Following his trip to England and France in the summer of 1952, Pauling returned to Caltech in September. However, even then he was not ready to plunge back fully into the DNA problem. A conversation he had with Crick in England that summer gave him an idea of how he could finally resolve the puzzle of the protein reflection at 5.1 angstroms. As often happens in science, Pauling and Crick solved that problem independently, each showing that the alpha-helices could form coiled, ropelike structures around one another, those giving rise to the enigmatic signature. This had a nice ring of finality to it, but even though Pauling didn’t know it at the time, the “race” to solving DNA was entering the homestretch.

Pauling’s visit to France provided him with an additional clue to the fact that it was probably DNA, after all, that was the primary genetic material. The American microbiologist Alfred Hershey presented the evidence in a talk at an international meeting on viruses in Royaumont, near Paris. Hershey and his collaborator, Martha Chase, labeled the DNA and protein of the T2 phage (a virus) with radioactive phosphorus and sulphur, respectively. They then allowed the phages to infect bacteria, and were able to demonstrate that the genetic material that infected the bacteria was most probably DNA and not protein. The viral protein coat remained outside the bacterial cell and played no role in the infection. But not everyone was convinced. Indeed, Hershey himself remarked cautiously that it was not yet clear whether his result had any fundamental significance. James Watson, on the other hand, who was also at Royaumont and who had DNA in his crosshairs, was fairly convinced.

Pauling finally came back to DNA work toward the end of November 1952. This return was spurred by an intriguing seminar at Caltech by biologist Robley Williams. Williams showed amazingly detailed electron-microscope images of a nucleic acid salt—a chemical relative of DNA. To Pauling, the images of the long, cylindrical strands, coupled with Astbury’s X-ray diffraction photos, seemed to provide definitive evidence, if he needed any, for a helical molecule. Pauling also knew from the work of the organic chemist Alexander Todd that the backbone of the DNA molecule contained repeating phosphate and sugar groups.

Armed with Astbury’s photos, which showed strong reflections with spacing of about 3.4 angstroms, Pauling started to perform calculations of DNA structure on November 26. Based on the density measurements of Astbury and Bell and the diameter of the strand as measured by Williams, he estimated that the length of one residue along the fiber axis was 1.12 angstroms—almost precisely one-third of the spacing in the X-ray photo (of 3.4 angstroms). This led him to a surprising conclusion: “The cylindrical molecule is formed of three chains, which are coiled about one another . . . each chain being a helix.” In other words, having convinced himself that a two-stranded helix would yield too low a density, Pauling opted for a three-stranded helical architecture. This structure became known as the triple helix.

The next problem that he had to tackle concerned the nature of the very core of the three-chain helical design—that part of the molecule closest to the axis. The question was, which of the three known components of the nucleotides (bases, sugars, or phosphate groups) formed the core? Pauling and Corey went through a mental process of elimination: