17th August, 1951

Dear Professor Pauling,

I am sending you a reprint of my recent paper together with my warmest congratulations to your 3.7 residue helix. As you know, your first paper with Corey and Branson led me to predict that your helix, if present in all fibrous proteins and artificial polypeptides of the alpha-keratin type, should give rise to a reflexion at 1.5A spacing when the fibre axis is inclined at an equal angle to the incident and diffracted rays. The fulfilment of this prediction and, finally, the discovery of this reflexion in haemoglobin has been the most thrilling discovery of my life. The reflexion has now been found in a variety of substances in addition to those mentioned in my paper, and there is no doubt that it is a universal feature at least of all fibres of the alpha type. Whether all crystalline proteins show it remain to be seen. Ribonuclease apparently does not.

Kendrew wrote to me that you disagree with my conclusions concerning your feather rachis structure. Perhaps I should have mentioned in my paper, not only the absence of the 1.5A reflexion, but also the strength of the well-known reflexion at 3.07A. This is much stronger in oscillation pictures than in the orthodox pictures with the beam normal to the fibre axis, and suggests that most of the fibre consists of chains with this repeat rather than a repeat of 1.5. The structure proposed by you and Corey would also be hard to reconcile with the perpendicular dichroism of the infra-red absorption bands reported by Elliott et al. Is it impossible to account for the equatorial reflexions in terms of a pure beta-structure? It seems much more likely to me that an explanation in these terms will eventually be found. I also wanted to draw your attention to a magnificent X-ray photograph of feather keratin which Bear showed me in April 1950 and which he does not seem to have published. This contained far more reflexions than any other picture I have seen. The spots on it are arranged as though they come from an array of two-dimensional lattice, all the row lines being spaced at roughly equal intervals of ξ. It seems to me that it should not be too difficult to account for this picture in terms of some model of the "pleated sheet" type.

With kind regards and congratulations to yourself and Dr. Corey.

Yours,

Max Perutz

P.S. I enclose a further sickle cell paper.