JEE Chemistry Chemistry In Everyday Life Notes Set 04

Drugs and Chemotherapy

A drug is a chemical of low molecular mass that interacts with a macromolecular target in the body and produces a biological response. When the response is useful in preventing, diagnosing, treating or curing a disease it is called a medicine. Chemotherapy means treating disease with chemicals that destroy disease-causing micro-organisms or cancer cells without harming the host.
Drugs are classified in four ways: on the basis of pharmacological effect (analgesics, antiseptics, antacids and so on), on the basis of drug action (for example antihistamines that block histamine), on the basis of chemical structure (sulphonamides, for example) and on the basis of molecular target (enzymes, receptors, nucleic acids, membrane lipids).

Drug-Target Interaction

Enzymes as targets. An enzyme binds its substrate at the active site through weak forces (ionic bonds, hydrogen bonds, van der Waals and dipole-dipole forces) and then provides a functional group that carries out the reaction. A drug that blocks this is an enzyme inhibitor. A competitive inhibitor resembles the substrate and competes for the active site. A non-competitive (allosteric) inhibitor binds at a different site and changes the shape of the active site so that the substrate no longer fits. A poison binds irreversibly (often covalently) to the active site and permanently disables the enzyme.
Receptors as targets. Receptors are proteins in the cell membrane that receive chemical messengers (hormones and neurotransmitters). When the messenger binds, the receptor changes shape and passes the message into the cell without the messenger entering. An antagonist binds to the receptor and blocks the natural messenger, which is useful when the message must be stopped. An agonist mimics the natural messenger and switches the receptor on, which is useful when the natural messenger is lacking.

Antacids, Antihistamines and Antiallergics

Antacids reduce stomach acidity. Simple antacids such as sodium hydrogencarbonate, magnesium hydroxide and aluminium hydroxide neutralise acid, but too much NaHCO3 makes the stomach alkaline and triggers more acid secretion. Metal hydroxides are insoluble, so they do not raise the pH above about 7 to 8. Cimetidine (Tagamet) and ranitidine (Zantac) prevent histamine from acting on the stomach-wall receptors, so less acid is released; omeprazole and lansoprazole inhibit the proton pump and also reduce acid release.
Histamine is a vasodilator. It contracts smooth muscle in the bronchi and gut, relaxes the walls of fine blood vessels, causes the nasal congestion of colds and allergy, and stimulates release of pepsin and HCl in the stomach. Antihistamines such as brompheniramine (Dimetapp), terfenadine (Seldane), pheniramine (Avil) and diphenhydramine act on the allergy receptors. Antacid and antiallergic drugs do not interfere with each other because the receptors they act on are different.

Neurologically Active Drugs

Tranquillisers relieve anxiety and mental tension and act on the central nervous system. Low noradrenaline causes depression, so antidepressants such as iproniazid and phenelzine inhibit the enzyme that degrades noradrenaline. Chlordiazepoxide, meprobamate (Equanil), valium and reserpine (an alkaloid from Rauwolfia serpentina) are used for tension and hypertension. Barbiturates (veronal, amytal, nembutal, seconal, luminal) are derivatives of barbituric acid that act as hypnotics (sleep-producing agents).
Analgesics relieve pain. Non-narcotic analgesics (aspirin, paracetamol, ibuprofen) are non-addictive, also reduce fever and, in the case of aspirin, prevent blood clotting, so it is used in small doses to prevent heart attacks. Aspirin is acetylsalicylic acid and paracetamol is 4-acetamidophenol. Narcotic analgesics (morphine, codeine, heroin) relieve pain and produce sleep; large doses cause stupor, coma, convulsions and death. They are used for post-operative, cardiac and terminal cancer pain.

Antimicrobials

Antimicrobials inhibit or kill bacteria, viruses, fungi and other micro-organisms. They include antibiotics, antiseptics and disinfectants.
Antibiotics are produced by micro-organisms (or synthetic modifications of such compounds). A broad-spectrum antibiotic acts on both Gram-positive and Gram-negative bacteria (ampicillin, amoxycillin, tetracycline, chloramphenicol, ofloxacin), while a narrow-spectrum one acts mainly on one group (penicillin G). Bactericidal antibiotics kill the organism (penicillin, aminoglycosides, ofloxacin), whereas bacteriostatic ones only stop its growth (erythromycin, tetracycline, chloramphenicol).
Penicillin is used for pneumonia and bronchitis; streptomycin for tuberculosis; chloramphenicol for typhoid, dysentery and meningitis; tetracyclines (terramycin, aureomycin) for typhoid and eye infections. Sulpha drugs are derivatives of sulphanilamide (sulphadiazine, sulphapyridine) and are synthetic antibacterials.

Antiseptics are applied to living tissue (wounds, cuts, ulcers, skin): furacin, soframycin, dettol (chloroxylenol plus terpineol), bithional (in soaps), tincture of iodine (2 to 3 percent iodine in alcohol-water), iodoform, dilute boric acid for the eyes, gentian violet and methylene blue. Disinfectants are applied to inanimate objects such as floors, drains and instruments. Some substances act as both depending on concentration: 0.2 percent phenol is an antiseptic and 1 percent phenol is a disinfectant. Chlorine at 0.2 to 0.4 ppm disinfects drinking water, and SO2 is another disinfectant.
Other classes. Antimalarials: quinine (from cinchona bark), chloroquine, primaquine. Antifungals: amphotericin B, griseofulvin. Antiamoebic: metronidazole, tinidazole. Antivirals: acyclovir, zidovudine, ribavirin. Antifertility drugs (oral contraceptives) contain synthetic estrogen and progesterone derivatives such as norethindrone and ethynylestradiol; mifepristone is a synthetic steroid that blocks progesterone and is used as the morning-after pill.

Chemicals in Food

Food additives include colours, flavours, sweeteners, emulsifiers, antioxidants, preservatives and nutritional supplements (minerals, vitamins, amino acids); only the last group has nutritive value.
Preservatives stop food spoilage by micro-organisms: sodium benzoate (0.06 to 0.1 percent, for juices, jams and pickles), parabens (esters of p-hydroxybenzoic acid), sorbates (salts of sorbic acid, for cheese and fish; they inhibit yeast), propionates (against mould in baked goods), sodium or potassium metabisulphite, and table salt and sugar. An ideal preservative does not react with the food, is effective for a long time and is not harmful.
Artificial sweeteners (sweetness relative to cane sugar): aspartame 100 (a dipeptide ester, unstable at cooking temperature, used in cold foods), saccharin 550 (the first, excreted unchanged in urine), sucralose 600 (a trichloro derivative of sucrose, stable at cooking temperature, no calories) and alitame 2000 (very potent, so sweetness is hard to control). They suit diabetics and calorie-conscious people.
Antioxidants slow the reaction of food with oxygen and the formation of free radicals; BHT (butylated hydroxytoluene) and BHA (butylated hydroxyanisole) are the common ones.

Cleansing Agents

Soaps are sodium or potassium salts of long-chain fatty acids (stearic, oleic, palmitic) and are made by saponification, heating fat with aqueous NaOH. Potassium soaps are soft. A soap molecule has a long hydrocarbon tail that is hydrophobic and an ionic head that is hydrophilic; in water the tails surround grease and the heads face the water, forming micelles that carry the dirt away. In hard water the Ca2+ and Mg2+ ions precipitate insoluble calcium and magnesium soaps (scum), which waste the soap and do not clean.
Synthetic detergents work in hard water because their calcium and magnesium salts are soluble. Anionic detergents are sodium salts of sulphonated long-chain alcohols or hydrocarbons (sodium lauryl sulphate, sodium dodecylbenzenesulphonate) and are used in household cleaning and toothpaste. Cationic detergents are quaternary ammonium salts such as cetyltrimethylammonium bromide; they are germicidal and expensive. Non-ionic detergents are esters of polyhydroxy alcohols such as the stearic acid and polyethylene glycol ester; liquid dishwashing detergents are of this type.
Highly branched hydrocarbon chains cannot be degraded by bacteria, so the detergent persists and causes foaming in rivers; modern detergents use unbranched, biodegradable chains. A soap solution is basic (salt of weak acid and strong base) whereas a detergent solution is neutral (salt of strong acid and strong base), so detergents are safe for wool and silk.

Quick Facts to Remember

1. Penicillin G is narrow spectrum; ampicillin and amoxycillin are broad spectrum semi-synthetic penicillins.
2. Ofloxacin, penicillin and aminoglycosides are bactericidal; erythromycin, tetracycline and chloramphenicol are bacteriostatic.
3. Cimetidine and ranitidine are antihistamines that reduce stomach acid; phenelzine is an antidepressant.
4. Octane number: n-heptane 0, isooctane 100.
5. DDT and gammexane are insecticides; DDT is non-biodegradable.
6. Rocket propellants combine a fuel and an oxidiser, for example liquid H2 with liquid O2.