Levene suspected that Jones was wrong in claiming that ‘animal’ nucleic acids contained a hexose, but it would take him another fifteen years to prove the point by isolating the sugar, which needed even gentler handling than ribose. In the meantime, he did not demean himself by arguing science with his rival. Nobody in his laboratory did – because, by order of Levene himself, it was forbidden even to mention the name of Walter Jones.
End of story?
As the Great War loomed in Europe, Levene had much to be proud of. As Albrecht Kossel’s natural successor, he had discovered that yeast nucleic acid contained ribose and worked out how this was attached to the phosphate and the base. In addition, he had coined a catchy new name for the unit that contained the base+sugar+phosphate building-blocks. He called this a ‘nucleotide’.
Levene had also come up with a neat idea about how nucleotides were joined together to make nucleic acids. He had analysed thymus nucleic acid from various sources and found this to contain roughly equal amounts of each of the four bases, adenine (A), guanine (G), cytosine (C) and thymine (T). From the simple equation A = G = C = T, Levene concluded that a single molecule of thymus nucleic acid must contain one molecule of each of the four bases, which meant that the nucleic acid molecule consisted of just four nucleotides, or a ‘tetranucleotide’.
In 1912, Levene published his first impression of what his tetranucleotide looked like: a chain of four ribose sugars, each with a different base tacked on to its side, linked together by phosphate (P) groups (Figure 7.2). It was an attractive notion, and as it originated from the greatest authority on nucleic acid chemistry, it quickly became dogma.
Figure 7.2 A ‘tetranucleotide’ structure for DNA, proposed by Levene during the late 1910s.
*
Meanwhile, the fortunes of that lucrative wonder drug, Protonuclein, had taken a nose-dive. The problem was the Pure Food and Drugs Act, forced through in 1906 to purge America of ‘misbranded and adulterated foods, drinks and drugs’ and the snake-oil salesmen who peddled them. An early casualty of the clampdown on ‘false therapeutic claims’ was Mrs Winslow’s Soothing Syrup, which miraculously calmed coughing in infants but also stopped some of them from breathing, due to its undisclosed content of morphine.
It was only a matter of time before the spotlight would swing on to Protonuclein, that astonishing panacea extracted from the very heart of the cell. And no matter what all those doctors had said, the claims of miracle cures for tuberculosis, liver cancer, impotence or anything else could not possibly survive close scrutiny. For the moment, though, Protonuclein clung on, thanks to the mystical appeal of the nucleic acids.
And so did Phoebus Levene’s tetranucleotide hypothesis.
8
CRYSTAL GAZING
What does a precious opal have in common with the youngest ever winner of a scientific Nobel Prize? Answer: both owe their glory to the physical phenomenon of diffraction, the scattering of waves on hitting obstacles. The waves can be wet or dry, deafening or inaudible, blindingly obvious or totally invisible. Diffraction operates across a billion-fold range of wavelengths; it bends ripples around a stick in a pond, fills every corner of a cathedral with a chorister’s top A, and makes the wings of the blue Morpho butterfly visible from half a kilometre away.
Interesting things happen if a train of waves meets a series of obstructions, regularly spaced at intervals that are close to the distance between the waves. The waves bounce off the obstructions and interfere with other waves; depending on the geometry, they may reinforce each other and create a bigger wave, or cancel each other out. This is where the opal comes in, as a lesson in physics as well as an object of beauty. Hold a piece of opal up to the light and it appears dull yellow, but if you look into it when bright sunshine hits it from the side, you will be dazzled by a vivid display of green, blue and fiery red. These colours are not some ethereal pigment, but the result of the opal’s submicro-scopic structure – tiny spheres of silica, tightly packed in layers like trays of oranges stacked on top of each other. The layers act as a ‘diffraction grating’. A beam of light entering the opal at an angle bounces off successive layers and, if that angle has the right relationship to the distance between the layers, the reflected rays will unite to form a new wave that creates those flashes of colour. If precious opal is beyond your budget, you can still enjoy diffraction by tilting a compact disc under a light; the microscopic grooves of the playing track also act as a diffraction grating.
Diffraction affects X-rays, which have wavelengths one-5,000th or less than that of visible light. These wavelengths are comparable to the size of individual atoms and the distances between them, which means that X-ray diffraction can be used to probe the structure of molecules – far beyond the reach of optical and even electron microscopes. The first substances to be deconstructed by X-rays were simple salts that form well-shaped crystals – hence the alternative name, ‘X-ray crystallography’.
From a standing start, this became one of the fastest-moving fields in science during the first quarter of the twentieth century, and one that produced several Nobel Prizes. And to make sense of the story of DNA, we need to know something about X-ray diffraction, because this was the technique that revealed the structure of the double helix.
X-ray diffraction had humble beginnings. It was conceived in the spring of 1912, from notes scribbled in pencil in a coffee-house in central Munich. The Café Lutz was famous for its cakes and open-air seating under the chestnut trees of the Hofgarten Park. It was also where members of the Physics Department at the nearby university went each afternoon to talk about work in progress. Their formulae, equations and graphs were not jotted down on paper, but – to the annoyance of the waitresses – on the white marble tabletops.
These particular scribbles concerned something so speculative that the professor had vetoed the idea of looking into it when the idea was aired during the departmental ski-trip at Easter. Now, behind the professor’s back, two of his junior researchers had been nudged into doing a pilot experiment. The results sketched out on the marble were uninterpretable, but looked as though they might be trying to say something important.
While walking home from the Hofgarten, the lecturer in physics who had persuaded the two juniors to defy the professor was struck by a bolt of inspiration. It was so startling that he remembered exactly where it happened: on the pavement outside 10 Siegfriedstrasse. As he thought about it, a series of equations which made sense of those peculiar results tumbled into his mind. He did more experiments over the next few days, and everything seemed to confirm his intuition. His explanation was not perfect, but it was good enough to impress a group of learned people who met in Stockholm just three years later, and who awarded him the Nobel Prize for Physics.
Max Laue, Lecturer in Physics, was just thirty-three years old when he walked past number 10 Siegfriedstrasse on that spring afternoon. Even before coming to Munich in 1909, he was clearly going places. He had done his PhD in Berlin with Max Planck, the giant of German physics who had discovered quantum theory – and who declared Laue to be his ‘favourite disciple’.
That pioneering experiment was inspired by a conversation about crystals with a student early in 1912. This set Laue thinking about how waves with an extremely short wavelength might interact with regular, submicroscopic lattices. X-rays were the obvious medium with which to test his ideas, especially as their discoverer, Wilhelm Roentgen (nicknamed ‘His Majesty’ by Laue), had been tucked away in the Physics Department in Munich since 1900.
The first experiment was a masterpiece of improvisation. A sheet of lead was folded to make a rectangular container the size of a large matchbox; a 3-millimetre hole was drilled in one side to admit a beam of X-rays; and a sheet of photographic paper was propped up inside the opposite wall. The target of the X-rays was a bright blue crystal of copper sulphate, stuck on to a metal rod in the middle of the lead box with a blob of wax. When the photograph was developed after several hours of bombardment, it showed
vague spots and streaks scattered around the exit wound where the X-rays had shot straight through the crystal. A zinc sulphide crystal, carefully lined up at right angles to the X-ray beam, gave a clearer picture with a symmetrical array of spots (Figure 8.1).
These were the results that Laue decrypted using the equations which came to him after he left the Café Lutz. The bolt of inspiration made him realise that the arrangement of the spots on the photographic film could reveal something extraordinary: the pattern in which zinc sulphide molecules were packed together to make up the crystal. X-rays could not only see into crystals, but could pick out individual molecules.
Within weeks of its birth, the strange new art of X-ray crystallography had become hard science. On 8 June 1912, Laue was back in the big lecture theatre in the Physics Department in Berlin, describing his experiments on the same spot where, in December 1900, Max Planck had announced the birth of the quantum era. And three years after that, in November 1915, Max von Laue* heard the wonderful news from Stockholm.
Figure 8.1 X-ray crystallography. A narrow beam of X-rays, made parallel by passing through a collimator, hits a crystal; the ‘pencils’ of diffracted X-rays are captured on photographic film (top). The X-rays are diffracted off the regular arrays of molecules in the crystal (bottom).
By then, the portcullis of the Great War had slammed down. Von Laue was sent to Würzburg to develop vacuum tubes for military communications, and the glitter and excitement of the Nobel Prize ceremonies was suspended while Europe was in lockdown. When the wartime prizes were finally awarded at a catch-up ceremony in the summer of 1920, it was an odd affair. X-ray crystallography had moved so fast that the second Nobel Prize for work in the field had been announced in the same month as von Laue’s. However, only one of the crystallographer laureates made the trip to Stockholm – and he would have been shaken if some fortune teller had told him what was going to happen to the gold Nobel medal which he took home.
Two of a kind
Max von Laue’s most serious competition was doubly formidable because there were two of them – a father and his son (Figure 8.2). Confusingly, both ended up as Sir William X Bragg, FRS and Nobel laureate, where X = Henry for the father, and Lawrence for the son. The father was always known as William; Bragg Junior was called ‘Willie’ at home and Lawrence by the outside world. Lawrence went on to preside over the final scramble to crack the double helical structure of DNA. By then, his father had been dead for over a decade, but their stories are so tightly interwoven that it is right to start with Bragg Senior.
William Bragg was born in Cumbria in 1862, when Friedrich Miescher was a schoolboy and still dreaming of becoming a doctor. A bright boy, William charged through school and into mathematics at Cambridge, where he graduated with a First and enough promise to be appointed in 1885 – at the age of twenty-three – to the Chair of Mathematics and Physics at the University of South Australia in Adelaide. This was a long way to go, but several weeks at sea gave him the chance to learn some physics, which he had avoided until then.
Research was not the university’s strongest suit, and its teaching was not much better. Bragg seized what opportunities he could, and soon revived the teaching, set up a highly popular series of public lectures, and started original research of his own. For good measure, he married Gwendoline, the nineteen-year-old daughter of the Postmaster General. Three children followed: Lawrence in 1890, followed by Robert (‘Bob’) a couple of years later and Gwendolen in 1907.
Figure 8.2 William Bragg, right, and his son Lawrence.
Bragg began serious research late in life, aged forty-two, and made rapid progress against long odds. The wheels of scientific enquiry are lubricated by daily contact with like-minded people and the constant bouncing of thoughts and ideas off them. None of this was possible when Bragg arrived in Adelaide, so he corresponded with eminent physicists in Britain and North America. Top of the list was Ernest Rutherford, whose ‘theory of atomic disintegration’ won him the Nobel Prize in Chemistry in 1908 and who was greatly impressed by all that Bragg achieved. At first, Bragg worked wonders with a primitive X-ray tube – the first to reach Australia – which pulled in record crowds for his public lectures, and also showed that six-year-old Lawrence had fractured his elbow when he fell off his tricycle. His experiments persuaded him that X-rays consisted of particles; this threw him into open conflict with J.C. Barkla, Professor of Physics at King’s College in London, who was convinced that X-rays behaved like waves. They slugged it out in the letters pages of Nature, until a weary editor guillotined further correspondence. These spirited exchanges, together with a torrent of papers between 1905 and 1908, brought Bragg his Fellowship of the Royal Society in 1907, and the Chair of Physics in Leeds two years later.
He had been ‘singularly happy’ in Adelaide but now swept into Leeds and began building up his new department. Everything was proceeding nicely when, in late June 1912, a letter was forwarded to Bragg while on holiday with his family. The letter contained a bombshell of bad news, together with some intelligence which knocked Bragg’s plans off course. It also pulled his son straight into the plot.
Off the old block
Like his father, Lawrence Bragg excelled academically from the start, undoubtedly helped by the ‘inspiring scientific atmosphere’ at home. At school, he was regarded as ‘a strange freak’, being hopeless at sports but excited by marine life (a new cuttlefish, Sepia braggi, was his first scientific discovery). Aged fifteen, he entered Adelaide University and emerged with a First in Mathematics in 1908, just as his father was preparing to leave for Leeds. Lawrence followed him to England, winning a scholarship to Trinity College, Cambridge and collecting another First. In 1911, he joined the world-famous Cavendish Laboratory as a research student with J.J. Thomson, Nobel prizewinner and discoverer of the electron. This should have been a thrilling ride but turned out to be ‘crude research’ into a dull subject.
In June 1912, he went to join his parents on holiday, and the letter arrived which changed the course of his life and his father’s. It was from Lars Vegard, a Norwegian physicist who had trained with William Bragg in Leeds, and was now at Würzburg University in Germany. Vegard reported the recent claim of a certain Max Laue that crystals could diffract X-rays, and helpfully included details of the experimental set-up. The implication was shockingly clear: by being diffracted, X-rays had to be waves, not particles. William was depressed at losing his battle with Barkla, but Lawrence saw more opportunity than threat and persuaded him that they should look together into the new phenomenon of X-ray diffraction.
After returning to Cambridge, Lawrence was hit by another of those unforgettable bolts from the blue. He realised that each spot on the Laue photograph was created by a ‘pencil’ of X-rays that bounced off identical structures in successive layers of the crystal lattice (Figure 8.1). The position of each spot could be predicted from the angle at which the original X-ray beam hit the target atom, and the distance between the layers of the lattice. The mathematical formula that described what happened, later known as ‘Bragg’s Law’, was much simpler than Laue’s complicated series of equations. Indeed, it was now obvious that Laue’s explanation was ‘unsatisfactory’.
Crucially, Bragg’s equation could be used to turn the pattern of spots into a map that showed the positions of individual atoms within the molecule of the substance that formed the crystal.
Up in Leeds, William Bragg had designed and built an instrument to detect the invisible pencils of diffracted X-rays that fanned out from the back of the target crystal. This was the X-ray spectrometer, in which two virtually weightless strips of gold leaf hanging inside a brass tube were pushed apart by electrical charges when the air around them was ionised by X-rays passing down the tube. The spectrometer was extremely accurate but laborious to use.
In the meantime, Lawrence had abandoned his ‘crude’ research project with J.J. Thomson and was trying out his new Law on Laue-style photographs of crystals – cubes of halite (rock salt) and
flat, glassy flakes of mica. These experiments confirmed that he was on to something much bigger and better than Laue’s interpretation. The results with mica were especially thrilling. With some trepidation, but shaking with excitement, Lawrence took the still-wet photograph to Thomson, who gravely inspected it for a few seconds before breaking into a huge smile of pleasure. Lawrence dashed off a letter to William, which began ‘Dear Father’ and ended, ‘Yours affectionately, W.L. Bragg’. The exhilaration was infectious, as Bragg Senior promptly took up his pen and boasted to a friend that ‘my boy has been getting beautiful X-ray reflections from mica sheets, just as simple as a reflection of light off a mirror’.
Lawrence first told the world what he had found in a lecture on ‘The diffraction of short electromagnetic waves by a crystal’ to the Cambridge Philosophical Society on 11 November 1912. This was barely five months after Laue had delivered his big talk in Berlin. Bragg’s paper on mica was published by Nature just a month later, with another on halite hard on its heels.
Over Christmas, Lawrence moved up to Leeds, and the combination of father and son, X-ray spectrometer and Bragg’s Law soon left Laue and other competitors far behind. As Lawrence put it, ‘a new crystallography is being developed’. In an ‘epoch-making’ burst of energy, they deduced the atomic structures of several salts, iron pyrites, fluorite and even diamond.† Later, Lawrence fondly recalled the perpetual excitement which lit up their laboratory, while the rest of Leeds trudged on through the darkness of winter:
Unravelling the Double Helix Page 12
