Conversely, oxo-ferryl stretching frequencies determined by resonance Raman spectroscopy (RR) indicate that both species have similar short Fe–O distances, suggesting a non-protonated ferryl oxygen. In general, protons are directly involved in the reactivity of peroxidases ( Scheme 1) and, as a consequence, ferryl centre protonation has been essential to justify the different Fe–O bond distances measured for compounds I and II by X-ray diffraction structural analysis. Computation indicates that the general oxidative properties of peroxidase intermediates, as well as their reactivity towards water and protons and Soret bands, are mainly controlled by the iron porphyrin and its proximal histidine ligand. It is demonstrated that this protonation is necessary to account for the experimental data, and computed Gibbs free energies reveal p K a values of compound II about 8.5–9.0. ![]() Protonation of the ferryl oxygen of compound II is discussed in terms of thermodynamics, Fe–O bond distances, and redox properties. Computed Gibbs free energies indicate that the corresponding aquo complexes are not thermodynamically stable, supporting the five-coordinate Fe(III) centre in native ferric peroxidases, with a water molecule located at a non-bonding distance. B3LYP and M06-2X density functionals with different basis sets were employed on a common molecular model of the active site (Fe-centred porphine and proximal imidazole). Shared geometries, spectroscopic properties at the Soret region, and the thermodynamics of peroxidases are discussed. ![]() Electronic structure calculations using the density-functional theory (DFT) have been performed to analyse the effect of water molecules and protonation on the heme group of peroxidases in different redox (ferric, ferrous, compounds I and II) and spin states.
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