Dear Dr. Pauling,
I thought you might like more information about the collagen work that Francis and I are carrying on. I am enclosing diagrams of polyglycine II, and collagen I, plus the approximate coordinates of collagen structures I and II.
The polyglycine II needs no comment. However, as there are two different types of triads of polypeptide chains as viewed in figure 1, this can give rise to two different types of models for collagen. Both of these models are based on a repeating sequence of –gly–pro–hypro–, where both pro and hypro sites can be substituted by other amino acids. In the collagen I structure, the hypro forms a hydrogen bond across to one of the other chains (collagen figure 2); in the collagen II structure, the hypro ring sticks out away from the axis, and forms no systematic inter chain bond.
We have taken optical transforms of both structures, with and without side chains and water. The results fit the collagen diffraction pattern reasonably well, although not perfectly.
The van der Waals contacts in both structures are comfortable, although the work in refining structure 1 is much greater than in structure II.
We are at the present time trying to make up our minds about the relative virtues of these two models. We feel that they fit the chemical sequence data quite well, and model I points to a specific role for the hydroxyproline residues.
John Rollett visited in Cambridge recently, and remarked that the orientation of the polypeptide backbone in these collagen models is similar to the configuration of the tripeptide leu–pro–gly which Dick Marsh has been working on. We would be eager to know whether he has carried along his refinement to the stage of approximate coordinates.
I would be eager to hear your comments about these structures for collagen.
You have probably noted that Ramachandran’s structure in the recent Sept-24th Nature has a van der Waals contact of 3.3Å between glycine Cα’s, as wel as a Cα to O contact of 2.6Å. I think that makes his model somewhat unfavorable.
I continue to see a great deal of Peter and Linda, both of whom are quite busy just now.
With best regards, yours sincerely