Modeling the haloperoxidases: Reversible oxygen atom transfer between bromide ion and an oxo-Mn(V) porphyrin

The manganese meso-dimethylimidazolium porphyrin complex Mn(III)[TDMImP] reacted with HOBr/OBr^- to generate the corresponding oxo-Mn(V)[TDMImP] species. The rate of this process accelerated with increasing pH. A forward rate constant, k_for, of 1.65 × 10⁶ M^-1 s^-1 was determined at pH 8. Under these conditions, the oxo-Mn(V) species is short-lived and is transformed into the corresponding oxo-Mn(IV) complex. A first-order rate constant, k_obs, of 0.66 s^-1 was found for this reduction process at pH 8. The mechanism of this reduction process, which was dependent on bromide ion, appeared to proceed via an intermediate Mn(III)-O-Br complex. Thus, both a fast, reversible Mn(III)-O-Br bond heterolysis and a slower homolytic pathway occur in parallel in this system. The reverse oxidation reaction between oxo-Mn(V)[TDMImP] and bromide was investigated as a function of pH. The rate of this oxo-transfer reaction (k_rev = 1.4 × 10³ M^-1 s^-1 at pH 8) markedly accelerated as the pH was lowered. The observed first-order dependence of the rate on [H⁺] indicates that the reactive species responsible for bromide oxidation is a protonated oxo-hydroxo complex and the stable species present in solution at high pH is dioxo-Mn(V)[TDMImP], [O{double bond, long}Mn(V){double bond, long}O]^-. The oxo-Mn(V) species retains nearly all of the oxidative driving force of the hypohalite. The equilibrium constant K_equi = k_for/k_rev for the reversible process was determined at three different pH values (K_equi = 1.15 × 10³ at pH 8) allowing the measurement of the redox potentials E of oxo-Mn(V)/Mn(III) (E = 1.01 V at pH 8). The redox potential for this couple was extrapolated over the entire pH scale using the Nernst relationship and compared to those of the manganese 2- and 4-meso-N-methylpyridinium porphyrin couples oxo-Mn(V)[2-TMPyP]/Mn(III)[2-TMPyP], oxo-Mn(V)[4-TMPyP]/Mn(III)[4-TMPyP], OBr^-/Br^- and H₂O₂/H₂O. Notably, the redox potential of oxo-Mn(V)/Mn(III) for the imidazolium porphyrin approaches that of H₂O₂/H₂O at low pH.