Iron porphyrins with different imidazole ligands. A theoretical comparative study

A theoretical comparative study of a series of five- and six-coordinate iron porphyrins, FeP(L) and FeP(L)(O(2)), has been carried out using DFT methods, where P = porphine and L = imidazole (Im), 1-methylimidazole (1-MeIm), 2-methylimidazole (2-MeIm), 1,2-dimethylimidazole (1,2-Me(2)Im), 4-ethylimidazole (4-EtIm), or histidine (His). Two ligated "picket-fence" iron porphyrins, FeTpivPP(2-MeIm) and FeTpivPP(2-MeIm)(O(2)), were also included in the study for comparison. A number of density functionals were employed in the computations to obtain reliable results. The performance of functionals and basis set effects were investigated in detail on FeP, FeP(Im), and FeP(Im)(O(2)), for which certain experimental information is available and there are some previous calculations in the literature for comparison. Many subtle distinctions in the effects of the different imidazole ligands on the structures and energetics of the deoxy- and oxy iron porphyrins are revealed. While FeP(2-MeIm) is identified to be high spin (S = 2), the ground state of FeP(1-MeIm) may be an admixture of a high-spin (S = 2) and an intermediate-spin (S = 1) state. The ground state of FeP(L)(O(2)) may be different with different L. A weaker Fe-L bond more likely leads to an open-shell singlet ground state for the oxy complex. The 2-methyl group in 2-MeIm, which increases steric contact between the ligand and the porphyrinato skeleton, weakens the Fe-O(2) bond, and thus iron porphyrins with 2-MeIm mimic T-state (low affinity) hemoglobin. The calculated FeP(2-MeIm)-O(2) bonding energy is comparable to the FeTpivPP(2-MeIm)-O(2) one, in agreement with the fact that the picket-fence iron porphyrin binds O(2) with affinity similar to that of myoglobin but different from the result obtained by the CPMD scheme. Im and 4-EtIm closely resemble His, the biologically axial base, and so future computations on hemoprotein models can be simplified safely by using Im.