Sarin

Sarin (NATO designation GB [short for G-series, "B"]) is an extremely toxic synthetic organophosphorus compound. A colourless, odourless liquid, it is used as a chemical weapon due to its extreme potency as a nerve agent. Exposure is lethal even at very low concentrations, where death can occur within one to ten minutes after direct inhalation of a lethal dose, due to suffocation from respiratory paralysis, unless antidotes are quickly administered. People who absorb a non-lethal dose, but do not receive immediate medical treatment, may suffer permanent neurological damage.
Sarin is generally considered a weapon of mass destruction. Production and stockpiling of sarin was outlawed as of April 1997 by the Chemical Weapons Convention of 1993, and it is classified as a Schedule 1 substance.
Health effects.
Like some other nerve agents that affect the neurotransmitter acetylcholine, sarin attacks the nervous system by interfering with the degradation of the neurotransmitter acetylcholine at neuromuscular junctions. Death will usually occur as a result of asphyxia due to the inability to control the muscles involved in breathing.
Initial symptoms following exposure to sarin are a runny nose, tightness in the chest, and constriction of the pupils. Soon after, the person will have difficulty breathing and they will experience nausea and drooling. As they continue to lose control of bodily functions, they may vomit, defecate, and urinate. This phase is followed by twitching and jerking. Ultimately, the person becomes comatose and suffocates in a series of convulsive spasms. Moreover, common mnemonics for the symptomatology of organophosphate poisoning, including sarin, are the "killer Bs" of bronchorrhea and bronchospasm because they are the leading cause of death, and SLUDGE – salivation, lacrimation, urination, defecation, gastrointestinal distress, and emesis (vomiting). Death may follow in one to ten minutes after direct inhalation.
Sarin has a high volatility (ease with which a liquid can turn into vapour) relative to similar nerve agents, therefore inhalation is very easy and even vapor may immediately penetrate the skin. A person's clothing can release sarin for about 30 minutes after it has come in contact with sarin gas, which can lead to exposure of other people.
Management.
Treatment measures have been described. Treatment is typically with the antidotes atropine and pralidoxime. Atropine, an antagonist to muscarinic acetylcholine receptors, is given to treat the physiological symptoms of poisoning. Since muscular response to acetylcholine is mediated through nicotinic acetylcholine receptors, atropine does not counteract the muscular symptoms. Pralidoxime can regenerate cholinesterases if administered within approximately five hours. Biperiden, a synthetic acetylcholine antagonist, has been suggested as an alternative to atropine due to its better blood–brain barrier penetration and higher efficacy.
Mechanism of action.
Specifically, sarin is a potent inhibitor of acetylcholinesterase, an enzyme that degrades the neurotransmitter acetylcholine after it is released into the synaptic cleft. In vertebrates, acetylcholine is the neurotransmitter used at the neuromuscular junction, where signals are transmitted between neurons from the central nervous system to muscle fibres. Normally, acetylcholine is released from the neuron to stimulate the muscle, after which it is degraded by acetylcholinesterase, allowing the muscle to relax. A build-up of acetylcholine in the synaptic cleft, due to the inhibition of acetylcholinesterase, means the neurotransmitter continues to act on the muscle fibre, so that any nerve impulses are effectively continually transmitted.
Sarin acts on acetylcholinesterase by forming a covalent bond with the particular serine residue at the active site. Fluoride is the leaving group, and the resulting phosphothioester is robust and biologically inactive.
Its mechanism of action resembles that of some commonly used insecticides, such as malathion. In terms of biological activity, it resembles carbamate insecticides, such as Sevin, and the medicines pyridostigmine, neostigmine, and physostigmine.
Diagnostic tests.
Controlled studies in healthy men have shown that a nontoxic 0.43 mg oral dose administered in several portions over a 3-day interval caused average maximum depressions of 22 and 30%, respectively, in plasma and erythrocyte acetylcholinesterase levels. A single acute 0.5 mg dose caused mild symptoms of intoxication and an average reduction of 38% in both measures of acetylcholinesterase activity. Sarin in blood is rapidly degraded either "in vivo" or "in vitro". Its primary inactive metabolites have "in vivo" serum half-lives of approximately 24 hours. The serum level of unbound isopropyl methylphosphonic acid (IMPA), a sarin hydrolysis product, ranged from 2–135 μg/L in survivors of a terrorist attack during the first four hours post-exposure. Sarin or its metabolites may be determined in blood or urine by gas or liquid chromatography, while acetylcholinesterase activity is usually measured by enzymatic methods.
A newer method called "fluoride regeneration" or "fluoride reactivation" detects the presence of nerve agents for a longer period after exposure than the methods described above. Fluoride reactivation is a technique that has been explored since at least the early 2000s. This technique obviates some of the deficiencies of older procedures. Sarin not only reacts with the water in the blood plasma through hydrolysis (forming so-called 'free metabolites'), but also reacts with various proteins to form 'protein adducts'. These protein adducts are not so easily removed from the body, and remain for a longer period of time than the free metabolites. One clear advantage of this process is that the period, post-exposure, for determination of sarin exposure is much longer, possibly five to eight weeks according to at least one study.
Toxicity.
As a nerve gas, sarin in its purest form is estimated to be 26 times more deadly than cyanide. The LD50 of subcutaneously injected sarin in mice is 172 μg/kg.
Sarin is highly toxic, whether by contact with the skin or breathed in. The toxicity of sarin in humans is largely based on calculations from studies with animals. The lethal concentration of sarin in air is approximately 28 – 35 mg per cubic meter per minute for a two-minute exposure time by a healthy adult breathing normally (exchanging 15 liters of air per minute, lower 28 mg/m3 value is for general population). This number represents the estimated lethal concentration for 50% of exposed victims, the LCt50 value. The LCt95 or LCt100 value is estimated to be 40 – 83 mg per cubic meter for exposure time of two minutes. Calculating effects for different exposure times and concentrations requires following specific toxic load models. In general, brief exposures to higher concentrations are more lethal than comparable long time exposures to low concentrations. There are many ways to make relative comparisons between toxic substances. The list below compares sarin to some current and historic chemical warfare agents, with a direct comparison to the respiratory LCt50:
Production and structure.
Sarin is a chiral molecule because it has four chemically distinct substituents attached to the tetrahedral phosphorus center. The "SP " form (the (–) optical isomer) is the more active enantiomer due to its greater binding affinity to acetylcholinesterase. The P-F bond is easily broken by nucleophilic agents, such as water and hydroxide. At high p"H", sarin decomposes rapidly to nontoxic phosphonic acid derivatives.
It is usually manufactured and weaponized as a racemic mixture (a 1:1 mixture of its two enantiomeric forms) as this involves a simpler synthetic process whilst providing an adequate weapon.
A number of production pathways can be used to create sarin. The final reaction typically involves attachment of the isopropoxy group to the phosphorus with an alcoholysis with isopropyl alcohol. Two variants of this process are common. One is the reaction of methylphosphonyl difluoride with isopropyl alcohol, which produces a racemic mixture of sarin enantiomers with hydrofluoric acid as a byproduct:
The second process, uses equal quantities of methylphosphonyl difluoride and methylphosphonic acid dichloride, a mixture "Di-Di" in this process, rather than just the difluoride. This reaction also gives sarin, but hydrochloric acid as a byproduct instead. The Di-Di process was used by the United States for the production of its unitary sarin stockpile.
The scheme below shows a generic example of Di-Di process; in reality, the selection of reagents and reaction conditions dictate both product structure and yield. The choice of enantiomer of the mixed chloro fluoro intermediate displayed in the diagram is arbitrary, but the final substitution is selective for chloro over fluoro as the leaving group. Inert atmosphere and anhydrous conditions (Schlenk techniques) are used for synthesis of sarin and other organophosphates.
As both reactions leave considerable acid in the product, consequently sarin produced in bulk by these methods has a short half life without further processing, and would be corrosive to containers and damaging to weapons systems. Various methods have been tried to resolve these problems. In addition to industrial refining techniques to purify the chemical itself, various additives have been tried to combat the effects of the acid, such as:
Another byproduct of these two chemical processes is diisopropyl methylphosphonate, formed when a second isopropyl alcohol reacts with the sarin itself. This chemical degrades into isopropyl methylphosphonic acid.
Degradation and shelf life.
The most important chemical reactions of phosphoryl halides is the hydrolysis of the bond between phosphorus and the fluoride. This P-F bond is easily broken by nucleophilic agents, such as water and hydroxide. At high pH, sarin decomposes rapidly to nontoxic phosphonic acid derivatives. The initial breakdown of sarin is into isopropyl methylphosphonic acid (IMPA), a chemical that is not commonly found in nature except as a breakdown product of sarin (this is useful for detecting the recent deployment of sarin as a weapon). IMPA then degrades into methylphosphonic acid (MPA), which can also be produced by other organophosphates.
Sarin with residual acid degrades after a period of several weeks to several months. The shelf life can be shortened by impurities in precursor materials. According to the CIA, some Iraqi sarin had a shelf life of only a few weeks, owing mostly to impure precursors.
Along with nerve agents such as tabun and VX, sarin can have a short shelf life. Therefore, it is usually stored as two separate precursors that produce sarin when combined. Sarin's shelf life can be extended by increasing the purity of the precursor and intermediates and incorporating stabilizers such as tributylamine. In some formulations, tributylamine is replaced by diisopropylcarbodiimide (DIC), allowing sarin to be stored in aluminium casings. In binary chemical weapons, the two precursors are stored separately in the same shell and mixed to form the agent immediately before or when the shell is in flight. This approach has the dual benefit of solving the stability issue and increasing the safety of sarin munitions.
History.
Sarin was discovered in 1938 in Wuppertal-Elberfeld in Germany by scientists at IG Farben who were attempting to create stronger pesticides; it is the most toxic of the four G-Series nerve agents made by Germany. The compound, which followed the discovery of the nerve agent tabun, was named in honor of its discoverers: Schrader, Ambros, Gerhard Ritter, and Hans-Jürgen von der Linde.
Use as a weapon.
In mid-1939, the formula for the agent was passed to the chemical warfare section of the German Army Weapons Office, which ordered that it be brought into mass production for wartime use. Pilot plants were built, and a high-production facility was under construction (but was not finished) by the end of World War II. Estimates for total sarin production by Nazi Germany range from 500 kg to 10 tons. Though sarin, tabun and soman were incorporated into artillery shells, Germany did not use nerve agents against Allied targets. Adolf Hitler refused to initiate the use of gases such as sarin as weapons.