The structure of the side groups in the circled portion of the molecule for amoxicillin (left), penicillin O (center), and cloxacillin (right)
These are only four of the ten or so different penicillins still in use today. (Many more exist that are no longer used clinically.) The structural modifications, at the same (circled) site on the molecule, can be very variable, but the four-membered β-lactam ring is always present. It is this piece of the molecular structure that may have saved your life, if you have had the occasion to need a penicillin antibiotic.
Although it is impossible to obtain accurate statistics of mortality during previous centuries, demographers have estimated average lifespans in some societies. From 3500 B.C. through to about A.D. 1750, a period of over five thousand years, life expectancy among European societies hovered between thirty and forty years; in classical Greece, around 680 B.C., it rose as high as forty-one years; in Turkey of A.D. 1400 it was just thirty-one years. These numbers are similar to those in the underdeveloped countries of the world today. The three main reasons for these high mortality rates—inadequate food supplies, poor sanitation, and epidemic disease—are closely interrelated. Poor nutrition leads to increased susceptibility to infection; poor sanitation produces conditions conducive to disease.
In those parts of the world with efficient agriculture and a good transportation system, food supply has increased. At the same time vastly improved personal hygiene and public health measures—clean water supply, sewage treatment systems, refuse collection and vermin control, and wholesale immunization and vaccination programs—have led to fewer epidemics and a healthier population more able to resist disease. Because of these improvements, death rates in the developed world have been dropping steadily since the 1860s. But the final onslaught against those bacteria that have for generations caused untold misery and death has been by antibiotics.
From the 1930s the effect of these molecules on mortality rates from infectious diseases has been marked. After the introduction of sulfa drugs to treat pneumonia, a common complication with the measles virus, the death rate from measles declined rapidly. Pneumonia, tuberculosis, gastritis, and diphtheria, all among leading causes of death in the United States in 1900, do not make the list today. Where isolated incidents of bacterial diseases—bubonic plague, cholera, typhus, and anthrax—have occurred, antibiotics contained what might otherwise have become a widespread outbreak. Today’s acts of bioterrorism have focused public concern on the possibility of a major bacterial epidemic. Our present array of antibiotics would normally be able to cope with such an attack.
Another form of bioterrorism, that waged by bacteria themselves as they adapt to our increasing use and even overuse of antibiotics, is worrying. Antibiotic-resistant strains of some common but potentially lethal bacteria are becoming widespread. But as biochemists learn more about the metabolic pathways of bacteria—and of humans—and how the older antibiotics worked, it should become possible to synthesize new antibiotics able to target specific bacterial reactions. Understanding chemical structures and how they interact with living cells is essential to maintaining an edge in the never-ending struggle with disease-causing bacteria.
11. THE PILL
BY THE MIDDLE of the twentieth century antibiotics and antiseptics were in common use and had dramatically lowered mortality rates, particularly among women and children. Families no longer needed a multitude of children to ensure that some reached maturity. As the specter of losing children to infectious diseases diminished, a demand for ways of limiting family size by preventing conception arose. In 1960 a contraceptive molecule emerged that played a major role in shaping contemporary society.
We are, of course, referring to norethindrone, the first oral contraceptive, usually known as “the pill.” The molecule has been credited with—or blamed for (depending on your point of view)—the sexual revolution of the 1960s, the women’s liberation movement, the rise of feminism, the increased percentage of women in the workplace, and even the breakdown of the family. Despite the varying opinions on the benefits or disadvantages of this molecule, it has played an important role in the enormous changes in society in the forty or so years since the pill was introduced.
Struggles for legal access to birth control information and supplies, fought in the early part of the last century by such notable reformers as Margaret Sanger in the United States and Marie Stopes in Britain, seem remote to us now. Young people today are often incredulous on hearing that just to give information on contraception was, in many countries during the initial decades of the twentieth century, a crime. But the need was clearly present: high rates of infant mortality and maternal death found in poor areas of cities often correlated with large families. Middle-class families were already using the contraceptive methods then available, and working-class women were desperate for the same information and access. Letters written to birth control advocates by mothers of large families detailed the despair they felt while facing yet another unwanted pregnancy. By the 1930s public acceptance of birth control, often couched in the more acceptable term family planning, was increasing; health clinics and medical personnel were involved in prescribing contraceptive devices, and laws, at least in some places, were being changed. Where restrictive statutes did remain on the books, prosecutions became less common, especially if contraception matters were handled discreetly.
EARLY ATTEMPTS AT ORAL CONTRACEPTION
Over the centuries and in every culture women have swallowed many substances in the hope of preventing conception. None of these substances would have accomplished this goal except, perhaps, by making the woman so ill she would be unable to conceive. Some of the remedies were fairly straightforward: brewed teas of parsley and mint, of leaves or bark of hawthorn, ivy, willow, wallflower, myrtle, or poplar. Mixtures containing spiders’ eggs or snake were also suggested. Fruits, flowers, kidney beans, apricot kernels, and mixed herbal potions were other recommendations. At one time the mule featured prominently in contraception, supposedly because a mule is the sterile offspring of a female horse and a male donkey. Sterility was allegedly assured if a woman ate the kidney or uterus of a mule. For male sterility the animal’s contribution was no less tasty; a man was to eat the burned testicles of a castrated mule. Mercury poisoning might have been an effective means of attaining sterility for a woman swallowing a seventh-century Chinese remedy of quicksilver (an old name for mercury) fried in oil—that is, if it did not kill her first. Solutions of different copper salts were drunk as contraceptives in ancient Greece and in parts of Europe in the 1800s. A bizarre method from the Middle Ages required a woman to spit three times into the mouth of a frog. It was the woman who would become sterile, not the frog!
STEROIDS
Though some of the substances smeared on various parts of the body to prevent pregnancy could possibly have had spermicidal properties, the advent of oral contraceptives in the middle of the twentieth century marked the first truly safe and effective chemical means of birth control. Norethindrone is one of a group of compounds known as steroids, a perfectly good chemical name that now is often applied to performance-enhancing drugs illegally used by some athletes. Such drugs are definitely steroids, but so are many other compounds that have nothing to do with athletic prowess; we will be using the term steroid in the wider chemical sense.
In many molecules very small changes in structure can have very large changes in effect. This is nowhere more pronounced than in the structures of the sex hormones: the male sex hormones (androgens), the female sex hormones (estrogens), and the hormones of pregnancy (progestins).
All compounds that are classified as steroids have the same basic molecular pattern, a series of four rings fused in the same way. Three of the rings have six carbon atoms each, and the fourth has five. These rings are referred to as the A, B, C, and D rings—the D ring always being five-membered.
The four basic rings of the steroid structure, showing the A, B, C, and D designations
Cholesterol, the mo
st widespread of all animal steroids, is found in most animal tissues, with especially high levels in egg yolks and human gall-stones. It is a molecule with an undeservedly bad reputation. We need cholesterol in our system; it plays a vital role as the precursor molecule of all our other steroids, including bile acids (compounds that enable us to digest fats and oils) and the sex hormones. What we do not need is a lot of extra cholesterol in our diet, as we synthesize enough of our own. The molecular structure of cholesterol shows the four basic fused rings as well as the side groups, including a number of methyl groups (CH3, sometimes written as H3C, just to fit more easily on the drawing).
Cholesterol, the most widespread animal steroid
Testosterone, the principal male sex hormone, was first isolated from ground-up bull testes in 1935, but it was not the first male sex hormone to be isolated. That hormone was androsterone, a metabolized and less potent variation of testosterone that is excreted in the urine. As you can see from a comparison of the two structures, there is very little difference between them, androsterone being an oxidized version where a double-bonded oxygen atom has replaced the OH of testosterone.
Androsterone varies from testosterone at only one position (arrowed).
The first isolation of a male hormone was in 1931 when fifteen milligrams of androsterone was obtained from fifteen thousand liters of urine collected from the Belgian police force, presumably an all-male group in those days.
The first sex hormone ever isolated was the female sex hormone estrone, obtained in 1929 from urine of pregnant women. As with androsterone and testosterone, estrone is a metabolized variation of the principal and more potent female sex hormone, estradiol. A similar oxidation process changes an OH on estradiol into a double-bonded oxygen.
Estrone varies from estradiol at only one position (arrowed).
These molecules are present in our bodies in very small amounts: four tons of pig ovaries were used to extract only twelve milligrams of the first estradiol that was isolated.
It is interesting to consider how structurally similar the male hormone testosterone and the female hormone estradiol are. Just a few changes in molecular structure make an enormous difference.
If you have one less CH3, an OH instead of a double-bonded O, and a few more C=C bonds, then at puberty instead of developing male secondary sex characteristics (facial and body hair, deep voice, heavier muscles), you will grow breasts and wider hips and start menstruating.
Testosterone is an anabolic steroid, meaning that it is a steroid that promotes muscle growth. Artificial testosterones—manufactured compounds that also stimulate the growth of muscle tissue—have similar structures to testosterone. They were developed for use with injuries or diseases that cause debilitating muscle deterioration. At prescription-level doses these drugs help rehabilitate with minimal masculinizing effect, but when these synthetic steroids, like Dianabol and Stanozolol, are used at a ten or twenty times normal rate by athletes wanting to “bulk up” the side effects can be devastating.
The synthetic anabolic steroids Dianabol and Stanozolol compared with natural testosterone
Increased risk of liver cancer and heart disease, heightened levels of aggression, severe acne, sterility, and shriveled testicles are just a few of the dangers from the misuse of these molecules. It may seem a bit odd that a synthetic androgenic steroid, one that promotes male secondary characteristics, causes the testes to shrink, but when artificial testosterones are supplied from a source outside the body, the testes—no longer needing to function—atrophy.
Just because a molecule has a similar structure to testosterone does not necessarily mean it acts like a male hormone. Progesterone, the main pregnancy hormone, not only has a structure closer to that of testosterone and androsterone than Stanozolol but is also more like the male sex hormones than the estrogens. In progesterone a CH3CO group (circled in the diagram) replaces the OH of testosterone.
Progesterone
This is the only variation in the chemical structure between progesterone and testosterone, but it makes a vast difference in what the molecule does. Progesterone signals the lining of the uterus to prepare for the implantation of a fertilized egg. A pregnant woman does not conceive again during her pregnancy because a continuous supply of progesterone suppresses further ovulation. This is the biological basis of chemical contraception: an outside source of progesterone, or a progesteronelike substance, is able to suppress ovulation.
There are major problems with the use of the progesterone molecule as a contraceptive. Progesterone has to be injected; its effectiveness is very much reduced when taken orally, presumably because it reacts with stomach acids or other digestive chemicals. Another problem (as we saw with the isolation of milligrams of estradiol from tons of pig ovaries) is that natural steroids occur in very minute quantities in animals. Extraction from these sources is just not practical.
The solution to these problems is the synthesis of an artificial progesterone that retains its activity when taken orally. For such a synthesis to be possible on a large scale, one needs a starting material where the four-ring steroid system, with CH3 groups at set positions, is already in place. In other words, synthesis of a molecule that mimics the role of progesterone requires a convenient source of large amounts of another steroid whose structure can be altered in the laboratory with the right reactions.
THE AMAZING ADVENTURE S OF RUSSELL MARKER
We have stated the chemical problem here, but it should be emphasized that we are seeing it from the perspective of hindsight. Synthesis of the first birth control pill was the result of an attempt to solve quite a different set of puzzles. The chemists involved had no idea that they would eventually produce a molecule that would promote social change, give women control over their lives, and alter traditional gender roles. Russell Marker, the American chemist whose work was crucial to the development of the pill, was no exception; the goal of his chemical experimentation was not to produce a contraceptive molecule but to find an affordable route to produce another steroid molecule—cortisone.
Marker’s life was a continual conflict with tradition and authority, perhaps fittingly for the man whose chemical achievements helped identify the molecule that would also have to battle tradition and authority. He attended high school and then college against the wishes of his sharecropper father, and by 1923 he had obtained a bachelor’s degree in chemistry from the University of Maryland. Although he maintained that he continued his education to “get out of farm work,” Marker’s ability and interest in chemistry must also have been factors in his decision to pursue graduate degrees.
With his doctoral thesis completed and already published in the Journal of the American Chemical Society, Marker was told that he needed to take another course, one in physical chemistry, to fulfill the requirements for a Ph.D. Marker felt this would be a waste of valuable time that could be more profitably spent in the laboratory. Despite repeated warnings by his professors about the lack of opportunity for a career in chemical research without a doctoral degree, he left the university. Three years later he joined the research faculty of the prestigious Rockefeller Institute in Manhattan, his talents obviously having overcome the handicap of not finishing his doctorate.
There Marker became interested in steroids, particularly in developing a method to produce large enough quantities to allow chemists to experiment with ways to change the structure of the various side groups on the four steroid rings. At this time the cost of progesterone isolated from the urine of pregnant mares—over $1,000 a gram—was beyond the means of research chemists. The small amounts extracted from this source were mainly used by wealthy racehorse owners to prevent miscarriages in their valuable breeding stock.
Marker knew that steroid-containing compounds existed in a number of plants, including foxglove, lily of the valley, sarsaparilla, and oleander. Though isolating just the four-ring steroid system had not been possible so far, the quantity of these compounds found in plants was much greater than in animal
s. To Marker this was obviously the route to follow, but once again he was up against tradition and authority. The tradition at the Rockefeller Institute was that plant chemistry belonged in the department of pharmacology, not in Marker’s department. Authority, in the person of the president of Rockefeller, forbade Marker to work on plant steroids.
Marker left the Rockefeller Institute. His next position was a fellowship at Pennsylvania State College, where he continued to work on steroids, eventually collaborating with the Parke-Davis drug company. It was from the plant world that Marker was eventually to produce the large quantities of steroids that he needed for his work. He began with roots of the sarsaparilla vine (used in flavoring root beer and similar drinks), which were known to contain compounds called saponins, so named because of their ability to make soapy or foamy solutions in water. Saponins are complex molecules, though nowhere near as large as polymer molecules like cellulose or lignin. Sarsasaponin, the saponin from the sarsaparilla plant, consists of three sugar units attached to a steroid ring system, which in turn is fused at the D ring to two other rings.
Structure of sarsasaponin, the saponin molecule from the sarsaparilla plant
It was known that removing the three sugars—two glucose units and a different sugar unit called rhamnose—was simple. With acid the sugar units split off at the point indicated by the arrow in the structure above.
It was the remaining portion of the molecule, a sapogenin, that presented problems. To obtain the steroid ring system from sarsasapogenin, it was necessary to remove the side grouping circled in the next diagram. The prevailing chemical wisdom of the day was that this could not be done, not without destroying other parts of the steroid structure.
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