Team:Bielefeld-CeBiTec/Chemicals

Abstract

We created a plant-based detection system for the detection of chemical weapon degradation products. While choosing suitable chemicals to work with, we had to consider a multitude of factors like safety hazards and risks while handling, potential availability or toxicity to plants, stability in the environment and of course price and availability. Therefore, we choose significantly less toxic chemical, which are safe to work with and which are similar to those chemical weapon components.

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Figure 1: Depiction of the chemical structure of mustard gas and thiodiglycol.

Thiodiglycol – mustard gas detection

Mustard gas was one of the first chemical warfare agents that was produced in an industrial scale during the first world war, where it was used to specifically attack trenches and the soldiers hiding in it, since it is heavier than air and therefore settles on the ground. The extensive use of this chemical weapon resulted in thousands of fatal and tens of thousands non-fatal casualties. Even though the exposure to mustard gas is usually not fatal, the adverse health effects depend on the time and the way of exposure [1]. A prolonged exposure typically results in strong skin irritation, destroyed lungs and blindness. Strongly affected limbs must often be amputated. Mustard gas is also cancerous because it alkylates DNA, especially in the bone marrow, where it also leads to decreased formation of blood cells [2]. Large quantities of mustard gas of Germany were disposed in the Baltic Sea, where the containers are leaking and occasionally solid yellow-brownish sulfur mustard and its degradation products on the beaches are mistaken for amber [3].

Thiodiglycol (TDG) is a non-toxic degradation product of mustard gas and marks an intermediate step in its synthesis [4]. Mustard gas is twice transformed into a cyclic compound with subsequent hydrolyzation in aqueous medium, resulting in exchange of chloride with hydroxyl groups, ultimately leading to formation of TDG and hydrochloric acid [5]. TDG is a small organic molecule with a central sulfur atom flanked by two hydroxyethyl groups [6]. TDG is found in urine of patients that were exposed to mustard gas [7]. TDG cannot be found in nature, so that every occurrence is linked to either chemical industry or related to the presence of mustard gas.

Figure 2: Depiction of the chemical structure of TNT and BTCA.

BTCA - a TNT surrogate

2,4,6-trinitrotoluene (TNT) is a highly explosive material, that was originally used as a dye and photography until its explosive attributes have been discovered. Since then, it has been used as explosive in shells, grenades, different kinds of bombs and mines. TNT was superior to other explosives, because it is relatively insensitive to the shock from the ejection of the shell when fired, while the detonation can be triggered by a detonator mechanism on impact on the target. The insensitivity to vibration made it the explosive of choice, due to a safe transport and storage. The impact TNT had on the warfare in the 20th century can still be seen in the fact, that since then, the force of an explosion is measured in TNT equivalents. To this day, many former production sites are contaminated with TNT, derivates and degradation products. The preparation of samples for detection is tedious work and must be performed on the site since nitroaromatic compound are volatile and are often taken up by plants and accumulate there to significant amounts [8, 9]. Besides its explosive properties, TNT is also toxic and cancerous.

Because of these properties we decided to seek an alternative chemical, that has a similar 3D structure. We chose the safe to work with 1,3,5-benzenetricarboxylic acid (BTCA) as simulant for TNT. In this molecule, the nitro groups have been replaced by carboxylic acids and the methyl group, which has little influence on a presumed receptor-ligand interaction, is missing when compared to TNT.

Figure 3: Depiction of the chemical structure of sarin and its degradation products/surrogates DIMP, DEMP and MPA.

Sarin and its byproducts

GB, or sarin is a highly toxic nerve agent produced for chemical warfare. It is a colorless, odorless und volatile organophosphate ester, that is an extremely potent acetylcholinesterase inhibitor due to its high affinity for the enzyme. Through an irreversible acetylcholinesterase inhibition, the retransmission of any nerve impulses in the affected organism is prevented. Upon exposure through inhalation, ingestion or dermal contact, sarin causes airway obstruction, weakness of the respiratory muscles, convulsions and respiratory failure and can lead to death in minutes or hours. After its discovery in 1938, sarin was used in Iraq, in terroristic attacks in Japan 1995 and in Syria as recently as 2017, altogether injuring and killing thousands of people. During and after World War II, sarin was produced and stockpiled by many nations. The shelf life of sarin can be a few weeks up to several months, with impurities shortening the shelf life drastically and stabilizers elongating it. Regarding sarin we found three similar chemicals to be suitable for the test of the sarin detection system.

Diisopropyl methylphosphonate (DIMP)

Diisopropyl methylphosphonate (DIMP) is a by-product and residue of sarin production. DIMP is soluble in water and thus can be found as a water and ground contamination at sarin storage sites, showing low volatility and slow biodegradation.

Diethyl methylphosphonate (DEMP)

Both DIMP and DEMP can function as a sarin surrogate due to similar chemical and physical properties. DEMP is used for the synthesis of sarin. It is a schedule 2 chemical in the sense of chemical weapons convention [10].

Methylphosphonic acid (MPA)

Methylphosphonic acid (MPA) is the final degradation product of sarin hydrolysis. In the initial and fast hydrolyzation step, sarin is degraded to isopropyl methylphosphonic acid (IMPA), which is further degraded at a much slower rate, to the stable product MPA.

References

[1] https://emergency.cdc.gov/agent/sulfurmustard/basics/facts.asp (access 22.10.)

[2] Amini, H. et al. Long-term Health Outcomes Among Survivors Exposed to Sulfur Mustard in Iran JAMA Netw Open. (2020).

[3] https://www.smithsonianmag.com/science-nature/decaying-weapons-world-war-II-threaten-waters-worldwide-180961046/ (access on 22.10.)

[4] Reddy G. et al. Toxicity Assessment of Thiodiglycol International Journal of Toxicology 24(6), 435-442 (2005).

[5] Brimfield A.A. et al. Thiodiglycol, the hydrolysis product of sulfur mustard: Analysis of in vitro biotransformation by mammalian alcohol dehydrogenases using nuclear magnetic resonance Toxicology and Applied Pharmacology 213(3), 207-215 (2006).

[6] https://pubchem.ncbi.nlm.nih.gov/compound/Thiodiglycol#section=InChI (access on 22.10.)

[7] https://pubmed.ncbi.nlm.nih.gov/16965944/ (access 22.10.)

[8] Klunk A. et al. Biologische Sanierung von Rüstungsaltlasten Umweltwissenschaften und Schadstoff-Forschung 8, 243-247 (1996).

[9] Görge E. et al. Aufnahme von 2,4,6-Trinitrotoluol in Pflanzen Umweltwissenschaften und Schadstoff-Forschung 7, 139-148 (1995).

[10] https://www.opcw.org/chemical-weapons-convention/annexes/annex-chemicals/schedule-2