Bromide oxidation mechanism by vanadium bromoperoxidase functional models with new tripodal amine ligands: A comprehensive theoretical calculations study

A density functional theory (DFT) study of the bromide oxidation mechanism catalyzed by a series of 14 oxo-peroxo vanadium(V) complexes containing tripodal amine ligands (hereafter called TPALs) is presented. The rate-determining step is the transfer of the protonated oxygen atom of the peroxide moiety to the bromide substrate via an SN2 mechanism within the respective complex. In all cases, a direct dependence of the calculated atomic charge on vanadium with the electronic character of the ligand was observed. The TPAL is decorated with a series of substituents exhibiting different donor groups and lengths which affect the barrier energy; the substituent effects on the barrier have been examined. A significant decrease of the barrier energy (by 1–6 kcal mol−1) is observed either upon protonation of the imine functional group or third substituent elongation of the TPAL. Increasing the electron donation ability of the ligands decreases the HOMO–LUMO energy gap of the peroxo complexes and increases the ligand-to-metal charge transfer (LMCT). The use of substituents with poor electron donation ability causes an activation of the complexes. The same effect is observed using TPAL substituents that are able to form intramolecular hydrogen bonds. Compared to the gas phase, the energy barriers are in general lower if solvent effects (acetonitrile) are taken into account. Compared to the previously used V-Hheida (heida = N-(2-hydroxyethyl) iminodiacetic acid), our calculations suggest that the new oxo-peroxo vanadium(V) V-peyc (N-(2-pyridylethyl-6-carbamoyl) iminodiacetic acid), V-Haada (H3aada: 2,2′-(2-aminoacetamido) diacetic acid) and V-pyc (H2pyc: N-(2-pyridylmethyl-6-carbamoyl) iminodiacetic acid) complexes are good functional models of vanadium-dependent bromoperoxidase (VBPO).

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