MACROMOLECULES 



115 



N 



H,0 



[sj Fe N 



H-O 



N 



CI 



N -- 



H3O 



N 



Fe- 



ci 



N 



+ H,0 



N 



Oxidation of the ferrous ion of a haemochromogen to a ferric ion will 

 give rise to a parahaematin. One may also obtain a parahaematin by the 

 combination of a ferri-porphyrin with nitrogen bases; however, the affinity 

 of ferri-porphyrins for nitrogen bases is much less than that of the ferro- 

 porphyrins. The parahaematins are slightly paramagnetic; they possess 1 

 unpaired electron ; the 6 coordination linkages of the complex are covalent 

 {dhp^) as in the ferricyanide ion, Fe(CN)6, which also has an unpaired 

 electron. Parahaematin has an octahedral form and can be written so : 



NR' 



4. Haemoglobin 



Combination of globin with a haem gives a haemochromogen of a 

 particular type : the ferrous ion retains all its 4 unpaired electrons so that 

 all the linkages in the complex are electrostatic in nature. 



Haemoglobin results from the union of protohaem and globin. The 

 linkage between the globin and the haem is between the ferrous ion and the 

 two imidazole groups of histidine residues in the globin, the latter replace 

 the two water molecules in the ferro-porphyrin. 



N 



; N / 



N- 



Fe-/'----N 



N' .^^ 

 N 



In the complex, haemoglobin, oxygen or carbon monoxide can displace 

 one of the imidazole groups; there is a consequent redistribution of 

 electrons (bringing 12 electrons to the ferrous ion) with formation of 



