Unravelling the Double Helix

by Gareth Williams

  Alpha male

  Linus Pauling (Figure 19.1) was born in Portland, Oregon, in 1901 and was inquisitive from the start. He was an impressionable thirteen-year-old when he started doing experiments with a friend’s chemistry set; just two years later, he was studying the subject at Oregon Agricultural College (later upgraded to Oregon State University). When he emerged with his degree in 1922, aged twenty-one, he had already decided to investigate how the atomic structure of compounds determines their physical and chemical properties.

  This led him to a PhD at Caltech in Pasadena, where he learned X-ray crystallography and wrote the first seven of his 1,200 papers. A Guggenheim Scholarship then sent him to Europe on a grand tour of the high priests of physics – Sommerfeld in Munich, Bohr in Copenhagen and Schrödinger in Zürich. Steeped in their way of thinking, he returned to Caltech where he began to apply quantum mechanics to chemistry. He succeeded so spectacularly that he was made a full professor at the age of twenty-nine – comfortably in time to carry off the Langmuir Prize, which the American Chemical Society awards to the most outstanding researcher under the age of thirty. By now, he had formulated ‘Pauling’s Five Rules’, to predict the crystal structure of ionic compounds; these were soon recognised to be as significant as Bragg’s Law.

  Figure 19.1 Linus Pauling.

  Pauling visited Europe again in 1930, where he was smitten by electron diffraction – analogous to X-ray diffraction, but able to dig even deeper into the structure of molecules. On returning to Caltech later that year, he built his own electron diffraction apparatus with Lawrence Brockway. He also sat down one evening in December to write a piece entitled The nature of the chemical bond’. This appeared as a forty-page essay in the Journal of the American Chemical Society; bolstered by six further instalments over the next couple of years, it evolved into the book of the same title. Published in 1939, this became a bestseller and, according to one authority, was a page-turner that outstripped detective novels. All this gave Linus Pauling his unparalleled knowledge of the intimate anatomy of the bonds that link atoms inside molecules. His predictions were astonishingly exact. He calculated the length of each bond to one-fiftieth of 1 Å, and the angles between adjacent bonds to within 2 degrees; all the components for his molecular models had to be precision-machined.

  Like everyone else, Pauling initially accepted Astbury’s ideas about the structures of keratin and other fibrous proteins – until Astbury visited Caltech during his American lecture tour in 1937. Pauling was left suspicious that something was badly wrong, but it took him over a decade to solve the problem. This was where the pencil and paper came in, helped by what must be the most productive bout of sinusitis in the history of science.

  In April 1948, while on a visiting professorship in Oxford, Pauling was stuck in bed with a head cold. Bored with reading science fiction, he decided to tackle the unsolved puzzle of what a linear sequence of amino acids in a simple polypeptide might look like in real life. From memory, he sketched out a hypothetical peptide, to scale and with the correct bond lengths and angles. He then cut out the strip of paper carrying the sketch and started playing with it, to see if he could translate the two-dimensional structure convincingly into the third dimension. By trial and error, he found that folding the strip with parallel creases, each cutting obliquely through the points of maximum flexibility in the molecule, brought together atoms that would be mutually attracted by hydrogen bonding – and so created a regular structure that would hold itself in shape. The effect of the parallel creases was to twist the flat strip of paper into a helix. In deference to Astbury’s time-honoured (but now erroneous) alpha-keratin, Pauling named this the ‘alpha-helix’. He quickly fleshed out the dimensions of the alpha-helix – radius 6 A, pitch 5.4 Å – and became convinced that this was a basic configuration into which stretches of varied amino acids would automatically snap, forming a common and important structural ‘motif’ in protein molecules.

  This was the biggest discovery of Pauling’s career, but he kept it quiet for a long time. This was because he was amassing evidence, and was in fierce competition to find the real-life shapes of proteins. His main rivals were what their boss called ‘a lot of clever young men and women’, working within ‘a corpus of X-ray crystallography, which was the strongest in the world’. The man making that grandiose claim was, appropriately enough, Lawrence Bragg, now chief of the Cavendish Laboratory in Cambridge.

  Pauling had already met Bragg and was not impressed. Bragg’s lab, then in Manchester, had been the first stop on his trip to Europe in 1930; he spent a few weeks there, and found the experience ‘a disappointment’. What they were doing failed to inspire him, and they were not interested enough in the discoverer of Pauling’s Five Rules to ask him to give a seminar. In May 1948, a month after the origami session that produced the alpha-helix, he again visited Bragg, now in the Cavendish. Bragg and his team were still locked into Astbury’s flat zigzags, and believed that the rounded contours of ‘globular’ molecules were made up of rodlike subunits of various lengths, stacked in piles to fill the mould. Pauling listened and watched, and said nothing. From what he observed, he could afford to bide his time.

  Back at Caltech, Pauling rubbed shoulders with Thomas Hunt Morgan’s colleagues from the Fly Room, Calvin Bridges and Alfred Sturtevant, as well as Theodosius Dobzhansky. Whatever they talked about, there was no reason for DNA to enter the conversation, because Pauling was interested in proteins – of which genes were made – and DNA was a molecule going nowhere.

  Crystal clear

  By the mid-1940s, X-ray crystallography had become a fully mature science. and one of those who had helped it through adolescence was the brilliant and erratic J.D. Bernal, who gave Bill Astbury his first taste of disappointment by beating him to the crystallography lectureship in Cambridge.

  Bernal’s time in Cambridge had been highly successful. He transformed ‘a few ill-lit and dirty rooms’ into a ‘fairy castle’ which he filled with ‘brilliance and boundless optimism’, and was promoted to Assistant Director of the Crystallography Laboratory. None of this made him palatable to the stuffier echelons of the university. One senior don observed that ‘No one with hair like that can be sound’, and it is unlikely that Bernal’s conspicuous Communist tendencies went down much better.

  At Cambridge, Bernal took the world’s first X-ray photograph of a crystalline protein, the enzyme pepsin.* This had recently been purified and was being studied in Uppsala, Sweden, where tiny but perfect crystals had formed in a tube of pepsin solution left in a refrigerator while its owner went skiing. These found their way to Bernal, who photographed them wet in their ‘mother liquor’ (the saturated solution in which they had grown). The result was a stunning X-ray diffraction pattern, so beautiful and so uninterpretable that a dazed Bernal spent the night ‘wandering about the streets of Cambridge, full of excitement’ at the possibility of using X-rays to look into the molecular structure of proteins. The paper about pepsin in Nature by Bernal and his PhD student Dorothy Crowfoot was the first ever publication about protein X-ray crystallography.

  In 1937, Bernal was elected FRS (three years before Astbury) and left Cambridge for the Chair of Physics at Birkbeck College in London. Birkbeck was no research powerhouse, but its egalitarian ethos – to enable anyone, irrespective of background, to obtain a university-level qualification – chimed with Bernal’s left-wing leanings. He had begun to rebuild his empire in a dilapidated house behind the main college building, when the war broke out. Despite being judged ‘as Red as the flames of hell’, he was asked to turn his hand to the defence of the realm. He discharged those duties with conspicuous distinction, but at a huge personal cost. ‘I was right out of science,’ he complained afterwards. When the war ended, he went right back into science, determined to make up for lost time.

  Bernal continued to respect the gentleman’s agreement that he had made with Astbury, sticking with ‘crystalline substances’ while Astbury occupied himself with ‘the amorphous
ones’. Astbury therefore took the fibrous DNA, while Bernal homed in on sub-components of DNA that could be crystallised. He began with the nucleosides, each of which consists of one of the four bases attached to the sugar deoxyribose. As it turned out, this was an astute choice.

  *

  Sven Furberg was a twenty-seven-year-old chemistry graduate from Oslo who joined Bernal’s group on a British Council scholarship in September 1947. His vision was broadened by this ‘extraordinary place’. Bernal’s department occupied 21 and 22 Torrington Square, a pair of four-storey town-houses in a Georgian terrace that had undergone extensive modification by the Luftwaffe; the blackboard in the top-floor lecture room of No. 22 covered a hole in the wall through which the rubble-carpeted void that had been No. 23 could be inspected. The labs were converted sitting- and bedrooms; on top of No. 21 was an acoustically transparent flat where Bernal lived and consummated his earthier passions, frequently and noisily. The atmosphere was, as Furberg remarked, ‘most stimulating’.

  Furberg spent his first few months learning X-ray crystallography and began his real assignment on April Fool’s Day 1948, when samples of cytidine (the nucleoside consisting of deoxyribose attached to the base cytosine) arrived from the remains of Masson Gulland’s lab in Nottingham. His brief was to work out the structure; Bernal’s strategy was to let him get on with it without interference. Furberg was ‘quiet, courteous and clever’, and attacked the problem with heroic energy. The cytidine crystals were tiny needles, just 3 mm long, and had to spend over five days in the X-ray apparatus; more heroism was needed to make sense of the images by dragging Fourier analysis into three dimensions.

  Indoctrinated by Astbury’s ‘pile of pennies’ model of DNA, Furberg had assumed that sugar and base were both flat and lay in the same plane, like two pieces of sheet metal welded together on a workbench. To his surprise, the X-ray diffraction photographs told a different story: deoxyribose and cytosine lay perpendicular to each other, like the covers of a half-opened book. Furberg’s preliminary findings appeared in Nature in July 1949, in a half-page note which included a drawing of the molecule and the promise to ‘publish later a more detailed account’.

  That eventually did happen, but not for years. To fill the rest of his two-year scholarship, Furberg busied himself with yards of copper wire, building skeleton models of cytidine (2 billion times life-size), and assembling them with the missing ingredient – phosphate groups – into a feasible shape for a stretch of the DNA molecule. He wrote up his work, was awarded his PhD and deposited a bound copy of An X-ray Study of some Nucleosides and Nucleotides in the Science Library of the University of London. Some of his findings were presented on 12 May 1950 at a meeting of the Faraday Society – by Harry Carlisle, Bernal’s second-in-command, because Furberg had already returned to Oslo and a new job that had nothing to do with DNA.

  Years later, Bernal confessed that Furberg’s work had been ‘grossly overlooked’, and accepted personal responsibility because ‘I was too preoccupied with other things’. In this regard, ‘Sage’ cannot be contradicted. Many of the ‘other things’ were ideological, not scientific, and nourished by his all-consuming admiration for everything Soviet. On a BBC radio debate in August 1948, Bernal had been non-committal about Lysenko’s new purge of 3,000 scientists accused of obstructing ‘proletarian science’; and when challenged directly about the imprisonment and death of Nikolai Vavilov, he had declined to comment.

  A year later, as Sven Furberg was preparing to leave London for Oslo, his supervisor was in Moscow as a VIP speaker at a peace conference (which took place a few days before the detonation of Russia’s first plutonium atom bomb). Bernal raised ‘stormy and prolonged applause’ (Pravda) for lauding Comrade Stalin as ‘the great protector of peace and science’, and for damning ‘capitalist science’, with its ‘weapons of mass destruction’, for bringing ‘no happiness, only torture and devastation’.

  ‘Sage’ rounded off his triumphant Russian tour with a visit to Trofim Lysenko, whom he praised as ‘a poet with a vivid imagination’ and ‘a scientist of the Darwin-Rutherford type’. And when Lysenko made it clear that he would continue to remove from office anyone with pro-Mendelian views, Bernal had nothing to say.

  The story so far (1950)

  The midpoint of the twentieth century was of more than numerological interest for the followers of genetics. Although the rediscovery of Mendel’s work had sprawled across a couple of years, 1950 was chosen as its fiftieth anniversary, and the event was marked in various ways.

  The Genetics Society of America organised a four-day jamboree in Columbus, Ohio. The Golden Jubilee of Genetics featured seventy-two scientific papers (‘on an unusually high plane’) by the likes of Mirsky and Muller, with respectful presentations on ‘The coming of Mendelism’ and ‘The heritage of Mendel’. There was also an exhibition of Mendel memorabilia curated by Hugo Iltis, his biographer – who forty years earlier had stood beside William Bateson in Mendelplatz, Brünn, at the unveiling of Mendel’s statue, paid for by Carl Correns’s fund-raising campaign.

  Looking back, a present-day historian has judged the Golden Jubilee ‘a publicity juggernaut’ and ‘an early example of Cold War political theatre’. Even at the time, it seemed over-extravagant, more of a glitzy showcase for Western genetics than a tribute to the neglected genius in his abbey garden.

  Those on the other side of the Iron Curtain responded in kind. Under Communist rule, Brünn had become Brno, in Czechoslovakia; abbeys had ceased to exist, and so had Mendel. On 20 September 1950, his statue was removed from its place on the square and dumped out of sight in a corner of the former abbey gardens.

  The last months of 1950 provided a career-defining honour for both Bill Astbury and Alfred Mirsky. Both were invited to follow in the footsteps of Phoebus Levene and Albrecht Kossel by giving one of the series of Harvey Lectures. Each performed true to form.

  Astbury went first, on 28 September 1950. En route, he had visited various American centres of excellence, including Erwin Chargaff at Columbia, but his lecture made no reference to that encounter. ‘Adventures in molecular biology’ was vintage Astbury: elegant, engaging and dotted with dreadful puns such as ‘spinning out’ the story of the ‘fabric of life’. He introduced himself as a physicist who had ‘gone all biological’ and been seduced by the ‘joys of molecular biology’ (‘a phrase I am fond of and have long tried to propagate, even if I did not invent it’).

  Virtually all of his forty-page lecture was devoted to the fabrics of nature – wool, the sailor’s eyeball Valonia, collagen – with excursions into permanently waved hair and garments woven from peanut protein. DNA merited less than a page, including the admission that ‘we have been unable to make as much progress as we should have liked’; what he had to say was much the same as it had been in 1946,1943 and 1938. He was much more excited to unveil his newest passion: the flagella of bacteria, which were bringing him great ‘molecular joy’. Astbury had moved on again, and DNA was receding into the past.

  Mirsky delivered his Harvey Lecture just before Christmas. He remarked that it was ‘striking’ how little impact chemistry had made on the chromosomal theory of heredity, but insisted that this was bound to change. His studies of DNA had extended across the animal kingdom – lungfish, geese, toads, man, sponges, worms and jellyfish – with detailed biochemical analyses of those cleverly isolated chromosomes. It was a comprehensive performance, but with some significant omissions. Strangely, Mirsky’s usual mantra that genes could only be proteins was missing. His only allusions to the composition of genes were the peculiar statements that ‘DNA is part of the germinal material’ and that ‘chromatin may form the nutritive material for the carriers of hereditary units’.

  Neither did he mention the studies published eighteen months earlier by W.P.G. Lamb of the new MRC Biophysics Laboratory at King’s College London, which showed (using electron microscopy) that Mirsky’s ‘isolated chromosomes’ were just shreds of chromatin junk that the kitchen blende
r had ripped out of smashed-up nuclei. And of course, Mirsky avoided any reference to Oswald Avery or his heretical notion that ‘genes = DNA’.

  Accessible but invisible

  Two young men could have rounded off this stage of the saga with an intriguing twist. Both were PhD students who were left in the lurch by their supervisors, and so never published or even presented some of their key findings. Their big ideas were ignored for three years in one case, and for half a century in the other.

  The first was Michael Creeth, one of Masson Gulland’s three last PhD students at University College, Nottingham. Creeth had little opportunity to savour his paper on the peculiar behaviour of DNA at extremes of pH. It was published during a time of catastrophe – Gulland’s abrupt resignation, then his death – and, with the disintegration of the research group, Creeth was desperately trying to find a new job. He applied to Cambridge, but they turned him down because his PhD was not from a proper university,† so he went to London to work on insulin.

  Creeth’s big idea could have been one of the most visionary offerings from the Nottingham group; instead, it remained invisible in the copy of his PhD thesis stored in the college library. Gulland may never have seen it, and Creeth’s PhD examiners presumably skipped over it. Certainly nobody was excited enough to encourage him to publish. Creeth had followed through the deduction that hydrogen bonds between bases were ‘instrumental in maintaining’ the structure of the DNA ‘particle’. He suggested that DNA was not a great pile of pennies with a phosphate backbone, but a series of short, straight stretches which partially overlapped. The result looked like a badly broken ladder (Figure 19.2). His flash of insight is contained in a simple sketch, squeezed in at the foot of a page, and a few lines of text: ‘The constituent chains are united down their common length by hydrogen bonding between the purines of one chain and the pyrimidines of the other, and vice versa.’ This was the first suggestion that DNA might be doubled-stranded – albeit broken up into short lengths – and that the strands could be bridged by hydrogen bonds linking a purine with an opposing pyrimidine.