MACRO MOLECULES 



113 



It is obvious that the reaction is pH-sensitive, in acid solution the 

 compound is completely dissociated. The loss of the two protons from the 

 porphyrin molecule leaves behind two negative charges which, by means of 

 the system of conjugated double bonds, are distributed by resonance over 

 the four nitrogen atoms. 



The ferrous ion is paramagnetic ; it has 4 unpaired electrons. Likewise, 

 the haem molecule is also paramagnetic and measurements of magnetic 

 susceptibility have shown that, like the ferrous ion, it has 4 unpaired 

 electrons; this shows that the nitrogen atoms of the porphyrin bound to 

 the ferrous ion do not share any electrons with it; the bond is ionic. In the 



CH 



CH, 



COOH GOGH 



Protohaem 



ferro-porphyrin complex, the ferrous ion has a coordination number of 6 : 

 in the complex, it retains two water molecules by means of ion-dipole bonds. 

 The 6 bonds of the complex are, therefore, electrostatic in nature. 



Ferro-porphyrin can be represented : 



N- 



N- 



Fe++ 



N- 



or 



N -- 



N 



I 

 I 



I 

 I 



I / 

 — Fe - 



H2O 



N 



H,0 



I 



N 



2. Haemochromogens 



The water molecules in the ferro-porphyrins are readily replaced by 

 nitrogen bases; the name "haemochromogens" is given to the new complexes 

 so formed. 



