Alkaline haematin and nitrogenous ligands.

1. Pyridine reacts with alkaline haematin to form a dipyridine dihydroxy dimeric haematin in which there is no competition between pyridine and OH(-) for coordination positions on the iron of haematin. 2. Imidazole competes directly with OH(-) to form the monomeric imidazole parahaematin in alkaline media. 3. The monomeric imidazole parahaematin aggregates readily to form a species that is precipitated on standing. 4. A structure is proposed for the haematin-pyridine compound in alkaline media.