Membrane transport - A molecular 'straw' Getting water across lipid membranes is not easy. In nature, molecules called aquaporins, discovered in the 1990s, move water from one side of a biological membrane to another, but the molecules are fragile and bulky. Now, researchers from the A*STAR Institute of Bioengineering and Nanotechnology have synthesized a much smaller molecule, which behaves like a �molecular drinking straw� and which may have applications in water purification and elsewhere.

For some years, Huaqiang Zeng of the Institute of Bioengineering and Nanotechnology has led a team aiming to produce tubular molecules that could pipe water across membranes. In 2012, they created molecules that stacked into a helical tube; unfortunately, this tube was not particularly good at holding water in its central tunnel.

Undeterred, Zeng team set out to modify that molecule. Substituting a carboxyphenyl group for a carboxybenzyl group was just what was needed � once again the molecules stacked into a helix, but this time it comfortably held a �string� of water molecules.

�The continuous one-dimensional �water chain� trapped by the molecules is indispensable for mediating water transport across a lipid membrane,� says Zeng.

But early experiments attempting to use osmotic pressure to drive water through the �straw� into a membrane-bound compartment (vesicle) drew a blank. �We repeatedly failed to demonstrate the water-transporting ability of the molecule when using a sodium chloride concentration gradient,� he says.

�Despite my skepticism, we proceeded to investigate whether a proton gradient could induce water transport. We were very surprised to find that it could.�

The system is the first known example of �proton gradient induced� water transport. �I am not aware of any other man-made or natural system that does this,� says Zeng. �It seems to be without precedent.�

Zeng explains that the team �molecular straw� beats the naturally occurring aquaporins in a number of ways. It is 40 times lighter, has a higher thermal stability, is expected to scale-up more easily for industrial applications and retains its functionality better when inserted into biomimetic membranes.

Zeng thinks this, and derivative molecules, may become �next-generation nanofiltration membranes for water purification applications, including sea-water desalination and waste-water reclamation.�

He says that osmotic agents often have to be at concentrations exceeding 100 millimolar to drive water movement in forward osmosis nanofiltration. �If a proton gradient is used as the driving force instead, the concentration difference needed would be exceedingly small." Zeng says. "This would translate into huge energy savings on an industrial scale.�