The Electronic Structure of Haemoglobin 



unpaired electron, 1-73 magnetons, and accordingly are considered to 

 contain essentially covalent bonds, which utilize six of the outer 

 orbitals. These compounds include ferrihaemoglobin cyanide (2-49), 

 ferrihaemoglobin azide (2-84), ferrihaemoglobin hydrosulphide (2-26), 

 ferrihaemoglobin imidazole (approximately 2), and the ferrihaemo- 

 globin hydroxide-ammonia complex (2-98). 3 ' 4 ' 5 



The moment of ferrihaemoglobin hydroxide (alkaline methaemo- 

 globin) is observed to be 4-45. This value is approximately equal to 

 the spin moment for three unpaired electrons, 3*88 magnetons, and 

 the structure of this complex has been considered to be one in which 

 five of the nine outer orbitals of the iron atom are involved in covalent 

 bond formation, leaving only four orbitals for occupancy by the five 

 unpaired electrons, corresponding to three unpaired electron spins in 

 the complex. 



THE POWER OF COMBINING WITH OXYGEN 

 AND CARBON MONOXIDE 



It has recently become possible to formulate a structural explanation 

 of the power of specific combination of oxygen and carbon monoxide 

 that is possessed by haemoglobin. 6 This explanation is based on a 

 new postulate, the postulate of the approximate electrical neutrality of 

 all atoms in stable compounds. 7 We assume that in a stable molecule, 

 complex ion, or crystal the electronic structure is such as to associate 

 with each atom the number of electrons that makes the residual elec- 

 trical charge of the atom zero or, at the most, a small fraction of an 

 electronic charge. Thus in the hexahydrated ferrous ion, [Fe(OH 2 ) 6 ] + + , 

 each iron atom forms six bonds with the six oxygen atoms of the 

 surrounding water molecules, and each of the six bonds has about 

 one-third covalent character and two-thirds ionic character, as given 

 by the electronegativity values of iron and oxygen, iron having a value 

 about 1-5 on the electronegativity scale. 8 The formation of six bonds 

 with one-third covalent character, using electron pairs originally 

 belonging to the water molecules, transfers a total negative charge 2 — 

 to the iron atom, which is just sufficient to neutralize the positive 

 charge originally present on the ferrous ion. Similarly the ionic 

 character of the oxygen-hydrogen bonds then transfers the total 

 positive charge to the twelve hydrogen atoms on the periphery of the 

 complex. In the ferrohaem group of ferrohaemoglobin the bonds be- 

 tween the iron atom and the four nitrogen atoms of the porphyrin 

 ring have about 50 per cent covalent character, corresponding to the 

 difference in electronegativity of iron and nitrogen. These four bonds 

 between iron and nitrogen accordingly transfer two negative charges 

 to the ferrous atom, neutralizing its charge exactly, in the same way 



59 



