23 December 1952
Dr. Jerry Donohue
Cavendish Laboratory
Free School Lane
Cambridge, England
Dear Jerry:
I am interested to learn about the helixes that you have been working
on.
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.
Dr. Donahue
Page 2 23/12/52
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
Sincerely yours,
Linus Pauling:W
P.S. I return your drawing herewith. LP
