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Letter from Linus Pauling to H. Tarver. April 3, 1944.
Pauling refutes Tarver’s suggestion that proteins are stabilized by a unique form of resonance, citing the asymmetrical bond structures and the nature of conjugated methylene groups as evidence against Tarver's claims.

Transcript

April 3. 1944

Dr. H. Tarver

Medical School

Division of Biochemistry

University of California

Berkeley 4, California

Dear Dr. Tarver:

I have thought some more about the structure which you suggest for proteins—that is, about your suggestion that a special sort of resonance say stabilize the molecule when the side chains R1 and R2 are alike. I do not think that there is any chance that this effect is significant. In the first place, the NHO hydrogen bond is already unsymmetrical, because the two atoms nitrogen and oxygen are different; hence there is no extra symmetry introduced in the bond by making the side chains R1 and R2 identical. I think that this fact alone rules out your suggestion. However, it is unlikely that any effect of great magnitude would result from the nature of the side chains R1 and R2, since these groups are in substituted methylene groups, which do not enter very effectively into conjugation or other interaction with adjacent groups. It is, of course, possible that the side chain would exert a small inductive effect on the adjacent NH and OO groups, and thus produce a small effect on the strength of the hydrogen bond. I do not see any reason for this small effect to be more important when the groups R1 and R2 are identical than when they are different. Hence, although I would like to accept some such simplifying assumption about the structure of proteins, I am afraid that your proposal can not be given theoretical support.

Sincerely yours,

Linus Pauling

LP:jr

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