Comparative study of physical and chemical properties of iron cobalt and nickel
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Abstract
Density functional calculations have been used to compare the geometric, electronic, and functional properties of the three important tetrapyrrole systems in biology, heme, coenzyme B12, and coenzyme F430, formed from iron porphyrin (Por), cobalt corrin (Cor), and nickel hydrocorphin (Hcor). The results show that the flexibility of the ring systems follows the trend Hcor > Cor > Por and that the size of the central cavity follows the trend Cor < Por < Hcor. Therefore, low-spin CoI, CoII, and CoIII fit well into the Cor ring, whereas Por seems to be more ideal for the higher spin states of iron, and the cavity in Hcor is tailored for the larger Ni ion, especially in the high-spin NiII state. This is confirmed by the thermodynamic stabilities of the various combinations of metals and ring systems. Reduction potentials indicate that the +I and +III states are less stable for Ni than for the other metal ions. Moreover, Ni–C bonds are appreciably less stable than Co-C bonds. However, it is still possible that a Ni–CH3 bond is formed in F430 by a heterolytic methyl transfer reaction, provided that the donor is appropriate, e.g. if coenzyme M is protonated. This can be facilitated by the adjacent SO3− group in this coenzyme and by the axial glutamine ligand, which stabilizes the NiIII state. Our results also show that a NiIII–CH3 complex is readily hydrolysed to form a methane molecule and that the NiIII hydrolysis product can oxidize coenzyme B and M to a heterodisulphide in the reaction mechanism of methyl coenzyme M reductase.