Haemoglobin (Edexcel A Level Chemistry): Revision Note

Haemoglobin

• Haemoglobin is one of nature's complexes using a transition metal ion

• The haem molecule is a complex with iron(II) at its centre

• Haemoglobin contains four haem groups; each haem group is a multidentate ligand that surrounds one Fe²⁺ ion

  • These consist of mostly carbon and hydrogen atoms

  • Each haem group uses four nitrogen atoms to form four dative covalent bonds to the Fe²⁺ ion, arranged in a plane around the iron

• There is a fifth dative bond from the protein (globin) to the Fe²⁺ ion

• Oxygen atoms form a dative covalent bond with the iron(II) which enables oxygen molecules to be transported around the body in the blood

The haem molecule with iron(II) at its centre

• Oxygen molecules are not very good ligands and bond weakly to the iron(II)

• The weak bonds allows them to break off easily and be transported into cells

Ligand Exchange in Haemoglobin

• Carbon monoxide is toxic because it is a better ligand than oxygen and binds strongly and irreversibly to the iron(II) preventing oxygen from being carried to the cells

• If oxygen attached to the haemoglobin (oxyhaemoglobin) is replaced by carbon monoxide (carboxyhaemoglobin), a bright cherry-red colour is produced in the haem complex

  • A sign of carbon monoxide poisoning

• The condition anaemia occurs when a person does not have enough haemoglobin in their blood due to a loss of blood or deficiency in iron

  • Deficiency in iron can be restored by taking iron sulfate tablets in the diet