by Isaac Asimov
Amazingly enough, rubber, though a soft, relatively weak substance, proved to be much more resistant to abrasion than wood or metal. This durability, coupled with its shock-absorbing qualities and the air-cushioning idea, introduced unprecedented riding comfort.
As the automobile increased in importance, the demand for rubber for tires grew astronomical. In half a century, the world production of rubber increased forty-two-fold. You can judge the quantity of rubber in use for tires today when I tell you that, in the United States, they leave no less than 200,000 tons of abraded rubber on the highways each year, in spite of the relatively small amount abraded from the tires of an individual car.
The increasing demand for rubber introduced a certain insecurity in the war resources of many nations. As war was mechanized, armies and supplies began to move on rubber, and rubber could be obtained in significant quantity only from the Malayan peninsula, far removed from the “civilized” nations most apt to engage in “civilized” warfare. (The Malayan peninsula is not the natural habitat of the rubber tree. The tree was transplanted there, with great success, from Brazil, where the original rubber supply steadily diminished.) The supply of the United States was cut off at the beginning of its entry into the Second World War when the Japanese overran Malaya. American apprehensions in this respect were responsible for the fact that the very first object rationed during the war emergency, even before the attack on Pearl Harbor, was rubber tires.
Even in the First World War, when mechanization was just beginning, Germany was hampered by being cut off from rubber supplies by Allied sea power.
By that time, then, there was reason to consider the possibility of constructing a synthetic rubber. The natural starting material for such a synthetic rubber was isoprene, the building block of natural rubber. As far back as 1880, chemists had noted that isoprene, on standing, tended to become gummy and, if acidified, would set into a rubberlike material. Kaiser Wilhelm II eventually had the tires of his official automobile made of such material, as a kind of advertisement of Germany’s chemical virtuosity.
However, there were two catches to the use of isoprene as the starting material for synthesizing rubber. First, the only major source-of isoprene is rubber itself. Second, when isoprene polymerizes, it is most likely to do so in a completely random manner. The rubber chain possesses all the isoprene units oriented in the same fashion: —uuuuuuuuu—. The gutta percha chain has them oriented in strict alternation: —unununununun—. When isoprene is polymerized in the laboratory under ordinary conditions, however, the u’s and the n’s are mixed randomly, forming a material which is neither rubber nor gutta percha. Lacking the flexibility and resilience of rubber, it is useless for automobile tires (except possibly for imperial automobiles used on state occasions).
Eventually, catalysts like those that Ziegler introduced in 1953 for manufacturing polyethylene made it possible to polymerize isoprene to a product almost identical with natural rubber, but by that time many useful synthetic rubbers, very different chemically from natural rubber, had been developed.
The first efforts, naturally, concentrated on attempts to form polymers from readily available compounds resembling isoprene. For instance, during the First World War, under the pinch of the rubber famine, Germany made use of dimethylbutadiene:
Dimethylbutadiene differs from isoprene only in containing a methyl group (CH3) on both middle carbons of the four-carbon chain instead of on only one of them. The polymer built of dimethylbutadiene, called methyl rubber, could be formed cheaply and in quantity. Germany produced about 2,500 tons of it during the First World War. While it did not stand up well under stress, it was nonetheless the first of the usable synthetic rubbers.
About 1930, both Germany and the Soviet Union tried a new tack. They used as the monomer, butadiene, which has no methyl group at all:
With sodium metal as a catalyst, they formed a polymer called Buna (from “butadiene” and Na for sodium).
Buna rubber was a synthetic rubber that could be considered satisfactory in a pinch. It was improved by the addition of other monomers, alternating with butadiene at intervals in the chain. The most successful addition was styrene, a compound resembling ethylene but with a benzene ring attached to one of the carbon atoms. This product was called Buna S. Its properties were very similar to those of natural rubber; and, in fact, thanks to Buna S, Germany’s armed forces suffered no serious rubber shortage in the Second World War. The Soviet Union also supplied itself with rubber in the same way. The raw materials can be obtained from coal or petroleum.
The United States was later in developing synthetic rubber in commercial quantities, perhaps because it was in no danger of a rubber famine before 1941. But after Pearl Harbor, it took up synthetic rubber with a vengeance. It began to produce buna rubber and another type of synthetic rubber called neoprene, built up of chloroprene:
This molecule, as you see, resembles isoprene except for the substitution of a chlorine atom for the methyl group.
The chlorine atoms, attached at intervals to the polymer chain, confer upon neoprene certain resistances that natural rubber does not have. For instance, it is more resistant to organic solvents such as gasoline: it does not soften and swell nearly as much as would natural rubber. Thus neoprene is actually preferable to rubber for such uses as gasoline hoses. Neoprene first clearly demonstrated that in the field of synthetic rubbers, as in many other fields, the product of the test tube need not be a mere substitute for nature but could be an improvement.
Amorphous polymers with no chemical resemblance to natural rubber but with rubbery qualities have now been produced, and they offer a whole constellation of desirable properties. Since they are not actually rubbers, they are called elastomers (an abbreviation of elastic polymer).
The first rubber-unlike elastomer had been discovered in 1918. This was a polysulfide rubber; its molecule was a chain composed of pairs of carbon atoms alternating with groups of four sulfur atoms. The substance was given the name Thiokol, the prefix coming from the Greek word for “sulfur.” The odor involved in its preparation held it in abeyance for a long time, but eventually it was put into commercial production.
Elastomers have also been formed from acrylic monomers, fluorocarbons, and silicones. Here, as in almost every field he or she touches, the organic chemist works as an artist, using materials to create new forms and improve upon nature.
Chapter 12
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The Proteins
Amino Acids
Early in their study of living matter, chemists noticed a group of substances that behaved in a peculiar manner. Heating changed these substances from the liquid to the solid state, instead of the other way round. The white of eggs, a substance in milk (casein), and a component of the blood (globulin) were among the things that showed this property. In 1777, the French chemist Pierre Joseph Macquer put all the substances that coagulate on heating into a special class that he called albuminous, after albumen, the name the Roman encyclopedist Pliny had given to egg white.
When the nineteenth-century organic chemists undertook to analyze the albuminous substances, they found these compounds considerably more complicated than other organic molecules. In 1839, the Dutch chemist Gerardus Johannes Mulder worked out a basic formula, C40H62O12N10, which he thought the albuminous substances had in common. He believed that the various albuminous compounds were formed by the addition to this central formula of small sulfur-containing groups or phosphorus-containing groups. Mulder named his root formula protein (a word suggested to him by the inveterate word-coiner Berzelius), from a Greek word meaning “of first importance.” Presumably the term was meant merely to signify that this core formula was of first importance in determining the structure of the albuminous substances; but as things turned out, it proved to be very apt for the substances themselves. The proteins, as they came to be known, were soon found to be of key importance to life.
Within a decade after Mulder’s work, Justus von Liebig had establis
hed that proteins are even more essential for life than carbohydrates or fats: they supply not only carbon, hydrogen, and oxygen but also nitrogen, sulfur, and often phosphorus, which are absent from fats and carbohydrates.
The attempts of Mulder and others to work out complete empirical formulas for proteins were doomed to failure at the time they were made. The protein molecule is far too complicated to be analyzed by the methods then available. However, a start had already been made on another line of attack that was eventually to reveal, not only the composition, but also the structure of proteins. Chemists had begun to learn something about the building blocks of which they are made.
In 1820, Henri Braconnot, having succeeded in breaking down cellulose into its glucose units by heating the cellulose in acid (see chapter 11), decided to try the same treatment with gelatin, an albuminous substance. The treatment yielded a sweet, crystalline substance. Despite Braconnot’s first suspicions, this turned out to be not a sugar but a nitrogen-containing compound, for ammonia (NH3) could be obtained from it. Nitrogen-containing substances are conventionally given names ending in -ine, and the compound isolated by Braconnot is now called glycine, from the Greek word for “sweet.”
Shortly afterward, Braconnot obtained a white, crystalline substance by heating muscle tissue with acid. He named this one leucine, from the Greek word for “white.”
Eventually, when the structural formulas of glycine and leucine were worked out, they were found to have a basic resemblance:
Each compound, as you see, has at its ends an amine group (NH2) and a carboxyl group (COOH). Because the carboxyl group gives acid properties to any molecule that contains it, molecules of this kind were named amino acids. Those that have the amine group and carboxyl group linked by a single carbon atom between them, as both these molecules have, are called alpha-amino acids.
As time went on, chemists isolated other alpha-amino acids from proteins. For instance, Liebig obtained one from the protein of milk (casein), which he called tyrosine (from the Greek word for “cheese”; casein itself comes from the Latin word for “cheese”):
The differences among the various alpha-amino acids lie entirely in the nature of the atom grouping attached to that single carbon atom between the amine and the carboxyl groups. Glycine, the simplest of all the amino acids, has only a pair of hydrogen atoms attached there. The others all possess a carbon-containing side chain attached to that carbon atom.
I shall give the formula of just one more amino acid, which will be useful in connection with matters to be discussed later in the chapter. It is cystine, discovered in 1899 by the German chemist K. A. H. Mörner. This is a double-headed molecule containing two atoms of sulfur:
Actually, cystine had first been isolated in 18lO by the English chemist William Hyde Wollaston from a bladder stone; hence, its name from the Greek word for “bladder.” What Mörner did was to show that this century-old compound is a component of protein as well as the substance in bladder stones.
Cystine is easily reduced (a term that, chemically, is the opposite of oxidized): that is, it will easily add on two hydrogen atoms, which fall into place at the S-S bond. The molecule then divides into two halves, each containing an -SH (mercaptan, or thiol) group. This reduced half is cysteine and is easily oxidized back to cystine.
The general fragility of the thiol group is such that it is important to the functioning of a number of protein molecules. A delicate balance and a capability of moving this way or that under slight impulse is the hallmark of the chemicals most important to life; the members of the thiol group are among the atomic combinations that contribute to this ability.
Altogether, nineteen important amino acids (that is, occurring in most proteins) have now been identified. The last of these was discovered in 1935 by the American chemist William Cumming Rose. It is unlikely that any other common ones remain to be found.
THE COLLOIDS
By the end of the nineteenth century, biochemists had become certain that proteins are giant molecules built up of amino acids, just as cellulose is constructed of glucose and rubber of isoprene units. But there is this important difference: whereas cellulose and rubber are made with just one kind of building block, a protein is built from a number of different amino acids, Hence, working out protein structure would pose special and subtle problems,
The first problem was to find out just how the amino acids are joined together in the protein-chain molecule, Emil Fischer made a start on the problem by linking amino acids together in chains, in such a way that the carboxyl group of one amino acid was always joined to the amino group of the next. In 1901, he achieved his first such condensation, linking one glycine molecule to another with the elimination of a molecule of water:
This is the simplest condensation possible. By 1907, Fischer had synthesized a chain made up of eighteen amino acids, fifteen of them glycine and the remaining three leucine. This molecule did not show any of the obvious properties of proteins, but Fischer felt that was only because the chain was not long enough. He called his synthetic chains peptides, from a Greek word meaning “digest,” because he believed that proteins broke down into such groups when they were digested. Fischer named the combination of the carboxyl’s carbon with the amine group a peptide link.
In 1932, the German biochemist Max Bergmann (a pupil of Fischer’s) devised a method of building up peptides from various amino acids. Using Bergmann’s method, the Polish-American biochemist Joseph Stewart Fruton prepared peptides that could be broken down, by digestive juices, into smaller fragments. Since there was good reason to believe that digestive juices would hydrolyze (split by the addition of water) only one kind of molecular bond, the bond between the amino acids in the synthetic peptides must therefore be of the same kind as the one joining amino acids in true proteins. The demonstration laid to rest any lingering doubts about the validity of Fischer’s peptide theory of protein structure.
Still, the synthetic peptides of the early decades of the twentieth century were very small and nothing like proteins in their properties. Fischer had made one consisting of eighteen amino acids, as I have said; in 1916, the Swiss chemist Emil Abderhalden went him one better by preparing a peptide with nineteen amino acids, but that held the record for thirty years. And chemists knew that such a peptide must be a tiny fragment indeed compared with the size of a protein molecule, because the molecular weights of proteins were enormous.
Consider, for instance, hemoglobin, a protein of the blood. Hemoglobin contains iron, making up just 0.34 percent of the weight of the molecule. Chemical evidence indicates that the hemoglobin molecule has four atoms of iron, so the total molecular weight must be about 67,000; four atoms of iron, with a total weight of 4 × 55.85, would come to 0.34 percent of such a molecular weight. Consequently, hemoglobin must contain about 550 amino acids (the average molecular weight of the amino acids being about 120). Compare that with Abderhalden’s puny nineteen. And hemoglobin is only an average-sized protein.
The best measurement of the molecular weights of proteins has been obtained by whirling them in a centrifuge, a spinning device that pushes particles outward from the center by centrifugal force (figure 12.1). When the centrifugal force is more intense than the earth’s gravitational force, particles suspended in a liquid will settle outward away from the center at a faster rate than they would settle downward under gravity. For instance, red blood corpuscles will settle out quickly in such a centrifuge, and fresh milk will separate into two fractions, the fatty cream and the denser skim milk. These particular separations will take place slowly under ordinary gravitational forces, but centrifugation speeds them up.
Figure 12.1. Principle of the centrifuge.
Protein molecules, though very large for molecules, are not heavy enough to settle out of solution under gravity; nor will they settle out rapidly in an ordinary centrifuge. But in 1923, the Swedish chemist Theodor Svedberg developed an ultracentrifuge capable of separating molecules according to their weight. This high-speed d
evice whirls at more than 10,000 revolutions per second and produces centrifugal forces up to 900,000 times as intense as the gravitational force at the earth’s surface. For his contributions to the study of suspensions, Svedberg received the Nobel Prize in chemistry in 1926.
With the ultracentrifuge, chemists were able to determine the molecular weights of a number of proteins on the basis of their rate of sedimentation (measured in svedbergs in honor of the chemist). Small proteins turned out to have molecular weights of only a few thousand and to contain perhaps not more than fifty amino acids (still decidedly more than nineteen). Other proteins have molecular weights in the hundreds of thousands and even in the millions, which means that they must consist of thousands or tens of thousands of amino acids. The possession of such large molecules put proteins into a class of substances that have only been studied systematically from the mid-nineteenth century onward.
The Scottish chemist Thomas Graham was the pioneer in this field through his interest in diffusion—that is, in the manner in which the molecules of two substances, brought into contact, will intermingle. He began by studying the rate of diffusion of gases through tiny holes or fine tubes. By 1831, he was able to show that the rate of diffusion of a gas was inversely proportional to the square root of its molecular weight (Graham’s law). (It was through the operation of Graham’s law that uranium 235 was separated from uranium 238, by the way.)
In following decades, Graham passed to the study of the diffusion of dissolved substances. He found that solutions of such compounds as salt, sugar, or copper sulfate would find their way through a blocking sheet of parchment (presumably containing submicroscopic holes). On the other hand, solutions of such materials as gum arabic, glue, and gelatin would not. Clearly, the giant molecules of the latter group of substances would not fit through the holes in the parchment.