Chemistry for Everyone
Weapons of Mass Destruction: It Is All about Chemistry Jessica Epstein Department of Chemistry, Saint Peters College, Jersey City, NJ 07306;
[email protected] A blast echoes in a crowded shopping mall. As people scramble for cover, no one knows what happened. A few minutes later, ambulances, police, and fire trucks arrive. As the first responders treat victims, the crowd pulls back. Some linger to watch, others talk to reporters, and others retreat to the safety of their homes. Within an hour federal and state authorities swing into action and begin combing the mall, parking lots, and streets. About two hours later, cleanup crews detect the presence of a radioactive isotope, cobalt-60. At this point, the cleanup will be more involved and the ramifications for each victim more complicated. Meanwhile, those who returned home have unintentionally tracked radioactive material well beyond the confines of the scene. Background Weapons of mass destruction (WMD) generally include nuclear, biological, chemical, and most recently, radiological weapons. The term first arose in 1937 in a Times article referring to the massive destruction wrought by a German Luftwaffe squadron during the Spanish Civil War (1). Following the bombing of Hiroshima and Nagasaki in 1945 at the end of World War II (WWII; 1939–1945), the term came to refer to the massive destruction wrought by a nuclear weapon. Although nuclear weapons certainly have the greatest potential for mass destruction, the term entered popular usage during the 1991 Gulf War in reference to nonconventional weapons (weapons that incorporate a chemical, biological, or nuclear payload). The topic of WMDs can be applied to many areas of chemistry. Nuclear chemistry is explored in most general chemistry courses. For example, most undergraduate general chemistry textbooks contain a chapter on isotopes and radioactivity, along with a section on nuclear fission and fusion. This is an excellent place to interject a discussion of nuclear weapons. Biological agents, while a very broad class, involve key concepts in biochemistry such as alkylating agents and enzyme inhibitors. Chemical weapons span several disciplines. These molecules are developed by organic chemists, but manifest their biological activity by disrupting key biochemical pathways. This is of particular interest to organic chemistry students who are pursuing careers in medicine. Furthermore, the delivery of chemical agents can
be covered in general chemistry where students learn concepts such as the gas laws and vapor pressure. This article will discuss chemical agents (Table 1), followed by a discussion of the distinction between nuclear and radiological weapons. It will also address the broad class of biological agents. Because it is difficult to mention all potential agents, the article will discuss only those likely to be encountered in popular press usage. It is difficult to predict every method of delivery and every scenario, but there are some general guidelines. Much of the information contained herein was obtained from the Centers for Disease Control (CDC; ref 2). Chemical Weapons A chemical agent is a small molecule, usually delivered in gas or liquid form. During exposure, these agents are inhaled, ingested, or absorbed through the skin. In 1899, two dozen countries signed the Hague Convention pledging not to use toxic gases or poisons as weapons. Chemical weapons were used in warfare on the battlefield during WWI (World War I; 1914–1918). By war’s end gases had caused 1.3 million injuries and 100,000 deaths (3). During WWI, chemical agents were made from phosgene and chlorine gas (Table 1). However, most stockpiles of chemical agents today are a solid or liquid at room temperature. For any sample of liquid, some molecules are volatile (in gas form), but most molecules must be put in an aerosol form for delivery. This is accomplished through a conventional explosion or through a device that will atomize (aerosolize) the molecules. The volatility of a substance is inversely related to its persistence, the tendency of a liquid to stay or persist as a hazard. An agent in the vapor phase is potentially easier to deliver but will dissipate more quickly. A more complicated delivery is required for a chemical agent that is a liquid at room temperature, but the agent continues to be a hazard by adhering to skin, clothes, and other surfaces (4). There are many types of potential chemical weapons. Some countries stockpile chemicals, although other agents may simply be used for industrial purposes. It is not possible to discuss every agent. Therefore, the discussion below is limited to the broad classes of chemical agents (Table 1) and how they manifest their activity (5–7).
Table 1. The Structures and Physical Properties of Several Chemical Agents Name Hydrogen cyanide
Sarin
Formula
Structure
HCN
H C N
M/(g/mol)
Appearance
mp/°C
bp/°C
colorless gas or pale blue liquid
−13.4
26
140.09
clear colorless liquid
−56
159.08
pale yellow to dark brown liquid
27.03
C4H10FO2P
158
a
Sulfur-mustard
C4H8Cl2S
Cl
S bacteria
Cl
14.4
217
O Phosgene
CCl2O
Cl
b C
Cl airborn spores can be inhaled
98.92
colorless gas
−118
spores germinate in the lung • Journal of Chemical Education © Division of Chemical Education • www.JCE.DivCHED.org • Vol. 86 No. 12 December 2009
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Cyanide Cyanides are widespread in many industrial processes. The term cyanide refers to the class of compounds that generate the cyanide anion (CN−). Examples include hydrogen cyanide, cyanogen chloride, and cyanogen bromide. Hydrogen cyanide is a colorless gas or a pale blue liquid that has the odor of bitter almonds, although most people cannot detect the odor. Following exposure, the cyanide ion is quickly distributed to all organs and tissues of the body. Ingestion, although rare, leads to particularly high levels in the liver. Hydrogen cyanide kills by binding to and disabling enzymes involved in cellular respiration. Specifically, it is an irreversible inhibitor of the enzyme cytochrome c, which is a key enzyme involved in electron transport (cellular respiration). This prevents the cells from continuing aerobic respiration and production of ATP. Cyanide prevents transfer of electrons to oxygen during the final stages. Upon full reduction of the heme (a3) that works in conjunction with copper (the Cub), cyanide causes a conformational change that constrains the ligand binding site by causing a weaker histidine interaction and a stronger heme electron withdrawing in the Cub–a3 complex (8, 9). Hydrogen cyanide is volatile at room temperature and rapidly disperses in an outdoor or well-ventilated environment. Hence, it was used with little success on the battlefield. However, hydrogen cyanide, also called Zyklon B, was used by the Nazis to conduct the mass murders in the gas chambers of the concentration camps (10). Nerve Agents Nerve agents, a particularly toxic group of compounds, were first synthesized in Germany in the late 1930s. These agents were originally intended for use as insecticides. Germany never used nerve agents on the battlefield (likely in fear of retaliation); however, they may have been used in the concentration camps (see above). A
H
Tabun is a nerve agent that is a colorless to amber-brown liquid with a slight fruity odor. Sarin and VX are also nerve agents; they both are usually colorless with no odor when pure. Sarin was the nerve agent released in the Tokyo subway in 1995, killing 12 people and injuring more (3). Nerve agents are liquids at room temperature and can be absorbed through intact skin, the digestive tract after ingestion of contaminated food, or the lungs if the agent is an aerosol. All nerve agents act by inhibiting the enzyme acetyl cholinesterase (11). Acetyl cholinesterase converts the neurotransmitter acetylcholine (a small molecule that helps transmit electrical impulses between nerve cells) to an inactive form. Inactivation of this enzyme leads to accumulation of acetylcholine between the nerve cells, resulting in continuous stimulation of the muscles and glands (Scheme I). Victims typically die because the diaphragm can no longer properly contract. Vesicants Sulfur-mustard (also called mustard) is a colorless to brown oily liquid that has the slight odor of garlic or mustard (7). German artillery shells first delivered sulfur-mustard in 1917 during WWI. It has a high persistence, but can also evaporate in warm, dry conditions. Sulfur-mustard works as an alkylating agent, meaning that it reacts with most biological molecules and alters their chemical structure by adding an alkyl group at various locations. When taken internally, alkylating agents can also form interstrand and intrastrand crosslinks with DNA. Exposure to the skin results in chemical burns and blisters that rupture easily; thus these molecules are also called “blister agents”. Medical data from WWI revealed that the mortality from exposure to sulfur-mustard was low. Most injuries were nonfatal, but a victim’s health could be compromised over time. Death, although uncommon, was usually due to overwhelming infection as a result of initial injuries.
histidine
O
N
HO
O
choline
O
N
O
N
O
H O
serine
O
H2O
O
O
O
H
OH
serine
serine
H
serine
acetic acid
B O P F O serine
O
HF
O H
P O
O O
serine stable P–O(serine) bond
H2O
P O
O OH
serine
OH acetic acid
reaction site not released
Scheme I. (A) Acetylcholine is converted to choline and acetate by the enzyme acetyl cholinesterase. (B) Sarin and other organophosphates bind to the serine residue at the active site, preventing further catalysis of acetylcholine (11).
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Chemistry for Everyone
Pulmonary Agents The German military first released chlorine gas in 1915 during WWI. The British military shortly responded in kind and later both sides expanded their arsenal to include phosgene and chloropicrin. Chlorine gas has a yellow or green color and an irritating odor similar to bleach. Chlorine is a powerful oxidant and is commonly used today as a disinfectant in swimming pools, drinking water, and industrial waste. It is also used in the manufacture of paper and cloth (for bleaching), pesticides, rubbers, and solvents. The gas is commonly pressurized and cooled to a liquid so it can be shipped and stored. When released, the dense gas stays close to the ground and spreads, accumulating in lowlying areas. Unfortunately, accidents are not uncommon. For example, in January 2005 a train accident in Graniteville, SC, resulted in a chlorine leak from a damaged tank car containing chlorine. Nine people died, dozens were hospitalized, and hundreds of local residents complained of respiratory irritation as a result of the chlorine gas released (12). Phosgene is a colorless gas with the odor of freshly mowed hay. Both phosgene and chlorine are absorbed through inhalation. Usually eye irritation is immediate; however, symptoms can be delayed by up to 24 hours. Once inhaled, pulmonary agents cause an increase in capillary permeability (osmosis). This shifts the flow of water between cells in the respiratory tract, causing fluid to collect in the lungs. Initial symptoms include difficulty breathing, tightness in the chest, and coughing. Over time, fluid collects, resulting in pulmonary edema (swelling) and ultimately respiratory failure; hence, these molecules are also called “choking agents”. Nuclear Weapons Nuclear testing to develop atomic weapons began in the 1940s. In 1945, the United States dropped the first atomic bombs on the Japanese cities of Hiroshima and Nagasaki, killing more than 120,000 people and injuring many more (3). The records surrounding the treatment of those victims provided the most extensive data for the medical repercussions of a full-scale atomic blast (13). A nuclear threat can take one of two forms: a nuclear explosion, or a radiological bomb, also called a dirty bomb (14–16). There are two types of nuclear explosions: nuclear fission and nuclear fusion. Both types take advantage of the massive quantities of energy contained within the nucleus of an atom. Atom bombs such as those used at the end of WWII are made from highly enriched uranium (uranium-235) or plutonium (plutonium-239). Elements such as uranium and plutonium exist naturally as several isotopes. Isotopes are forms of the element differing only by the number of neutrons in their nucleus. A nuclear explosion can be sustained by only one particular isotope. The energy for nuclear fission, also called a nuclear chain reaction, comes from the energy that holds an atom together. During a nuclear chain reaction, the energetic particles generated from the splitting of one atom strike other atoms, causing them to split and release more energetic particles. This process of striking, splitting, and generating more energetic particles is called a chain reaction. To sustain a nuclear chain reaction a certain critical mass or quantity of a particular isotope is required. Because these isotopes do not occur in a pure form in nature, isolating them requires a full nuclear weapons program (16, 17).
The second type of nuclear explosion, nuclear fusion, also takes advantage of the energy that holds the nucleus of an atom together. The energy released from a hydrogen bomb is generated by bringing together two less-stable isotopes to form a more stable atom. While on an atomic level the reverse of fission, this process has the potential to release even more energy than fission. A nuclear explosion (from fission or fusion) results in a massive explosion, intense heat, electromagnetic pulse effect, and intense radiation near the detonation site. Death occurs from trauma, burns, or radiation sickness from the direct ionizing radiation. Radioactive isotopes such as those used in a nuclear bomb emit a powerful form of radiation, γ-rays, that can pass through most material. Alternatively, the dirty bomb is a conventional explosion that releases radioactive isotopes. Isotopes such as cobalt-60, which is used in radiography, or strontium-90, which is used in industrial heating devices, are examples. This type of radiation dispersal is unlikely to cause radiation fatalities but results in high cleanup costs. The immediate danger is from flying objects, and the severity of injury depends on proximity to the blast, duration of exposure, and type of radioactive material used. Biological Weapons Biological weapons are a large class of agents and can include bacteria, viruses, or toxins. Biological agents can be derived from natural strains of bacteria and viruses or may be mutated in the laboratory to express certain characteristics. Bacteria are single-cell organisms capable of self-replication. Bacteria invade host tissue and replicate. If a person’s immune system does not suppress it, an untreated bacterial infection can lead to sepsis and ultimately death. A virus is genetic material (DNA or RNA) in a protein coat. A virus is unable to replicate without a host cell. A virus invades a host cell and uses that cell’s replicative and metabolic machinery to propagate itself. A toxin is a poison or other harmful substance produced by bacteria, animals, or plants. The medical field makes a distinction between infectious and contagious. An infectious agent can cause disease whereas a contagious agent (in addition to being infectious) may be passed from one person to another. A discussion of one of each type of agent follows: bacterial (anthrax), virus (smallpox), and toxin (ricin). The CDC has information about other biological agents (2). Anthrax Robert Koch discovered anthrax (Bacillus anthracis) along with the pathogens causing tuberculosis and cholera. In 1905 he was awarded the Nobel Prize for elucidation of the causal relationship between microbes and disease. Most pathogens need to keep their host (the infected organism) alive long enough to spread to the next host. However, anthrax bacteria quickly kill their host and revert to a dormant form called a spore. Spores can remain dormant for years. Anthrax is deadliest when inhaled into the lungs where the spores will germinate, reproduce, and release lethal toxins (Figure 1). Beginning on September 18, 2001, letters containing a dry powder of anthrax spores were sent to several news media offices and two U.S. senators; 5 people were killed and 17 others were infected (18, 19). The anthrax genome has been entirely sequenced. DNA analysis of the strain used in the 2001 U.S. postal attacks identified the Ames strain, used in research (20).
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a
bacteria
b
airborn spores can be inhaled
spores germinate in the lung
spores Figure 1. Life cycle of anthrax bacterium: (a) In host tissue, bacteria will replicate and release lethal toxins. (b) In the absence of nutrients, bacteria revert to a dormant form called a spore. Once re-introduced into host tissue, spores can germinate.
An anthrax infection, while not contagious, has a mortality of 90–100% if left untreated. The incubation period is 1–7 days from initial exposure. Early symptoms of infection include fever, malaise, fatigue, cough, and difficulty breathing. Later symptoms include toxemia, cyanosis, and terminal shock (2). Smallpox A highly contagious virus causes smallpox. There are two forms of the virus: the more common and severe Variola major and the less severe Variola minor (21). The virus has an incubation period of 7–17 days after initial exposure. A smallpox infection has a mortality rate of about 30% if left untreated. The vaccine for smallpox was once part of standard childhood immunization until the 1970s when it was believed that the virus had been completely eradicated. Today, there are two official repositories of the variola virus: the CDC in Atlanta, GA, and the VECTOR institute in the former Soviet Union (Koltsovo, Russia) (22). Early symptoms of smallpox include malaise, fever, chills, vomiting, headache, and backache. Approximately 2–4 days after the onset of initial symptoms, a rash of flat red spots appear and progress to pus-filled lesions on the skin and lining of the mouth and throat. Smallpox is most contagious during the first 7–10 days following the onset of the rash. The variola virus is transmitted from person to person by aerosolized droplets expressed from the nose and mouth (by coughing, sneezing, or talking) or from contact with contaminated bedding or clothing. The variola virus is very stable outside a host. In an early example of biological warfare (23), British soldiers deliberately distributed blankets from smallpox patients to Native Americans during the French and Indian War (1754–1767). Presumably, smallpox could be used as a WMD in a similar manner or by sending infected individuals into close contact with the general population. Ricin Ricin is a protein with two subunits, an A-chain of 30 kDa and a B-chain of 32 kDa. The ricin toxin derives from castor beans. Indigenous to eastern Africa, the castor plant is now distributed worldwide. Ricin is typically made from the waste left from the processing of castor beans to extract castor oil, using 1380
standard protein purification techniques (24). Ricin can be delivered in the form of a powder, mist, or dissolved in water (25). The protein inactivates ribosomes by site-specifically cleaving the glycosidic bond (5´-AGUACGAGAGGA-3´) of the 28S rRNA of eukaryotic ribosomes. Once the ribosomes are inactivated, protein synthesis is arrested, ultimately leading to cell death. The ricin toxin can cause lethal damage to the gastrointestinal tract if swallowed, or lungs if inhaled. The incubation period is approximately 18–24 hours from initial exposure and mortality is variable. While ricin has never been used on a large scale, it was implicated in the assassination of Bulgarian dissident Georgi Markov in 1978 and has been studied as a weapon since the 1940s (26). Ricin was in the news as recently as 2008 when a man was hospitalized for ricin poisoning and later plead guilty to possessing an outlawed substance (27). Classroom Activities WMDs can be an anxiety-provoking subject, but clear understanding can remove much of the fear. Students enjoy anything topical and relevant. In addition, it is a subject that affects us all whether we are chemists or not. At the same time, even those students who go on to pursue careers outside of chemistry can leave a chemistry course with a better understanding of the many applications of chemistry. This subject can be used as the basis for classroom activities. In a general chemistry course:
• Discuss how boiling point and vapor pressure relate to the danger of the chemical agent. How would these dangers differ in a closed environment such as a subway versus an open environment such as a battlefield?
• Discuss lab safety. What if your lab partner inhaled fumes and started coughing uncontrollably? What should you do when something corrosive is spilled on your skin? Most chemical agents require a similar approach to that of lab accidents or industrial chemical accidents.
• How does chlorine gas react with water in the eyes and lungs? Does this explain the immediate burning sensation upon exposure?
− HOCl + H+ + Cl
Cl2 + H2O
• Discuss why nuclear weapons require a full-scale program whereas the dirty bomb is most likely the weapon of a small terrorist group.
In an organic chemistry course:
• Discuss the ethics of chemical weapons. Some of the original synthetic pathways were developed for use as insecticides, although others were developed specifically for warfare.
• Have students propose a synthesis for pralidoxime, an antidote for organophosphate nerve agents (2). Pralidoxime attacks the phosphate of the nerve agent, freeing it from the enzyme. O P
N
N
pralidoxime
OH
O
O
serine inhibited complex
• Review lab safety in the context of chemical weapons. Why is working under a fume hood so important?
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In a biochemistry course:
• Cyanide is an inhibitor of cytochrome c in oxidative phosphorylation. There are several examples of compounds that interrupt the flow of electrons in oxidative phosphorylation: retonone, antimycin A, CO, and CN−. Discuss the controlled use of inhibitors in in vitro studies to work out a biochemical pathway. Most textbooks have detailed mechanisms of electron transport and some information on specific inhibitors (9).
• Discuss antidotes for cyanide poisoning (28). Scavengers such as Fe3+ or Co3+ compounds compete with the oxidase complex for cyanide. In detoxification, an enzyme catalytically detoxifies cyanide as in the case of thiosulfate-cyanide sulfur transferase (TCST) by converting cyanide to thiocyanate. Biochemical antidotes such as antihistamines or the α-andrenergic blocking agent phenoxybenzamine work through an unexplained mechanism.
• Have students propose an enzyme mechanism for inhibition by an organophosphate such as sarin. Students can also suggest a mechanism for the antidote pralidoxime.
• Discuss the difference(s) between bacteria and viruses.
• Discuss the difference between contagious and infectious.
• Discuss the implications of an agent that inhibits protein synthesis. Many inhibitors used to block protein synthesis are either antibiotics or toxins. Their mechanism of action includes the interruption of peptide-chain elongation, blocking the A site of ribosomes, or misreading of the genetic code. Some of them may also prevent the attachment of oligosaccharide side chains to glycoproteins. The antibiotic tetracycline targets the prokaryotic 30S ribosome, whereas ricin targets eukaryotic ribosomes. Why is it important for an antibiotic to specifically target prokaryotic ribosomes?
• In the case of ricin, which site-specifically cleaves rRNA, students can research compounds that work in a similar manner. For example, the chemotherapy agent bleomycin cleaves DNA at GT or GC.
• Discuss health effects of radiation exposure. Explain how radiation leads to mutations during DNA replication. Relate radiation to cancer treatment. Radiation compromises rapidly dividing cells, which can specifically target certain types of cancers. However, radiation also damages normal rapidly proliferating cells such as the intestinal endothelium, bone marrow, and hair follicles, causing side effects of nausea, loss of natural immunity, and hair loss.
Literature Cited 1. Archbishop of Canterbury. Archbishops Appeal. Times (London), Dec 28, 1937, p 9. 2. The Centers for Disease Control. Emergency Preparedness and Response: Bioterrorism Agents and Diseases. http://www.bt.cdc. gov/ (accessed Oct 2009). 3. Simons, L. M. Weapons of Mass Destruction. National Geographic, Nov 2002, pp 2–35. 4. Evison, D.; Hinsley, D.; Rice, P. BMJ 2002, 324, 332–335. 5. U.S. Army Medical Research Institute of Chemical Defense. Medical Management of Chemical Casualties Handbook, 2nd ed.; Chemical Casualty Care Office: Aberdeen Proving Ground, MD, 1999. http://www.fas.org/nuke/guide/usa/doctrine/army/mmcch/ index.html (accessed Oct 2009).
6. Wells, C. F. J. Chem. Educ. 1928, 5, 929–932. 7. Duchovic, R. J.; Vilensky, J. A. J. Chem. Educ. 2007, 84, 944– 948. 8. Kim, Y.; Babcock, G. T.; Surerus, K. K.; Fee, J. A.; Dyer, R. B.; Woodruff, W. H.; Berling, W. A. Biospectroscopy 1998, 4, 1–15. 9. Nelson, D. A.; Cox, M. M. Principles of Biochemistry, 4th ed.; W. H. Freeman: New York, 2005; pp 696–701. 10. Pressac, J. C. The Machinery of Mass Murder at Auschwitz. In Anatomy of the Auschwitz Death Camp, Gutman, Y., Berenbaum, M., Eds.; Indiana University Press: Bloomington, IN, 1994; pp 183–245. 11. Marrs, T. C. Pharmacol. Ther. 1993, 58, 51–66. 12. National Transportation and Safety Board. Testimony. http:// www.ntsb.gov/speeches/s060613.htm (accessed Oct 2009) 13. Ehrlich, P. R.; Harte, J.; Harwell, M. A.; Raven, P. H.; Sagan, C.; Woodwell, G. M.; Berry, J.; Ayensu, E. S.; Ehrlich, A. H.; Eisner, T. Science 1983, 23, 293. 14. Jaworowski, Z. Physics Today 1999, 52, 24–29. 15. Sime, R. L. Scientific American 1998, 278 (11), 80–85. 16. Glaser, A.; vonHippel, F. Scientific American 2006, 294 (2), 56–63. 17. Broad, W. J.; Sanger D. E. As Crisis Brews, Iran Hits Bumps on Atomic Path. NY Times, Mar 5, 2006, pp A1, A11. 18. Chamberlain, C. My Anthrax Survivor’s Story. http://www.msnbc. msn.com/id/14785359/ (accessed Oct 2009) 19. Engleberg, S.; Miller, J. Daschle Letter Called First Use of Anthrax as Weapon. NY Times, Oct 17, 2001, p A1. 20. Read, T. D. et al. Nature 2003, 423, 81–86. 21. Fenner, F.; Henderson, D. A.; Arita, I.; Jezek, Z.; Ladnyi, I. D. Small Pox and Its Eradication; World Health Organization: Geneva, 1988; Chapters 1 and 4. 22. Preston, R. The Demon in the Freezer. The New Yorker, July 12, 1999, pp 44–61. http://cryptome.org/smallpox-wmd.htm (accessed Oct 2009) 23. Henderson, D. A.; Inglesby, T. V.; Bartlett, J. G.; Ascher, M. S.; Eitzer, E.; Jahrling, J. D.; Haver, J.; Layton, M.; McDade, J.; Osterholm, M. T.; O’Toole, T.; Parker, G.; Perl, T.; Russel, P. K. JAMA 1999, 281, 2127–2137. 24. Craig, H. L.; Alderks, O. H.; Corwin, A. H.; Dieke, S. H.; Karel, C. Preparation of Toxic Ricin. U.S. Patent 3,060,165, 1952. 25. Associated Press. Researchers Create Human Ricin Vaccine. NY Times, Jan 31, 2006, p A17. 26. CRS Report for Congress. http://fas.org/irp/crs/RS21383.pdf (accessed Oct 2009). 27. Freiss, S. In Accord, Ricin Owner Enters Plea of Guilty. NY Times, Aug 5, 2008, p A7. 28. Bhattacharya, R. Indian J. Pharm. 2000, 32, 94–101.
Supporting JCE Online Material
http://www.jce.divched.org/Journal/Issues/2009/Dec/abs1377.html Abstract and keywords Full text (PDF) with links to cited URLs and JCE articles Figure 1 in color Supplement: Signs and symptoms of exposure JCE Featured Molecules for December 2009 (see p 1470 for details) Structures of some of the molecules discussed in this article are available in fully manipulable Jmol format in the JCE Digital Library at http://www.JCE.DivCHED.org/JCEWWW/Features/ MonthlyMolecules/2009/Dec/.
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