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Letter from Linus Pauling to Irving Langmuir. June 2, 1939.
Pauling responds to Langmuir's questions regarding the polypeptide chain theory, discussing each question in detail. He suggests that most answers to Langmuir's questions can be found in The Nature of the Chemical Bond and, accordingly, Pauling is having a copy sent.


June 2, 1939

Dr. Irving Langmuir

Associate Director

Research Laboratory

General Electric Company

1 River Road

Schenectady, New York

Dear Dr. Langmuir:

Some of the questions raised by your letter of May 17 can be discussed conveniently with reference to my book on "The Nature of the Chemical Bond," a copy of which is being sent to you by the Cornell University Press. I shall discuss your points one by one, with the paragraphs of this letter corresponding to those of your letter. The first question, regarding the general accuracy of the bond energy treatment, is a difficult one to answer. It is found empirically that a single value for a bond energy usually holds to within about 1 kcal/mol for all non-resonating molecules, but the carbonyl bond shows greater variability probably because of the large partial ionic character of the bond, as discussed on page 123 of the book. The instability of cyclols is so very great that this uncertainty can hardly be important.

I have assumed that a hydrogen atom forming a hydrogen bond between two oxygen atom is closer to one oxygen atom than to the other because there is strong experimental evidence showing this to be so for many substances, as discussed in Chapter IX of my book, and no evidence at all indicating that the hydrogen atom is midway between two oxygen atoms in any hydrogen bond. The structures of the type which you discuss, involving resonance of a covalent bond from one side to the other of a symmetrically placed hydrogen atom, have been proposed by Sidgwick, Gillette, and Sherman, and others, but the evidence is all against them.

Dr. Irving Langmuir -2- June 2, 1939

It seems to me that the entropy factor can hardly work to stabilize a cyclol, because the completely ordered arrangement of the cyclol corresponding to low entropy and hence to instability.

This is also the reason that the discussion of the relative energy values of polypeptide chain and cyclol structures is significant with respect to their free energies, since a polypeptide chain structure would presumably be favored by the entropy factor with respect to the cyclol structure.

It seems to me that there is a fallacy in your argument regarding the stability of a chain and a fabric. If the products of breakdown of the two are the same, then we know from thermodynamics that the chain will be the more stable if its free energy is less than that of the fabric.

The effect of symmetry on melting points is very interesting, and is presumably due in part to an entropy factor. I think that the cases quoted by you, the dihydroxy benzenes and trihydroxy benzenes, are instead to be attributed largely to hydrogen bond formation within the molecule for molecules such as pyrocatechol, as discussed on page 289. There is an effect on the boiling points as well as the melting points.

I think that in proteins there is some tendency for hydrophilic groups to be on the outside and hydrophobic groups be on the inside, but that this is not carried to the extreme shown by the cyclol cage-like structures.

Although interatomic forces do decrease rapidly with distance, effects may be transmitted from group to group, as in a crystal, which retains a regular structure over long distances because of the coordinating action of each atom or molecule or its immediate neighbors. This might lead to a distribution of side chains corresponding to the symmetry of the molecules.

Dr. Irving Langmuir -3- June 2, 1939

It would seem surprising to me then that the insulin crystal would not reflect the tetrahedral symmetry of the cyclol molecule. With respect tothe polypeptide chain, I feel that there is evidence for periodicity of some residues but that the evidence does not show that all residues correspond to this regularity.

I do not see any difficulty with the postulate which Mirsky and I made regarding the uniquely defined configurations of polypeptide chains.

Sincerely yours,

Linus Pauling


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