On November 25, 1952, three months after returning from England,
Pauling finally made a serious stab at a structure for DNA. The
immediate spur was a Caltech biology seminar given by Robley
Williams, a Berkeley professor who had done some amazing work
with an electron microscope. Through a complicated technique he
was able to get images of incredibly small biological
structures. Pauling was spellbound. One of Williams's photos
showed long, tangled strands of sodium ribonucleate, the salt of
a form of nucleic acid, shaded so that three-dimensional details
could be seen. To Pauling the strands appeared cylindrical. He
guessed then, looking at these black-and-white slides in the
darkened seminar room, that DNA was likely to be a helix. No
other conformation would fit both Astbury's x-ray patterns of
the molecule and the photos he was seeing. Even better, Williams
was able to estimate the sizes of structures on his photos, and
his work showed that each strand was about 15 angstroms across.
Pauling was interested