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Comparison of the Chemical Properties of Iron and Cobalt Porphyrins and Corrins
Author(s) -
Jensen Kasper P.,
Ryde Ulf
Publication year - 2003
Publication title -
chembiochem
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.05
H-Index - 126
eISSN - 1439-7633
pISSN - 1439-4227
DOI - 10.1002/cbic.200200449
Subject(s) - corrin , porphyrin , chemistry , cobalt , imidazole , ligand (biochemistry) , crystallography , octahedron , stereochemistry , metal , photochemistry , inorganic chemistry , crystal structure , organic chemistry , biochemistry , receptor
Density functional calculations have been used to compare various geometric, electronic and functional properties of iron and cobalt porphyrin (Por) and corrin (Cor) species. The investigation is focussed on octahedral M II/III complexes (where M is the metal) with two axial imidazole ligands (as a model of b and c type cytochromes) or with one imidazole and one methyl ligand (as a model of methylcobalamin). However, we have also studied some five‐coordinate M II complexes with an imidazole ligand and four‐coordinate M I/II complexes without any axial ligands as models of other intermediates in the reaction cycle of coenzyme B 12 . The central cavity of the corrin ring is smaller than that of porphine. We show that the cavity of corrin is close to ideal for low‐spin Co III , Co II , and Co I with the axial ligands encountered in biology, whereas the cavity in porphine is better suited for intermediate‐spin states. Therefore, the low‐spin state of Co is strongly favoured in complexes with corrins, whereas there is a small energy difference between the various spin states in iron porphyrin species. There are no clear differences for the reduction potentials of the octahedral complexes, but [Co I Cor] is more easily formed (by at least 40 kJ mole −1 ) than [Fe I Por]. Cobalt and corrin form a strong CoC bond that is more stable against hydrolysis than iron and porphine. Finally, Fe II/III gives a much lower reorganisation energy than Co II/III ; this is owing to the occupied d z 2orbital in Co II . Altogether, these results give some clues about how nature has chosen the tetrapyrrole rings and their central metal ion.