November 04 2017 by Brantley Henson
It’s not often that scientists are able to outperform nature, especially when it comes to protein functionality. After all, evolution has had millions upon millions of years to hone natural proteins to perfection. However, a group of researchers from Lawrence Livermore National Laboratory, Northeastern University in Boston, and UC Merced have recently managed to make more efficient cell membrane protein analogs using synthetic carbon nanotubes (CNTs).
High Efficiency Porins
The scientists were successful in creating a high functioning synthetic aquaporin. Aquaporins are proteins found in cell membranes that serve as channels to transport water and some small ions into and out of the cell. The efficiency with which aquaporins are able to selectively transport water motivated researchers to develop a synthetic bioinspired water channel. The end result: development of narrow CNTs that, when integrated into a lipid bilayer like those found in a cell membrane, has a water permeability 6 times greater than that of the naturally occurring water transport protein Aquaporin-1 (AQP1). Their work was published in the journal Science.
Nano's Advantage over Natural Proteins
So how exactly can a CNT outperform AQP1? As water nears the 0.3 nm pore formed by AQP1, a hydrophobic narrow channel forces water molecules to move through it in a single file line. At the molecular level, this requires each water molecule to reduce the number of hydrogen bonds it has with neighboring molecules to facilitate an easy flow through the channel. However, this system is not perfect. Proteins by their nature are composed of amino acid residues that interact with water. As water passes through the protein channel, momentary hydrogen bonds form and break between H2O and amino acid residues found in the aquaporin structure. This slows the rate at which water can pass through the aquaporin. In the synthetic CNT porin the channel is similarly narrow at 0.8 nm and hydrophobic, but is “atomically smooth”. The smoothness of the channel prevents hydrogen bonding from occurring between the water and the CNT itself. It is this smoothness that sets the synthetic channel apart from its naturally occurring analog, resulting in a higher permeability.
A single chain of water molecules lines the cavity inside a carbon nanotube porin, which is embedded in a lipid bilayer. Image by: Y. Zhang and Alex Noy/LLNL.
The researchers found that narrow CNTs not only transport water more efficiently, but also had the capability for selective ion transport. A charge potential can be applied to the opening of the CNT channel causing unidirectional selective transport of ions across the membrane. This implies the CNTs could also be used as a switchable ionic diode.
The applications of this technology are far reaching, including the promise for improved water purification. The authors of this study point out that the CNT porins could improve the efficiency and scalability of high-flux desalination membranes used in the production of fresh water. This would potentially mitigating global fresh water shortages due to rising demands.
- Tunuguntla, Ramya H., et al. "Enhanced water permeability and tunable ion selectivity in subnanometer carbon nanotube porins." Science 357.6353 (2017): 792-796. DOI: 10.1126/science.aan2438