23 December 1952
Dr. Jerry Donohue
Free School Lane
I am interested to learn about the helixes that you have been working
As to the helix 3.010, this is the one that is described in our paper with Branson, as corresponding to the rotational angle 120° (which gives
3.0 residues per turn), end as having the same orientation as the α helix. Dr. Branson concluded from his very complicated
analytical treatment that the hydrogen bonds are badly bent. Also, the crude model that he made indicated similar bad bending,
although our new models show that the strain in the hydrogen bond is small - there is a bending by about 40° at the oxygen
atom, which, however, is of course allowed. Our van der Waals models indicate that the helix is too tight, introducing steric
hindrance along the axis, but of course van der Waals radii are not very reliable. Also, I am reasonably sure that this is
the structure that Huggins discussed, as having three residues per turn; it is, I think, the one that he thought, at the time
of the New York meeting in the fall of 1951, to be the α helix. He did not discuss the α helix.
I am sure that the helix 4.416 the one that Barbara low has described. We had made an investigation of it, and some months ago I
asked Dr. Yakel to calculate its radial distribution function, using the
helicometer. I do not think that he has yet made this calculation - he
decided to rebuild the helicometer, and has put it into operation only
recently. In our considerations about 4.416, carried out before we
know it was Barbara Low's structure, we decided that the strain should be relieved mainly by twisting the amide group from
the planar configuration, rather than increasing the α -carbon angle. Probably the strain is relieved partially in each
of these places.
We have not made any study of 4.414, which is like the gamma helix, but twisted more tightly. (The series of α helices has subscript 3n+4, and the series of
gamma helixes has subscript 3n+5.) The amount of strain is so great - you say 7° on the α -carbon angle and 6° twist of the
amide group - that this configuration should, we think, be ruled out. I think, although I am not sure, that it is mentioned
under the angle 108° in our table - this corresponds to 3.314, but we were putting all of the strain into the hydrogen bonds. Perhaps 108° represents 3.311, with 3.314 left out because of its deviation from our assumed structural features.
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As to your publishing a description or discussion of these, I would say that it might be well worth while for a complete discussion
to be published, about all of the α helixes (N3n+4) and all of the gamma helixes (N3n+5). I would say that an effort should be made to find the best configuration for each of these, dividing the strain among the
different structural features in the way that minimizes the strain energy. In my paper for the Ninth Solvay Congress, which
should be available soon (Bragg will have a copy, probably in about one month, or I can send you part of the manuscript),
I give energy expressions for stretching and bending covalent bonds, twisting the amide group (the same expression as before),
van der Waals interactions - at any rate van der Waals attraction. I think that it is van der Waals repulsion, steric hindrance,
that might be the hardest to take into consideration. I would say that the time has passed now for piece-meal discussion
of these structures. Of the three that you mention, 4.414, which is probably the worst of the group, is the only one that has [sic]
I doubt that a discussion of these three structures is worth a letter to the J.A.C.S. - that is, I think that a more detailed
discussion should be published, at sometime, rather than a letter. Since so much discussion of polypeptide chains has been
published in the Proceedings of the National Academy of Sciences, I suggest that you consider submitting a paper there, that
is, sending the manuscript to be submitted to the Proceedings. Your paper could be up to six printed pages long. Why don't
you consider expanding it, to include all of the α helices that are at all reasonable, and all of the gamma helixes that are
at all reasonable?
I think that it would be fine for you to publish radial distribution curves, in each case with two alternative positions for
the β carbon atom, unless one is clearly ruled out by steric hindrance, as it seems to be for 3.010.
Filey has pointed out that our radial distribution curves for the α helix are not very accurate. I haven't tracked
down the error, but I think that we may have smoothed the points out a bit too much. You might check with Riley and Arndt,
rather than with us, to see whether you get agreement for the α helix.
Dr. Corey and I have been rather disturbed by the delay in publishing our communication to Nature about the structure of the
α -keratin proteins. I wrote to the editors of Nature on 2 October, saying that we were anxious to have our manuscript published
quickly, and asking if Nature would be interested. The editors replied at once that they would be, and we submitted the
manuscript on 14 October, eight days before Crick's manuscript was submitted. I have now written the editors asking what
has gone wrong, and saying that it seems to me that editorial policy should have led to the publication of the two communications
in the same issue of Nature, or that, at any rate, the one that was submitted earlier should not be published at a considerably
later tine. Dr. Corey and I are hoping that ours will not be delayed so long that it will have a 1953 date.
Dr. Donahue Page 3 23/12/52
As to your idea of coming back here, in order to increase the chance of your getting a permanent appointment in California,
I think that it can be arranged. Dr. Corey and I are embarking on a new project, the determination of the structure of nucleic
acids and nucleoproteins. We hope to have a short paper on the structure of nucleic acids published within two months -
we are not going to send it to Nature. In connection with this work, we want to have some precise structure determinations
made of phosphate diesters, nucleosides, nucleotides, and so on. I have written to Bragg about this matter, in order that
we shall not be making structure determinations on the same substances as are under investigation in the Cavendish. We have
not yet got a grant for increasing this activity next year, but I think that we shall get it, and we should know within two
or three months. As soon as we hear, we can make a definite offer of appointment to you, as Senior Research Fellow in Chemistry,
at salary fit the rate of $5400 per year, with the obligation of working on the structure of these substances.
With best regards, I am
P.S. I return your drawing herewith. LP