|The Secret of Photo 51
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.