L. Pauling, The oxygen equilibrium of hemoglobin and its structural interpretation, Proc.
Natl. Acad. Sci. USA, 21, 186 – 191 (1935).
L. Pauling, C. D. Coryell, The magnetic properties and structure of hemoglobin,
oxyhemoglobin and carbonmonoxyhemoglobin, Proc. Natl. Acad. Sci. USA, 22,
210 – 216 (1936) .
Having read that Faraday found blood to be diamagnetic despite iron and oxygen being
paramagnetic, he told his student Charles Coryell to measure the magnetic
susceptibility of haemoglobin in the presence and absence of oxygen. In 1936, Coryell
found oxyhaemoglobin to be diamagnetic, and deoxyhaemoglobin to be paramagnetic
with a spin of S=2. Forty years later I asked Pauling what made him think of this
experiment which later turned out to have been crucial for an understanding of
haemoglobin's function. Pauling replied that it had not been clear whether oxygen formed a
chemical bond with the iron in haemoglobin or whether it was merely being adsorbed, and he
thought that formation of a chemical bond might be accompanied by a magnetic change.
He chose to study hemoglobin because blood was easy to obtain from
animals and it could be crystallized. This indicated that the hemoglobin’s structure
could possibly be solved with x-ray diffraction (Hager, 1995).
First, Pauling decided to study the heme portion (red iron
containing pigment) of hemoglobin by investigating its magnetic behavior. Pauling and his
colleague, Charles Coryell, found that when oxygen loses its unpaired electrons it
specifically, and covalently binds to iron in hemoglobin. They also discovered that when
iron binds to oxygen in the hemoglobin, its bonds switched from ionic to covalent in its
prophyrin or ring like cage. Their discovery proved that physical chemistry could be
applied to problem solving in biochemistry, and Pauling became even more renown
In 1931, Pauling introduced a magnetic criterion of bond type for
transition metal coordination compounds, together with hybrid atomic orbitals for
stereochemically defined bonding. In the ‘ionic’ complexes of the transition metals
all five of the d-orbitals were available for occupation by unpaired d-electrons with
parallel spins, whereas in the corresponding ‘covalent’ complexes one or two of the
d-orbitals were unavailable, being employed in square-planar or octahedral hybrid
formation. Accordingly, for a number of d-electron configurations, measurements of the
magnetic moment arising from spinparallel d-electrons distinguish the ‘ionic’ from the
‘covalent’ complexes of a given transition metal ion.
Magnetochemical measurements directed by
Pauling at Caltech in 1935 showed that the iron (n) complex haemoglobin of red blood cells
had four parallel-spin electrons per haem unit, corresponding to an ionic complex of
ferrous iron (with the db configuration). The addition of either carbon monoxide or oxygen
produces a covalent complex, with all electrons spinpaired. This is a remarkable result
for oxyhaemoglobin, since a molecule of free oxygen carries two unpaired electrons. The
electronic structures of both the haem and the oxygen are profoundly reorganised on
binding. Thereafter Pauling and his coworkers investigated
further types of haemoglobin derivatives, and those of related biomolecules, myoglobin,
haemocyanin, and the cytochromes, moving on to the problem of the chain-folding of the
globulins and other proteins.