“Selective Chromium(VI) Ligands Identified Using Combinatorial Peptoid Libraries.” Knight, A. S., Zhou, E. Y., Pelton, J. G., Francis, M.B. J. Am. Chem. Soc. 2013. 135, 17488–93.
Abby Knight is a fifth-year PhD student in the Francis Group at UC Berkeley, and she and Julia Roberts share a mutual acquaintance: hexavalent chromium.
Roberts may or may not remember the chemical’s name, but it was her nemesis in the 2000 film Erin Brokovich, when she played a single mother agitating for PG&E to pay for contaminating her town’s water. Brokovich successfully forced the industry giant to stop polluting, and that was the end of the movie. But that wasn’t the end of the story, because although the chromium was stopped at the source, no method exists to remove what was already in the water. That’s where Knight comes in.
“There’s no good way to clean up chromium contamination in groundwater,” Knight says. “Right now, the EPA strategy is to just say ‘don’t drink this water’ and wait for it to diffuse. “
That’s not an ideal solution, especially when drinking water is in short supply. But it’s very hard to remove a problematic metal, like chromium, from natural waters because they have lots of other ions that get in the way. But Knight was up for the challenge: She set out to come up with a method to selectively remove chromium and other heavy metals from complex solutions—like groundwater, or human blood—while leaving the natural and necessary elements.
Drawing inspiration from within
Knight and her adviser, Matthew Francis, looked to nature for a chemical solution to this problem. They knew humans and animals have naturally occurring proteins that are able to remove small doses of heavy metals from the blood.
“Proteins are kind of your body’s natural therapeutic response to low doses of heavy metal poisoning,” Knight says. Proteins grab on to (chelate), the metal ions and get them out of your bloodstream, but they’re easily degraded by enzymes and so don’t work well for environmental applications. But Knight thought maybe she could use a compound with a similar structure to perform the same function.
She narrowed in on a specific class of molecules called peptoids, a recently invented class of synthetic peptidomimetic (peptide-like) molecules. They are similar to naturally occurring peptides—a backbone of carbon, nitrogen and oxygen with side branches of varying structure attached along the spine—but in peptoids the side branches are attached to nitrogen atoms in the backbone, while in peptide they attach to the carbon.
Knight chose peptoids as her binders of choice for their stability and customizability.
“The scientist who invented peptoids is right here at Lawrence Berkeley National Lab,” Knight says, “So I was able to get a lot of help.” This was important because peptoids are so new that no one else in her lab had ever worked with them before.
Dyeing beads and building a library
Thousands of different peptoids have been synthesized, and there are infinite ways in which they could be modified. To find the best chromium binder, Knight created a library of peptoids with varying side chains. After she built her library, the only way to find out which molecules best grabbed on to chromium was through trial and error.
Knight coated small polystyrene resin beads with each peptoid she synthesized. She then added the coated beads to a chromium solution, and painted them with a dye that turns pink when it reacts with chromium. Then Knight and her undergrad research assistant, Effie Zhou, painstakingly removed the pinkest beads—those that had most successfully chelated chromium—and made more of the peptoid on that bead. Then they would test that peptoid again.
Through this iterative process Knight identified a few winners. Then it was time for a real world test. Knight and Zhou collected natural water from Ocean Beach in San Francisco and Strawberry Canyon on the UC Berkeley campus.
Moment of truth
Fortunately, the ocean and creek water in the Bay Area don’t have high levels of chromium contamination, so Knight and Zhou had to pollute the samples of the? water before trying to clean it. They added chromium, in amounts 10 and 100 times higher than the EPA safety limit. After the water was sufficiently contaminated, the beads, armed with carefully selected peptoids, were sent in to do their job. They let the solutions incubate, and then measured the chromium concentrations to see if the peptoids had successfully removed the metal.
The results: 80-90% reduction in chromium concentrations. “We drastically out-performed the commercially available [non-selective] resin,” Knight says proudly.
There is still room for improvement—they started to lose efficiency and selectivity at lower chromium levels. But overall, she says this looks like a very promising method to clean up contaminated water.
A new toolbox
The peptoids were so good at removing chromium from groundwater, Knight decided to apply the same technique to new applications, and new metals. Her next project was getting cadmium out of human blood, and based on her preliminary results with human serum, this application seems succesful as well.
But Knight and peptoids are parting ways. She is graduating in May, and starts a post-doc at UC Santa Barbara in the fall. She’ll be working in the Hawker lab to create new, self-assembling materials. It’s a new problem, with new chemistries to explore, but Knight looks forward to the challenge.
“What I love about the day to day activities of ‘being a scientist’ is the opportunity to find new and creative solutions to problems,” Knight says.