112 UNITY AND DIVERSITY IN BIOCHEMISTRY 



fill the orbitals which have the lowest energy level, so that in iron, the s and 

 p orbitals of the M shell are saturated, making 8 electrons in all (4 pairs). 

 The 6 remaining electrons are in d orbitals and they tend to occupy as 

 many of these as possible : one orbital contains two paired electrons and 

 the four others each contain one unpaired electron. So iron possesses 4 

 unpaired electrons. 



Each unpaired electron, on account of its spin, behaves as a small 

 magnet : it becomes oriented in a magnetic field so as to oppose the 

 field. Measurements of magnetic susceptibility allow the number of un- 

 paired electrons to be determined. Substances which, like iron, contain 

 unpaired electrons are said to be paramagnetic and those which contain 

 only paired electrons are called diamagnetic. 



The valency electrons are usually those in the outer shell. However, in 

 the case of iron, the energy level of the d orbitals of the M shell is almost the 

 same as the energy level of the s orbitals of the N shell, so that the d 

 orbitals of the M shell take part in the formation of iron complexes. 



The iron atom loses two electrons and becomes a ferrous ion; the two 

 electrons which are lost are two paired electrons of the 5 orbital of the N 

 shell; the ferrous ion then, like iron itself, contains 4 unpaired electrons. 



The loss of an additional electron to give the ferric ion is at the expense 

 of the six electrons in the d orbital of shell M ; the 5 remaining electrons 

 redistribute themselves among the 5 d orbitals and ferric iron contains 

 5 unpaired electrons. 



In ferrous or ferric coordination complexes containing covalent links, 

 the complexing groups bring two electrons per bond. Four electrons can 

 be introduced into the d orbitals of the M shell (saturating them in the case 

 of ferrous complexes and leaving one unpaired electron in the case of ferric 

 complexes) and 8 other electrons will be required to saturate the 5 and p 

 orbitals of the N shell, conferring upon this shell the very stable octet 

 structure. It can be seen that the coordination number of the ferrous ion, as 

 well as that of the ferric ion, is 6 : two bonds established through the M shell 

 {d orbitals) and 4 bonds through the N shell {s and p orbitals) of the ion. 

 These hybrid bonds (hybrid, because two electronic shells are involved) are 

 denoted by the symbols d^sp^. The complex has the form of an octahedron 

 with the centre occupied by the ion ; the coordination linkages which are 

 covalent (2 shared electrons per bond) are directed towards the 6 corners. 



1. Haems 



The porphyrin molecule can combine with heavy-metal ions such as 

 ferrous, ferric, cupric ions, etc. 



Haem refers to the ferro-porphyrin which results from the combination 

 of a porphyrin with a ferrous ion : 



porphyrin + Fe++ 5± ferro-porphyrin + 2H+ 



