Crick and Watson's first attempt to solve the structure of DNA in the fall of 1951 was brief and unsuccessful. Thinking like
Pauling, they quickly came up with a model of three DNA strands wound around each other in a helix, phosphates at the core.
It seemed to fit the density data, the x-ray data was compatible with anything from two to four strands per molecule, and
it solved a theoretical problem. If DNA was the genetic material then it had to say something specific to the body; it had
to have a language that could be translated somehow into the making of proteins. It was already known that the sugars and
phosphates were simple repeating units, unvarying along the DNA strands. The bases were the variables. The bases varied, but
the x-ray pattern indicated a repeating crystalline structure; ergo, the core - the part of the structure giving rise to the
repeating patterns - must contain the repeating subunits, the sugars or phosphates, with the bases sticking out where they
would not get in the way.
The only major problem was explaining how one could pack phosphates into the middle when at normal pH they would be generally
expected to carry a negative charge. All those negative charges at the core would repel each other, blowing the structure
apart. The triple helix they had devised was so pretty, though, and fit so much of the data that Crick and Watson figured
there had to be a place for positive ions at the core to cancel out the negative charges. They grabbed a copy of Pauling's
The Nature of the Chemical Bond, searched for inorganic ions that would fit their needs, and found that magnesium or calcium might fit. There was no good
evidence for the presence of these positive ions, but there was no good evidence against it, either. They were trying to think
like Pauling, after all, and Pauling would certainly have assumed - as he had with the alpha helix - that the structure came
first and the minor details fell into place later.