Quantum-chemical simulation of the active center of vitamin B12

The electronic structure of porphin and corrin complexes of cobalt differing with respect to the oxidation state of the central ion has been investigated by the MO-LCAO-SCF-CNDO method with the Kai-Nishimoto parameters for transition metals. On the basis of an analysis of the distribution of the electron density and the structure of the energy spectrum, it has been shown that the oxidation-reduction processes of the complexes are accompanied by restructuring of the energy spectrum, and the differences between the electronic structures of porphin and corrin complexes have been discussed. It has been established that cobalt(I) porphin has stronger nucleophilic properties than does cobalt(I) corrin. The electronic structure of hexacoordinate complexes in which an imidazole molecule and a molecule of L (L = H₂O, CH₃ ⁺, CN^–) are axially coordinated has been calculated. The mechanisms of the dissociation of cobalt alkyl complexes and the differences between the processes of the heterolytic dissociation of porphin and corrin complexes have been discussed. It has been shown that the elimination of a CH₃ ⁺ cation, which plays an important role in biomethylation reactions, is more favorable in corrin complexes.Translated from Teoreticheskaya i Éksperimental'naya Khimiya, Vol. 22, No. 4, pp. 400–409. July–August, 1986.