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L'incontro 'magnetico' con l'emoglobina


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.

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