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Letter from Alexander Rich to Linus Pauling. August 8, 1955.
Rich writes to inform Pauling of the details of a collagen structure that he and Francis Crick formulated.

Transcript

Dear Dr. Pauling,

I want to tell you of a structure for collagen that Francis Crick and I have made.

We started with three polyglycine chains, each with a left handed three-fold screw and hydrogen bonded together as in the polyglycine structure which I described to you in my previous letter. We rotated these to produce a coiled-coil model, this time in a right handed manner. The original repeat (3.1 Å) then falls to 2.86 Å along the axis. This structure repeats after 28.6 Å, each chain having then gone one third the way around. There are ten residues in each chain or a total of thirty residues in the crystallographic repeat. One chain repeats identically after 85.8 Å.

The structure which we have built has the sequence –gly–pro–hypro– repeating. The glycine nitrogen makes a hydrogen bond to another chain through the center of the structure the hydroxyproline OH makes a bond on the periphery of the model. The sequence predictions from the model are such that gly is necessary, and the hypro is preferred in a site just before the gly in order to make its hydrogen bond to one of the other chains. Proline can be between the gly and hypro but any other amino acid can fit in as well. The same is true for the hypro site, since only the van der Waals contact between the -carbon atom in that position and the adjacent chain is used in producing the coiling of the coils.

I am enclosing two photographs of the model which we have built. The pentagonal cards are the proline residues. I must apologize for the photographs, as even I find them somewhat confusing. However, I will send you a proper description in a short time.

We think the model will give the proper diffraction pattern, as we believe this is similar to the Ramachandran-Kartha structure, although modified. We have heard that their model gives an approximately correct diffraction pattern.

We have looked closely at the infra-red data and do not think there is a conflict as long as there is not too much proline and hydroxyproline.

We have heard that someone in the laboratory in Pasadena is working on a proline peptide. We would like to have the dimensions for the proline residue if they are available, or perhaps your estimate of what its dimensions would be.

I will send you a fuller account of the model shortly.

I hope that things are going pleasantly in Pasadena.

With Best regards,

Yours sincerely,

Alex

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