Biomarkers of exposure to chlorine gas identified in plant tissue

When used as a chemical weapon, chlorine gas disperses so rapidly that evidence of its release can be hard to find.

“With most nerve agents, we have biomarkers that are stable in the environment that we look for to confirm [the agents’] use, but no biomarkers have been verified and accepted by the scientific community for the case of chlorine,” Katelyn E. Mason, a researcher in the Forensic Science Center at the Lawrence Livermore National Laboratory, told C&EN. “It’s a major gap in forensics.”

At the American Chemical Society Spring 2021 meeting, Mason reported that she and her colleagues have identified biomarkers in plant tissue that might be forensic tools for confirming chlorine-gas release. She presented their findings on Wednesday in a session organized by the Division of Analytical Chemistry.

To identify potential chlorine-containing molecules that could serve as biomarkers, Mason and her coworkers exposed Aegilops tauschii, a type of grass that’s native to parts of Asia, including Syria, to chlorine gas at various concentrations and for various durations. The chlorine visibly damages the tissue immediately. The researchers pulverized the plant tissue, used methanol to isolate its proteins and small molecules and analyzed each of the two fractions by mass spectrometry.

They found chlorine-containing compounds in both fractions. They originally thought the reaction with chlorine would occur on the plant surface, so they expected that chlorine-containing small molecules would be mostly lipids. But the chlorine penetrated the tissue, so they found a variety of chlorinated sugars, metabolites, and lipids in the small-molecule fraction. For now, the team’s lead small-molecule biomarker—which they found in all conditions—is 2,4,6-trichlorophenol; the plant does not make this, so they propose that this is a likely indicator of chlorine exposure. In the protein fraction, they found a chlorinated peptide from the protein ribulose-1,5-bisphosphate carboxylase-oxygenase, which is also known as rubisco and is common to all green-plants’ tissues. The peptide had a chlorinated tyrosine amino acid.

Crister Åstot, a researcher at the Swedish Defence Research Agency, said the findings are in line with those from other labs responding to the alleged use of chlorine gas in Syria. “However, it’s difficult to assess if a chemical marker is unambiguous for chlorine since the chlorine element is very abundant in the environment,” he said.

“In this study, we were really just trying to establish that these biomarkers exist, but understanding how long they persist in the plant material after the exposure event is extremely important for this to be forensically applicable,” Mason said. Future work will involve determining whether measurements of these biomarkers can be used in combination with atmospheric models to tell where and when a release of chlorine gas occurred.