2020/02/01 by Thoru Pederson · 1 voice
Arts and Humanities · Psychology · #Twentieth Century Scientific Developments #Science Education and Perceptions
paper · pdf · doi:10.1096/fj.202000119
Last December a friend of mine attended a showing of the film The Secret of Photo 51 at the New York Academy of Medicine and conveyed to me the lively audience discussion that ensued, of which she was a co-moderator. As many readers know, this film, produced by the Public Broadcasting System for its acclaimed NOVA series, described the rapid, momentous events that led to the idea, published April 25, 1953, that DNA is a double helix. (I deploy the word “idea” as it was not much more than such at the time—though certainly a spellbinding one). The film derives its name from a particular X-ray diffraction photo obtained in May, l952 at King's College, London by the gifted crystallographer Rosalind Franklin. This photo was, months later, surreptitiously conveyed to her competitor James Watson, working with Francis Crick in the Cavendish Laboratory in Cambridge, and was critically important to their evolving ideas and model building. This part of the story is familiar to many, and the photo has become iconic, both as regards the information it held but also as a symbol of the dashing manner by which Watson and Crick were conducting themselves. But the story is more complex than this photo, important though it was. There was a second event that was even more catalytic for the Cavendish group's success. How the DNA double helix was “discovered” has been treated meticulously by numerous historians, most ably in my opinion by Robert Olby.1 In the context of this editorial, we need only to consider the key elements, and the central one is Rosalind Franklin (Figure 1).2, 3 A native of London, she had done superb work on the structure of coal and graphite in Paris from 1947 to 1949 but soon sought to broaden her horizons. At the time, she already enjoyed a high reputation as an exceptionally able crystallographer, based on a suite of seven important papers she published in a short period, all in excellent journals at the time in this field.4-10 She applied for a position at Birkbeck College in London where biological structure was an active program but was turned down. Searching for other positions in England she learned of the biologically oriented structural work being conducted at King's College in London. If there was one person in her nascent career who deserves credit for encouraging her it was Charles Coulson there. In March 1950, she visited King's at his invitation and was introduced to John Randall (Figure 2), the head of the Medical Research Council's Biophysics Unit. Randall was duly impressed and invited her to come, for the express purpose of working on DNA, a project that was currently headed by Maurice Wilkins and his student Raymond Gosling. Randall did not inform Wilkins of Franklin's appointment nor that he had told her that the DNA project would be hers and, moreover, that she would take over advising Wilkins' student. Adding to this inexplicable set of events and the predictably vexed ambience it would create, was the fact that Wilkins was on vacation when Franklin arrived and got set up. An irony here is that among those who had initially encouraged Randall to interview Franklin was none other than Wilkins himself. Upon joining King's, in a short time Franklin obtained excellent X-ray diffraction pictures of DNA fibers, better than almost any that had been gotten before by Wilkins or others elsewhere, and then, made a major discovery. Knowing from her previous graphite work that the relative humidity can often influence the crystalline structure of molecules, and thus the diffracted X-ray pattern, she explored the variable of humidity and found that the DNA fibers were bimorphic, existing as either so-called A- or B-forms. Francis Crick's graduate work in physics had been interrupted by World War II. Among his assignments was one in which his astuteness for understanding structure shone and portended things to come: divining the shapes of German ships' hulls by analyzing the wakes they made as recorded in British aerial reconnaissance photos. After the war he joined the Cavendish to pursue a PhD on the structure of hemoglobin, then a major effort of the laboratory. In the fall of 1951, the American James Watson arrived, having decided that this was the best place in the world for the study of biological structure by X-ray diffraction (without question correct). As has been widely chronicled, the two “connected,” to say the least. Crick had a gifted eye for structure as enabling function (having once said that if one can't figure out function, studying structure will lead there) and Watson, trained in the genetics of bacteriophage, had a roaring, inflamed desire to get the structure of the gene. They set out to learn everything they could about DNA and tossed around all sorts of ideas and then annihilating most of them through intense Socratic warfare, trying to deduce the structure by the sheer dint of their cerebral foreplay and constitutional brilliance, the latter present at uncommon levels. The only lab work they did, and it was important, was to try and build models of DNA that accommodated both the extant information as well as ongoing results that they could access. It did not go well at first. Notwithstanding their own fits and starts, they were also terrified that Linus Pauling at Caltech would beat them. He was one of the greatest chemists in the world and, having discovered the protein alpha helix, and also considered as the greatest structural biologist. Pauling indeed promptly published a model of anhydrous DNA but it was immediately seen as implausible on certain stereochemical grounds, as well as the fact that water molecules interacting with the phosphate groups, as in vivo, would give a lower density of DNA than Pauling used. (As far as I am aware, historians have never come up with an explanation of how Pauling made this obvious blunder.) Soon, Watson, and Crick also built (but did not publish) a model. Like Pauling's, it had three chains and the bases sticking out. Franklin and Wilkins and other members of the King's team were invited to come and see it. Franklin's biographer Brenda Maddox wrote “The gross miscalculation was obvious. Rosalind wasted no time on pleasantries”.2 This denouement was more than just an embarrassment for Watson and Crick, it played a major kinetic role in the race. This is because the director of the Cavendish, Sir Lawrence Bragg, was so humiliated that he forbade further DNA work. (Adding to this Shakespearean drama was the fact that Bragg had not recovered from Pauling scooping the Cavendish on the alpha helix). Throughout the remainder of 1952 Franklin continued to study both the A- and B-forms. In November, she gave an in-house seminar at King's, attended by Watson. By this time, he and Crick had become keen about the possibility that DNA, whatever its number of chains, might well be helical. Watson took no notes at the seminar but did recall certain details, which he verbally shared with Crick. However, the historical record is clear that nothing in Franklin's seminar was critically important to their model building. But soon, the key event took place. In January 1953, Franklin told her student Raymond Gosling to give what has now become known as photo 51 to Maurice Wilkins. The previous summer she had informed Randall that she would be leaving on January 1. Gosling would not be moving with her and would still need a thesis, likely returning to Wilkins as adviser. Wilkins, having worked on the X-ray diffraction of DNA for years was not only struck by Franklin's generous sharing this photo given their ice-cold relationship, but of course could instantly see (Figure 3) that the pattern was demonstrative of a helix. (Franklin had been decidedly anti-helical throughout much of 1952 but by November had decided that at least the B-form was helical, with bets still off on the A-form). Franklin's passage of photo 51 to Wilkins, via Gosling, was an act of considerable generosity, perhaps eased by the fact that she was soon moving on to the structure of tobacco mosaic virus at Birkbeck College (where she had once been turned down) and may not have felt the same burning desire to get the “structure of the gene” as her competitors. What she did not know, was that Wilkins would soon show it to Watson. If he and Crick had been leaning toward a helical structure, this photo was certainly confirmation. But it turns out that photo 51 did not contain the most critical piece of information. Randall had recently been requested to prepare a report of his unit's work for a committee that had been appointed to take a look at work in the field of biophysics at the several MRC-funded sites around England. He assembled the reports of his unit, including that of Franklin, into a document and provided it to the committee during their visit on December 15. One of the committee members was Max F. Perutz at the Cavendish Laboratory (Figure 4), a pioneering X-ray crystallographer of hemoglobin. Sometime in the second week of February 1953 Perutz showed this report to Crick. I shall return to this act itself but first let's see what Crick learned. The section of the report on Franklin's work contained her determination of the relative humidity at which either the A- or B-form predominate, and how this changes the fiber repeat period. Franklin's report also gave added momentum to the view that the phosphates lie on the outside (which earlier electrochemical titrations had suggested). But neither of these parts of Franklin's report were news to Crick, in the least. But something else was revelatory. Crystals, whether in solid form or a wet fiber (as in the case of DNA studied by X-ray diffraction) can come in at least of 230 different conformations, also known as space groups. Their nomenclature and geometric depiction is beyond the scope of this essay. A large set of these forms have regions that are parallel or anti-parallel and as regards DNA, whether the chains (if 3 as in the rejected Pauling and first Watson and Crick models, or any other number) run in the same or opposite directions was unknown. But right there in Franklin's report, were space group values (Table 1) that immediately told Crick that the structure was anti-parallel. This is because the data given describe a unit cell that has a geometry that possesses what is termed called face-centered monoclinic C2 symmetry. Crick's leap required the assumption that this symmetry came from the chains and not other parts of the molecule but given the high plausibility of this assumption, it was virtually certain that the chains ran in opposite directions. That, and that alone, was the breakthrough for it laid the stereochemical foundation for the base-pairing, which Watson would then promptly get (vide infra). And there is irony here. The very reason Crick was familiar with the C2 symmetry and it being a telltale sign of anti-parallel strands, was because he had encountered such a region in his ongoing studies of hemoglobin, this occurring in an intramolecular fold-back region, but nonetheless a helix. Indeed, Crick had laboriously worked out how to solve the so-called phase problem when diffraction is from a helix, since the standard Patterson functions do not apply. Thus, technically a graduate student at the time, he had the good fortune to be in a perfect position to understand what Franklin could not possibly have seen in the pattern. Armed with Crick's insight, all that remained was for Watson to get the base-pairing (after a misstep corrected by a lab member). The anti-parallel structure they got from Franklin's section of the MRC report allowed for the base pairs to be neatly accommodated internally, with each of the two purine:pyrimidine pairs occupying such a similar amount of space as to not pucker the two strands of the helix. And of course, they and virtually everyone one else who looked at the model they built in early April, 1953 saw the templating for replication. It had all come together in just a few weeks. I have long pondered these events and have here emphasized the MRC report because photo 51 has always held center stage. Both were important but I contend the MRC report was far more enabling. I have also thought more and more over the years about key players and offer the following opinions. As to Franklin: She was enormously gifted as a crystallographer and made spectacular progress on DNA in very short order, this being the first biological molecule she had encountered. Her experimental deftness was clear from the rapidity of her progress as well as her intellectual power to deeply penetrate an entirely new field so rapidly. She had been told by Randall that the DNA project was hers, and hers alone. Any detractors of her role, and there are some (including Watson), must recognize that. By November 1952, she had given up her anti-helix stance, and was very close to getting the double helix (for the B-form). She lost for two reasons. She had not encountered the monoclinic C2 space group, as had Crick. The second reason is even more poignant. Due to the personalities of her competitors, she innocently did not know that she even had competitors until late in the game, or that they were conducting a different kind of science. As to Randall: Historians have long labored over the manner in which he “installed” Franklin. From all accounts, and this is an essential part of the story, Wilkins thought he would be welcoming a new team member and, as I mentioned earlier, had indeed recommended that Randall recruit her.11 That Wilkins was shy, perhaps especially with a very smart, bold woman like Franklin, (perhaps unlikely for him to have encountered in his world) should not obscure the fact that he recognized her talent. This is an element of the story that is often lost. But why did Randall then leap so far beyond Wilkins' suggestion and install Franklin to take over the King's DNA work? And without there being any record of him sharing this decision with Wilkins. One here has the sense of an additional part of the story, perhaps now beyond the maw of any historian. The most definitive account of the situation at King's is one by Horace Judson,12 in which, inter alia, he emphasizes that Franklin was by no means the first talented female scientist to be appointed by Randall. I think getting to the roots of Randall's decision to displace Wilkins on the DNA work, and even reassign his student (!) might be latent in records at King's and that this would be a fine PhD thesis for a motivated student. As to Perutz. There are many mysteries here. He likely received the MRC report before the December 15, 1952 site visit but perhaps didn't read it until arriving there that day. (If so, he would not have been the first committee member to adopt that practice of brushing up at the very last minute). It was 2 months before he decided, for whatever reason, to show the report to Crick. Why? We don't know but I suspect that during that period he had become acutely aware, as most all at the Cavendish were, that Watson and Crick were hot on the scent. He was their colleague, had known Crick for years and had become, I suspect, most impressed with Watson in the shorter period. He also was a member of this center that, by 1952, many throughout the world of science saw as the kingdom of structural biology, as subsequent Nobel Prizes would soon attest. Perhaps not as aggressive as Watson and Crick, he was nonetheless a member of the Cavendish. An expatriate of Vienna, and not sharing British boys school competition experiences or other such traditions with many of his Cambridge colleagues, it is not unreasonable to think that Perutz hoped the structure of DNA would be solved there, and not at King's. Years later, in a Scientific American letter13 and a contemporaneous one in Science,14 having sought counsel from many friends and colleagues, Perutz tried to explain. His main contention was that the report was not confidential. Given that Randall had responded to the MRC's request for a report on his unit, at least he, Randall, knew that it would describe unpublished work and Perutz certainly knew this too, upon inspection. And yet, it was not marked “Confidential” and Perutz clearly did not regard it as such. But another claim of Perutz was that the report did not contain anything Watson and Crick didn't already know, either from the King's seminar Watson attended, or via information Crick may have gotten from Wilkins. But that is not true. Until the moment Crick's eyes saw the report, neither he nor Watson knew of the monoclinic C2 symmetry and, as I have emphasized, that was everything. There is one final point. In the aftermath, Watson wrote that he suspected that Crick had asked Randall to show him the report. Clearly, this is speculation. I would say that a request vs. Perutz proactively offering it is different. But Francis is gone and we shall not likely ever know. Here I have reflected on (pun intended), but hopefully not deflected, two cardinal elements: the brilliance of Rosalind Franklin and the perhaps one time in a life of unsullied conduct15 when Max Perutz erred to a degree, for whatever reason.