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 sed on octahedral M-II/III complexes (where M is the metal) with 0 axial imidazole ligands (as a model of b and c type cytochromes) or with one imidazole and one methyl ligand (as a model of methylcobolamin). 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 1 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 Co-C bond that is more stable against hydrolysis than iron and porphine. Finally, Fe-II/III gives a much lower reorganization energy than Co-II, this is owing to the occupied d(z) orbital in Co-II. Altogether, these results give some clues about how nature has chosen the tretrapyrrole rings and their central metal ion.
- Density functional calculations