Figure 12

About six weeks later, Pauling visited the Cavendish Laboratory at Cambridge, and what he saw there impressed him deeply. “They have about five times as great an outfit as ours,” he wrote to his assistant at Caltech, “with facilities for taking nearly 30 X-ray pictures at the same time.” Concerned that there was something still wrong with his model, and at the same time anxious that the Cavendish group might beat him to analyzing it, Pauling remained silent about the alpha-helix. Even during a discussion with the famous chemist Max Perutz, in which the latter showed him exciting new results on the structure of the hemoglobin crystal, Pauling decided to keep his ideas to himself.

The problem, however, continued to haunt him. Upon his return to Pasadena, Pauling immediately asked a visiting professor of physics, Herman Branson, to inspect his calculations carefully. Pauling was particularly interested to know whether Branson could find a third helical structure that would satisfy the restrictions of a planar peptide bond and maximum hydrogen bonding for stability. Branson and one of Pauling’s research assistants, Sidney Weinbaum, went over Pauling’s computations with a fine-tooth comb for about a year and concluded that there were truly only two structures—the alpha-helix and the gamma-helix—that satisfied all the constraints. Branson and Weinbaum also confirmed that the alpha-helix, which was the tighter of the two helices, was characterized by a distance of 5.4 angstroms between turns.

Pauling was now presented with the choice of whether simply to ignore the incongruity with the X-ray data and to publish his model or to delay publication until that conundrum was resolved fully. A paper submitted for publication to the Proceedings of the Royal Society of London on March 31, 1950, helped him decide.

The paper, entitled “Polypeptide Chain Configurations in Crystalline Proteins,” was written by an illustrious trio: Lawrence Bragg, Nobel Prize for Physics laureate in 1915, and two molecular biologists who eventually shared the 1962 Nobel Prize for Chemistry—John Kendrew and Max Perutz—all from the Cavendish Laboratory at Cambridge. At the time, this famous lab was the world’s leading center for X-ray crystallography. This method for analyzing crystals was largely Bragg’s baby; he and his father, Sir William Henry Bragg, together worked out the mathematics underlying the physical phenomenon and developed the experimental technique.