The mechanism for the reversible oxygen addition to heme. A theoretical CASPT2 study

By performing CASPT2 calculations, the lowest energy pathway for oxygen addition to an isolated heme center of a hemeprotein is evaluated and found to be reversible (the oxyheme compound is just 14.9 kcal mol(-1) more stable than the deoxyheme + O-2 reactants, and the energy barriers to dissociation are even smaller).