation of the dipolar oriented (electrically neutral) wire structure of water in nanoscale channels, including supported lipid bilayers, similar to aquaporins.

“I would call this the first real observation of a water wire,” Petersen said. “We’re not just seeing the oxygen [atoms], we see the protons, as well. It’s the first observation of the hydrogen bonding in a water wire.”

This work employs some of the methods Petersen’s lab used in work published in 2017, in which it reported the first observation of a chiral water superstructure surrounding a biomolecule (DNA).

“We can do our chiral method, just like we did for the DNA research, and see that they also form a chiral structure,” Petersen said. “They have a helicity to them – they sort of spiral through.”

One of the key findings: The net dipolar orientation of water molecules in confined channels induced specific polarization of the channel, which likely acts as a driving force for water permeation through the membranes.

The best current method for desalinating seawater is reverse osmosis – fluid passing through a membrane in the direction opposite of naturally occurring osmotic pressure. The group feels that identifying the chirality of water structures within these artificial channels will inspire design of more efficient purification technologies in the future.

“Scientists are trying to make structures that mimic aquaporin – that’s the gold standard for water transport,” Petersen said. “And for aquaporin, we know it’s all about the alignment of the water molecules, and their helical nature, that is important for water transport.”

Source: Cornell University

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