To add to the mystery, none of these agents affected the oxygen equilibrium of myoglobin or of isolated subunits of hemoglobin.
We now know that all the cooperative effects disappear if the hemoglobin molecule is merely split in half, but this vital clue was missed.
The inconsistency between the 5.4 and 5.1 Å continued to worry me until one morning about two years later when Francis Crick arrived at the lab with two rubber tubes around which he had pinned corks with a helical repeat of 3.6 corks per turn and a pitch of 5.4 centimeters.
He showed me that the two tubes could be wound around each other to make a double helix such that the corks neatly interlocked. Perutz works at the Medical Research Council's Laboratory of Molecular Biology, Cambridge CB2 2QHQ, UK, with support from the U.
As soon as I put the light on I found a strong reflection at 1.5 Å spacing, exactly as demanded by Pauling and Corey's a-helix.
The reflection did not by itself prove anything, but it excluded all alternative models that had been put for-ward by ourselves and others and was consistent only with the a-helix.
The zoologist explained how disturbances introduced into these regular patterns pointed to their formation being governed by some kind of gradient.
Bragg listened attentively and then exclaimed: Your disturbed gradient behaves like a stream of sand running downhill and encountering an obstacle.
And how could as heterogeneous a collection of chemical agents as protons, chloride ions, carbon dioxide, and diphosphoglycerate influence the oxygen equilibrium curve in a similar way?
It did not seem plausible that any of them could bind directly to the hemes or that all of them could bind at any other common site, although there again it turned out we were wrong.