Hydrogen Peroxide Decomposition by a Non‐Heme Iron(III) Catalase Mimic: A DFT Study

Abstract

Non-heme iron(III) complexes of 14-membered tetraaza macrocycles have previously been found to catalytically decompose hydrogen peroxide to water and molecular oxygen, like the native enzyme catalase. Here the mechanism of this reaction is theoretically investigated by DFT calculations at the (U)B3LYP/6-31G* level, with focus on the reactivity of the possible spin states of the FeIII complexes. The computations suggest that H2O2 decomposition follows a homolytic route with intermediate formation of an iron(IV) oxo radical cation species (L.+FeIV==O) that resembles Compound I of natural iron porphyrin systems. Along the whole catalytic cycle, no significant energetic differences were found for the reaction proceeding on the doublet (S=1/2) or on the quartet (S=3/2) hypersurface, with the single exception of the rate-determining O--O bond cleavage of the first associated hydrogen peroxide molecule, for which reaction via the doublet state is preferred. The sextet (S=5/2) state of the FeIII complexes appears to be unreactive in catalase-like reactions.

Substituent effects on geometric and electronic properties of iron tetraphenylporphyrin: a DFT investigation

“…The hybrid Becke-Lee-Yang-Parr (B3LYP) functional [21,22] and the 6-31 G(d) basis set were used for complete geometry optimization without any symmetry constraint. The level has been proved suitable for geometry optimization of iron porphyrins [23][24][25][26][27][28][29]. The following frequency analysis revealed that all the structures obtained in this work had no imaginary vibration, indicating that the lowest energy structures for all of the complexes were truly local minima in the potential energy surfaces.…”

Catalases versus peroxidases: DFT investigation of H2O2 oxidation in models systems and implications for heme protein engineering

“…Since the BP86 is known to underestimate the energy barrier for hydrogen atom transfers [37], the energy values were further refined using the hybrid B3LYP functional [38][39][40]. The B3LYP functional has been found to give accurate energies for energy barriers in the catalytic cycle of a non-heme catalase mimic [41]. To this aim, single point energy calculations were performed with the ORCA program [42] on the CPMD optimized structures of reactants and transition states using both B3LYP and BP86 and the TZVP basis set [43].…”

Case Studies of ONIOM(DFT:DFTB) and ONIOM(DFT:DFTB:MM) for Enzymes and Enzyme Mimics

“…64,65 The present investigation follows the reaction pathway in reference 64 . A flaw in that study is that the additional axial (proximal) ligand was not taken into account.…”

“…The reason for this electron transfer is probably the lack of the distal ligand. 65 The spin on iron couples ferromagnetically with the spin of the unpaired electron on of the ligand to form a quartet state. The present assignment agrees with the results in reference 64 (unpaired electrons in d z2 , d xz and the ligand b 1u orbitals).…”