Structure and bonding analysis of intermediate model heme-imidazole and heme-thiolate enzymes complexed with formate, acetate and nitrate: A theoretical study

Structure and bonding analysis of the FeO bonds in the model heme-imidazole and heme-thiolate complexes [(por)Fe(Im)(R)] (R = HC(O)O−, 1FeIm, R = H3CC(O)O−, 2FeIm, R = NO3−, 3FeIm) and [(por)Fe(SMe))(R)]− (R = HC(O)O−, 4FeSMe, R = H3CC(O)O−, 5FeSMe, R = NO3−, 6FeSMe) were investigated at the DFT/TZP level of theory using density functionals BP86, PW91, PBE, TPSS and B3LYP. The FeO bond distances at BP86/TZP in the complexes [(por)Fe(Im)(R)] (1.858 Å in 1FeIm, 1.868 Å in 2FeIm and 1.919 Å in 3FeIm) and [(por)Fe(SMe)(R)]− (1.955 Å in 4FeSMe, 1.981 Å in 5FeSMe and 1.991 Å in 6FeSMe) are longer than those expected for a FeO single bond estimated on the basis of covalent radii predictions (FeO = 1.79 Å). The FeO bond distances in [(por)Fe(Im)(R)] 1FeIm–3FeIm are shorter than those in [(por)Fe(SMe)(R)]− 4FeSMe–6FeSMe. The thiolate is more trans-directing ligand than the imidazole ligand. The results of optimized geometries, NPA charges, Wiberg bond indices, NHO analysis and BDEs clearly indicate that the formate, acetate and nitrate ligands bind more weakly in heme-thiolate complexes than in heme-imidazole complexes. The reaction intermediate of chloroperoxidase enzyme would be more reactive with organic substrates than the myeloperoxidase (MPO) enzyme. The quartet and the sextet spin states are less stable than the doublet spin state. The optimized bond distances are shortest for doublet spin state.