Substitution at the Porphyrin Ring

All haem-containing compounds are derivatives of porfihin (Fig. 1), which does not itself occur naturally. AO. porphyrins and metalloporphyrins which are found in living systems are fully substituted at the pyrrole carbon 

Note The haem of the cytochrome a series is not quite so well defined but has one aldehyde substituent. 

The porphyrin ring can be regarded as an 18-atom aromatic system, whose properties will be dependent on the electron-donating and -withdrawing characteristics of the substituents. 

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