Metal ions can be found in almost every environment, including wastewater, chemical waste and electronic recycling waste. Properly recovering and recycling valuable metals from various sources is crucial for sustainable resource management and contributes to environmental cleanup. Because of the scarcity of some of these metals, such as rare earth elements or nickel, scientists are working to find ways to remove these ions from the waste and recycle the metals. One method used to remove these metals is to bind them to other molecules known as chelators or chelating agents. Chelators have multiple molecular groups that combine to form binding sites with a natural affinity for binding metal ions, making them a natural choice to extract metals from toxic waste. Ethylenediaminetetraacetic acid, or EDTA, is a chelator commonly used in metal removal and many other applications, including medicine. “EDTA is used to treat heavy-metal poisoning,” JILA graduate student Lane Terry explained. “So, if you have lead poisoning, you can take EDTA, which binds to the lead and then safely passes through your system. It's also used as a food preservative. So EDTA is everywhere. It's in one of my topical creams, etc.” EDTA is also commonly used in various laboratories, including many within JILA.
To understand how EDTA binds to these metal ions and water molecules, Madison Foreman, a former JILA graduate student in the Weber group, now a postdoctoral researcher at the University of California, Berkeley, Terry, and their supervisor, JILA Fellow J. Mathias Weber, studied the geometry of the EDTA binding site using a unique method that helped to isolate the molecules and their bound ions, allowing for more in-depth analyses of the binding interactions. They published a series of three papers on this topic. In their first paper, published in the Journal of Physical Chemistry A, they found that the size of the metal ion changes where it sits in the EDTA binding site, which affects other binding interactions, especially with water.
Binding to Metal Ions
EDTA is a chemical commonly found in a chemistry or biology laboratory. “EDTA is employed in many different contexts,” explained Weber. “Whenever you want to get rid of a metal ion in a solution, you throw EDTA into the solution. EDTA will bind to pretty much any metal ion across the periodic table. That's what makes it so widely used in chemistry and biochemistry.” Because of this, EDTA as a model system can reveal more about similar binding behaviors in proteins, including some found in the human body. “People are using it as a model for the binding sites of metal ions in proteins,” said Weber.
However, actually observing the mechanics of EDTA binding is rather tricky. “So, to see exactly what’s going on, you must isolate your target complex from other species,” explained Weber. “That's why we bring these ions into the gas phase, where we can control the number of solvent molecules they interact with, first without any solvent, then selectively start adding solvent one molecule at a time to see what changes.” To do this, the EDTA ions were coaxed into a gas phase. “We then cool them in a cryogenic ion trap to about 50 Kelvin,” Foreman added. “After that, we attach weakly bound nitrogen molecules, which act as messengers telling us later that a photon has been absorbed. We only let those [tagged EDTA] molecules into the second half of the experiment. So there's nothing else, and we have only one sort of ion.”
These tagged ion clusters were then bombarded with light from a tunable laser, which helped detect the target clusters. “We hit that nitrogen-tagged EDTA complex with a photon, which ejects the nitrogen tag,” added Foreman. “So now we have these two fragments flying along, the complex ion and the nitrogen, as well as some amount of undissociated cluster that still has the nitrogen on it.” Thanks to this nitrogen eviction, the researchers can detect that light was absorbed. “After this, we do a second mass spectrometry step to distinguish the undissociated parent ions from the fragment ions,” Weber clarified. “We selectively only measure the intensity of those fragment ions as we tune our laser. That’s how we measure a photo-dissociation spectrum which is the analog of the infrared absorption spectrum of that complex.”
The infrared absorption spectrum of these complexes is something physicists and chemists often refer to, but because multiple atoms and molecules tend to contaminate a sample, this spectrum can be hard to isolate. With their gas-phase method, Weber and his team were able to create an analogous process to the infrared absorption measurements and understand more about the molecular behavior of EDTA. “Now, we can analyze the absorption features from that infrared spectrum to tell us something about the molecular structure,” added Weber. “So encoded in this infrared spectrum is how the EDTA molecule interacts with that metal ion, how its functional groups are oriented, and how that orientation changes as you attach water to it or bring it into solution.”
Binding to Water Molecules
As there is usually water around EDTA and proteins, as in the human body, Weber and his team were curious to understand how EDTA’s behavior changes when interacting with water. “These binding sites in proteins bind to metal ions like calcium or magnesium with similar functional groups as those in EDTA,” Weber explained. “And in proteins, the interaction between the metal ion and the protein binding pocket often does not allow lots of water molecules around it. Instead, it allows one or two in the vicinity. So, one could argue that the behavior of EDTA in the gas phase is actually a good model for trying to understand how these binding sites work.”
In one experiment, published in the Journal of Physical Chemistry Letters, the researchers added water to the metal-EDTA complex one molecule at a time to see how small amounts of water affected the EDTA. “Here you start with just the EDTA metal complex, and then you add one water molecule and see where it binds and how it deforms the metal-EDTA complex as a whole,” Weber added. “Then you can add the second water molecule and see how it influences the complex. In our research, we contrasted it with full solvation, full hydration.”
Studying how EDTA binds metals while in the presence of water can also help researchers better understand the binding processes happening within the human body. “One of the main proteins that EDTA is used to emulate is calmodulin, as its binding pockets are kind of similar,” Foreman explained. “Calmodulin is part of a larger class of proteins. They're all over the body serving all sorts of different functions. But the primary function of calmodulin is as a calcium mediator, so it reacts to the presence of calcium and signals other proteins to perform their functions. This can have effects on everything from hormones to muscle contraction.” Because calmodulin usually binds more to calcium than magnesium in water, the researchers wanted to see if EDTA mimicked this behavior in solution. “When we then look at EDTA, in solution, we see a similar trend in binding affinity, [where EDTA] would prefer to bind calcium than magnesium,” stated Foreman. “So then, by looking at it in the gas phase, or with just a few water molecules, we can see that the structure of the EDTA metal complex does change between magnesium and calcium. And that gives us a hint as to why these proteins might be more selective to some ions than others.”
Recycling Metal Ions
Weber and his team first studied how the molecule binds to alkaline earth metals (such as magnesium, calcium, strontium, or barium) to understand EDTA's interaction with different metal ions. In a second paper, published in 2023 in the Journal of Physical Chemistry A, the researchers found geometric differences in bindings between transition metals, like manganese, cobalt, and nickel, and alkaline earth metals, like calcium or magnesium. “The alkaline earth ions are simple ions. They present a spherically symmetric charge distribution to the outside world,” Weber elaborated. “So they're really round. The transition metals we published in the paper, their electronic structure brings directionality to their bonding with other molecules; they do not look like a spherically symmetric charge distribution. I usually phrase this where the alkaline earth metals are round and the transition metals are spiky. Their electronic structure produces “arms” or “spikes” in a structural template that allows other molecules to bind to them in a very structured way.”
Understanding how EDTA binds to various metals can give Weber and other scientists insight into using molecules that are similar to EDTA in wider applications, such as metal recycling. “Imagine nickel, cobalt, or rare earth metals, everything that you need for things from electric vehicles to batteries to your cell phone,” stated Weber. “These metals need to be removed from electronics waste during recycling; then they need to be purified. One way to do that is to grab them with something [like EDTA] …Lane gathered background information on using chelators for rare earth metal recycling. She actually wrote a proposal on that process. And there are other, very different kinds of ion receptors, too.” They’re hopeful that their results can help other scientists and engineers improve current metal chelation applications.
Written by Kenna Hughes-Castleberry, JILA Science Communicator