Because of their varied nature, the purine-pyrimidine group [the bases] cannot be packed along the axis of the helix in such a way that suitable bonds can be formed between the sugar residues and the phosphate groups . . . It is also unlikely that the sugar groups constitute the core of the molecule . . . the shape . . . is such that close packing of these groups along a helical axis is difficult, and no satisfactory way of packing them has been found . . . We conclude that the core of the molecule is probably formed of the phosphate groups [emphasis added].

The arrangement now looked like this: The phosphate groups were arranged about the axis of the helix, with the sugars surrounding them and the bases projecting radially outward. The three-stranded molecule was held together by hydrogen bonds between the phosphate groups of different strands.

This structure looked promising, but Pauling still saw some problems. The center of the molecule now appeared to be so jam-packed by the three chains of phosphates that it resembled the “telephone booth squash”—the competition to cram as many people as possible into a telephone booth. Pauling knew that the phosphate ion was tetrahedral in shape, with the central phosphorus atom surrounded by four oxygen atoms positioned at the vertices of a pyramid. Throughout the month of December, he, Corey, and chemist Verner Schomaker were continuously trying to squeeze, warp, and twist those tetrahedra so that they would fit better. In this process, Pauling was following the same instincts that had led him previously to triumph with the alpha-helix. He believed that if he could find a structural chemistry solution that was generally consistent with the X-ray data, all other problems would sort themselves out later. For instance, there was a question of how the model allowed for the existence of a sodium salt of DNA, since there was definitely no room for sodium ions in the core. Pauling did not have an answer, but he assumed that one would be found once the main architecture was figured out. The pace of the work was frantic. Pauling even had a small group of scientists in his lab for an informal presentation of the model on Christmas Day. By the end of the month, he thought he had gotten it to be essentially right. Pauling and Corey submitted the paper, “A Proposed Structure for the Nucleic Acids,” for publication on the last day of 1952. The paper started, “The nucleic acids, as constituents of living organisms, are comparable in importance to the proteins.” A few phrases with a more cautious tone followed:

We have now formulated a promising structure for the nucleic acids . . . This is the first precisely described structure for the nucleic acids that has been suggested by any investigator. The structure accounts for some of the features of the x-ray photographs; but detailed density calculations have not yet been made, and the structure cannot be considered to have been proved to be correct.

In other words, even if some of the wrinkles still had to be ironed out, Pauling wanted to establish priority.