This was the kind of success that had made Avery’s reputation. By contrast, the lab’s intermittent research into the ‘established but poorly understood fact’ of pneumococcal transformation seemed dull and was not going anywhere obvious. Avery now conceded that the phenomenon existed but was unexcited by it, even after Lionel Alloway had isolated the all-powerful precipitate. The rest of the world was even less excited. In 1933, Wilhelm Baurhenn in Heidelberg had repeated Dawson and Sia’s experiments and confirmed their findings. Otherwise, there was a deafening silence; nobody even bothered to try out Alloway’s improved recipe.
The ‘transforming principle’ responsible for all the non-fuss was curious stuff. It could be filtered, precipitated and dissolved, just like any other chemical. But this was no ordinary compound, because after being forced to jump through all those hoops, it retained its extraordinary power to change forever the characteristics of a living organism, so indelibly that the change was transmitted faithfully to subsequent generations. In higher animals, the instinct would have been to think that the transforming principle had somehow altered the genes. In the 1930s, though, things that happened to bacteria were of no interest to geneticists; it was not even agreed whether or not bacteria possessed genes.
The biggest question posed by the transforming principle was simple: what is it? Today, the answer is obvious; but at the start of 1935, it would take another decade of stop-start research before its identity was revealed – thanks to what was later called ‘the pivotal discovery of twentieth-century biology’.
13
UP NORTH
Anyone assuming that Bill Astbury was thrilled to return to his roots in northern England would have been mistaken. Winning his lectureship in Textile Physics in Leeds did little to offset the disappointment of having lost the post in Cambridge to J.D. Bernal, his friend and sparring partner from the Royal Institution. Bernal had scooped the plum job, leaving Astbury with crumbs in the provinces and a high-risk post lashed together with money from (of all things) the Worshipful Company of Clothmakers.
In September 1928, Astbury wrote to Bernal in Cambridge with the ‘sad news’ of his imminent departure into the ‘wilderness’, adding that ‘it seems possible that I have abandoned crystallography’. In the conventional sense of the word, he had. His new career hung by a thread, seemingly as tenuous as the one which had started this whole unfortunate venture: the human hair which he had put into the X-ray camera for Sir William Bragg’s lecture.
Astbury’s misgivings were shared by Bernal, who was ‘shocked’ that Astbury was going into such a ‘completely complex and mundane field’. It was challenging enough to sort out what was going on inside the geometrically precise boundaries of proper crystals; Bernal thought it ‘premature’ of Astbury to try to make sense of fibres, whose very flexibility argued against a highly ordered structure. At the same time, Bernal admired his friend’s ‘essentially pioneering spirit’ and his unquenchable ‘impulse to wander into the unknown’. If anyone could do it, Astbury would be the man.
To mark the parting of their ways, they reached a gentleman’s agreement about their future research. Bernal would concentrate on real crystals, and Astbury on fibres and other amorphous materials.
Woolly thinking
At first sight, the Department of Textiles must have fulfilled all the prophecies of doom for the new lecturer. This was one of the biggest departments at Leeds University but was essentially a finishing school for those going into the textile industry; research was a small and neglected part of its portfolio. Astbury’s embryonic empire consisted of a bare room in a Victorian mansion near the cricket ground at Headingley, with money for an assistant and basic equipment. He soon discovered that this was not virgin territory: a junior researcher in the Chemistry Department had taken X-ray photographs of wool and presented his findings at a big meeting in Leeds the previous year. Never one to waste time on diplomacy, Astbury made it crystal clear that he was ‘highly disconcerted’; the interloper was swiftly moved to another project, leaving Astbury to lead the university’s new research programme into the molecular structure of natural fibres.
He began by taking his own look at wool, a commodity of huge economic importance to the region and a topic crying out for proper science. This turned out to be an excellent choice. Wool produced a string of high-impact papers and made him think about how the shape of molecules determined their function – the foundations for the novel field that he later christened ‘molecular biology’.
Like all mammalian hair, wool consists of the protein keratin, which also makes fingernails, hedgehog spines, porcupine quills, the horns of cattle and rhinos, and even whalebone. The ignorant had written keratin off as ‘lifeless and structurally dull’, but Astbury was quickly bewitched. The ability of wool to stretch reversibly fascinated him, and made him determined to find out what happened at the molecular level. He built his own X-ray camera, incorporating refinements that he had invented during his years at the Royal Institution. Hundreds of X-ray photographs later, wool had begun to reveal new facts about itself, and about proteins in general.
Unstretched wool showed a distinctive X-ray pattern, with a repeating structure every 5.1 A along the fibre. Astbury called this the ‘alpha’ configuration of keratin. Fully stretched wool also had a repeating pattern but with a shorter interval of 3.32 A, which he christened the ‘beta’ configuration. On releasing the tension, the wool returned to its resting length and the alpha pattern reappeared. This transformation excited Astbury, because it proved that stretching wool was more complicated than doing the same to a spring. The sequence of amino acids in the keratin molecule could not have changed, but the disappearance of the alpha repeat and its replacement by the shorter beta interval meant that the molecular structure of keratin had undergone a fundamental, yet reversible overhaul.
This was his first glimpse of the research theme that he pursued for the rest of his life – that their shape, not their chemical formula, is what enables molecules to carry out their particular functions. Astbury sketched out a two-dimensional zigzag structure for alpha-keratin and suggested that parts of it could snap open when stretched to produce a different configuration with a shorter repeat interval. This notion looked plausible, and was only proved wrong after it had helped to make Astbury’s reputation.
Keratin kept Astbury busy for several years. He found the molecular explanation for the permanent wave in hairdressing salons, and used the same trick to put a new bend into a cow’s horn. Then he discovered that birds and reptiles shared their own brand of keratin, structurally distinct from the mammalian version, which was identical in the feathers, claws and beak of a hen, the scales of a snake, and the shell of a tortoise. Astbury noted the molecular hint that birds and reptiles might share a common ancestor.
From there, he diversified into other fibres, natural and man-made. He was intrigued by the skin of the ‘sailor’s eyeball’ (Valonia ventricosa), a single-celled marine organism which is ‘most remarkable’ because it is as big as a grape. The skin consisted of cellulose fibres woven into a beautiful pattern that ran in different directions in adjacent layers, as clever as anything from the textile wizards of Leeds. Next came the tough and unstretchable collagen, which took him from shoe leather to the tendons around arthritic joints, the outer skin of earthworms and the stinging filaments of jellyfish. Not forgetting the egg-white protein albumen, which when poached turned into fibres with an X-ray diffraction pattern similar to that of beta-keratin. Astbury followed up this observation with a remarkable feat – transforming a soluble protein from cotton seed into insoluble fibres that could be spun like wool.
Astbury largely succeeded in his aim to inject intellectual rigour into textile physics and to make this one of the ‘fullest scientific activities of the university’. He built a large research group which brought excitement and credibility to this previously utilitarian field, with dozens of papers in top journals on the X-ray structures of natural fibres and their wider imp
lications for the biological properties of protein molecules.
He owed much to lucky timing, because he filled the vacuum caused by the implosion of his only serious competitor. The Kaiser Wilhelm Institut für Faserstoffchemie (Fibre Chemistry) had been established in Dahlem, Berlin in 1920. It was part of a network of over twenty Kaiser Wilhelm Institutes (KWIs) which included those for Chemistry, Physics and Biology, directed respectively by Fritz Haber, Max Planck and Carl Correns. During the 1920s, the KWI for Fibre Chemistry had monopolised the field and had used X-ray diffraction to probe the structure of silk and cotton. It had seemed unstoppable, until the Nazis began purging Jews from academia. The KWI lost its best researchers, and Astbury prospered. This brought him to the attention of the Rockefeller Foundation in New York, which had previously funded research at the KWIs. Astbury had failed to excite the English wool industry in his work; now, the Rockefeller began to pump in money.
During the 1930s, the Department of Textile Physics in Leeds became the world’s leading centre for the study of natural fibres – and was later described by the future Nobel laureate Max Perutz, one of those Jewish scientists purged from Germany, as ‘the Vatican of X-ray crystallography’.
Bill Astbury was a fascinating compendium of the traits that make brilliant scientists – and bad ones. He was a master of what might now be called ‘fuzzy logic’, with a knack for seeing what imperfect data were trying to tell him. An early example was his ‘brilliant idea’ (Bernal’s words) that a given molecule could exist in completely different shapes, each of which had distinct physical properties and biological functions.
All of Astbury’s colourful voyages were inspired, driven and rewarded by the thrill of discovery. However, this was risky behaviour, because he was a butterfly who flitted from one topic to the next; the irresistible curiosity which fired his creativity in so many directions also distracted him at crucial moments and stopped him from seeing that he was about to throw away a big trick. As Bernal said, ‘It was part of his personality to be so excited and delighted with the things he was finding out, to want to tell everyone about them and to publish them in all kinds of journals in all countries.’ Others were more critical: a ‘cream-skimmer’ who hoped to chance upon ‘something fairly spectacular’, and ‘an artistic amateur, not a scientific professional’.
Astbury’s personality was similarly an ebullient, maddening mixture of contradictions. He was a devoted family man who brought fun, ping-pong and music to his research group; but he also declared ‘I am alpha and omega, the beginning and end of the whole thing’ and thought that women scientists were often ‘marvellously conscientious and thorough’ but lacked the ‘creative spark’ of the male. His ‘unsinkable enthusiasm’ infected and inspired those around him, but also pushed him into places ‘where more anxious scientists might have feared to tread’. Bernal said that he was ‘always brimful of ideas but often these were rather difficult to understand’, and that ‘most people thought he spoke nonsense’. Like another product of the Trent Valley,* Astbury was either loved or hated. To Bernal, he was ‘someone who made you glad to be alive’. His detractors dismissed him as ‘possessive, inflexible and over-confident’.
One of Astbury’s most enduring qualities was a profound, almost religious willingness to surrender himself to the marvels of whatever he encountered. When he wrote ‘the wonder of it all flooded over me’, he could have been describing anything from the fine structure of the praying mantis’s egg-case to Beethoven’s Ninth Symphony. In this instance, it was his daughter knitting a cardigan from a novel ‘molecular yarn’, spun from a soluble peanut protein which he had tricked into forming fibres. ‘Ardil’, the weavable peanut protein, could almost have been a metaphor for Astbury’s research – a brilliant concept which did not quite realise its potential. It could be turned into an overcoat that looked terrific when modelled by a suave chap on a dry afternoon but it underperformed in real life; the seat of Astbury’s Ardil trousers quickly wore thin, while his son’s Ardil sweater would only have remained a good fit if he had also shrunk in the wash.
A question of preparation
The mid-1930s found Astbury approaching the long plateau of his career, content with his science, his reputation and the good things of life – and still oblivious to his weaknesses. In 1935, the arrival of a small package postmarked Giessen, Germany gave him the chance to demonstrate all his character traits, positive and negative.
The package was from Professor W.G. Schmidt and contained a peculiar fibrous material that Astbury had never seen before – thymonucleic acid. Astbury mounted a fibre in the X-ray diffraction camera and found an indistinct pattern, with a smear indicating that a structure of some kind was repeated every 3.34 Å along the molecule. He was unexcited and promptly returned to the familiar territory of fibrous proteins, about to be explored by Florence Bell, a recently arrived PhD student. Bell had a formidable pedigree: Girton College, Cambridge followed by crystallography training with both J.D. Bernal at Cambridge and Lawrence Bragg in Manchester. Like Kathleen Yardley at the Royal Institution, Bell was bright, spoke her mind and refused to be intimidated by Astbury. He referred to her as his ‘vox diabolica’ (devil’s advocate) and valued her stabilising influence as well as the technical prowess which had impressed Bragg.
Bell had almost completed the first part of her PhD on ‘protein multilayers’ when another sample of thymonucleic acid arrived in late 1937. This time, Astbury had asked for it, because new developments in the ‘chromosome business’ had recently seized his attention. Using an ultraviolet microscope, a young Swedish scientist called Torbjörn Caspersson had shown that nucleic acid levels in individual chromosomes increased dramatically as living cells divided. Caspersson and Rudolf Signer in Bern had also deduced new facts about the shape of thymonucleic acid from its physical and optical properties: it was an immensely long, thin cylinder, whose length was about 300 times its diameter, containing repeating structures stacked at right angles to its long axis. Careful analysis indicated that the stacked structures could only be the bases: adenine, guanine, cytosine and thymine.
This was exciting stuff, but far from Astbury’s interest. The thing that made him react was a letter from J.D. Bernal, revealing that he had performed X-ray diffraction on thymonucleic acid and found a longitudinal repeat with a periodicity of 3.34 A. Astbury wrote back, pointing out that this was an infringement of their gentleman’s agreement that Bernal would stick to crystals, and adding that he was ‘rather amused’ that Bernal had confirmed his own finding of ‘three or four years ago’, which he now intended to investigate ‘in more detail’. Having seen off Bernal, Astbury persuaded Caspersson and Signer to send him some of their highly purified thymonucleic acid and, even though it had nothing to do with ‘protein multilayers’, told Florence Bell to make it Part II of her PhD.
This was the purest thymonucleic acid yet obtained, thanks to a painstaking, carefully controlled extraction. It formed attractive ‘snow-white fibres of a peculiar consistency, like gun-cotton’, and was so viscous in concentrated solutions that it could not be poured. Bell came up with an ingenious method to stretch out those long, thin molecules side by side to give the X-rays the best chance of peering into their structure. Using a fan, she dried a puddle of the solution on a glass plate, then applied another layer of solution and repeated the process. This gave her a ‘beautifully opalescent’ film of thymonucleic acid which she cut into 2-millimetre strips with a razor blade; these were stretched up to twice their length and fixed in the X-ray camera. Her efforts, and the purity of the preparation, paid off. The X-ray photographs showed the same 3.34 A periodicity along the axis of the fibre, but stronger and clearer than before.
Astbury and Bell milked these ‘striking, though still rather obscure’ photographs as comprehensively as they could. Their findings tied in neatly with Caspersson’s predictions of a long, cylindrical molecule made up of hundreds of subunits – the bases – stacked on top of each other. The X-ray photographs now ad
ded detail to that picture: the bases, coupled to the sugar deoxyribose, formed the flat subunits which stuck out at right angles every 3.34 Å along the long axis of the molecule. The whole thing looked like ‘a pile of pennies’ – or more accurately, pairs of pennies (the sugar and base molecules) soldered together – suspended in zero gravity, and all 3.34 A apart. The third component of thymonucleic acid, the phosphate groups, would be joined into a strand that ran down the cylinder from top to bottom and formed the ‘backbone’ which held the whole contraption together (Figure 13.1).
From the known dimensions of the bases, Astbury and Bell were able to calculate the molecule’s vital statistics. The thymonucleic acid molecule was just 20 A in diameter but stood 6,000 A tall and contained almost 2,000 nucleotides (base+sugar+phosphate units). Its molecular weight – between 500,000 and 1 million – was much higher than Phoebus Levene had calculated, and comparable with the value which Caspersson had worked out from its physical properties.
These results – clean, believable, novel and consistent with independent data – were good enough for Nature. The paper by Astbury and Bell on ‘X-ray structure of thymonucleic acid’ was published on 23 April 1938, just three months after Caspersson’s paper on ‘Molecular shape and size of thymonucleic acid’ – and just three months before Astbury and Bell’s paper on the protein multilayers that had featured in Part I of her PhD.
Figure 13.1 The ‘pile of pennies’ structure proposed for DNA by Bill Astbury and Florence Bell in 1938.
Unravelling the Double Helix Page 18