ROYAL INSTITUTION OF GREAT BRITAIN
WEEKLY EVENING MEETING
Friday, February 27, 1948

E. SLADE, M.C., D.Sc.
Treasurer and Vice-President, in the Chair

LINUS C. PAULING, Sc.D., Ph.D., Hon.M.R.I.
Professor of Chemistry.in the California Institute of Technology

THE NATURE OF FORCES BETWEEN LARGE
“MOLECULES OF BIOLOGICAL
INTEREST

As I look at a living organism—at one of you or myself—I see
reminders of many questions that need to be answered. Not all
of these questions are obviously important, nor would their
answers be useful—but we want them answered, anyway.
Thomas Wright in 1601 said, ‘Nothing is so curious and
thirsty after knowledge of dark and obscure matters as the
nature of man’.—of scientists especially, he might have said.
What is skin, fingernail? How do fingernails grow? How do
I feel things—how are nerves built and how do they function?
How do I see things? How can I smell things, and why does
-benzene have one smell and iso-octane another? Why is sugar
sweet and vinegar sour? How does the hemoglobin in my
blood do its job of carrying oxygen from the lungs to the
tissues? How do the enzymes in my body break up the food that
I eat, burn it to keep me warm and to permit me to do work,
and build new tissues for me from the food fragments? Why do
I catch cold when exposed through contact with an ailing
person, get pneumonia, and then recover after treatment with a
specific antiserum or a sulfa drug? How does penicillin carry
out its wonderful function of fighting disease? Why am I
immune to measles, whooping cough, poliomyelitis, small-pox,
whereas some other people are not? And finally, why is it that
my children, as they grow and develop, become human beings,
and show characteristics similar to mine, and their mother’s—
how have these characters been transmitted to them?
The basic answers of all of these questions are not to be
found in books. Even though Chaucer said

‘ For out of olde feldes, as men seith,
Cometh al this newe corn fro yeer to yere;
And out of olde bokes, in good feith,
Cometh al this newe science that men lere.’

FRO Baran . I
"Denko a aoe od tah,
PROFESSOR LINUS C. PAULING
he was before long corrected by Francis Bacon:
‘Books must follow sciences, and not sciences books.’

To understand all of these great biological phenomena we need
to understand atoms, and the molecules that they form by bond-
ing together. And we must not be satisfied with an understand-

ing of simple molecules—nitrous oxide, NNO, that Davy used
to produce hysteria; benzene, C,H,, discovered by Faraday;-
penicillin, containing 40 atoms in its molecule—we must also
learn about the structure of the giant molecules in living organ-
isms, such as insulin, with 2,000 atoms in its molecule, heemo-
globin, with about 10,000, the disease-producing viruses, a
thousand times larger still.

Just 2,000 years ago the Roman poet Lucretius wrote

‘Wine flows easily because its particles are smooth and round
and roll easily over one another, whereas the sluggish olive oil
hangs back because it is composed of particles more hooked and
entangled one with another’.

This is essentially the modern point of view, expressed by
Lucretius, of course, only as a surmise, whereas to us it is a
fact, of which we have detailed understanding. Lucretius went
beyond us in interpreting the taste also of substances in terms
of the shape of their molecules, writing:

‘There is this too that the liquids of honey and milk give a
pleasant sensation of the tongue, when rolled in the mouth; but
on the other hand, the loathsome nature of wormwood and biting
centaury set the mouth awry by their noisome taste; so that you
may easily know that those things which can touch the senses
pleasantly are made.of smooth and round bodies, but that on the
other hand all things which seem to be bitter and harsh, these are
held bound together with particles more hooked.’

I shall now present a detailed discussion of the shapes of
molecules, and the forces between molecules, in order that you
may see the extent to which this surmise too of Lucretius is
substantiated.

Even though hemoglobin is spoken of as a large molecule,
the molecule is very small in comparison with our usual scale.
If I look at my hand or watch a few feet away, the atoms
subtend the same angle that oranges would on the surface of the
moon. The molecules might thus be described as an aggregate
of oranges, held together by bonds which in actual molecules
are electrons circulating between the atoms. On this scale
of 250,000,000-fold linear magnification the nitrous oxide

2
LARGE MOLECULES OF BIOLOGICAL INTEREST

molecule of Davy’s laughing gas consists of three atoms in
line, N-N-O, the distance between the two nitrogen atoms
being about 1.12 inches and between the nitrogen and oxygen
atoms 1.19 inches (in the molecule themselves 1.12 a. and
1.19 A., where one A. is 1/100,000,000 of a centimetre).
Faraday’s benzene molecule consists of twelvé atoms, six
carbon atoms arranged at the corners of a regular hexagon,
with the carbon-carbon distance 1.39 A., and six hydrogen
atoms, in the same plane, at the corners of a larger hexagon,
the distance of each hydrogen atom to the carbon atom to
which it is attached being 1.06 a. The structure of penicillin,
determined by Mrs. Hodgkin and her collaborators, is also
known, as are the structures of many hundreds of simpler
molecules.

As yet the detailed atomic structure of no protein molecule
has been determined; but the vigour of the attack that is being
made and the increasing power of the marvellous methods of
investigation used in structural study permit the confident hope
that the next decade or two will bring the solution of this great
problem. ;

Much information is already available, however, about the
general sizes and shapes of large molecules of bialogical
importance, and also about their function. The most striking
general property of these molecules is the specificity of their
activity. For example, haemoglobin, the red protein in the red
cells of the blood, has the specific property of combining with
_ oxygen in the lungs, and then releasing the oxygen in the tissues,
thus making it available for the oxidation of foods. There is no
other substance known that is able to combine with oxygen
and release so large a fraction of the combined oxygen
reversibly with the small change in partial pressure of oxygen
existing between the lungs and the tissues. Then there are
the enzymes, such as those involved in the catalysis of the
processes of degradation and oxidation of food stuffs. These
substances all show a high degree of specificity in their action;
in general, a particular enzyme serves the purpose of speeding
up a particular single chemical reaction in the body.

Specificity is shown in a striking way by antibodies. These
are substances produced in the living body in response to its
invasion by foreign organic material, such as viruses or bacteria.
They have the power of combining specifically with the
material (the antigen) that led to their production, and of neutral-
izing or incapacitating the antigén. Thus an antitoxin is able

3
PROFESSOR LINUS C. PAULING

to neutralize its homologous toxin, an antibody against a virus
or bacterium is able to prevent the virus or bacterium from
reproducing, and thus to aid in the control of disease. An
unfortunate incidental effect of this protective mechanism of
the body is the occasional production of protein sensitization,
leading to asthma and hay fever. Sufferers from protein
sensitization, such as those people who are sensitive to egg
white (ovalbumin), can testify to the specificity of the reaction
and to its delicacy.

The specificity of serological reactions can be shown by
simple serological tests carried out in the laboratory. Thus if
a minute amount of ovalbumin is injected into a rabbit, it is
found that after a week or two the serum of the rabbit’s blood
will produce a precipitate when mixed with a solution of
ovalbumin, whereas the serum of an uninoculated rabbit will
not produce such a precipitate. The precipitate contains
ovalbumin from the ovalbumin solution that is added to the
serum, and antibody molecules from the serum of the rabbit.
These antibody molecules are protein molecules, with molecular
weight about 160,000, corresponding to the presence in the
molecule of about 20,000 atoms. These anti-ovalbumin mole-
cules have the power of combining with molecules of ovalbumin,
but in general not with molecules of other substances. Only if
another substance is extremely closely similar in its structure to
the ovalbumin with which the rabbit was originally injected
can the antibody combine with it. Thus a precipitate, somewhat
smaller in amount than with hen ovalbumin, is formed by
antibodies produced by hen ovalbumin when mixed with a
solution of duck ovalbumin, but no precipitate is formed when
these antibodies are mixed with a solution of ovalbumin from
the eggs of birds of other species. .

Dr. Karl Landsteiner showed by his experiments that animals
are able to manufacture antibodies specific to chemical groups
that do not exist in nature. The power of formation of anti-
bodies able to combine only with an injected antigen is hence

- not a power that was developed in the course of evolution, in
response to interaction with this antigen at some earlier age,
and in a particular individual called into operation by the
inoculating injection, but is instead a general power, which
enables the animal to cope with a foreign material, even of
completely foreign nature to the animal and its ancestors
throughout its course of evolution.

I became interested in the problem of the structure of

4
LARGE MOLECULES OF BIOLOGICAL INTEREST

antibodies in 1936, as the result of conversations with Dr.
Landsteiner. I found that the complex phenomena of immun-
ology could be clarified and brought into order by a theory of
the structure of antibodies based upon the idea of the folding
of the basic polypeptide chain of the protein précursor of the
antibody into the most stable of the configurations accessible to
it. This theory of the structure and process of formation of
antibodies involves the acceptance of the suggestion that
antibody and antigen have molecular structures that are
mutually complementary, originally made by Breinl and
Haurowitz, Jerome Alexander, and Stuart Mudd. If the
molecule of gamma globulin that might become an antibody
is considered to consist of a chain that might fold up into any
one of a large number of configurations, with essentially the
same stability, then in the presence of a molecule of antigen
that configuration would be selected which is stabilized by
interaction with the antigen. The configuration that would be
stabilized is the one in which there is the greatest force of
attraction ‘between the folded polypeptide chain and the
molecule of antigen, and this greatest force of attraction would
result from the assumption by the folding polypeptide chain of
a configuration which permits the parts of the chain to approach
as closely as possible to a portion of the surface of the antigen
molecule, which also brings positive charges in the chain into
close proximity to negative charges in the antigen, and brings
hydrogen-bond-forming groups in the chain into juxtaposition
with their complementary hydrogen-bond-forming groups in
the antigen. A very high degree of specificity can be obtained
if the surface area over which the complementariness of
structure is exercised is great enough to include a good number
of interacting structural units.

Experiments carried out with antibodies and antigens have
shown that the configuration of the antibody molecules is very
closely complementary to that of the surface of the homologous
antigen; the antibody reflects or reproduces, in a negative way,
the shape of the surface of the immunizing antigen to within
about 1 A., that is, to within about one-half or one-quarter of the
diameter of an atom. The forces that lead to the production of
the specific bonds bétween antibody and antigen are thus
interatomic forces that operate between atoms essentially in
contact with one another, and are not long-range specific forces
of attraction operating through great distances (on the atomic
scale) through space.
PROFESSOR LINUS C. PAULING

There is a highly specific phenomenon of the chemistry of
simpler substances that is closely analogous in its nature and
its cause to the highly specific phenomenon of serological
interaction; namely, the phenomenon of crystallization.
Chemists are accustomed to using the process of crystallization
as a method of purification: a crystal growing in a complex
mixture of molecules is able to select from the mixture just the
molecules of one kind, rejecting all others. Thus pure crystals
of sugar may deposit from a jam in which there are molecules
of thousands of different substances. The specificity of
crystallization is the result of the same striving toward com-
plementariness and the operation of the same interatomic and
intermolecular forces that are responsible for the specificity of
antibodies. A molecular crystal has the structure that gives it
the greatest stability, which would result from the maximum
amount of attraction for each molecule in the crystal and the
surrounding molecules. Each molecule in the crystal is then
in a cavity that conforms in shape to the shape of the molecule
itself. The molecule may be described as complementary in
structure to the remainder of the crystal, and other molecules,
with different shape and structure, would not fit into this cavity
nearly so well, and’ in general would not be incorporated in the
growing crystal. Hence we may say that life has borrowed from
inanimate processes the same basic mechanism used in pro-
ducing these striking structures that are crystals, with their

beautiful plane faces, their unfailingly constant interfacial
angles, and their wonderfully complex geometrical forms.

I believe that the same mechanism, dependent on a detailed
complementariness in molecular structure, is responsible for all
biological specificity. I think that enzymes are molecules that
are complementary in structure to the activated complexes of
the reactions that they catalyze, that is, to the molecular
configuration that is intermediate between the reacting sub- —
stances and the products of reaction for these catalyzed
processes. The attraction of the enzyme molecule for the
activated complex would thus lead to a decrease in its energy,
and hence to a decrease in the energy of activation of the
reaction and to an increase in the rate of the reaction.
Although convincing evidence is not yet at hand, I believe
that it will be found that the highly specific powers of self-
duplication shown by genes and viruses are due to the same
intermolecular forces, dependent upon atomic contact, and the
same processes of replica-formation through complementariness

6
LARGE MOLECULES OF BIOLOGICAL INTEREST

in structure as are operative in the formation of antibodies
under the influence of an antigen. I believe that it is molecular
size and shape, on the atomic scale, that are of primary
importance in these phenomena, rather than the ordinary
chemical properties of the substances, involving their power of
entering into reactions in which ordinary chemical bonds are
broken and formed.

Even though the general picture of some important biological
processes is becoming clear, our present knowledge of the
detailed structure of the complex substances of biological
importance is vague. We may expect that as more precise
information about the structure of these molecules is obtained in
the future a more penetrating understanding of biological
reactions will develop, and that this understanding will lead
to great progress in the fields of biology and medicine.

[L.C.P.]