Single Molecule Channels Water

Chinese researchers have made the first nanoscale artificial water channels that selectively transport water across lipid membranes. Such a system could be used for applications such as water purification and desalination.

Jun-Li Hou and coworkers at Fudan University, in Shanghai, constructed the channels from single pillar[5]arene molecules with hydrazide-containing side chains (J. Am. Chem. Soc., DOI: 10.1021/ja302292c). Intramolecular hydrogen bonds between the side chains curve the molecules into tubes.

Hou and coworkers inserted these tubes into vesicles to form membrane-spanning, water-transporting channels. The researchers controlled transport via salt concentration and osmosis, with higher salt concentrations inside the vesicles. “The osmotic pressure difference prevents water inside the vesicles from coming back out,” Hou says.

Although several groups had previously reported water-trapping molecules, none had demonstrated a synthetic channel that could selectively transport water, says Huaqiang Zeng of the National University of Singapore. The new work of Hou and coworkers “represents a very important step toward realizing the full potential of water-transporting molecules for their eventual uses as nanofiltration membranes and devices for water purification,” Zeng says.

The hydrogen-bonding pattern is reminiscent of the interactions between peptides in aquaporins, proteins that transport water across cell membranes, says Peter J. Cragg of the University of Brighton, in England. Studying such synthetic molecules could help researchers better understand the workings of biological channels as well, he notes.

“Understanding how any transmembrane channel-forming molecule works is of fundamental importance, and we still know so little about how these systems select chemical species or switch their transport on and off in a process known as gating,” Cragg says. “Observing ion transport by model transmembrane channels will help to identify those chemical motifs that are responsible for selectivity and gating.”

Hou and his coworkers are working on tweaking the pillar[5]arene backbone to improve transport efficiency and to better control the process.