Linus Pauling and the Race for DNA: A Documentary History Narrative  
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The Secret of Photo 51
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There has been much speculation that the May denial of a passport cost Pauling his chance to solve the structure of DNA. In fact, the passport issue caused a minor delay with little likely effect. After significant public protest, Pauling was granted a limited passport to travel to England just ten weeks later. He went, visited the same places he would have in May, talked with the same people, and had the same opportunities. But he paid no attention to DNA at all. Proteins were still in the forefront of Pauling's mind. During a month in England, Pauling thought so little about DNA that he did not even make an effort to visit King's College to see Wilkins and Franklin's increasingly valuable x-ray photographs. The reason was twofold, he later remembered: He was preoccupied with proteins, and he still assumed that Wilkins did not want to share his data.

It was a historic mistake. Franklin had new pictures now, photos 51 and 52, crisp, focused patterns from DNA in its pure, extended, wet form, clearly showing both twofold symmetry - thus ruling out three-stranded structures-and the cross-like reflections of a helix. If Pauling had seen these - and there was no reason to think she would not have shown him; she had, after all, shown Corey her work during his May visit - if he had talked to Franklin, who was not shy about presenting her strong ideas about DNA, and had capsized an early Crick-Watson model based on three intertwining strands, he would undoubtedly have changed the nature of his later approach. At the very least, a visit with Franklin would have impressed upon him that Astbury's earlier photos, the ones he was using, showed a mixture of two forms of the molecule.

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Crystallographic photo of Sodium Thymonucleate, Type B. "Photo 51." May 1952.

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Letter from Linus Pauling to Alexander Todd. May 12, 1952.

"Compared with all previous B patterns that Franklin had obtained, these two pictures were vivid, No. 51 especially so. The overall pattern was a huge blurry diamond. The top and bottom points of the diamond were capped by heavily exposed, dark arcs. From the bull's-eye, a striking arrangement of short, horizontal smears stepped out along the diagonals in the shape of an X or a maltese cross. The pattern shouted helix."

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