114 



UNITY AND DIVERSITY IN BIOCHEMISTRY 



This substitution reaction is accompanied by a complete rearrangement 

 of the structure of the complex. Measurements of susceptibiHty now indi- 

 cate that the haemochromogen no longer possesses any unpaired electrons : 

 it is diamagnetic. The two molecules of nitrogen base and the four por- 

 phyrin nitrogen atoms have shared their electrons (12 in all) with the ferrous 

 ion; there are no remaining unpaired electrons and the 6 coordination 

 bonds of the complex are covalent (d^sp^). The haemochromogen has the 

 shape of an octahedron with the ferrous ion at the centre and the nitrogen 

 atoms at the 6 corners. The 4 nitrogen atoms of the porphyrin molecule are 

 in the same plane. The structure of the haemochromogens is exactly the 

 same as that of the ferrocyanide ion, Fe (CN)6, which is also diamagnetic. 



A haemochromogen can be written thus : 



N 



NR 



N 



3. Haematins and parahaematins 



Oxidation of the ferrous ion of haem gives rise to a ferri-porphyrin which 

 is given the name "haematin". An oxygen molecule can bring about this 

 oxidation. 



The ferric ion has 5 unpaired electrons. Ferri-porphyrin also possesses 

 5 unpaired electrons ; hence all the bonds in the complex are of an electro- 

 static character. The complex, in which the ferric ion has a coordination 

 number of 6, can be represented thus : 



N 



N 



OH 



N 



HjO 



• Fe N + 



N 



H^ 



I 



I 



I -' 



.1/ 



HoO 



N Fe N 



H3O 



N 



The equilibrium depends upon the pH; addition of HCl will displace it 

 to the right with the formation of the chloride of ferri-porphyrin which can 

 be represented as a mixture of the two forms in the following equilibrium : 



