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
