The structures and stabilities of the complexes of biologically available ligands with Fe(II) porphine: an Ab initio study.

Fe(II) porphine complexes were investigated at the “MP2/LB”//B3LYP/SB level of theory where SB and LB denote the small and large basis sets, 6-31+G(d) and 6-311+G(2df,2p), respectively. Solvation effects due to water and benzene were approximated by the IEFPCM procedure. Ligands included H2O, Cl-, and OH- and Im(imidazole), CH3NH2, (CH3)2S, CH3CO2-, CH3 S-, and CH3PhO- as models of the side chains of His, Lys, Met, Asp/Glu, Cys, and Tyr residues, respectively. Fe(II)porphine, 2, and the complexes 2(H2O), 2(Im), 2(CH3)2S), 2(CH3NH2), and 2(H2O)2 have triplet ground states. All pentacoordinated complexes of 2 with negatively charged ligands have high spin quintet ground states, while all hexacoordinated complexes with at least one Im ligand have low spin singlet ground states. With the exception of 2(Im)2 and 2(Im)((CH3)2S), no hexacoordinated complexes are stable in water or benzene.Redox properties are sensitive to the nature of the environment and the ligand(s)attached to the iron center. With the exception of the parent system, 2+/2, all complexes are predicted to have a negative reduction potential relative to the standard hydrogen electrode in water. With neutral ligand(s),the reduction potential is higher in the nonpolar environment. The opposite is true with negatively charged ligands. The redox activity of cytochromes, peroxidases, and catalases is discussed in the context of the model parent systems.