Mechanisms for the formation of six-coordinate Fe(III)TPP-peroxide complexes studied by simultaneous ESR and optical measurements

Models of the Low-Spin Iron(III) Hydroperoxide Intermediate of Heme Oxygenase: Magnetic Resonance Evidence for Thermodynamic Stabilization of the dxy Electronic State at Ambient Temperatures

“…The color of the solution changes to a cherry red. The electronic absorption spectrum recorded at −78 °C in this “dip probe” cell is very similar to that reported for [TPPFe(OCH 3 )(OO t Bu)] - at 77 K . Similar experiments allowed us to determine that the molar proportions needed to obtain electronic absorption spectra characteristic of the different alkyl peroxide complexes at −78 °C are 1 TPPFeCl (0.2 mM):200 OH - :100 t BuOOH for [TPPFe(OCH 3 )(OO t Bu)] - , and 1 TPPFeCl (0.2 mM):200 OH - :600 t BuOOH for [TPPFe(OO t Bu) 2 ] - .…”

“…Synthesis of Alkyl Peroxide Porphyrinate Complexes. Alkyl peroxide complexes of TPPFe were synthesized by a modification of the synthetic procedures reported by Tajima and co-workers. − These investigators reported the synthesis and characterization (EPR and electronic absorption spectra) of [FeTPP(OCH 3 ) 2 ] - , [TPPFe(OCH 3 )(OO t Bu)] - , and [TPPFe(OO t Bu) 2 ] - in frozen glasses at 77 K. An important aim of the investigations reported here is the study of these complexes by 13 C NMR spectroscopy in solution. Consequently, modifications were necessary to synthesize and characterize the complexes at temperatures above the melting point of CH 2 Cl 2 .…”

“…Consequently, modifications were necessary to synthesize and characterize the complexes at temperatures above the melting point of CH 2 Cl 2 . As a first step toward this goal, conditions were explored that allowed us to reproduce the electronic absorption spectra, previously obtained from frozen glasses at 77 K, − in solutions thermostated at 195 K. To facilitate these experiments, the cell shown in Figure was constructed out of glass. To assemble the cell, the “dip probe” (a) is inserted through the cap (b) and secured with the O-ring (c) and teflon washer (d).…”

“…Some years ago Tajima and co-workers showed that synthetic hemes in the presence of alkyl peroxides and a variety of sixth ligands, including methoxide, − imidazolate, or a second alkyl peroxide, as well as heme proteins with histidine , or cysteinate sixth ligands, yield very similar EPR spectra with compressed g anisotropy (∑ g 2 ≈ 14). These spectra are very similar to those obtained for the Fe III -OOH complexes of various heme protein enzymes, which were prepared by cryoreduction and then annealing of the corresponding Fe II -O 2 complexes. , The g -values of the complexes of Tajima and co-workers (2.32, 2.16, 1.95, methoxide, tert -butylperoxide; 2.25, 2.15, 1.96, bis- tert -butyl-peroxide; 2.32, 2.19, 1.94, imidazolate, hydroperoxide 20 ) are very similar to those of annealed hemoglobin-hydroperoxide (2.31, 2.18, 1.94), heme oxygenase-hydroperoxide (2.37, 2.19, 1.93), and cytochrome P450-hydroperoxide (2.29, 2.16, 1.96) .…”

Heme oxygenase, steering dioxygen activation toward heme hydroxylation

“…It was also noted [82] that model Fe III -porphyrinates axially coordinated by an alkyl peroxide and a variety of sixth ligands, including methoxide [94][95][96], imidazolate [97], or a second alkyl peroxide [98], exhibit EPR spectra with compressed heme anisotropy, P g 2 $ 14. In addition, the sum of the squared g values in the EPR spectra of the Fe III -OOH complexes of cytochrome P450 cam [66] and myoglobin [67] is also approximately 14, suggesting that the axial coordination of an alkylperoxide or hydroperoxide ligand in low-spin ferric porphyrinates would induce a (d xy ) 1 electron configuration.…”