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
LP/jr
