LINUS PAULING 



as six iron-oxygen bonds. It is interesting to note that the structure 



of the porphyrin molecule is such as to involve resonance among a 



number of structures, each of which places a double bond and a single 



bond with adjacent carbon atoms for two of the nitrogen atoms, and 



two single bonds to carbon for the other two nitrogen atoms ; each 



nitrogen atom is then given zero charge by forming a bond with 50 



per cent covalent character with the iron atom. 



Inasmuch as the iron atom of the ferrohaem group has achieved 



electrical neutrality through its formation of the four bonds with 



nitrogen, it will tend to resist the formation of further bonds in which 



electron pairs are donated to it by groups to be attached to it, even 



though there are two coordination positions, above and below the 



plane of the porphyrin group, available for additional ligands. Water 



molecules, chloride ions, hydroxide ions, and similar groups would 



in this way be kept from combining with the iron atoms of ferro- 



haemoglobin. Certain molecules, however, including oxygen, carbon 



monoxide, cyanide ion, and the alkylisocyanides, have such a structure 



as to permit them to combine with this group without destroying the 



electrical neutrality of the iron atom. These molecules can form a 



double bond with the iron atoms, using two electrons of their own 



and two electrons of the iron atom. The electronic structures that 



would be written for the complexes of these molecules with the 



ferrohaem group are the following : 



NN NN NN NN 



\/.. \/ .. \/ .. \/ .. 



— Fe = (X _Fe=C=0: — Fe=C=N: — Fe=C=N v 



/\ N>: /\ /\ /\ X R 



NN'-NN NN NN 



THE HAEM-LINKED ACID GROUPS 



Our knowledge of the haem-linked acid groups of haemoglobin is 

 summarized in Table J (from Ref. 10). The explanation given above 

 of the ability of ferrohaemoglobin to form compounds with only a 

 few molecules also provides an explanation of the linking between the 

 haem group and an acidic group, the imidazole ring of a histidine side 

 chain, which has been suggested as the important haem-linked 

 group 1 ' 9 ' 10 leading to pK 2 = 7-83 for ferrohaemoglobin and 6-80 for 

 oxyhaemoglobin and carbonmonoxyhaemoglobin. In ferrohaemo- 

 globin itself there would be only a small tendency of an iron atom to 

 form a covalent bond with a nitrogen atom of the imidazole ring, 

 because its electrical charge has been made zero by the formation of 

 the four bonds in the ferrohaem complex. However, when a new 

 double bond is formed, with carbon monoxide or the oxygen molecule, 



60 



