I was thunderstruck by Pauling and Corey’s paper. In contrast to Kendrew’s and my helices, theirs was free of strain; all the amide groups were planar and every carboxyl group formed a perfect hydrogen bond with an amino group four residues further along the chain. The structure looked dead right. How could I have missed it? Why had I not kept the amide groups planar? Why had I stuck blindly to Astbury’s 5.1 angstrom repeat? On the other hand, how could Pauling and Corey’s helix be right, however nice it looked, if it had the wrong repeat? My mind was in a turmoil. I cycled home to lunch and ate it oblivious of my children’s chatter and unresponsive to my wife’s inquiries as to what the matter was with me today.

Thinking a bit more about Pauling’s model, Perutz realized that the alpha-helix resembled a helical staircase, in which the amino acid residues (marked by “R” in figure 12) were forming the “steps.” The height of each step was about 1.5 angstroms. Bragg’s X-ray diffraction theory therefore predicted the existence of never-before-reported X-ray reflection signatures, separated by 1.5 angstroms, from planes perpendicular to the fiber axis. None of the models of Bragg’s group would have produced such a mark, while this would have been a distinct “fingerprint” of Pauling’s alpha-helix.

Just as he was about to conclude that the lack of such reflections in Astbury’s data was sufficient to refute Pauling’s model, Perutz suddenly recalled that Astbury’s particular experimental setup—with the fibers oriented such that their long axes were perpendicular to the beam of X-rays—would not have really allowed for the detection of the 1.5 angstrom signature. Rather, calculations predicted that the optimal conditions to observe the reflection would have required inclining the fibers at an angle of about 31 degrees.

Perutz felt absolutely compelled to make the crucial test right away. He cycled back to the lab, grabbed a horsehair he had in a drawer, inserted it into the apparatus at the angle he calculated to be favorable for detecting the reflection, put a film around it (as opposed to Astbury’s flat-plate camera, which was too narrow and could have missed reflections deflected at large angles), and fired the X-ray beam. The few hours that passed before he could develop the film were sheer agony, but finally Perutz had the answer. The strong reflection predicted by the alpha-helix at a spacing of 1.5 angstroms stuck out unambiguously!

Perutz showed the X-ray photograph to Bragg first thing on Monday morning. Bragg wondered what it was that suddenly gave Perutz the idea to conduct this crucial test. Perutz replied that he was madly furious with himself for not having thought of the alpha-helix. Bragg retorted with what has by now become an immortal phrase: “I wish I had made you angry earlier!”