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Francis only learned of DNA’s monoclinic space group through reading a nonconfidential King’s progress report sent to Max Perutz in mid-February. By then, through a new burst of model building, I had found that a sugar-phosphate backbone of 20-­angstrom diameter optimally repeats every 34 angstroms, the repeat distance measured in B-form DNA. Francis now argued, in light of Rosalind’s space group, that the two chains must run in opposite directions. But I didn’t initially buy this assertion, not understanding the underlying crystallographic symmetry argument. Until I knew how the centrally located bases bonded to each other, I didn’t want to worry about backbone directions. Then, unknown to me, my model building was being hindered by faulty textbook descriptions of the structures of guanine and thymine. Using such false configurations, I had become momentarily excited about a pairing scheme similar to that found in crystals of adenine.

That scheme, however, would have given a ­17-­angstrom repeat along the helical axis, not the ­34-­angstrom figure observed by Rosalind. Happily, the Caltech structural chemist Jerry Donohue, then spending his sabbatical year in Cambridge, set me on the right track by arguing that the guanine and thymine hydrogens should have keto rather than the textbook-ascribed enol configurations. Needing only a day to incorporate Jerry’s reasoning, I changed the locations of the hydrogen atoms on my paper-cutout models of thymine and guanine. Almost instantly I found myself forming the A-T and G-C base pairs we now know to exist in DNA. Coming a half-hour later into our office that Saturday morning, Francis took only a few minutes to conclude that the symmetry of the base pairs demanded that the chains run in opposite directions. Rosalind’s monoclinic space group was in a true sense a prediction of a model derived by Francis and me from purely stereochemical arguments. The double helix had to be correct. All that remained to be done was to build a backbone segment and measure its atomic coördinates to show that all the bond lengths and angles in our model agreed with those previously found in smaller molecules. This task, which for the first time in months took Francis away from his desk, took less than three days to complete. The double helix was ready to let loose upon the world.

Breaking the news to Wilkins that we very likely had solved the DNA structure was bound to cause his heart to spasm. A day after we had verified appropriate coördinates for all the atoms, a letter from him arrived informing Francis that Rosalind was out of King’s and that Maurice was about to resume work on DNA. Perhaps to soften the blow, John Kendrew, not Francis, called Maurice to report that Francis and I had a promising novel structure for DNA. Coming up the next day, Maurice instantly recognized the double helices’ elegant simplicity and agreed that it was likely too good not to be true. Knowing that we would not have found the DNA structure without knowledge of x-ray results from King’s, Francis and I suggested to Maurice that his name also be on the manuscript we planned to send to Nature. Without hesitation, he declined, possibly not knowing how to deal with Rosalind Franklin’s and Raymond Gosling’s equally important contributions. The April 25, 1953, issue of Nature, besides containing the 900-word description of our model, also included separate continuing contributions from the two warring DNA groups at King’s. Maurice was later to write that his refusal to publish jointly with the two of us was the biggest mistake of his life.

In every sense, solving the double helix was a problem in chemistry. Alex Todd facetiously told me that Francis and I were good organic chemists, not wanting to admit that a major objective in chemistry had been solved by nonchemists. In reality, Francis and I would not have been first to see the structure if Todd’s fellow chemists had not done botched jobs. Linus had all the keys to unlock the DNA structure but inexplicably didn’t use them that fall of 1952. Rosalind Franklin would have seen the double helix first had she seen fit to enter the model-building race and been better able to interact with other scientists. If she had accepted rather than rejected Maurice as a collaborator, the two of them could not have failed to realize the significance of the monoclinic space group. Dorothy Hodgkin’s Oxford put-down of Rosalind as a crystallographer would not have been the fatal wound that it seems in retrospect.

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Credit: Andreas Feininger/Time-Life Pictures/Getty Images

Tagged: Biomedicine

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