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On 2 September, German Chancellor Angela Merkel revealed that Alexei Navalny, a Russian opposition politician, had been poisoned with a nerve agent “identified unequivocally in tests” as a Novichok—one of a family of exotic Soviet-era chemical weapons. Merkel, a chemist by training, did not reveal the nature of the tests, conducted in a military lab in Munich. But scientists familiar with Novichoks have a good idea how the toxicological sleuths went about it—and are impressed by how fast the culprit was unmasked.

Navalny fell ill on 20 August after drinking a cup of tea at a Siberian airport. He lapsed into a coma and was flown to Berlin 2 days later; in a statement yesterday, the hospital treating him said he is out of the coma and “responding to verbal stimuli.” Navalny’s supporters have accused Russian operatives of slipping poison into the tea—a charge that seems credible in light of Russia’s recent record of using toxic substances to silence critics.

Novichok A234 was the weapon of choice for settling a score with a former Russian spy, Sergei Skripal, in Salisbury in the United Kingdom in March 2018. In a botched operation, two Russian intelligence officers left a trail of evidence in the attempted assassination of Skripal, whose daughter Yulia also fell ill after exposure to A234. They survived, but a woman who later came across a perfume bottle containing the substance died.

The Salisbury scandal brought Novichoks out of the shadows. After a Russian chemist in 1992 divulged some details about the exquisitely toxic nerve agents—there are at least seven of them—the U.S. government and allies clamped down on open discussion; Novichoks were classified as secret. A234’s brazen use in the United Kingdom led to a public reckoning. In October 2019, parties to the Chemical Weapons Convention agreed to add Novichoks to the treaty’s list of toxic chemicals, bringing them under the convention’s verification regime and paving the way for research on the mechanism of action of these “fourth generation” nerve agents, as well as on countermeasures and treatments.

The diplomatic progress hardly deterred Navalny’s unknown assailants. As doctors in Berlin fought to save him, scientists at the Bundeswehr Institute of Pharmacology and Toxicology in Munich set out to unravel the mysterious cause of his symptoms.

They had clear targets to hunt for. Like other nerve agents, Novichoks bind to acetylcholinesterase (AChE), an enzyme that breaks down the neurotransmitter acetylcholine when it is released into synapses. Common symptoms of Novichok poisoning include nausea, trouble breathing, and seizures; without medical intervention, victims can slip into a coma. Red blood cells have AChE anchored to their membranes, so a blood sample could yield a conjugate formed when a Novichok latches onto AChE, which scientists could detect using mass spectrometry, says Palmer Taylor, a pharmacologist at the University of California, San Diego (UCSD).

Another possibility is a Novichok conjugate of serum albumin, the most abundant protein in the blood. Nerve agent conjugates with serum albumin “are very useful markers” that can be detected for at least a couple of weeks after a poisoning, says Stefano Costanzi, a chemist and nonproliferation analyst at American University in Washington, D.C.

Novichok structures are unique. Really different from other nerve agents.

A third candidate is a conjugate of butyrylcholinesterase (BChE), an enzyme that scavenges nerve agent molecules in the bloodstream. It would be straightforward to use an anti-BChE antibody to latch onto the conjugate and then digest the protein. Most of the Novichok molecule would remain linked to one of the fragments and would be easy to detect by mass spectrometry, says Oksana Lockridge, a toxicologist at the University of Nebraska Medical Center. “I have no doubt that the Bundeswehr group used this method,” she says. “The part of the Novichok that stays attached to the enzyme is way larger than that of any other nerve agent,” says UCSD chemist Zoran Radić. Detection of such a conjugate would make identification of a Novichok “100% certain,” says Lockridge, who is developing BChE as a prophylactic for exposure to nerve agents.

The Salisbury probe presumably yielded closely held insights into Novichoks. U.K. authorities now “know much more about toxicity, detection, and general behavior of Novichoks,” says Kamil Kuča, a toxicologist at the University of Hradec Králové in the Czech Republic. “They could share their results with ‘friends’” such as Germany, he says. And chemical detective work undertaken by investigators at the U.K. defense lab Porton Down may have sped up analyses at the Munich lab, Radić says, by “providing a suitable set of protocols to follow.”

It may also be possible to directly detect the parent compound—the Novichok itself—in Navalny’s body. “One could easily assume that they accumulate in lipids,” Radić says. In an article earlier this year in the journal Heliyon, a team at the U.S. Army Combat Capabilities and Development Command Chemical Biological Center showed three Novichok compounds are much more stable than other nerve agents, with the most durable, A234, about 1000-fold more stable than the nerve agent sarin.

That stability may be a big factor in why the handful of known Novichok victims respond so poorly to treatment: It could take weeks for lipids in fat cells to relinquish their Novichok reserves. Another insidious characteristic of Novichoks is their death grip on AChE: They bind to and block not one site in the active center—as other nerve agents do—but two. “Novichok structures are unique. Really different from other nerve agents,” Radić says. For that reason oximes, an antidote that pries nerve agents off AChE and leads to reactivation of the enzyme, may be far less effective against Novichoks than against classical nerve agents like sarin. “There’s no declassified information on reactivation” in Novichok victims, Radić says. One researcher says pralidoxime, an antidote carried by U.S. soldiers at risk of nerve agent exposure, helped the Salisbury victims—but not by reactivating AChE.

Regardless of who perpetrated the attack on Navalny, the brazen incident and revelations about the Novichoks’s durability heighten concerns about the threat they pose. And while most nerve agents are stockpiled as liquids, some Novichoks are stable as an ultra-fine powder. “They can be hidden and stored much more easily than classical nerve agents,” says Radić, who notes that makes them more likely to end up on the black market.