In November 1952, after returning from England, Linus Pauling attended a Caltech biology seminar where Robley Williams presented electron microscope images of sodium ribonucleate strands that appeared cylindrical. Estimating each strand at about 15 angstroms across, Pauling concluded that DNA was likely helical and, using Williams's density estimate, calculated exactly three polynucleotide chains per unit, leading him to propose a triple‑strand model with the phosphates on the inside. He noted that he still lacked decent x‑ray images and firm structural data on the base‑sugar‑phosphate building blocks, but proceeded with the available information. The seminar sparked his interest in pursuing a structural model for DNA.
After developing the model with sketchy and sometimes incorrect data, Pauling became convinced of the triple helix structure. Years later he expressed astonishment at his own work, stating: 'I am now astonished that I began work on the triple helix structure, rather than on the double helix.' He reflected that he had initially expected two strands if the fibres represented genes, but the density calculation led him to three. This acknowledgment shows that he recognized the model as a mistake and wished he had pursued the double helix instead.
The historical note observes that Pauling 'had no Rosalind Franklin to set him right,' implying that Franklin’s understanding of DNA’s structure was correct and could have prevented his error, although the supplied text does not detail Franklin’s own evidence or when she arrived at the helical insight. It only suggests that her insight would have corrected him, without describing the basis of her conclusion. The note does not provide any direct description of Franklin’s experimental work, such as her famous Photo 51, nor does it indicate when she formulated the correct helical model.