Kinetic and molecular orbital studies on the rate of oxidation of monosubstituted phenols and anilines by horseradish peroxidase compound II

Abstract

The second-order rate constant (k4) for the oxidation of a series of aromatic donor molecules (monosubstituted phenols and anilines) by horseradish peroxidase (HRP) compound II was examined with a stopped-flow apparatus. The electronic states of these substrates were calculated by an ab initio molecular orbital method. It was found that in both phenols and anilines log k4 values correlate well with the highest occupied molecular orbital (HOMO) energy level and the lowest unoccupied molecular orbital (LUMO) energy level, but not with the net charge or frontier electron density on atoms of these molecules. The HOMO and LUMO energy levels of phenols and anilines further showed linear relationships with Hammett's sigma values with negative slopes. Similar results were obtained in the oxidation of substrates by HRP compound I, except that the rate of reaction was much higher than in the case of HRP compound II. In addition, the rates of oxidation of phenols by compound I or II were found to be about 1000 times higher than those of anilines with similar HOMO energy levels. On the basis of these results, the mechanism of electron transfer from the substrate to the heme iron of HRP compound II is discussed.

Characterisation of a haem active‐site mutant of horseradish peroxidase, Phe41 → Val, with altered reactivity towards hydrogen peroxide and reducing substrates

“…The highest-occupied-molecular-orbital energy levels are expected to correlate with the standard redox potential of the substrate [46]. It would seem that aromatic donors interact at a generalized binding site on the distal side of the haem active site [47,481. This binding site in HRP has not been unambiguously located.…”

“…Recent NMR experiments with HRP-C have implicated the haem %methyl group [39], a methyl group from an Ile side chain [49] and one of a pair of interacting Phe residues (Phe A) [48], in the binding of the inhibitor benzhydroxamic acid. Amino acid sequence comparison and NMR comparison of several plant peroxidases favours the assignment of these Phe resonances to Phe142 and Phe143 (Phe A and Phe B) [47,48].…”

Chemical kinetics and interactions involved in horseradish peroxidase-mediated oxidative polymerization of phenolic compounds

“…Despite early work by Bordeleau et al 1972 indicating a correlation between compound II reactivity and atomic charge on the substrate's phenolic oxygen, Hosoya et al 1983 andSakurada et al 1990 were unable to confirm significant correlation. Job et al 1976, Dunford et al 1986, Sakurada et al 1990, and Gilabert et al 2004 proteins in which the active pocket was opened, and they determined that HRP reactivity (i.e. k cat ) was reasonably correlated (R 2 = 0.81) with predicted binding distances.…”

“…Correlations between k cat and the energy of the lowest unoccupied molecular orbital (E LUMO ) have also produced mixed results for substituted phenols (Sakurada et al, 1990, Brewster et al, 1991, Hosoya et al, 1983, Colosi et al, 2006. Sakurado et al 1990 andBrewster et al 1991 r eported fairly strong correlations with coefficient of 0.86 a nd 0.89 r espectively between compound II reactivity and the E LUMO for sets of substituted phenols.…”

Characterization of Dimers of Hydroquinone Glucosides Produced by Peroxidase-Catalyzed Polymerization

“…9,10) In order to explain the low accumulation levels of the oligomers, the electrochemical reactivity was compared in terms of the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO), calculated by the density functional theory. 11) In this study, under generalized gradient approximation, the DNP basis set (comparable to the 6-31G Ã basis set) and the BLYP functional were used.…”