This simulated cross-section shows how the lipid-like peptoids interact to form a membrane. Each peptoid has two sections: a fatty-like region that interacts with its neighbors via benzene rings (shown in pink) to form a sheet. And a water-loving region that juts above or below the flat sheet. Each region can be designed to have specific functions. Image: Chun-Long Chen/PNNL.Materials scientists have created a new material that can spontaneously form a membrane similar to the cell membranes found in nature. Such synthetic cell membranes have long been sought after for use in applications as varied as water purification and drug delivery.
Referred to as a lipid-like peptoid, the material can assemble itself into a sheet that is thinner, but more stable, than a soap bubble, the researchers report in a paper in Nature Communications. The assembled sheet can withstand being submerged in a variety of liquids and can even repair itself after damage.
"Nature is very smart. Researchers are trying to make biomimetic membranes that are stable and have certain desired properties of cell membranes," said chemist Chun-Long Chen at the US Department of Energy's Pacific Northwest National Laboratory. "We believe these materials have potential in water filters, sensors, drug delivery and especially fuel cells or other energy applications."
Cell membranes are amazing materials. Made from thin sheets of fatty molecules called lipids, they are at least ten times thinner than an iridescent soap bubble and yet allow cells to come together to form organisms as diverse as bacteria, trees and people.
Cell membranes are very selective about what they let pass through into the cell, using tiny embedded proteins as gatekeepers. They can also repair dings to their structure automatically and change thickness to pass signals from the outside environment to the cell's interior, where most of the action is.
Scientists would like to be able to take advantage of some of these properties, such as gatekeeping and signalling, to make advanced filters and sensors. For example, mimicking a cell membrane's efficient gatekeeping could result in water purifying membranes that don't require a lot of pressure or energy to push the water through.
Synthetic molecules called peptoids, which are cheap, versatile and customizable, offer a potential way to do this. They are like natural proteins, including those that embed themselves in cell membranes, and can be designed to have very specific forms and functions. So Chen and his colleagues decided to see if they could design peptoids to make them more lipid-like.
Lipid molecules are long and mostly straight: they have a fatty end that prefers to hang out with other fats, and a water-loving end that prefers the comfort of water. Because of this chemistry, lipid molecules arrange themselves such that their fatty ends point toward each other, away from any surrounding liquid solution, while their water-loving ends point out into the liquid solution. This forms a lipid bilayer that envelops the contents of a cell and can incorporate proteins or carbohydrate molecules within it.
Inspired by this, Chen and his colleagues designed peptoids in which each base peptoid was a long molecule with one water-loving end and one fat-loving end, and chose other chemical features that they hoped would encourage the individual molecules to pack together. They examined the resulting structures using a variety of analysis methods, including some at the Advanced Light Source and the Molecular Foundry, two DOE Office of Science User Facilities at Lawrence Berkeley National Laboratory.
The team found that after putting the lipid-like peptoids into a liquid solution, the molecules spontaneously crystallized and formed what the scientists call nanomembranes – straight-edged sheets as thin as cell membranes – floating in the beaker. These nanomembranes maintained their structure in water or alcohol, at different temperatures, and in solutions with a high or low pH or high concentrations of salts, a feat that few cell membranes could accomplish.
To better understand the nanomembranes, the team simulated how single peptoid molecules interacted with each other using molecular dynamics software. They found that the simulated peptoids formed a membrane reminiscent of a lipid bilayer: the fat-loving ends lined up in the middle, while the water-loving ends pointed out either above or below.
To test whether their synthetic membranes had the signaling ability of cell membranes, the researchers added a touch of sodium chloride salt, which is involved in the last step in many signaling sequences and causes real cell membranes to thicken up. And thicken up the peptoids did. The more salt the researchers added, the thicker the nanomembranes became, reaching about 125% of their original thickness over the range of applied salt concentrations.
Real membranes also house proteins that have specific functions, such as letting water into the cell. Chen's group tested the ability of peptoids to do this by adding a variety of side chains; these are essentially small molecules of different shapes, sizes and chemical natures attached to the longer lipid-like peptoids. They tried 10 different designs. In each case, the peptoids still assembled into nanomembranes with an intact core structure. The team were also able to add a carbohydrate into the nanomembranes, showing the material can be designed to have versatile functions.
Finally, the team tested whether the nanomembranes could repair themselves, a useful feature for membranes that can get scratched during use. After cutting slits in a membrane, they added more of the lipid-like peptoid. Viewed under a microscope over the course of a few hours, the scratches filled up with more peptoid and the nanomembrane became complete again.
Taken together, the results showed the researchers that they are on the right path to making synthetic cell membrane-like materials. However, there are still some challenges to be addressed; for example, the researchers would like to understand better how the membranes form, so they can make them in a range of different sizes.
The next step, Chen said, is to build biomimetic membranes by incorporating natural membrane proteins or other synthetic water channels such as carbon nanotubes into these sheet matrices. The team is also looking at ways to make the peptoid membranes conductive for energy uses.
This story is adapted from material from the Pacific Northwest National Laboratory, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.