The ABCs of Chemistry: Part 2
Chug, chug, chug. The journey continues with several more interesting stops.
The good thing about chemistry is that there are so many different interesting aspects, not just molecules.
D is for DEET
DEET is short for N,N-Diethyl-m-toluamide, the active ingredient in many insect repellents. It is a pale yellow oil. The US Army developed it in 1946, and it was registered for use by the public in 1957. The exact mechanism of its effectiveness is still unknown. This much is known, however: DEET blocks the olfactory neurons in mosquitoes, so that it can’t smell otherwise attractive odors, like humans, those fountains of yummy blood. It is thought to decrease the sensation of 1-octen-3-ol, a compound found in human breath and sweat. Another hypothesis is that mosquitoes actually dislike the smell of DEET.
Concerns have arisen regarding the safety of DEET. It can irritate the skin and so should not be applied where the skin is damaged. Some people have had full-blown allergic reactions to it. DEET can interfere with the hydrolysis of acetylcholine, which is involved with muscle control. The acetylcholine builds up and can cause death by asphyxiation. Employees in the Everglades National Park who had greater exposure to DEET showed an increased incidence of insomnia and impaired mental function.
Still, DEET can be useful if you don’t want to look like you’re having a case of chicken pox, especially if you’re someone who has a sign over your head reading, “All-you-can-eat buffet!” written in Mosquitoese. Twenty- to thirty- percent solutions are common and should provide protection for the duration of most outdoor activities, about three to six hours. Here are some guidelines from the EPA:
- Don’t apply DEET under clothes; it should be on exposed skin or clothing only.
- Don’t spray in an enclosed area.
- Don’t douse yourself with it.
- Don’t allow young children to apply it.
- Wash skin with soap and water when you come back in.
- Wash affected clothing before wearing it again.
E is for Epoxide
Epoxides are a class of compounds characterized by a three-member ring consisting of two carbon atoms and one oxygen atom. This triangular molecule is roughly shaped like an equilateral triangle, with angles of 60 degrees. The optimal bond angle for the carbons in the molecule is 109.5 degrees, so the ring is very strained and opens easily. One way of opening the ring is a process called hydroxylation, which produces a dialcohol molecule, which is better known as a glycol. This is the process used to make automotive antifreeze.
Epoxides and epoxy glues are related. An epoxy contains a “prepolymer,” which contains a molecule with an epoxide ring and another linker molecule. A popular prepolymer combines bisphenol A with epichlorohydrin. The bracketed part with the “n” is the repeated unit in the polymer. The prepolymer is treated with a curing agent to link chains together at the free alcohol (-OH) groups, converting epoxides to alcohol groups. The prepolymer and curing agent are kept in separate tubes so that the reaction does not start until needed. The resulting polymer is strong, resisting many types of attacks.
Epoxies have many uses beyond glues. They are used to “powder coat” white objects such as washing machines and dryers. Another industrial use is making durable molds and castings. Because epoxies are excellent electrical insulators, they are used in circuitry. The marine uses epoxies in boat repairs, covering them with paint for UV protection. After Gorilla Glue failed, my mom used a two-part marine epoxy to fix her food processor. Kayaks are often coated with protective epoxy resins.
F is for Fischer Projection
There are many different ways to draw molecules because different projections help visualize different aspects of molecules. Here is a molecule of glucose in a typical projection used in organic chemistry. The solid triangles indicate a bond coming out of your screen. The dashed lines show a bond coming out the back of your screen:
This is a legitimate way to draw it. There are some problems, however. The stereochemistry (orientation of groups in three-dimensional space) can be difficult to observe. There are molecules that are similar but with a slightly different arrangement of groups, and telling the difference between molecules is difficult with this projection. Enter the Fischer projection. This was produced in 1891 by the organic chemist Emil Fischer. It shows the carbon backbone as a straight vertical line. The terminal carbons’ groups are in the plane of the backbone. The groups of the middle carbons are drawn perpendicular to the backbone, but it is understood that they are coming out of the plane of the backbone. Here is the glucose molecule in a Fischer projection:
Isn’t that nicer to see? Now identifying different sugars is easy. Swap the hydroxide and hydrogen on the third carbon from the top so that all the hydroxides are on the right. You now have a different sugar. Look it up in a table; it’s allose.
This projection helps to determine the handedness of sugar molecules. Sugar molecules’ handedness is determined by the location of the hydroxide on second-from bottom carbon. If the hydroxide is on the right side of the carbon, it’s a D (dexter, right) sugar. If it’s on the left, it’s an L (laevus, left) sugar. The hydroxide on both sugar molecules is on the right side, so they’re both D-sugars. I’m not sure why this and not R and S are used. This projection and nomenclature are also used for amino acids.
G is for GABA
GABA is the shortened form of gamma-aminobutyric acid. The “gamma” before the amino means that the amine (-NH3) group is attached to the third carbon away from the carbon double bonded to the oxygen. This is synthesized in the brain from glutamic acid. GABA is a fairly flexible molecule, so it can bind to different receptor sites. The molecule functions, in the mature brain, as an inhibitory neurotransmitter.
It carries out this function by binding to specific receptors, which, in turn opens ion channels in the neurons, allowing chloride anions (negatively-charged ions) to flow into the cell or potassium cations (positively-charged ions) out of it. This changes the electric potential of the cell. Neurons that produce GABA are called GABAergic neurons. Certain of this type of cells are called “chandelier cells.”
In the developing brain, GABA is an excitatory neurotransmitter because it pumps chloride out of the cell. In addition to this, it regulates the growth of neurons and their axons and dendrites. GABA governs the differentiation and migration of nerve cells. It holds sway over embryonic stem cells and, with brain-derived neurotrophic factor (BDNF), governs neural progenitor cells.
Drugs that increase its availability result in a person becoming more relaxed and less anxious. There are even anticonvulsive effects. One study found that GABA had a hand in controlling the pituitary gland, which keeps the body running on an even keel. The receptor sites also have an effect on people. The GABA type B receptor 1 has been linked to OCD because the gene that codes for it is in region on chromosome 6 that also has been linked to it.
There are so many different, interesting things to discover in chemistry, from materials to methods. I took four years of chemistry classes, and I feel that there are things I still don’t know. I hope you are enjoying these discoveries as much as I am.
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"The Insect Repellent DEET." Pesticides: Topical & Chemical Fact Sheets. EPA, 10 Sep 2012. Web. 21 Mar 2013.
McCann, SM, and V Rettori. "Gamma Amino Butyric Acid (GABA) Controls Anterior Pituitary Hormone Secretion." Advances in Biochemical Psychopharmacology. 42. (1986): 173-189. Web. 21 Mar. 2013.
McMurry, John. Organic Chemistry. 7th ed. Belmont, CA: Brooks/Cole, 2008. Print.
Syed, Zainulabeuddin, and Walter Leal. "Mosquitoes Smell and Avoid the Insect Repellent DEET." Proceedings of the National Academy of Sciences of the United States of America. 1005.36 (2008): 13598–13603. Web. 21 Mar. 2013.
Zwai, Gwyenth, Arnold Paul, et al. "Evidence for the Gamma-Amino-Butyric Acid Type B Receptor 1 (GABBR1) Gene as a Susceptibility Factor in Obsessive-Compulsive Disorder." American Journal of Medical Genetics. 134B.1 (2005): 25-29. Web. 21 Mar. 2013.
PubChem Sketcher v. 2.4 used for some molecular drawings