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Nerve Gases, Part I

Dateline: 03/23/98

By Alan Bruzel

The chemistry of the nerve gases is inextricably intertwined with the chemistry of the enzyme acetylcholinesterase. Disabling this crucial component of nerve transmission compromises the entire nervous system. The outcome is decidedly unpleasant, and often fatal. "For, even the preachers have begun to tell us that God is radium, or ether or some scientific compound, and that the worst we wicked ones may expect is a chemical reaction." Some chemical reactions exceed even the punishment merited by the prosperous-looking gentlemen of O. Henry's "An Unfinished Story" (quoted above). This article presents the nerve gases presently stockpiled; next week's article explores their reactivity with acetylcholinesterase.

Nerve gases are an outgrowth of organophosphorous pesticide research. They were first synthesized in Germany before and during World War II, but not used for fear of retaliation in kind. Tabun (1936), Sarin (1939), and Soman (1944) were made by Gerhard Schrader. VX (1952) was produced independently by Ranaji Goshem and Lars-Erik Tammelin.

Sarin (Isopropyl methylphosphonofluoridate) Lethal adult human dose = 1700 mg [on skin]	Soman (Pinacolyl methylphosphonofluoridate) Lethal adult human dose = 350 mg [on skin]

Tabun (Ethyl N,N- dimethylphosphoramidocyanidate) Lethal adult human dose = 4000 mg [on skin]	VX (O-Ethyl-S-[2-diisopropylaminoethyl] methylphosphonothiolate) Lethal adult human dose = 10 mg [on skin]

The organophosphorous pesticides below show their close structural relationship to the nerve gases, but a much lower toxicity permits commercial use.

Orthene (O,S-Dimethyl acetylphosphoramidothioate)	Trichlorofon (O,O-Dimethyl- 1-hydroxy- 2,2,2-trichloroethylphosphonate)

The nerve "gases" are actually liquids, ranging from the fairly volatile Sarin with a vapor pressure of 2.9 mm mercury at 25^oC (compare water's vapor pressure of 17.5 mm mercury at 20^oC), to the almost nonvolatile VX with a vapor pressure of 0.0007 mm mercury at 20^oC. But as gas, aerosol, or liquid, these are deadly chemicals. (There are worse natural toxins; see this site's The Poison List.) With this in mind, it is reasonable to store reactants separately, allowing them to mix and to generate the active nerve agent only when needed. This is the principle behind binary munitions. To produce Sarin requires adding methylphosphoryldifluoride (DF) to a mixture of isopropyl alcohol and isopropylamine (OPA). The US Army stores canisters of DF and canisters of OPA at separate arsenals. Before military operations, one canister of each is placed in a 155 mm shell. When fired from a howitzer, the canisters break open, the contents mix, and Sarin forms in the spinning shell. Soman and VX are similarly generated using binary components.

Binary weapons occur in living systems, as well. Here, the reactants exist in one location, break down enzymatically in a second location, and toxic moieties deploy from a flexible muzzle. Bombardier beetles (genus Brachinus), carry a reservoir of hydrogen peroxide and hydroquinones. Muscular contractions by the annoyed insect push these reactants through a one-way valve into an internal reaction chamber. Enzymes (catalases and peroxidases) in this chamber break down the hydrogen peroxide and oxidize the hydroquinones into odoriferous quinones. The resultant 100^oC mixture bursts from the tip of the beetle's abdomen using the pressure generated by the exothermic reactions. The target does not linger to admire the engineering design.

The creationists argue that the bombardier beetle possesses a mechanism composed of units that separately are useless and therefore would not be retained during the course of evolution. They conclude that components of this defensive mechanism must have been purposively assembled (created) at one particular time. This contention is countered by positing a logical sequence of micro-events that, over time, permitted the step-wise development of the beetle's delivery system using previously acquired and still functioning organ systems. For instance, other beetles in the same suborder as Brachinus have secretory glands emptying into a reservoir. Add hydrogen peroxide (formed from amino acid and fatty acid degradation), an adjoining chamber with the requisite enzymes (which are present in all cells, anyway), and some anatomical modifications, and you have a mechanism that uses existing materials in a new way.

On March 20, 1995, the Aum Shinrikyo – an inconspicuous sect hidden among the approximately 183,000 cults in Japan – grabbed attention by placing Sarin on five subway trains traveling toward Kasumigaseki station. This subway stop is a nexus for Tokyo government offices. Twelve commuter deaths and over 5,000 injuries resulted. A deadly June 1994 Sarin attack in Matsumoto City, and an assassination with VX were subsequently linked to the Aum Shinrikyo. Cult leader Shako Asahara, charged with these and other crimes, including nerve gas manufacture, is now serving a life sentence.

It is fruitless to expect formal agreements with terrorist cells, but organized governments are moving toward consensus. Signatories to the Chemical Weapons Convention agree, over a specified period, to destroy their manufactured nerve gases and binary components. Incineration, using activated charcoal to adsorb waste gases, is one mode of destruction. A proposed methodology would use the enzyme organophosphorous hydrolase from Pseudomonas diminata. A crystallized and cross-linked derivative of this enzyme is under development. Its manufacturers hope to prepare a stable catalyst capable of operating in organic solvents and at high temperatures. (See this site's Mirror Image Chemistry, Part III for other uses of this technology in enatiospecific synthesis.)

Next week's article will focus on the reaction of these nerve gases with acetylcholinesterase – a key enzyme regulating normal transmission of nerve impulses. Protective chemicals and antidotes used to inhibit or reverse nerve gas poisoning will also be discussed.

Recommended Web resources for additional information:

Chemical Terrorism
Canadian Security Intelligence Service provides assessment and historical instances of chemical weapons use.

Nerve Gases, Part II
An article from this Web site describing biochemistry and therapy.

Organophosphorous Insecticides
Insect toxicology lectures from University of California, Riverside.

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