by Frank Ryan
The first creative step was to realize that the answer lay with DNA. To be more accurate, they realized that somehow chemical structure must parallel function: so the answer to the great conundrum lay in the three-dimensional chemical structure of DNA. But nobody really knew what shape or form this structure took. To the minds of Crick and Watson at that particular moment in time, it would have seemed nothing more than a ghost in the mist.
New discoveries in science will usually involve a lengthy period of laboratory labor, with knowledge growing by hard-won increments, often involving contributions from several, or a good deal more than several, different sources. In many ways, the struggle to get to grips with the mysteries of heredity followed exactly such a course. But the mundane sweat of the laboratory aspects, the growth of knowledge by hard-won increments, would not fall to Watson and Crick. These would be left to others. The Crick–Watson symbiosis would be founded on a second, equally important ingredient of scientific advance—one that has commonalities with the advances in the arts and humanities: this is the quintessentially human gift we call “creativity.”
Within the hierarchy of the lab, Crick and Watson were the lowest contributing level. In Crick's words, “I was just a research student and Jim was just a visitor.” They read very widely, imbibing the fruits of the hard work of others. They talked and talked, thinking out loud, probing one another's ideas and knowledge, often with Crick playing devil's advocate. In fact they gossiped and argued so much they were given a room to themselves—to avoid their interrupting the thoughts of their more senior colleagues—within the crowded structure of the old Cavendish Laboratory. The X-ray laboratory, with its heavy machinery and radiation dangers, was located in the basement. Jim and Francis would also share a cheap and cheerful lunch, of shepherd's pie or sausage and beans, at the local pub, the Eagle—a grubby establishment in a cobblestoned courtyard—where the creative debate would simply continue.
What little they knew about DNA was made even more uncertain by the fact that Crick believed that much of what was generally assumed to be the case with DNA and heredity was almost certainly wrong. It had been this attitude that had got him into trouble with Bragg. It meant that he didn't even trust the work of his seniors here in the lab. But the real reason behind Bragg's anger was his resentment of the fact that the chemist, Pauling, had discovered the alpha helix of protein. Meanwhile, Crick was convinced that the reason why the Cavendish had missed out on this was because they were assuming the accuracy of some earlier experimentation on the X-ray interpretation of the skin protein, keratin, which is the main ingredient of our human nails and a raptor's claws. The way in which Crick's mind worked can be gleaned from a remembered conversation:
The point is [so-called] evidence can be unreliable, and therefore you should use as little of it as you can. We have three or four bits of data, we don't know which one is reliable…[What if] we discard that one…then we can look at the rest and see if we can make sense of that.
Watson joined the Cavendish the same year, 1951, in which Linus Pauling published his paper on the protein “alpha helix.” This discovery so rattled Watson that all of the time he was working with Crick on the structure of DNA, he was looking over his shoulder in Pauling's direction.
He had good reason for seeing Pauling as the supreme rival in such an exploration; awarded the Nobel Prize in Chemistry in 1954, Pauling was already being hailed by scientific historians as one of the most influential chemists in history. His master work, though he contributed a great deal more, was to apply a quantum theory perspective to the chemical bonds that bind atoms within the structure of molecules, extending this basic science to the complex organic molecules that are the chemical building blocks of life.
The twentieth century has amazed us with its achievements in astronomy, in which scientists have plotted the stars and galaxies, and the forces, such as black holes, that govern the universe. Equally important, though not so easily recognized as such by the ordinary man and woman, have been the achievements of the chemists and biochemists in exploring the micro-universe of atoms and molecules. Two forces in particular play a key role in the way that atoms bind to one another to make up life's particular molecules. One of these is called the covalent bond; the other is called the hydrogen bond. Pauling applied the science of quantum mechanics to the forces involved in these two very different chemical bonds.
We have no need to concern ourselves with the complex mathematics of the applied physics. We just need to grasp the basic mechanics. And where better to look than at the familiar molecule of water.
Everybody knows that the chemical formula for water is H2O. This tells us that a molecule of water comprises one atom of oxygen and two atoms of hydrogen. But how do they link with one another to form the stable compound that we handle and consume every day of our lives? The molecule of water might be compared to a planet, oxygen, with two encircling moons of hydrogen. In such a situation, we can readily imagine how the force of gravity would hold the hydrogen moons to their orbits around the oxygen planet. In molecular terms, the forces holding the two hydrogen atoms to the oxygen atom are called “covalent bonds.” At the ultramicroscopic level of atoms, the nucleus of each hydrogen atom contains a single positively charged proton while circling around the nucleus is a single negatively charged electron. Meanwhile, the oxygen atom has eight positively charged protons within its nucleus and eight balancing, negatively charged electrons in orbits around it. These electrons occupy two orbits—two electrons taking up an inner orbit and six taking up an outer orbit. In coming together to form a molecule of water, the two electrons in orbit around each of the two hydrogen nuclei have paired with two of the six electrons of the oxygen outer orbits. The paired electrons share their attraction to the protons of the two parent nuclei, so the paired electrons are now equally attracted to the oxygen nucleus and the hydrogen nuclei. This sharing of attraction creates a stable “covalent” bond between the three atoms, just as gravity created stable orbits for the two moons rotating around our imaginary planet of oxygen.
Hydrogen bonds are something else.
Once again, we might take water as our example. But here we are looking at the chemical interactions between whole water molecules—the H2Os reacting with one another. There are forces of attraction, albeit rather weaker and less stable than covalent bonds, between certain molecules that contain both hydrogen and heavier atoms such as nitrogen, oxygen, or fluorine. Since water contains hydrogen and oxygen, these hydrogen bonds can form between molecules of water—it is this sticking together of water molecules that explains the difference between water vapor, or steam; liquid water; and solid water, or ice. In ice, most of the molecules are attached to one another by hydrogen bonds to form something like a crystal; in liquid water varying amounts are attached to one another; and in steam, as a result of the addition of energy through heating, the hydrogen bonds linking water molecule to water molecule are broken down, but the covalent bonds linking atoms to atoms remain intact.
We see that hydrogen bonds are weak and thus unstable when heated, but covalent bonds are stable. These same two bonds, covalent and hydrogen bonds, are important ingredients in the structure of organic chemicals such as proteins. And they are also important in the structure of DNA.
Between 1927 and 1932, Pauling published some fifty scientific papers in which he conducted X-ray diffraction studies, coupled with quantum mechanical theoretical calculations, leading him to postulate five rules, known as Pauling's rules, that would help science to predict the nature of the bonds that held together atoms within molecules. At least three of these rules were based on Bragg's own work, the purloining of which provoked Bragg to fury. It was now inevitable that there would be ongoing scientific rivalry between the two scientists. Pauling's work into the nature of chemical bonding was so original, and pioneering, that he was awarded the Nobel Prize in Chemistry in 1954. Meanwhile, this new level of understanding enabled Pauling to visualize the precise shape and dimensions of
molecules in three-dimensional space. Working at Caltech, Pauling applied this to the huge molecules of proteins, using the techniques of X-ray diffraction analysis pioneered by the Braggs. He showed, for example, that the hemoglobin molecule—the focus of Perutz's research—changed its physical structure when it gained or lost an oxygen atom. And Pauling continued to apply his rules to researching the molecular structure of proteins.
Pioneering X-ray pictures of fibrous proteins had been obtained some years before at the University of Leeds by William Thomas Astbury, the physicist who had attended Wilkins's talk in Naples, but it was assumptions based on these X-ray diffraction pictures that Crick was now questioning at the Cavendish Laboratory. For many years, Pauling had tried to apply quantum mechanics calculations to Astbury's X-ray pictures, but he found that things just didn't add up. It would take him and two collaborators, Robert Corey and Herman Branson, fourteen years before they made the necessary breakthrough.
All proteins have a primary structure that is made up of an amino acid code, with the letters made up of twenty different amino acids. The chemical bonds that join up the amino acids into the primary chain are called “peptide bonds.” Pauling and his collaborators now realized that peptides bonded together in a flat two-dimensional plane—they called this “a planar bond.” A problem with outdated equipment had caused Astbury to make a critical error in taking his X-ray pictures: the protein molecules became tilted away from their natural planes, skewing the mathematical extrapolations of their structure. Once they had corrected Astbury's error, Pauling and co discovered that as the chain of amino acids grew, to form the primary structure of proteins, it naturally followed the shape of a coiled spring, twisting to the right—the so-called “alpha helix.” This was the discovery that had excited Watson on his return trip from Naples.
Back in Cambridge, Sir Lawrence Bragg was bitterly disappointed when Pauling's group beat his to the discovery of the primary structure of proteins. But there was a silver lining to the cloud: Perutz now used Pauling's breakthrough to reappraise his own work on the hemoglobin molecule, a reappraisal that would solve the structural puzzle of hemoglobin and garner his Nobel Prize in Chemistry in 1963. Pauling's discovery also alarmed Watson who, from his arrival at Cambridge, had assumed that they had a very knowledgeable and powerful rival in what was now a race to discover the three-dimensional structure of DNA.
But the problem, as Crick would point out in their day-to-day sharing of thoughts and incessant debate, was that they couldn't even assume that Pauling's data was right. In Crick's words, “Data can be wrong. Data can be misleading.” So Crick and Watson attempted to construct their physical model with a skeptical eye on prevailing experimental data. To put it another way, they relied just as heavily on creative leaps of their own imagination as on existing experimental data.
Crick and Watson were now asking themselves if DNA, like proteins, had a helical structure, and Watson in particular was convinced that they should also take their cue from Pauling, who liked to construct three-dimensional models of the molecules he was attempting to envisage. To do so they would have to think, as Pauling did, about the atomic structures that made up the chemistry of DNA—to fit the molecules, with their component atoms, and the bonds between them, into a complex three-dimensional jigsaw. They knew that they were dealing with the four nucleotides—guanine, adenine, cytosine, and thymine—together with the molecule of the sugar, called ribose, and the inorganic chemical, phosphate, all of which, when correctly fitted together, must somehow make up the mysterious three-dimensional jigsaw puzzle.
Two relevant questions now loomed. Firstly, if the structure was helical, what kind of helix was involved? And secondly, where did the phosphate molecule fit into the structure? Calcium phosphate is the mineral of bones, shells, and rocks formed from the remains of living marine organisms—limestone. The presence of phosphate suggested some kind of strengthening of the DNA chain—a chemical scaffold—maybe a spine? But where did this spine lie in relation to the presumptive and as yet unknown spiral? And where, or how, did the sugar fit in? The code itself must surely lie with the nucleotides, acting perhaps as something like letters. Each was a key ingredient, but how on earth did the whole thing assemble in a way that made sense?
An important clue must come from the X-ray diffraction patterns. That meant they needed the help of Maurice Wilkins and Rosalind Franklin—“Rosy,” as Watson referred to her in his autobiography—who were conducting X-ray analyses of DNA fibers at the King's College London laboratory.
Rosalind Elsie Franklin was born in London to a prosperous Jewish family in 1920. From an early age she showed both a brilliantly incisive mind and the stubbornness necessary to make a distinguished mark for herself. She also showed an aggressively combative side to her personality that might prove a mixed blessing in overcoming the prevailing prejudices against Jews in society, as well as against women being in higher education and the scientific workplace. It didn't help that her father, who appeared to be a similarly combative character to his daughter, opposed her notion of a career in science. In her second year at Newnham College, Cambridge, he threatened to cut off her fees, urging that she switch to some practical application in support of the war effort. Only when he was dissuaded by her mother and aunt did he relent and allow her to continue her course.
Franklin studied physical chemistry, which involved lectures, extensive reading and laboratory experience in physics, chemistry, and the mathematics that applied to these disciplines. One of the mandatory texts she read was Linus Pauling's The Nature of the Chemical Bond.
The youthful Rosalind Franklin was disappointed when she ended up with a good second, and not a first, “bachelor's” degree in 1941. Even then, such was the lingering prejudice against female graduates in science that she was forced to wait in an unseemly uncertainty, one shared with all previous female graduates of Newnham, until her due qualification was formally granted, retrospectively, in 1947.
Like Francis Crick, Franklin was seconded to National Service during the Second World War, studying the density and porosity of coal for a PhD in which she helped to classify different types of coal in terms of fuel efficiency. Post-war, she followed this up with a research stint working under the direction of Jacques Mering at the Laboratoire Central des Services Chimique de l’Etat in Paris. Here, Mering introduced her to the world of X-ray crystallography which he used to study the structure of fibers, such as rayon. “With his high tartar cheekbones, green eyes and hair combed rakishly over his bald spot,” Franklin was surprised to discover that Mering was Jewish, as well as being “the archetypal seductive Frenchman.” The still youthful, and perhaps naïve, Rosalind Franklin appears to have fallen in love with Mering, who was already married, but whose wife was “nowhere in evidence.”
Brenda Maddox, one of Franklin's biographers, would draw attention to the fact that Franklin's most imaginative and productive research was conducted when she was teamed up with male scientists of Jewish background. Mering also appeared to be attracted to the trim, slender young woman, with the lustrous dark hair and glowing eyes. They would spend entire days and on into the evenings deep in discussion and argument over likely meanings of X-ray plates and atomic structures.
However, Franklin's infatuation with Mering would be painfully halted when, in January 1951, she took up a post as research associate at King's College London in the Medical Research Council Biophysics Unit, directed by John Turton Randall. Her appointment happened to coincide with a major post-war rebuilding within the department, designed to accommodate new ambitions within the nascent field of biophysics. The precise nature and purpose of her appointment has since become the subject of debate. In part some confusion has arisen because Randall changed the scope of her appointment in between first confirming it and Franklin taking up the post. She had initially agreed to carry out X-ray diffraction studies of proteins, but Randall wrote to her before she took up her appointment, suggesting that she change direction to the study of
DNA. According to Maurice Wilkins, this was at his suggestion. Whether at Wilkins's suggestion or Randall's own idea, Franklin agreed. She was offered the assistance of a promising graduate student, Raymond Gosling, to work with. But there was an inherent problem with this new direction.
Wilkins, who was deputy director of the MRC Unit based at King's College, was the same scientist who had first lit the fuse of inspiration for Watson in the 1950 Naples lecture. Wilkins had initiated the research into DNA in the department but happened to be deputizing once again for Randall in America at the time of Franklin's appointment. Up to now, Gosling had been working with Wilkins on DNA; even after his return from America, Randall failed to inform Wilkins about the terms he now proposed for Franklin's job description. This led to what Franklin's later research colleague, Aaron Klug, would describe as “an unfortunate ambiguity about the respective positions of Wilkins and Franklin, which later led to dissension between them and about the demarcation of the DNA research at King's.”
This is a short quote from the typed letter from Randall to Franklin, specifying her working conditions:
…as far as the experimental X-ray effort is concerned there will be at the moment only yourself and Gosling, together with the temporary assistance of a graduate from Syracuse, Mrs. Heller…
While this clearly suggests that Franklin was expected to take on the X-ray diffraction work, the qualification “at the moment” is too vague to interpret. But there is nothing in this letter to suggest that Franklin should ignore the work performed by Wilkins, or that she should refuse to collaborate with the rest of the department in her approach to the DNA problem.
Wilkins, working with Gosling, had initiated the X-ray diffraction studies on DNA in the department and, in particular, had obtained the best resolution diffraction photographs that existed up to this date. They had demonstrated a key property of DNA—that it had a regular, crystal-like molecular structure. In Paris, Franklin had learned, and improved upon, X-ray diffraction techniques for dealing with substances of limited order. But even Klug, an ardent supporter of Franklin, admitted that in relation to the work conducted by Franklin in Paris, “It is important to realize…Franklin gained no experience of such formal X-ray crystallography.”