uric acid, white, odorless, tasteless crystalline substance formed as a result of purine degradation in man, other primates, dalmatians, birds, snakes, and lizards. The last three groups of animals also channel all amino acid degradation into the formation of glycine, aspartic acid, and glutamine, which combine to form purines and finally uric acid; these so-called uricotelic organisms thus excrete uric acid as the major end-product of the metabolism of all nitrogen-containing compounds. Uric acid is a very weak organic acid that is barely soluble in water and insoluble in alcohol and ether. The urates are its salts. Uric acid is present in human urine only in extremely small amounts but constitutes a large part of the body waste matter of birds (see guano) and of reptiles. It collects sometimes in the human kidneys or bladder in calculi, or stones, and is responsible, when present in tissues or deposited upon bones in the form of urates, for gouty conditions (see gout). It occurs also in normal human blood. The pure acid is obtained from guano and other similar substances. Upon decomposition urea is obtained. A common test for the presence of the acid in urine depends upon the formation of murexide (an ammonium salt), which is an intense reddish purple. Nitric acid is added to the urine, which is then evaporated. If uric acid is present, murexide is formed when ammonia is added to the residue.

tricarboxylic acid cycle: see Krebs cycle.

trans-butenedioic acid, IUPAC name for fumaric acid.

tetrahydrofolic acid: see coenzyme.

tartaric acid, HO₂CCHOHCHOHCO₂H, white crystalline dicarboxylic acid. It occurs as three distinct isomers, the dextro-, levo-, and meso- forms. The dextro- and levo- forms are optically active; the meso- form is optically inactive, as is racemic acid, a mixture of equal parts of the dextro- and levo- forms. Tartaric acid is found in many plants, e.g., grapes; this natural acid is chiefly the dextrorotatory d-tartaric acid, called also d-2,3-dihydroxysuccinic acid or l-2,3-dihydroxybutanedioic acid. This form can be partially converted to the others by heating it with an aqueous alkali, e.g., potassium hydroxide. Tartaric acids can be synthesized from maleic acids or fumaric acids by reaction with aqueous potassium permanganate. The various isomeric forms differ in such physical properties as boiling point. Tartaric acid is used chiefly in the form of its salts, e.g., cream of tartar and Rochelle salt.

sulphuric acid: see sulfuric acid.

sulfuric acid, chemical compound, H₂SO₄, colorless, odorless, extremely corrosive, oily liquid. It is sometimes called oil of vitriol.

Concentrated Sulfuric Acid

When heated, the pure 100% acid loses sulfur trioxide gas, SO₃, until a constant-boiling solution, or azeotrope, containing about 98.5% H₂SO₄ is formed at 337°C;. Concentrated sulfuric acid is a weak acid (see acids and bases) and a poor electrolyte because relatively little of it is dissociated into ions at room temperature. When cold it does not react readily with such common metals as iron or copper. When hot it is an oxidizing agent, the sulfur in it being reduced; sulfur dioxide gas may be released. Hot concentrated sulfuric acid reacts with most metals and with several nonmetals, e.g., sulfur and carbon. Because the concentrated acid has a fairly high boiling point, it can be used to release more volatile acids from their salts, e.g., when sodium chloride (NaCl), or common salt, is heated with concentrated sulfuric acid, hydrogen chloride gas, HCl, is evolved.

Concentrated sulfuric acid has a very strong affinity for water. It is sometimes used as a drying agent and can be used to dehydrate (chemically remove water from) many compounds, e.g., carbohydrates. It reacts with the sugar sucrose, C₁₂H₂₂O₁₁, removing eleven molecules of water, H₂O, from each molecule of sucrose and leaving a brittle spongy black mass of carbon and diluted sulfuric acid. The acid reacts similarly with skin, cellulose, and other plant and animal matter.

When the concentrated acid mixes with water, large amounts of heat are released; enough heat can be released at once to boil the water and spatter the acid. To dilute the acid, the acid should be added slowly to cold water with constant stirring to limit the buildup of heat. Sulfuric acid reacts with water to form hydrates with distinct properties.

Dilute Sulfuric Acid

Dilute sulfuric acid is a strong acid and a good electrolyte; it is highly ionized, much of the heat released in dilution coming from hydration of the hydrogen ions. The dilute acid has most of the properties of common strong acids. It turns blue litmus red. It reacts with many metals (e.g., with zinc), releasing hydrogen gas, H₂, and forming the sulfate of the metal. It reacts with most hydroxides and oxides, with some carbonates and sulfides, and with some salts. Since it is dibasic (i.e., it has two replaceable hydrogen atoms in each molecule), it forms both normal sulfates (with both hydrogens replaced, e.g., sodium sulfate, Na₂SO₄) and acid sulfates, also called bisulfates or hydrogen sulfates (with only one hydrogen replaced, e.g., sodium bisulfate, NaHSO₄).

Production of Sulfuric Acid

There are two major processes (lead chamber and contact) for production of sulfuric acid, and it is available commercially in a number of grades and concentrations. The lead chamber process, the older of the two processes, is used to produce much of the acid used to make fertilizers; it produces a relatively dilute acid (62%-78% H₂SO₄). The contact process produces a purer, more concentrated acid but requires purer raw materials and the use of expensive catalysts. In both processes sulfur dioxide is oxidized and dissolved in water. The sulfur dioxide is obtained by burning sulfur, by burning pyrites (iron sulfides), by roasting nonferrous sulfide ores preparatory to smelting, or by burning hydrogen sulfide gas. Some sulfuric acid is also made from ferrous sulfate waste solutions from pickling iron and steel and from waste acid sludge from oil refineries.

Lean Chamber Process

In the lead chamber process hot sulfur dioxide gas enters the bottom of a reactor called a Glover tower where it is washed with nitrous vitriol (sulfuric acid with nitric oxide, NO, and nitrogen dioxide, NO₂, dissolved in it) and mixed with nitric oxide and nitrogen dioxide gases; some of the sulfur dioxide is oxidized to sulfur trioxide and dissolved in the acid wash to form tower acid or Glover acid (about 78% H₂SO₄). From the Glover tower a mixture of gases (including sulfur dioxide and trioxide, nitrogen oxides, nitrogen, oxygen, and steam) is transferred to a lead-lined chamber where it is reacted with more water. The chamber may be a large, boxlike room or an enclosure in the form of a truncated cone. Sulfuric acid is formed by a complex series of reactions; it condenses on the walls and collects on the floor of the chamber. There may be from three to twelve chambers in a series; the gases pass through each in succession. The acid produced in the chambers, often called chamber acid or fertilizer acid, contains 62% to 68% H₂SO₄. After the gases have passed through the chambers they are passed into a reactor called the Gay-Lussac tower where they are washed with cooled concentrated acid (from the Glover tower); the nitrogen oxides and unreacted sulfur dioxide dissolve in the acid to form the nitrous vitriol used in the Glover tower. Remaining waste gases are usually discharged into the atmosphere.

Contact Process

In the contact process, purified sulfur dioxide and air are mixed, heated to about 450°C;, and passed over a catalyst; the sulfur dioxide is oxidized to sulfur trioxide. The catalyst is usually platinum on a silica or asbestos carrier or vanadium pentoxide on a silica carrier. The sulfur trioxide is cooled and passed through two towers. In the first tower it is washed with oleum (fuming sulfuric acid, 100% sulfuric acid with sulfur trioxide dissolved in it). In the second tower it is washed with 97% sulfuric acid; 98% sulfuric acid is usually produced in this tower. Waste gases are usually discharged into the atmosphere. Acid of any desired concentration may be produced by mixing or diluting the products of this process.

Uses of Sulfuric Acid

Sulfuric acid is one of the most important industrial chemicals. More of it is made each year than is made of any other manufactured chemical; more than 40 million tons of it were produced in the United States in 1990. It has widely varied uses and plays some part in the production of nearly all manufactured goods. The major use of sulfuric acid is in the production of fertilizers, e.g., superphosphate of lime and ammonium sulfate. It is widely used in the manufacture of chemicals, e.g., in making hydrochloric acid, nitric acid, sulfate salts, synthetic detergents, dyes and pigments, explosives, and drugs. It is used in petroleum refining to wash impurities out of gasoline and other refinery products. Sulfuric acid is used in processing metals, e.g., in pickling (cleaning) iron and steel before plating them with tin or zinc. Rayon is made with sulfuric acid. It serves as the electrolyte in the lead-acid storage battery commonly used in motor vehicles (acid for this use, containing about 33% H₂SO₄ and with specific gravity about 1.25, is often called battery acid).

History of Sulfuric Acid

Although sulfuric acid is now one of the most widely used chemicals, it was probably little known before the 16th cent. It was prepared by Johann Van Helmont (c.1600) by destructive distillation of green vitriol (ferrous sulfate) and by burning sulfur. The first major industrial demand for sulfuric acid was the Leblanc process for making sodium carbonate (developed c.1790). Sulfuric acid was produced at Nordhausen from green vitriol but was expensive. A process for its synthesis by burning sulfur with saltpeter (potassium nitrate) was first used by Johann Glauber in the 17th cent. and developed commercially by Joshua Ward in England c.1740. It was soon superseded by the lead chamber process, invented by John Roebuck in 1746 and since improved by many others. The contact process was originally developed c.1830 by Peregrine Phillips in England; it was little used until a need for concentrated acid arose, particularly for the manufacture of synthetic organic dyes.

sulfonic acid, organic compound containing the functional group RSO₂OH, which consists of a sulfur atom, S, bonded to a carbon atom that may be part of a large aliphatic or aromatic hydrocarbon, R, and also bonded to three oxygen atoms, O, one of which has a hydrogen atom, H, attached to it. The hydrogen atom makes the compound acidic, much as the hydrogen of a carboxylic acid (see carboxyl group) makes it acidic (see acids and bases). However, while carboxylic acids are weak (with dissociation constants of about 10^-5), sulfonic acids are considered strong acids (with dissociation constants of about 10^-2). Because sulfonic acids are so acidic, they generally exist as their salts and thus tend to be quite soluble in water. Sulfonic acid groups are often introduced into organic molecules such as dyes to stabilize them for use in aqueous dye baths. Sulfonic acid groups also improve the washfastness of wool and silk dyes by enabling the dye to bind more tightly to the fabric. The most important use of sulfonic acid salts (sulfonates) is in the detergent industry. Sodium salts of long-chain aliphatic or aromatic sulfonic acids are used as detergents. Unlike ordinary soaps, which contain carboxylic acid salts, soaps containing sulfonates do not form a scum in hard water because the calcium and magnesium ions present in the hard water do not form insoluble precipitates with sulfonates as they do with carboxylates. Some sulfonic acid derivatives, e.g., the sulfa drugs, are important as antibiotics.

succinic acid: see Krebs cycle.

sialic acid: see glycoprotein.

salicylic acid or 2-hydroxybenzoic acid, C₆H₄(OH)CO₂H, a colorless, crystalline organic carboxylic acid that melts at 159°C;; it is soluble in ethanol and ether but is only slightly soluble in water. It is prepared commercially by heating sodium phenolate (the sodium salt of phenol) with carbon dioxide under pressure to form sodium salicylate, which is treated with sulfuric acid to liberate salicylic acid. Salicylic acid and its derivatives are toxic when consumed in large amounts. Sodium salicylate is used to a small extent as a food preservative and as an antiseptic in mouthwashes and toothpastes. The major use of salicylic acid is in the preparation of its ester derivatives; since it contains both a hydroxyl (-OH) and a carboxyl (-CO₂H) group, it can react with either an acid or an alcohol. The hydroxyl group reacts with acetic acid to form the acetate ester, acetylsalicylic acid (see aspirin). Several useful esters are formed by reaction of the carboxyl group with alcohols. The methyl ester, methyl salicylate (also called oil of wintergreen since it produces the fragrance of wintergreen), is formed with methanol; it is used in food flavorings and in liniments. The phenyl ester, phenyl salicylate, or salol, is formed with phenol; it is used in medicine as an antiseptic and antipyretic. This ester hydrolyzes, not in the acidic stomach, but in the alkaline intestines, releasing free salicylic acid. The menthyl ester, menthyl salicylate, which is used in suntan lotions, is formed with menthol.

racemic acid: see tartaric acid.

pyroligneous acid, a dark liquid that is essentially a mixture of acetic acid and methanol (wood alcohol) and is obtained in the destructive distillation of wood. It once served as a commercial source of acetic acid.

pteroylglutamic acid: see vitamin.

prussic acid: see hydrogen cyanide.

propanoic acid, 2-methyl-, IUPAC name for isobutyric acid (see butyric acid).

propanetricarboxylic acid, 2-hydroxy-1,2,3-, IUPAC name for citric acid.

picric acid or 2,4,6-trinitrophenol, C₆H₂(NO₂)₃OH, a toxic yellow crystalline solid that melts at 122°C; and is soluble in most organic solvents. Picric acid is a derivative of phenol. It reacts with metals to form metal picrates, which like picric acid itself are highly sensitive explosives that can be detonated by heat, flame, shock, or friction. The high explosives lyddite and melinite are composed mostly of compressed or fused picric acid. Picric acid is often used as a booster to detonate another, less sensitive explosive, such as TNT (trinitrotoluene). Although picric acid can be synthesized by nitration of phenol, higher yields are obtained if chlorobenzene is used as a starting material; the latter method involves several steps and the formation of several intermediate products. In addition to its use in explosives, picric acid has been used as a yellow dye, as an antiseptic, and in the synthesis of chloropicrin, or nitrotrichloromethane, CCl₃NO₂, a powerful insecticide.

phosphoric acid, any one of three chemical compounds made up of phosphorus, oxygen, and hydrogen (see acids and bases). The most common, orthophosphoric acid, H₃PO₄, is usually simply called phosphoric acid. Two molecules of it are formed by adding three molecules of water, H₂O, to one molecule of phosphorus pentoxide (phosphoric anhydride, P₂O₅). It occurs as rhombic crystals or as a viscous liquid; both are deliquescent. The crystals melt at about 42°C;. It has specific gravity 1.834 at 18°C;, is soluble in alcohol, and is very soluble in water. It is a tribasic acid and forms orthophosphate salts with either one, two, or all three of the hydrogens replaced by some other positive ion. When it is heated to about 225°C;, it dehydrates to form pyrophosphoric acid, H₄P₂O₇; at still higher temperatures metaphosphoric acid, HPO₃, is formed. Salts of pyrophosphoric acid are pyrophosphates; salts of metaphosphoric acid are metaphosphates. Phosphoric acid is prepared commercially by heating calcium phosphate rock with sulfuric acid; purer grades may be prepared by treating red phosphorus with nitric acid. It is used in pickling and rust-proofing metals, in acidifying jellies and beverages, and in preparing phosphate salts.

phosphatidic acid: see phospholipid.

para-aminobenzoic acid: see vitamin.

pantothenic acid: see coenzyme; vitamin.

oxalic acid or ethanedioic acid, HO₂CCO₂H, a colorless, crystalline organic carboxylic acid that melts at 189°C; with sublimation. Oxalic acid and oxalate salts are poisonous. Oxalic acid is found in many plants, e.g., sorrel and rhubarb, usually as its calcium or potassium salts. Oxalic acid is the only possible compound in which two carboxyl groups are joined directly; for this reason oxalic acid is one of the strongest organic acids. Unlike other carboxylic acids (except formic acid), it is readily oxidized; this makes it useful as a reducing agent for photography, bleaching, and ink removal. Oxalic acid is usually prepared by heating sodium formate with sodium hydroxide to form sodium oxalate, which is converted to calcium oxalate and treated with sulfuric acid to obtain free oxalic acid.

nucleic acid, any of a group of organic substances found in the chromosomes of living cells and viruses that play a central role in the storage and replication of hereditary information and in the expression of this information through protein synthesis. In most organisms, nucleic acids occur in combination with proteins; the combined substances are called nucleoproteins. Nucleic acid molecules are complex chains of varying length. The two chief types of nucleic acids are DNA (deoxyribonucleic acid), which carries the hereditary information from generation to generation, and RNA (ribonucleic acid), which delivers the instructions coded in this information to the cell's protein manufacturing sites.

A substance that he called nuclein (now known as DNA) was isolated by 1869 by Friedrich Miescher, but it was only in the last half of the 20th cent. that that research revealed its significance as the material of which the gene is composed, and thus its function as the chemical bearer of hereditary characteristics. RNA was first made by laboratory synthesis in 1955. In 1965 the nucleotide sequence of tRNA was determined, and in 1967 the synthesis of biologically active DNA was achieved. The amount of RNA varies from cell to cell, but the amount of DNA is normally constant for all typical cells of a given species of plant or animal, no matter what the size or function of that cell. The amount doubles as the chromosomes replicate themselves before cell division takes place (see mitosis); in the ovum and sperm the amount is half that in the body cells (see meiosis).

DNA

The chemical and physical properties of DNA suit it for both replication and transfer of information. Each DNA molecule is a long two-stranded chain. The strands are made up of subunits called nucleotides, each containing a sugar (deoxyribose), a phosphate group, and one of four nitrogenous bases, adenine, guanine, thymine, and cytosine, denoted A, G, T, and C, respectively. A given strand contains nucleotides bearing each of these four. The information carried by a given gene is coded in the sequence in which the nucleotides bearing different bases occur along the strand. These nucleotide sequences determine the sequences of amino acids in the polypeptide chain of the protein specified by that gene.

Between the genes, or coding loci, on the DNA of higher organisms, there are long portions of DNA, often referred to as "junk" DNA, that code no proteins. Sometimes junk DNA occurs within a gene; when this occurs, the coding portions are called exons and the noncoding (junk) portions are called introns. Junk DNA makes up 97% of the DNA in the human genome. Little is known of its purpose.

In 1953 the molecular biologists J. D. Watson, an American, and F. H. Crick, an Englishman, proposed that the two DNA strands were coiled in a double helix. In this model each nucleotide subunit along one strand is bound to a nucleotide subunit on the other strand by hydrogen bonds between the base portions of the nucleotides. The fact that adenine bonds only with thymine (A—T) and guanine bonds only with cytosine (G—C) determines that the strands will be complementary, i.e., that for every adenine on one strand there will be a thymine on the other strand. It is the property of complementarity between strands that insures that DNA can be replicated, i.e., that identical copies can be made in order to be transmitted to the next generation.

RNA and Protein Synthesis

In order to be expressed as protein, the genetic information must be carried to the protein-synthesizing machinery of the cell, which is in the cell's cytoplasm (see cell). One form of RNA mediates this process. RNA is similar to DNA, but contains the sugar ribose instead of deoxyribose and the base uracil (U) instead of thymine. To initiate the process of information transfer, one strand of the double-stranded DNA chain serves as a template for the synthesis of a single strand of RNA that is complementary to the DNA strand (e.g., the DNA sequence AGTC … will specify an RNA sequence UCAG …). This process is called transcription and is mediated by enzymes.

The newly synthesized RNA, called messenger RNA, or mRNA, moves quickly to bodies in the cytoplasm called ribosomes, which are composed of two particles made of protein bound to ribosomal RNA, or rRNA. Each ribosome is the site of synthesis of a polypeptide chain. Several ribosomes attach to a single mRNA so that many polypeptide chains are synthesized from the same mRNA; each cluster of an mRNA and ribosomes is called a polyribosome or polysome. The nucleotide sequence of the mRNA is translated into the amino acid sequence of a protein by adaptor molecules composed of a third type of RNA called transfer RNA, or tRNA. There are many different species of tRNA, with each species binding one of 20 amino acids.

In protein synthesis, a nucleotide sequence along the mRNA does not specify an amino acid directly; rather, it specifies a particular species of tRNA. For example, in coding for the amino acid tyrosine, a nucleotide sequence of mRNA is complementary to a portion of a tyrosine-tRNA molecule. As each specified tRNA associates with its complementary space on the mRNA, the amino acid is added onto the lengthening protein chain and the tRNA is released. When the protein chain is complete, it is released from the ribosome.

The particular sequence of amino acids in each polypeptide chain is determined by the genetic code. Starting at one end of the mRNA strand, each 3-nucleotide sequence, or codon, specifies, via complementary tRNA sequences, one amino acid, and the series of such codons in the mRNA specifies a polypeptide chain. Although a "vocabulary" of 64 words, or specifications, is theoretically possible with 4 different nucleotides taken three at a time, there are only 20 amino acids to be specified. However, several triplets may code for the same amino acid; for example UAU and UAC both code for the amino acid tyrosine. In addition, there are some codons that do not code for amino acids but code for polypeptide chain initiation and polypeptide chain termination. The code is also nonoverlapping; i.e., a nucleotide in one codon is never part of either adjacent codon. The code seems to be universal in all living organisms.

The determination of the mechanism of protein synthesis has increased understanding of many genetic processes and permitted such developments as bioengineering. Some mutagens, or mutation-inducing agents, cause the substitution of one nucleotide for another in an mRNA strand; other mutagens cause deletion or addition of nucleotides. Decoding, or reading, of such strands will be altered.

Metabolic regulation has been studied to determine how the genes that control enzyme synthesis can be switched on and off when certain substances are present. For example, in the process known as induction, bacteria synthesize the enzyme β-galactosidase only when lactose is present. Induction has been linked to the activity at a so-called operator site on a chromosome. When the operator site is open, the genes it controls function freely; when it is blocked, as by a repressor molecule, the genes it controls also do not function.

nitric acid, chemical compound, HNO₃, colorless, highly corrosive, poisonous liquid that gives off choking red or yellow fumes in moist air. It is miscible with water in all proportions. It forms an azeotrope (constant-boiling mixture) that has the composition 68% nitric acid and 32% water and that boils at 120.5°C;. The nitric acid of commerce is typically a solution of 52% to 68% nitric acid in water. Solutions containing over 86% nitric acid are commonly called fuming nitric acid. White fuming nitric acid (WFNA) is similar to the anhydrous variety, and red fuming nitric acid (RFNA) has a reddish brown color from dissolved nitrogen oxides. When treated with hydrogen fluoride, both varieties form inhibited fuming nitric acid, which has increased corrosion resistance in metal tanks, e.g., when used as an oxidizer in liquid fuel rockets.

Nitric acid is a strong oxidizing agent. It ionizes readily in solution, forming a good conductor of electricity. It reacts with metals, oxides, and hydroxides, forming nitrate salts. Chief uses of nitric acid are in the preparation of fertilizers, e.g., ammonium nitrate, and explosives, e.g., nitroglycerin and trinitrotoluene (TNT). It is also used in the manufacture of chemicals, e.g., in making dyes, and in metallurgy, ore flotation, etching steel, photoengraving, and reprocessing of spent nuclear fuel. It is produced chiefly by oxidation of ammonia (the Ostwald process). Small amounts are produced by the treatment of sodium nitrate with sulfuric acid. Nitric acid was known to the alchemists as aqua fortis; the name is used in commerce for impure grades of it. Aqua regia is a mixture of nitric and hydrochloric acids. Niric acid is a component of acid rain.

nicotinic acid: see coenzyme; vitamin.

muriatic acid: see hydrogen chloride.

methanoic acid, IUPAC name for formic acid.

malic acid: see Krebs cycle.

maleic acid: see fumaric acid.

lysergic acid diethylamide: see LSD.

lipoic acid: see coenzyme.

lactic acid, CH₃CHOHCO₂H, a colorless liquid organic acid. It is miscible with water or ethanol. Lactic acid is a fermentation product of lactose (milk sugar); it is present in sour milk, koumiss, leban, yogurt, and cottage cheese. The protein in milk is coagulated (curdled) by lactic acid. Lactic acid is produced in the muscles during intense activity by the breakdown of glucose, and may be used by muscle cells as a source of energy. Calcium lactate, a soluble lactic acid salt, is used as a source of calcium in the diet. Lactic acid is produced commercially for use in pharmaceuticals and foods, in leather tanning and textile dyeing, and in making plastics, solvents, inks, and lacquers. Although it can be prepared by chemical synthesis, production of lactic acid by fermentation of glucose and other substances is a less expensive method. Chemically, lactic acid occurs as two optical isomers, a dextro and a levo form; only the levo form takes part in animal metabolism. The lactic acid of commerce is usually an optically inactive racemic mixture of the two isomers.

isobutyric acid: see butyric acid.

indicators, acid-base, organic compounds that, in aqueous solution, exhibit color changes indicative of the acidity or basicity of the solution. Common indicators include p-nitrophenol, which is colorless from pH 1 to 5 and yellow from pH 5 to 9; methyl orange, yellow in basic and neutral solutions and reddish below pH 3.7; phenolphthalein, colorless in acid and neutral solutions, pink at about pH 8.5, and purplish at pH 10; and litmus. Most indicators are also used in large amounts for dyeing; small quantities are nonetheless invaluable for use as indicators in chemical laboratories.

hydrosulfuric acid: see hydrogen sulfide.

hydrocyanic acid: see hydrogen cyanide.

hydrochloric acid: see hydrogen chloride.

hydrobromic acid: see bromide.

hyaluronic acid: see mucopolysaccharide.

glutamic acid, organic compound, one of the 20 amino acids commonly found in animal proteins. Only the l-stereoisomer occurs in mammalian proteins. Like aspartic acid, glutamic acid has an acidic carboxyl group on its side chain which can serve as both an acceptor and a donor of ammonia, a compound toxic to the body. Once glutamic acid has coupled with ammonia, it is called glutamine and can as such safely transport ammonia to the liver, where the ammonia is eventually converted to urea for excretion by the kidneys. Free glutamic acid (that not incorporated into proteins) can also be converted reversibly to α-ketoglutaric acid, an intermediate in the Krebs cycle, and as such can be degraded to carbon dioxide and water, or transformed into sugars. The acidic side chain of glutamic acid confers one negative charge under most conditions to proteins in which this amino acid is found, thus increasing the water solubility of the protein. Monosodium glutamate (MSG), the monosodium salt of l-glutamic acid, is widely used as a condiment. The amino acid was isolated from wheat gluten in 1866 and chemically synthesized in 1890. It is not essential to the human diet, since it can be synthesized in the body from the common intermediate α-ketoglutaric acid.

gallotannic acid: see tannin.

gallic acid or 3,4,5-trihydroxybenzoic acid, C₆H₂(OH)₃CO₂H, colorless crystalline organic acid found in gallnuts, sumach, tea leaves, oak bark, and many other plants, both in its free state and as part of the tannin molecule (see tannin). Since gallic acid has hydroxyl groups and a carboxylic acid group in the same molecule, two molecules of it can react with one another to form an ester, digallic acid. Gallic acid is obtained by the hydrolysis of tannic acid with sulfuric acid. When heated above 220°C;, gallic acid loses carbon dioxide to form pyrogallol, or 1,2,3-trihydroxybenzene, C₆H₃(OH)₃, which is used in the production of azo dyes and photographic developers and in laboratories for absorbing oxygen.

fumaric acid or trans-butenedioic acid, HO₂CCH--CHCO₂H, unsaturated dicarboxylic acid that melts at 287°C;. Maleic acid, or cis-butenedioic acid, is a geometric isomer of fumaric acid; it melts at about 140°C;. Of the two isomers fumaric acid is the more stable and can be prepared from maleic acid by heating it. Fumaric acid can be prepared by catalytic oxidation of benzene or by bacterial action on glucose. It is found in small amounts in a variety of plants and is essential to the respiration of animal and vegetable tissue. Fumaric acid is used as a substitute for tartaric acid in beverages and baking powder. It is used as a mordant in dyeing and in the manufacture of synthetic resins and polyhydric alcohols.

formic acid or methanoic acid, HCO₂H, a colorless, corrosive liquid with a sharp odor; it boils at 100.7°C; and solidifies at 8.4°C;. It has the lowest molecular weight and is the simplest of the carboxylic acids. Functionally, it is both an acid and an aldehyde. Like other acids, it reacts with most alcohols to form esters and decomposes when heated; like other aldehydes, it is easily oxidized. Formic acid occurs in the bodies of red ants and in the stingers of bees. It can be made by the oxidation of formaldehyde; it is prepared commercially by heating carbon monoxide and sodium hydroxide to form sodium formate which, when carefully treated with sulfuric acid, yields formic acid. Formic acid is used industrially in textile dyeing, in leather tanning, and in coagulating latex rubber.

folic acid: see coenzyme; vitamin.

fatty acid, any of the organic carboxylic acids present in fats and oils as esters of glycerol. Molecular weights of fatty acids vary over a wide range. The carbon skeleton of any fatty acid is unbranched. Some fatty acids are saturated, i.e., each carbon atom is connected to its carbon atom neighbors by single bonds; and some fatty acids are unsaturated, i.e., contain at least one carbon-carbon double bond (see chemical bond). When fats and oils are hydrolyzed with an alkali, the fatty acids are liberated as their metal salts; these salts are soaps. Butyric acid is a fatty acid found in butter.

ethanedioic acid, IUPAC name for oxalic acid.

digallic acid: see gallic acid.

citric acid cycle: see Krebs cycle.

citric acid or 2-hydroxy-1,2,3-propanetricarboxylic acid, HO₂CCH₂C(OH)(CO₂H)CH₂CO₂H, an organic carboxylic acid containing three carboxyl groups; it is a solid at room temperature, melts at 153°C;, and decomposes at higher temperatures. It is responsible for the tart taste of various fruits in which it occurs, e.g., lemons, limes, oranges, pineapples, and gooseberries. It can be extracted from the juice of citrus fruits by adding calcium oxide (lime) to form calcium citrate, an insoluble precipitate that can be collected by filtration; the citric acid can be recovered from its calcium salt by adding sulfuric acid. It is obtained also by fermentation of glucose with the aid of the mold Aspergillus niger and can be obtained synthetically from acetone or glycerol. Citric acid is used in soft drinks and in laxatives and cathartics. Its salts, the citrates, have many uses, e.g., ferric ammonium citrate is used in making blueprint paper. Sour salt, used in cooking, is citric acid.

cis-butenedioic acid, IUPAC name for maleic acid; see fumaric acid.

chloric acid: see chlorate.

carboxylic acid: see carboxyl group.

carbonic acid, H₂CO₃, a weak dibasic acid (see acids and bases) formed when carbon dioxide dissolves in water; it exists only in solution. Carbonic acid forms carbonate and bicarbonate (or acid carbonate) salts (see carbonate) by reaction with bases. It contributes to the sharp taste of carbonated beverages.

carbolic acid: see phenol.

butyric acid or butanoic acid, CH₃CH₂CH₂CO₂H, viscous, foul-smelling, liquid carboxylic acid; m.p. about -5°C;; b.p. 163.5°C;. It is miscible with water, ethanol, and ether. It is a low molecular weight fatty acid that is present in butter as an ester of glycerol; the odor of rancid butter is due largely to the presence of free butyric acid. Butyric acid is used in the manufacture of plastics. Isobutyric acid, or 2-methylpropanoic acid, (CH₃)₂CHCO₂H, is a geometric isomer of the butyric acid described above; it has different physical properties but similar chemical properties.

butanoic acid, IUPAC name for butyric acid.

boric acid, any one of the three chemical compounds, orthoboric (or boracic) acid, metaboric acid, and tetraboric (or pyroboric) acid; the term often refers simply to orthoboric acid. The acids may be thought of as hydrates of boric oxide, B₂O₃. Orthoboric acid, H₃BO₃ or B₂O₃·3H₂O, is colorless, weakly acidic, and forms triclinic crystals. It is fairly soluble in boiling water (about 27% by weight) but less so in cold water (about 6% by weight at room temperature). When orthoboric acid is heated above 170°C; it dehydrates, forming metaboric acid, HBO₂ or B₂O₃·H₂O. Metaboric acid is a white, cubic crystalline solid and is only slightly soluble in water. It melts at about 236°C;, and when heated above about 300°C; further dehydrates, forming tetraboric acid, H₄B₄O₇ or B₂O₃·H₂O. Tetraboric acid is either a vitreous solid or a white powder and is water soluble. When tetraboric or metaboric acid is dissolved it reverts largely to orthoboric acid. The major uses of the boric acids are in forming other boron compounds and in borate salts, e.g., borax. A dilute water solution of boric acid is commonly used as a mild antiseptic and eyewash. Boric acid is also used in leather manufacture, electroplating, and cosmetics. Boric acid can be crystallized from an acidified borax solution. It occurs as the mineral sassolite in the Tuscan region of Italy, where it is also recovered from hot springs and vapors. In the United States boric acid is recovered from brines from Searles Lake in California.

boracic acid: see boric acid.

benzoic acid, 3,4,5-trihydroxy-, IUPAC name for gallic acid.

benzoic acid, C₆H₅CO₂H, crystalline solid organic acid that melts at 122°C; and boils at 249°C;. It is the simplest aromatic carboxylic acid (see aryl group and carboxyl group). In addition to being synthesized from a variety of organic compounds (e.g., benzyl alcohol, benzaldehyde, toluene, and phthalic acid), it may be obtained from resins, notably gum benzoin. It is used largely for making its salts and esters, most notably sodium benzoate, which is widely used as a preservative in foods and beverages and as a mild antiseptic in mouthwashes and toothpastes.

aspartic acid, organic compound, one of the 20 amino acids commonly found in animal proteins. Only the l-stereoisomer participates in the biosynthesis of proteins. Its acidic side chain adds a negative charge and hence a greater degree of water-solubility to proteins in neutral solution and has been shown to be near the active sites of some enzymes (see pepsin). Aspartic acid is not essential to the human diet. It was discovered in protein in 1868.

ascorbic acid: see vitamin.

amino acid, any one of a class of simple organic compounds containing carbon, hydrogen, oxygen, nitrogen, and in certain cases sulfur. These compounds are the building blocks of proteins. They are characterized by the presence of a carboxyl group (COOH) and an amino group (NH₂) attached to the same carbon at the end of the compound. The 20 amino acids commonly found in animals are alanine, arginine, asparagine, aspartic acid, cysteine, glutamic acid, glutamine, glycine, histidine, isoleucine, leucine, lysine, methionine, phenylalanine, proline, serine, threonine, tryptophan, tyrosine, and valine. More than 100 less common amino acids also occur in biological systems, particularly in plants. Every amino acid except glycine can occur as either of two optically active stereoisomers, d or l; the more common isomer in nature is the l-form. When the carboxyl carbon atom of one amino acid covalently binds to the amino nitrogen atom of another amino acid with the release of a water molecule, a peptide bond is formed. Amino acids are released in the intestinal tract by the digestion of food proteins and are then carried in the bloodstream to the body cells, where they are used for growth, maintenance, and repair. Cellular catabolism breaks amino acids down into smaller fragments. Many of the amino acids necessary in metabolism can be synthesized in the human or animal body when needed; these are called nonessential. Others cannot be synthesized in sufficient quantities; these are termed essential and must be provided in the diet.

acid-base indicators: see indicators, acid-base.

acid rain or acid deposition, form of precipitation (rain, snow, sleet, or hail) containing high levels of sulfuric or nitric acids (pH below 5.5-5.6). Produced when sulfur dioxide and various nitrogen oxides combine with atmospheric moisture, acid rain can contaminate drinking water, damage vegetation and aquatic life, and erode buildings and monuments. Automobile exhausts and the burning of high-sulfur industrial fuels are thought to be the main causes, but natural sources, such as volcanic gases and forest fires, may also be significant. It has been an increasingly serious problem since the 1950s, particularly in the NE United States, Canada, and W Europe, especially Scandinavia.

Acid rain became a political issue in the 1980s, when Canada claimed that pollutants from the United States were contaminating its forests and waters. Since then regulations have been enacted in North America and Europe to curb sulfur dioxide emissions from power plants; these include the U.S. Clean Air Act (as reauthorized and expanded in 1990) and the Helsinki protocol (1985), in which 21 European nations promised to reduce emissions by specified amounts. To assess the effectiveness of reductions a comprehensive study, comparing data from lakes and rivers across N Europe and North America, was conducted by an international team of scientists in 1999. The results they reported were mixed: while sulfates (the main acidifying water pollutant from acid rain) were lower, only some areas showed a decrease in overall acidity. It remained to be determined whether more time or a greater reduction in sulfur emissions was needed to reduce freshwater acidity in all areas. See air pollution; forest; pollution.

acid anhydride, chemical compound that reacts with water to form an acid (see acids and bases). Anhydrides of inorganic acids are usually oxides of nonmetallic elements. Carbon dioxide, CO₂, is the anhydride of carbonic acid, H₂CO₃ . Nitrogen pentoxide, N₂O₅, is the anhydride of nitric acid, HNO₃ . Phosphorus pentoxide, P₂O₅, is the anhydride of phosphoric acid, H₃PO₄ . Sulfur dioxide, SO₂, is the anhydride of sulfurous acid, H₂SO₃ . Sulfur trioxide, SO₃, is the anhydride of sulfuric acid, H₂SO₄ . Anhydrides of organic acids, like the acids themselves, contain the carbonyl group, CO. Organic anhydrides include acetic anhydride or ethanoic anhydride, (CH₃C--O)₂O, and benzoic anhydride, (C₆H₅C--O)₂O.

acetylsalicylic acid, acetate ester of salicylic acid. See aspirin.

acetic acid, CH₃CO₂H, colorless liquid that has a characteristic pungent odor, boils at 118°C;, and is miscible with water in all proportions; it is a weak organic carboxylic acid (see carboxyl group). Glacial acetic acid is concentrated, 99.5% pure acetic acid; it solidifies at about 17°C; to a crystalline mass resembling ice. Acetic acid is the major acid in vinegar; as such, it is widely used as a food preservative and condiment. For industrial use concentrated acetic acid is prepared from the oxidation of acetaldehyde. Acetic acid is also a product in the destructive distillation of wood. It reacts with other chemicals to form numerous compounds of commercial importance. These include cellulose acetate, used in making acetate rayon, nonflammable motion-picture film, lacquers, and plastics; various inorganic salts, e.g., lead, potassium, and copper acetates; and amyl, butyl, ethyl, methyl, and propyl acetates, which are used as solvents, chiefly in certain quick-drying lacquers and cements. Amyl acetate is sometimes called banana oil because it has a characteristic banana odor.

Heterocyclic compound of the purine type, the end product of metabolism of the purines in nucleic acids in many animals, including humans. It is excreted by reptiles and birds as the chief nitrogenous end product of protein breakdown. Small quantities are normally found in human blood; in gout, levels are abnormally high. Uric acid is used industrially in organic synthesis.

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or Krebs cycle or citric-acid cycle

Last stage of the chemical processes by which living cells obtain energy from foodstuffs. Described by Hans Adolf Krebs in 1937, the reactions of the cycle have been shown in animals, plants, microorganisms, and fungi, and it is thus a feature of cell chemistry shared by all types of life. It is a complex series of reactions beginning and ending with the compound oxaloacetate. In addition to re-forming oxaloacetate, the cycle produces carbon dioxide and the energy-rich compound ATP. The enzymes that catalyze each step are located in mitochondria in animals, in chloroplasts in plants, and in the cell membrane in microorganisms. The hydrogen atoms and electrons that are removed from intermediate compounds formed during the cycle are channeled ultimately to oxygen in animal cells or to carbon dioxide in plant cells.

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or tannic acid

Any of a group of pale yellow to light brown amorphous substances widely distributed in plants and used chiefly in tanning leather, dyeing fabric, and making ink. Their solutions are acid and have an astringent taste. They are isolated from oak bark, sumac, myrobalan (an Asian tree), and galls. Tannins give tea astringency, colour, and some flavour. Tannins are used industrially to clarify wine and beer, reduce viscosity of oil-well drilling mud, and prevent scale in boiler water; they have also had medical uses.

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or oil of vitriol

Dense, colourless, oily, corrosive liquid inorganic compound (H₂SO₄). A very strong acid, it forms ions of hydrogen or hydronium (H⁺ or H₃O⁺), hydrogen sulfate (HSO₄⁻), and sulfate (SO₄²⁻). It is also an oxidizing (see oxidation-reduction) and dehydrating agent and chars many organic materials. It is one of the most important industrial chemicals, used in various concentrations in manufacturing fertilizers, pigments, dyes, drugs, explosives, detergents, and inorganic salts and acids, in petroleum refining and metallurgical processes, and as the acid in lead-acid storage batteries. It is made industrially by dissolving sulfur trioxide (SO₃) in water, sometimes beyond the saturation point to make oleum (fuming sulfuric acid), used to make certain organic chemicals.

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White, crystalline solid organic compound used chiefly to make aspirin and other pharmaceutical products, including methyl salicylate (oil of wintergreen, for medicines and flavourings), phenyl salicylate (for sunburn creams and pill coatings), and salicylanilide (a cutaneous fungicide). Its molecular structure, with the formula C₆H₄(OH)COOH, consists of a six-membered aromatic ring (see aromatic compound) having a hydroxyl group (singlehorzbondOH) and a carboxyl group (singlehorzbondCOOH) bonded to adjacent carbon atoms; as such, it is both a phenol and a carboxylic acid. It and certain derivatives occur naturally in some plants, particularly species of Spiraea and Salix (willow). Large amounts are used in producing certain dyes.

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in full ribonucleic acid

One of the two main types of nucleic acid (the other being DNA), which functions in cellular protein synthesis in all living cells and replaces DNA as the carrier of genetic information in some viruses. Like DNA, it consists of strands of repeating nucleotides joined in chainlike fashion, but the strands are single (except in certain viruses), and it has the nucleotide uracil (U) where DNA has thymine (T). Messenger RNA (mRNA), a single strand copied from a DNA strand that acts as its template, carries the message of the genetic code from DNA (in chromosomes) to the site of protein synthesis (on ribosomes). Ribosomal RNA (rRNA), part of the building blocks of ribosomes, participates in protein synthesis. Transfer RNA (tRNA), the smallest type, has fewer than 100 nucleotide units (mRNA and rRNA contain thousands). Each nucleotide triplet on mRNA specifies which amino acid comes next on the protein being synthesized, and a tRNA molecule with that triplet's complement on its protruding end brings the specified amino acid to the site of synthesis to be linked into the protein. Various minor types of RNA also exist; at least some act as catalysts (ribozymes), a function long ascribed only to proteins.

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or folate

Organic compound essential to animal growth and health and needed by bacteria as a growth factor. Part of the vitamin B complex, folic acid is necessary for synthesis of nucleic acids and formation of the heme component of hemoglobin in red blood cells. To prevent neural tube defects in babies, it should ideally be taken by women starting at least a month before conception. Dietary folate sources include leafy and dark green vegetables, citrus fruits, cereals, beans, poultry, and egg yolks, but free folic acid is available only in supplements. Low intake leads to folic acid deficiency anemia.

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Organic compound, essential in animal metabolism. The nature of the bound form was clarified through the discovery and synthesis (1947–50) of the compound pantetheine, which contains pantothenic acid combined with the compound thioethanolamine. Pantetheine is part of two larger compounds (coenzyme A and acyl-carrier protein) that promote a large number of metabolic reactions essential for the growth and well-being of animals. A dietary deficiency severe enough to lead to clear-cut disease has not been described in humans; however, when a person is severely malnourished, deficiency of the vitamin appears to contribute to the observed weakness and mental depression.

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Colourless, crystalline, toxic carboxylic acid found in many plants, especially rhubarb, wood sorrel, and spinach. Because it forms soluble chelates with iron, some of the iron in these plants is not available nutritionally. However, this property makes it useful for removing blood and rust stains, cleaning metals other than iron, and flushing car radiators. Oxalic acid and its salts (oxalates) are used in many chemical processes.

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Any of the organic compounds making up the genetic material of living cells. Nucleic acids direct the course of protein synthesis, thereby regulating all cell activities. Their transmission from one generation to the next is the basis of heredity. The two main types, DNA and RNA, are composed of similar materials but differ in structure and function. Both are long chains of repeating nucleotides. The sequence of purines and pyrimidines (bases)—adenine (A), guanine (G), cytosine (C), and either thymine (T; in DNA) or uracil (U; in RNA)—in the nucleotides, in groups of three (triplets, or codons), constitutes the genetic code.

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Inorganic compound, colourless, fuming, highly corrosive liquid, chemical formula HNO₃. A common laboratory reagent, it is important in the manufacture of fertilizers and explosives (including nitroglycerin), as well as in organic syntheses, metallurgy, ore flotation, and reprocessing of spent nuclear fuel. A strong acid, it is toxic and can cause severe burns. It attacks most metals and is used for etching steel and photoengraving.

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or nicotinic acid or vitamin B₃

Water-soluble vitamin of the vitamin B complex, essential to growth and health in animals, including humans. It is found in the body only in combined form as a coenzyme, nicotinamide adenine dinucleotide (NAD), which is involved in the metabolism of carbohydrates and the oxidation of sugar derivatives and other substances. One of the most stable vitamins, it survives most cooking and most preserving processes. It is widely found in dietary sources, especially lean meat. Deficiency causes pellagra. It is used as a drug to reduce high cholesterol levels in the blood.

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Simplest carboxylic acid, chemical formula HCOOH. It is secreted by some insects, especially red ants (its name comes from the Latin word for ant), in their bite or sting. It has many industrial uses, in textile and leather manufacture, as an industrial solvent, and as an intermediate.

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in full lysergic acid diethylamide

Highly potent hallucinogenic drug. An organic compound, LSD can be derived from the alkaloids ergotamine and ergonovine, found in the ergot fungus, but most LSD is produced synthetically. It can block the action of the neurotransmitter serotonin and produces marked deviations from normal perceptions and behaviour lasting 8–10 hours or longer. Mood shifts, time and space distortions, and impulsive behaviour may progress to paranoia and aggression. Flashbacks to LSD-induced hallucinations can occur years later. LSD is not an approved drug, and no clinically valuable uses have been found for it.

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Carboxylic acid found in certain plant juices, in blood and muscle, and in soil. In blood it occurs in the form of its salts (lactates) when glycogen is broken down in muscle; it can be reconverted to glycogen in the liver. Stiffness and soreness after prolonged heavy exercise are due to accumulated lactic acid in the muscles. The end product of bacterial fermentation, lactic acid is the most common acidic constituent of fermented milk products (e.g., sour milk and cream, cheese, buttermilk, yogurt). It is used in other foods as a flavouring or preservative and industrially in tanning leather and dyeing wool and as a raw material or catalyst in many chemical processes.

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or ascorbic acid

Water-soluble organic compound important in animal metabolism. Most animals produce it in their bodies, but humans, other primates, and guinea pigs need it in the diet to prevent scurvy. It is essential in collagen synthesis, wound healing, blood-vessel maintenance, and immunity. Some studies have found a moderate benefit of vitamin C in reducing the duration and severity of the common cold. It works as an antioxidant in the body and is used as a preservative. It is easily destroyed by oxygen. Excellent sources are citrus fruits and fresh vegetables.

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One of the nonessential amino acids, closely related to glutamine. The two constitute a substantial fraction of the amino acids in many proteins (10–20percnt in many cases and up to 45percnt in some plant proteins). An important metabolic intermediate as well as a neurotransmitter molecule in the central nervous system, glutamic acid finds uses in medicine and biochemical research. Its sodium salt is  the food flavour enhancer monosodium glutamate (MSG).

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Most important carboxylic acid (CH₃COOH). Pure (“glacial”) acetic acid is a clear, syrupy, corrosive liquid that mixes readily with water. Vinegar is its dilute solution, from fermentation and oxidation (see oxidation-reduction) of natural products. Its salts and esters are acetates. It occurs naturally as a metabolic intermediate in body fluids and plant juices. Industrial production is either synthetic, from acetylene, or biological, from ethanol. Industrial chemicals made from it are used in printing and as plastics, photographic films, textiles, and solvents.

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Organic compound that is an important component of lipids in plants, animals, and microorganisms. Fatty acids are carboxylic acids with a long hydrocarbon chain, usually straight, as the fourth substituent group on the carboxyl (singlehorzbondCOOH) group (see functional group) that makes the molecule an acid. If the carbon-to-carbon bonds (see bonding) in that chain are all single, the fatty acid is saturated; artificial saturation is called hydrogenation. A fatty acid with one double bond is monounsaturated; one with more is polyunsaturated. These are more reactive chemically. Most unsaturated fats are liquid at room temperature, so food manufacturers hydrogenate them to make them solid (see margarine). A high level of saturated fatty acids in the diet raises blood cholesterol levels. A few fatty acids have branched chains. Others (e.g., prostaglandins) contain ring structures. Fatty acids in nature are always combined, usually with glycerol as triglycerides in fats. Oleic acid (unsaturated, with 18 carbon atoms) is almost half of human fat and is abundant in such oils as olive, palm, and peanut. Most animals, including mammals, cannot synthesize some unsaturated “essential” fatty acids; humans need linoleic, linolenic, and arachidonic acids in their diet.

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Method developed by the British geneticist Alec Jeffreys (born 1950) in 1984 for isolating and making images of sequences of DNA. The procedure consists of obtaining a sample of cells containing DNA (e.g., from skin, blood, or hair), extracting the DNA, and purifying it. The DNA is then cut by enzymes, and the resulting fragments of varying lengths undergo procedures that permit them to be analyzed. The pattern of fragments is unique for each individual. DNA fingerprinting is used to help solve crimes and determine paternity; it is also used to locate gene segments that cause genetic diseases, to map the genetic material of humans (see Human Genome Project), to engineer drought-resistant plants (see genetic engineering), and to produce biological drugs from genetically altered cells.

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or deoxyribonucleic acid

One of two types of nucleic acid (the other is RNA); a complex organic compound found in all living cells and many viruses. It is the chemical substance of genes. Its structure, with two strands wound around each other in a double helix to resemble a twisted ladder, was first described (1953) by Francis Crick and James D. Watson. Each strand is a long chain (polymer) of repeating nucleotides: adenine (A), guanine (G), cytosine (C), and thymine (T). The two strands contain complementary information: A forms hydrogen bonds (see hydrogen bonding) only with T, C only with G. When DNA is copied in the cell, the strands separate and each serves as a template for assembling a new complementary strand; this is the key to stable heredity. DNA in cells is organized into dense protein-DNA complexes (see nucleoprotein) called chromosomes. In eukaryotes these are in the nucleus, and DNA also occurs in mitochondria and chloroplasts (if any). Prokaryotes have a single circular chromosome in the cytoplasm. Some prokaryotes and a few eukaryotes have DNA outside the chromosomes in plasmids. Seealso Rosalind Franklin; genetic engineering; mutation; Maurice Wilkins.

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Colourless, crystalline organic compound (C₆H₈O₇), one of the carboxylic acids. It is present in almost all plants (especially citrus fruits) and in many animal tissues and fluids. It is one of a series of compounds involved in the physiological oxidation (see oxidation-reduction) of fats, proteins, and carbohydrates to carbon dioxide and water (see tricarboxylic acid cycle). It has a characteristic sharply sour taste and is used in many foods, confections, and soft drinks. It is added to certain foods to improve their stability in metal containers. Industrially, it is used as a water conditioner, cleaning and polishing agent, and chemical intermediate.

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Any organic compound with the general chemical formula singlehorzbondCOOH in which a carbon (C) atom is bonded to an oxygen (O) atom by a double bond to make a carbonyl group (singlehorzbondCdoublehorzbondO; see functional group) and to a hydroxyl group (singlehorzbondOH) by a single bond (see bonding). The fourth bond on the carbon links it to a hydrogen (H) atom (for formic acid), a methyl (singlehorzbondCH₃) group (for acetic acid), or another natural or synthetic monovalent group. Carboxylic acids occur widely in nature. In fatty acids, the fourth group is a hydrocarbon chain. In aromatic acids (see aromatic compound), it is a ring-structured hydrocarbon. In amino acids, it contains a nitrogen atom. Carboxylic acids participate in chemical reactions as acids, usually fairly weak. Many carboxylic acids (acetic acid, citric acid, lactic acid) are intermediates in metabolism and can be found in natural products; others (e.g., salicylic acid) are used as solvents and to prepare many chemical compounds. Important carboxylic-acid derivatives include esters, anhydrides, amides, halides (see halogen), and salts (see soap).

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One of the nonessential amino acids, found in many proteins and closely related to asparagine. It is used in medical and biochemical research, as an organic intermediate, and in various industrial applications. It is one of the two components of aspartame.

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Any of a class of organic compounds in which a carbon atom has bonds to an amino group (singlehorzbondNH₂), a carboxyl group (singlehorzbondCOOH), a hydrogen atom (singlehorzbondH), and an organic side group (called singlehorzbondR). They are therefore both carboxylic acids and amines. The physical and chemical properties unique to each result from the properties of the R group, particularly its tendency to interact with water and its charge (if any). Amino acids joined linearly by peptide bonds (see covalent bond) in a particular order make up peptides and proteins. Of over 100 natural amino acids, each with a different R group, only 20 make up the proteins of all living organisms. Humans can synthesize 10 of them (by interconversions) from each other or from other molecules of intermediary metabolism, but the other 10 (essential amino acids: arginine, histidine, isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan, and valine) must be consumed in the diet.

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Division of igneous rocks on the basis of their silicate mineral content, these minerals usually being the most abundant in such rocks. Rocks are described as felsic, intermediate, mafic, and ultramafic, in order of decreasing silica content, and, in general, the gradation from felsic to mafic corresponds to an increase in colour (i.e., light to dark). Due to the erroneous belief that silica present in rock magmas occurred in the form of silicic acid, high- and low-silica rocks were once known as acid and basic rocks, respectively.

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Any precipitation, including snow, that contains a heavy concentration of sulfuric and nitric acids. This form of pollution is a serious environmental problem in the large urban and industrial areas of North America, Europe, and Asia. Automobiles, certain industrial operations, and electric power plants that burn fossil fuels emit the gases sulfur dioxide and nitrogen oxide into the atmosphere, where they combine with water vapour in clouds to form sulfuric and nitric acids. The highly acidic precipitation from these clouds may contaminate lakes and streams, damaging fish and other aquatic species; damage vegetation, including agricultural crops and trees; and corrode the outsides of buildings and other structures (historic monuments are especially vulnerable). Though usually most severe around large urban and industrial areas, acid precipitation may also occur at great distances from the source of the pollutants.

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Any substance that in water solution tastes sour, changes the colour of acid-base indicators (e.g., litmus), reacts with some metals (e.g., iron) to yield hydrogen gas, reacts with bases to form salts, and promotes certain chemical reactions (e.g., acid catalysis). Acids contain one or more hydrogen atoms that, in solution, dissociate as positively charged hydrogen ions. Inorganic, or mineral, acids include sulfuric acid, nitric acid, hydrochloric acid, and phosphoric acid. Organic acids include carboxylic acids, phenols, and sulfonic acids. Broader definitions of acids cover situations in which water is not present. Seealso acid-base theory.

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Common name of acetylsalicylic acid, an organic compound introduced in 1899. The ester of salicylic acid and acetic acid, it inhibits production of prostaglandins in the body. Its analgesic, fever-reducing, and anti-inflammatory effects make it useful in treating headaches, muscle and joint aches, arthritis pain, and the symptoms of mild fevers and infections. It also has anticoagulant activity and is taken in low doses by coronary heart disease patients to prevent heart attack. Prolonged use may cause stomach bleeding and peptic ulcer, and its use in children with fever has been linked to Reye syndrome. Seealso acetaminophen; ibuprofen; NSAID.

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