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.