On the reversible O 2 binding of the Fe–porphyrin complex

Electronic mechanism of the reversible O 2 binding by heme was studied by using Density Functional Theory calculations. The ground state of oxyheme was calculated to be open singlet state [Fe(S =1/2) + O 2 (S = 1/2)]. The potential energy surface for singlet state is associative, while that for triplet state is dissociative. Because the ground state of the O 2 + deoxyheme system is triplet in the dissociation limit [Fe(S = 2) + O 2 (S = 1)], the O 2 binding process requires relativistic spin‐orbit interaction to accomplish the intersystem crossing from triplet to singlet states. Owing to the singlet‐triplet crossing, the activation energies for both O 2 binding and dissociation become moderate, and hence reversible. We also found that the deviation of the Fe atom from the porphyrin plane is also important reaction coordinate for O 2 binding. The potential surface is associative/dissociative when the Fe atom locates in‐plane/out‐of‐plane. © 2006 Wiley Periodicals, Inc. J Comput Chem 4: 426–433, 2006