Pauling put DNA aside until November, when he saw an article on its structure by Edward
Ronwin. It started him thinking again about how DNA might be built. The four building
blocks of DNA, called nucleotides, were known to consist of a sugar attached to a
phosphate group and a large, flat ring structure called a base. Unfortunately, no
one had yet published a good structure for any nucleotide.
Ronwin in his proposed structure put the phosphates down the middle of the molecule,
with the flat bases sticking out to the sides. This was certainly possible - Astbury's
x-ray photos did not rule out such an arrangement and it would solve a major problem.
The four bases of DNA came in two different sizes: two larger purines and two smaller
pyrimidines. Say that it was a helix, Pauling thought, as Astbury's photos indicated
it might be. Trying to arrange the different-sized bases on the inside of a long helical
molecule would create all sorts of fitting and stacking problems. Facing the bases
out would make the molecule easier to work with.
Ronwin, Pauling quickly saw, had made a basic mistake concerning his phosphates at
the core. His structure was not possible. But the idea of bases facing out, phosphates
in the middle, was appealing and fit the available x-ray data.
Pauling got that far, then stopped.