Ferryl haem protonation gates peroxidatic reactivity in globins.
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ABSTRACT:
SUBMITTER: Silaghi-Dumitrescu R
PROVIDER: S-EPMC1876371 | biostudies-other | 2007 May
REPOSITORIES: biostudies-other
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Project description:Ferryl porphyrins, P-Fe(IV)═O, are central reactive intermediates in the catalytic cycles of numerous heme proteins and a variety of model systems. There has been considerable interest in elucidating factors, such as terminal oxo basicity, that may control ferryl reactivity. Here, the sulfonated, water-soluble ferryl porphyrin complexes tetramesitylporphyrin, oxoFe(IV)TMPS (FeTMPS-II), its 2,6-dichlorophenyl analogue, oxoFe(IV)TDClPS (FeTDClPS-II), and two other analogues are shown to be protonated under turnover conditions to produce the corresponding bis-aqua-iron(III) porphyrin cation radicals. The results reveal a novel internal electromeric equilibrium, P-Fe(IV)═O ⇆ P(+)-Fe(III)(OH2)2. Reversible pKa values in the range of 4-6.3 have been measured for this process by pH-jump, UV-vis spectroscopy. Ferryl protonation has important ramifications for C-H bond cleavage reactions mediated by oxoiron(IV) porphyrin cation radicals in protic media. Both solvent O-H and substrate C-H deuterium kinetic isotope effects are observed for these reactions, indicating that hydrocarbon oxidation by these oxoiron(IV) porphyrin cation radicals occurs via a solvent proton-coupled hydrogen atom transfer from the substrate that has not been previously described. The effective FeO-H bond dissociation energies for FeTMPS-II and FeTDClPS-II were estimated from similar kinetic reactivities of the corresponding oxoFe(IV)TMPS(+) and oxoFe(IV)TDClPS(+) species to be ∼92-94 kcal/mol. Similar values were calculated from the two-proton P(+)-Fe(III)(OH2)2 pKa(obs) and the porphyrin oxidation potentials, despite a 230 mV range for the iron porphyrins examined. Thus, the iron porphyrin with the lower ring oxidation potential has a compensating higher basicity of the ferryl oxygen. The solvent-derived proton adds significantly to the driving force for C-H bond scission.
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Project description:Oxygen-containing mononuclear iron species--iron(III)-peroxo, iron(III)-hydroperoxo and iron(IV)-oxo--are key intermediates in the catalytic activation of dioxygen by iron-containing metalloenzymes. It has been difficult to generate synthetic analogues of these three active iron-oxygen species in identical host complexes, which is necessary to elucidate changes to the structure of the iron centre during catalysis and the factors that control their chemical reactivities with substrates. Here we report the high-resolution crystal structure of a mononuclear non-haem side-on iron(III)-peroxo complex, [Fe(III)(TMC)(OO)](+). We also report a series of chemical reactions in which this iron(III)-peroxo complex is cleanly converted to the iron(III)-hydroperoxo complex, [Fe(III)(TMC)(OOH)](2+), via a short-lived intermediate on protonation. This iron(III)-hydroperoxo complex then cleanly converts to the ferryl complex, [Fe(IV)(TMC)(O)](2+), via homolytic O-O bond cleavage of the iron(III)-hydroperoxo species. All three of these iron species--the three most biologically relevant iron-oxygen intermediates--have been spectroscopically characterized; we note that they have been obtained using a simple macrocyclic ligand. We have performed relative reactivity studies on these three iron species which reveal that the iron(III)-hydroperoxo complex is the most reactive of the three in the deformylation of aldehydes and that it has a similar reactivity to the iron(IV)-oxo complex in C-H bond activation of alkylaromatics. These reactivity results demonstrate that iron(III)-hydroperoxo species are viable oxidants in both nucleophilic and electrophilic reactions by iron-containing enzymes.
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