 In helices E and F of haemoglobin, histidine influences binding of dioxygen as well as carbon monoxide.This interaction enhances the affinity of Fe(II) for O2 but destabilizes the binding of CO, which binds only 200 times stronger in haemoglobin, compared to 20,000 times stronger in free haem.The tautomerism and acid-base properties of the imidazole side chain has been characterized by 15N NMR spectroscopy.