The Electronic Structure of Haemoglobin 



LINUS PAULING 



// is possible to explain the power of specific combination with 

 oxygen and carbon monoxide possessed by ferrohaemoglobin, by 

 means of the postulate of the approximate electrical neutrality of 

 all atoms in stable compounds. It is shown that the iron atom in 

 ferrohaemoglobin itself is made approximately neutral by the bonds 

 to the nitrogen atom of the porphyrin group, and that accordingly 

 only a molecule that can form covalent bonds with the iron atom 

 without transferring a large amount of electrical charge to this 

 atom would be expected to combine with ferrohaemoglobin. Oxygen, 

 carbon monoxide, cyanide ion, and the alky I isocyanides have such 

 structure as to permit them to combine with ferrohaemoglobin 

 without a large change in the electrical charge of the iron atom, 

 and accordingly these molecules, and not substances such as water 

 molecules, chloride ions, hydroxide ions, etc, are expected to form 

 ferrohaemoglobin compounds. 



During the past twenty-five years great progress has been made in 

 the development of a detailed theory of molecular structure, both by 

 the application of experimental methods of determining the arrange- 

 ment of atoms in molecules and crystals (the methods of spectroscopy 

 and of x-ray diffraction and electron diffraction) and by the application 

 of quantum mechanics to the problem of the electronic structure of 

 molecules and the nature of the chemical bond. In addition, much 

 new experimental information has been obtained about haemoglobin 

 and haemoglobin compounds, which has been combined with the 

 theory of molecular structure in an effort to explain the chemical 

 properties of haemoglobin in terms of its structure. Let us consider 

 to what extent this effort has been successful. 



THE ENVIRONMENT OF THE IRON ATOMS 



Each of the iron atoms in haemoglobin occupies a position at the centre 

 of a square formed by the four nitrogen atoms in the porphyrin ring 

 system. It is accordingly in a position to complete its Werner co- 

 ordination complex by ligating to itself two more atoms, one above 

 and the other below the plane of the porphyrin molecule, as was 

 suggested by J. B. Conant 1 . Investigation of the magnetic properties 

 of haemoglobin and haemoglobin derivatives has verified this structure. 

 It was found 2 that haemoglobin (ferrohaemoglobin) is paramagnetic 

 and has approximately the same magnetic moment as the hydrated 



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