20 January 1953
Dr. Jerry Donohue
The holidays and various activities have kept me from answering your letter of 16 December until now.
I asked Yakel to check over the calculations of the radial distribution functions for the a helix, and he has done so. He
has reported to me that the inter-atomic distances that were used seem to agree with those that he has calculated, to within
about 0.1 A. The coordinates that were used in the calculations are, in fact, not those given in our published table (which
refers to a 3.6—residue helix), but those obtained about a year earlier. They correspond to a 3.67—residue helix - the helix
with 11 residues in 3 turns.
I think that the smoothing function that we used was one that I devised on the basis of information that you gave me. It
involved spreading each point out over about 0.5 A to either side. I think that it is in between yours for B = 2 and B =
4. I sent details about our smoothing function to Riley, and it is likely that it does not differ much from the one that
he used. It is not clear to me whether the difference between our calculated curves and yours (and Riley's) is due to a
difference in the coordinates, or to something else. Perhaps we missed the double peak at 5 A for β carbon 2 because of an
error in one of our points.
Yakel, Schatz, and Rack have completed their work on the cylindrical distribution function. They have evaluated the cylindrical
distribution function for a keratin and also for collagen from the experimental fiber diagrams, and they have evaluated the
cylindrical distribution function for the a helix, using the coordinates for the 3.67-residue helix as the starting point.
There is only rough agreement between the theoretical function and the experimental function for a keratin. There is of
course, disagreement for collagen.
I expect that you have seen our article in NATURE, on the a-keratin proteins. We have made more detailed calculations, but
have not yet written the material up for publication in the PROCEEDINGS.
Did I tell you that Corey and I have sent a paper on the structure of nucleic acids off to the PROCEEDINGS for publication?
It will appear In the February issue.
As to Carlisle, I went over his material on ribonuclease with him and Bernal this summer. I reached the conclusion that
the Fourier diagrams that he obtains, and adduces as evidence against a helixes and in favor of a ribbon structure, are pure
artifacts - that there is no justification for assuming the signs of Fourier terms in the way that he does. There is, of
course, the possibility that
I misunderstood the argument that he presented, but I do not think that I did.
I hope to see you in Cambridge about 6 April - I plan to stop off for the weekend while on the way to Brussels.
With best regards, I am