Low-spin Ferric Complexes Research Articles

Reactivity of inorganic sulfide species towards a pentacoordinated heme model system

The reactivity of inorganic sulfide towards ferric bis(N-acetyl)- microperoxidase 11 in sodium dodecyl sulfate has been explored by means of visible absorption and resonance Raman spectroscopies. The reaction has been previously studied in buffered solutions at neutral pH and in the presence of excess sulfide, revealing the formation of a moderately stable hexacoordinated low spin ferric sulfide complex that yields the ferrous form in the hour's timescale. In the surfactant solution, instead, the ferrous form is rapidly formed. The spectroscopic characterization of the heme structure in the surfactant milieu revealed the stabilization of a major ferric mono-histidyl high spin heme, which may be ascribed to out of plane distortions prompting the detachment of the axially ligated water molecule, thus leading to a differential reactivity. The ferric bis(N-acetyl)- microperoxidase 11 in sodium dodecyl sulfate provides a model for pentacoordinated heme platforms with an imidazole-based ligand.

Structural and Spectroscopic Characterization of a High-Spin {FeNO}(6) Complex with an Iron(IV)-NO(-) Electronic Structure.

Although the interaction of low-spin ferric complexes with nitric oxide has been well studied, examples of stable high-spin ferric nitrosyls (such as those that could be expected to form at typical non-heme iron sites in biology) are extremely rare. Using the TMG3 tren co-ligand, we have prepared a high-spin ferric NO adduct ({FeNO}(6) complex) via electrochemical or chemical oxidation of the corresponding high-spin ferrous NO {FeNO}(7) complex. The {FeNO}(6) compound is characterized by UV/Visible and IR spectroelectrochemistry, Mössbauer and NMR spectroscopy, X-ray crystallography, and DFT calculations. The data show that its electronic structure is best described as a high-spin iron(IV) center bound to a triplet NO(-) ligand with a very covalent iron-NO bond. This finding demonstrates that this high-spin iron nitrosyl compound undergoes iron-centered redox chemistry, leading to fundamentally different properties than corresponding low-spin compounds, which undergo NO-centered redox transformations.

EPR Analysis of Imidazole Binding to Methanosarcina acetivorans Protoglobin

Protoglobins form an unusual class of globin proteins with surprising structural features, such as a highly ruffled heme. Electron paramagnetic resonance spectra of the frozen solution of the ferric imidazole complex of Methanosarcina acetivorans protoglobin reveal the existence of two low-spin ferric heme complexes. Hyperfine sublevel correlation spectroscopy was used to study the imidazole complex of this protoglobin as well as the imidazole complex of horse heart myoglobin for comparison. The spin Hamiltonian values obtained for the dominant contribution of the imidazole-ligated protoglobin agree with a heme conformation in which the plane of the exogenous imidazole is turned more than 30° versus the imidazole plane of the proximal histidine ligand. In turn, the electron paramagnetic resonance data of the minority complex indicate a second conformer, in which the two ligand planes are aligned almost parallel to each other with a lengthened iron–nitrogen bond for the exogenous imidazole ligand.

Probe the Heme Iron Ligand and Conformational Change of Misfolded States of Cytochrome C through EPR Spectroscopy

Cytochrome c is a heme-protein existing in the membrane of mitochrondria which plays a pivotal role in apoptosis. Besides its important biological functions, it is a model system for protein folding and unfolding studies. Recently, Soffer et al. [Soffer et al. Biochemistry 2013, 52, 1397−1408] claimed that a meta-stable misfolded state of horse heart ferricytochreome c could be obtained after exposing the protein to the ferricyanide solution at very alkaline pH for an extended period of time, and it did not undergo any remarkable change in its secondary and tertiary structures upon adjusting the solution to folding promoting conditions at neutral pH. Yet the axial coordination of the heme iron as well as the electronic structures for the misfolded protein remained undetermined. For this purpose, in this work, electron paramagnetic resonance (EPR) spectra of the misfolded protein in the solution at different pH values were recorded, and DFT calculation was conducted to predict the EPR g-tensor at different axial ligand configurations. Our EPR data indicate that the misfolded protein at very alkaline condition exhibits a hexacoordinated low spin state, while at pH 5 the protein adapt the configuration similar to the native-like low-spin ferric heme complex which possesses a histidine/methionine coordination environment. For pH value between 6 and 7, the EPR spectra appear to be the superposition of the two species. With the aid of DFT calculation, also checked by the EPR and other spectroscopic data, the possible hexacoordinated low spin state with a hydroxyl ion as the proximal ligand and the pentacoordinated quantum mixed state of the heme iron for misfolded protein at different folding conditions were discussed. This work was partially supported by Hundred Talents Program of Chinese Academy of Sciences.

Reactivity of inorganic sulfide species toward a heme protein model.

The reactivity of inorganic sulfide species toward heme peptides was explored under biorelevant conditions in order to unravel the molecular details of the reactivity of the endogenous hydrogen sulfide toward heme proteins. Unlike ferric porphyrinates, which are reduced by inorganic sulfide, some heme proteins can form stable Fe(III)-sulfide adducts. To isolate the protein factors ruling the redox chemistry, we used as a system model, the undecapeptide microperoxidase (MP11), a heme peptide derived from cytochrome c proteolysis that retains the proximal histidine bound to the Fe(III) atom. Upon addition of gaseous hydrogen sulfide (H2S) at pH 6.8, the UV-vis spectra of MP11 closely resembled those of the low-spin ferric hydroxo complex (only attained at an alkaline pH) and cysteine or alkylthiol derivatives, suggesting that the Fe(III) reduction was prevented. The low-frequency region of the resonance Raman spectrum revealed the presence of an Fe(III)-S band at 366 cm(-1) and the general features of a low-spin hexacoordinated heme. Anhydrous sodium sulfide (Na2S) was the source of sulfide of choice for the kinetic evaluation of the process. Theoretical calculations showed no distal stabilization mechanisms for bound sulfide species in MP11, highlighting a key role of the proximal histidine for the stabilization of the Fe(III)-S adducts of heme compounds devoid of distal counterparts, which is significant with regard to the biochemical reactivity of endogenous hydrogen sulfide.

Heterocyclic dithiocarbazate iron chelators: Fe coordination chemistry and biological activity

The iron coordination and biological chemistry of a series of heterocyclic dithiocarbazate Schiff base ligands is reported with regard to their activity as Fe chelators for the treatment of Fe overload and also cancer. The ligands are analogous to tridentate heterocyclic hydrazone and thiosemicarbazone chelators we have studied previously which bear NNO and NNS donor sets. The dithiocarbazate Schiff base ligands in this work also are NNS chelators and form stable low spin ferric and ferrous complexes and both have been isolated. In addition an unusual hydroxylated ligand derivative has been identified via an Fe-induced oxidation reaction. X-ray crystallographic and spectroscopic characterisation of these complexes has been carried out and also the electrochemical properties have been investigated. All Fe complexes exhibit totally reversible Fe(III/II) couples in mixed aqueous solvents at potentials higher than found in analogous thiosemicarbazone Fe complexes. The ability of the dithiocarbazate Schiff base ligands to mobilise Fe from cells and also to prevent Fe uptake from transferrin was examined and all ligands were effective in chelating intracellular Fe relative to known controls such as the clinically important Fe chelator desferrioxamine. The Schiff base ligands derived from 2-pyridinecarbaldehyde were non-toxic to SK-N-MC neuroepithelioma (cancer) cells but those derived from the ketones 2-acetylpyridine and di-2-pyridyl ketone exhibited significant antiproliferative activity.

Oxidation of Glutathione by [FeIV(O)(N4Py)]2+: Characterization of an [FeIII(SG)(N4Py)]2+ Intermediate

The mechanism of glutathione (GSH) oxidation by a nonheme ferryl species has been investigated. The reaction of [Fe(IV)(O)(N4Py)](2+) (1) with GSH in an aqueous solution leads to the rapid formation of a green intermediate, characterized as the low-spin ferric complex [Fe(III)(SG)(N4Py)](2+) (2) by UV-vis and electron paramagnetic resonance spectroscopies and by high-resolution time-of-flight mass spectrometry. Intermediate 2 decays to form the final products [Fe(II)(OH(2))(N4Py)](2+) and the disulfide GSSG over time. The overall reaction was fit to a three-step process involving rapid quenching of the ferryl by GSH, followed by the formation and decay of 2, which are both second-order processes.

On the formation, nature, stability and biological relevance of the primary reaction intermediates of myoglobins with hydrogen peroxide

The reaction between hydrogen peroxide and myoglobin (or haemoglobin) ferric haem is a two-electron redox process, yet the stable product is ferryl haem, retaining only one oxidizing equivalent. We have used SVD (singular value decomposition) and global spectroscopic analysis to examine the transient primary spectral intermediates in this reaction, which have been reported as either "compound 0"(ferric peroxide) or "compound I"(ferryl and porphyrin cation radical) types and which may precede the formation of ferrylmyoglobin. To test the hypothesis that the distal histidine facilitates ferryl formation we studied the myoglobin-like haemoglobin from the gastropod mollusc Aplysia fasciata, where this histidine is replaced by valine and its hydrogen bonding role is taken up by a non-homologous arginine. In this protein, consistent with the distal histidine hypothesis, a compound 0 intermediate is formed identified by an EPR spectrum typical of low spin ferric haem complexes. It is significantly more stable than any species seen with mammalian myoglobin. Thirdly, as ferryl haems and associated free radicals may play a role in disease, we have studied the action of myoglobin-peroxide mixtures towards external reductants. Even at a low pH, where ferrylmyoglobin is protonated and in its most reactive state, pre-incubation with reducing donors, including one-electron donors such as ferrocyanide, prior to peroxide addition renders both oxidizing equivalents available. The physiological antioxidant vitamin, ascorbate, is also able to trap both reactive species. Myoglobin can therefore act as a true ascorbate peroxidase. Ascorbate in vivo may be critical in controlling and preventing toxic side reactions of this and related haem proteins.

Characterization of low-spin ferric hydroperoxo complexes with N,N’-dimethyl- N,N’-bis(2-pyridylmethyl)-1,2-diaminoethane

Hydroperoxo intermediates formed by the interaction of [FeII(BPMEN)(MeCN)2](ClO4)2 [BPMEN = N,N’-dimethyl-N,N’-bis-(2-pyridylmethyl)-1,2-diaminoethane] with H2O2 in a CH2Cl2–MeCN mixture were characterised using EPR spectroscopy.

Analysing low-spin ferric complexes using pulse EPR techniques: a structure determination of bis (4-methylimidazole)(tetraphenylporphyrinato)iron(iii)

Continuous-wave (CW) EPR has been extensively used as a characterization tool for low-spin ferric complexes. Using the bis(4-methylimidazole) complex of iron(III) tetraphenylporphyrin as an example we show how the combination of CW EPR and pulsed EPR techniques allows for a detailed structure analysis of such ferric complexes. Both proton HYSCORE and combination-peak experiments indicate that the imidazole ligand planes are (nearly) parallel to the gx axis (±20°). Simulations of the porphyrin nitrogen contributions to the HYSCORE spectra allow a determination of the g-axes frame in the molecular frame. Combination of these spectral results and the counter-rotation principle show that the gx and gy axes are rotated 5° (±5°) away from the Np–Fe–Np axes and that the imidazole ligand planes are counter-rotated over 5° (±5°). The nitrogen hyperfine and nuclear-quadrupole interactions of the pyrrole and imidazole nitrogens are determined and discussed as a function of the electronic structure.

Characterization of a Direct Oxygen Sensor Heme Protein fromEscherichia coli: EFFECTS OF THE HEME REDOX STATES AND MUTATIONS AT THE HEME-BINDING SITE ON CATALYSIS AND STRUCTURE

A protein containing a heme-binding PAS (PAS is from the protein names in which imperfect repeat sequences were first recognized: PER, ARNT, and SIM) domain from Escherichia coli has been implied a direct oxygen sensor (Ec DOS) enzyme. In the present study, we isolated cDNA for the Ec DOS full-length protein, expressed it in E. coli, and examined its structure-function relationships for the first time. Ec DOS was found to be tetrameric and was obtained as a 6-coordinate low spin ferric heme complex. Its alpha-helix content was calculated as 53% by CD spectroscopy. The redox potential of the heme was found to be +67 mV versus SHE. Mutation of His-77 of the isolated PAS domain abolished heme binding, whereas mutation of His-83 did not, suggesting that His-77 is one of the heme axial ligands. Ferrous, but not ferric, Ec DOS had phosphodiesterase (PDE) activity of nearly 0.15 min(-1) with cAMP, which was optimal at pH 8.5 in the presence of Mg(2+) and was strongly inhibited by CO, NO, and etazolate, a selective cAMP PDE inhibitor. Absorption spectral changes indicated tight CO and NO bindings to the ferrous heme. Therefore, the present study unequivocally indicates for the first time that Ec DOS exhibits PDE activity with cAMP and that this is regulated by the heme redox state.

Mössbauer and EPR studies of the photoactivation of nitrile hydratase.

The alphabeta dimer of active nitrile hydratase from Rhodococcus sp. R312 contains one low-spin ferric ion that is coordinated by three Cys residues, two N-amide groups from the protein backbone, and one OH(-). The enzyme isolated from bacteria grown in the dark is inactive and contains the iron site as a six-coordinate diamagnetic Fe-nitrosyl complex, called NH(dark). The active state can be obtained from the dark state by photolysis of the Fe-NO bond at room temperature. Activation is accompanied by the conversion of NH(dark) to a low-spin ferric complex, NH(light), exhibiting an S = (1)/(2) EPR signal with g values of 2.27, 2.13, and 1.97. We have characterized both NH(dark) and NH(light) with Mössbauer spectroscopy. The z-axis of the 57Fe magnetic hyperfine tensor, A, of NH(light) was found to be rotated by approximately 45 degrees relative to the z-axis of the g tensor (g(z) = 1.97). Comparison of the A tensor of NH(light) with the A tensors of low-spin ferric hemes indicates a substantially larger degree of covalency for nitrile hydratase. We have also performed photolysis experiments between 2 and 20 K and characterized the photolyzed products by EPR and Mössbauer spectroscopy. Photolysis at 4.2 K in the Mössbauer spectrometer yielded a five-coordinate low-spin ferric species, NH(A), which converted back into NH(dark) when the sample was briefly warmed to 77 K. We also describe preliminary EPR photolysis studies that have yielded new intermediates.

19F NMR study of protein-induced rhombic perturbations on the electronic structure of the active site of myoglobin.

A novel C2-symmetric ring-fluorinated hemin, 13,17-bis(2-carboxyethyl)-2,8,12,18-tetramethyl-3,7-difluoroporphyrin atoiron(III), has been synthesized and was incorporated into sperm whale apomyoglobin to investigate protein-induced rhombic perturbations on the electronic structure of the active site of myoglobin (Mb) using 19F NMR spectroscopy. NMR signals for 19F atoms introduced as substituents on the present heme in ferrous low-spin and high-spin and ferric low-spin complexes have been observed and their shifts sharply reflect not only the electronic nature of the heme iron, but also in-plane asymmetry of the heme electronic structure. The two-fold symmetric electronic structure of the ring-fluorinated hemin is clearly manifested in the 19F and 1H NMR spectra of its dicyano complex. The chemical equivalence of the two fluorine atoms of the heme is removed in the active site of myoglobin and the splitting of the two 19F NMR signals provides a quantitative probe for characterizing the rhombic perturbation of the heme electronic structure induced by the heme-protein interaction. The in-plane asymmetry of heme electronic structures in carbon-monoxy and deoxy Mbs have been analyzed for the first time on the basis of the shift difference between the two 19F NMR signals of the heme and is interpreted in terms of iron-ligand binding and/or the orbital ground state of the heme. A potential utility of 19F NMR, combined with the use of a symmetric fluorinated hemin, in characterizing the heme electronic structure of myoglobin in a variety of iron oxidation, spin, and ligation states, is presented.

Catalytic and Spectroscopic Properties of Imidazole(N)-Fe(III)porphyrin Complexes in Dioxygenation of Tryptophan Derivatives.

It has been proposed that a heme-peroxide complex is formed as an intermediate state in the enzymatic reaction. Since the axial position of the heme in tryptophan 2, 3-dioxygenase (TDO) is occupied with a nitrogenous ligand from histidine residue, we studied the heme-peroxide complexes with an axial nitrogenous ligand at the fifth position in a chemical model for TDO. Addition of N-methyl-imidazole to a ferric high-spin porphyrin exhibited ESR spectrum due to a formation of ferric low-spin complex (complex A). Following addition of tetramethylammonium hydroxide to the reaction mixture, the signal due to a different ferric low-spin complex (complex B) appeared. When 3-methylindole (skatole; SK) was added to the resulting solution followed by introduction of dry oxygen gas, a new ESR signal due to a ferric low-spin complex (complex C) was detectable. A similar ESR signal was observed when N-acetyl-tryptophan ethyl ester was used in place of Sk (complex D). Bohan plotting of the crystal-field parameters as a rhombicity-tetragonality diagram obtained from ESR g-values for the complexes (A, B, C, D) revealed that these heme iron complexes can be classified into three groups having N-Fe-N (complex A), N-Fe-O (complex B), and N-Fe-OO (complexes C, D) coordinations. Product analyses for the same reaction systems demonstrated the conversion of each substrate to the dioxygenated product.

Change in Electron Configuration of Ferric Ion in Bis(cyanide)(meso-tetraalkylporphyrinatoiron(III)), [Fe(TRP)(CN)2]-, Caused by the Nonplanarity of the Porphyrin Ring

The synthesis and characterization of a series of bis(cyanide)(meso-tetraalkylporphyrinatoiron(III)), [Fe(TRP)(CN)2]- where R is H, Me, Et, and iPr, are reported. The 1H NMR spectrum of the unsubstituted [Fe(THP)(CN)2]- shows a pyrrole signal at δ = −23.19 ppm (−25 °C) in CD2Cl2, which is quite typical as a low spin ferric complex. As the bulkiness of the meso substituent increases, the pyrrole signal moves to lower magnetic field; 0.34, −2.26, and 11.94 ppm for [Fe(TMeP)(CN)2]-, [Fe(TEtP)(CN)2]-, and [Fe(TiPrP)(CN)2]-, respectively. Corresponding to the pyrrole proton signal, the cyanide carbon signal also exhibits a large downfield shift. The difference in chemical shifts between [Fe(THP)(CN)2]- and [Fe(TiPrP)(CN)2]- reaches as much as 1443 ppm at −25 °C. The substituent dependent phenomena are also observed in EPR spectra taken in frozen CH2Cl2 solution at 4.2 K. While the unsubstituted complex gives a so called large gmax type signal at 3.65, the alkyl substituted complexes exhibit axial type spectra; the EPR parameters for [Fe(TiPrP)(CN)2]- are g⊥ = 2.43 and g∥ = 1.73. These results clearly indicate that the electronic ground state changes from the usual (dxy)2(dxz, dyz)3 to the unusual (dxz, dyz)4(dxy)1 as the substituent becomes bulkier. Analysis of the EPR g values reveals that the orbital of the unpaired electron has more than 90% dxy character in the alkyl substituted complexes. The unusual electron configuration is ascribed to the destabilization of dxy orbital and/or stabilization of dxz and dyz orbitals caused by the S4 ruffled structure of the alkyl substituted porphyrin ring. Thus, in a strongly ruffled low spin complex such as [Fe(TiPrP)(L)2]±, electron configuration of iron is presented by (dxz, dyz)4(dxy)1 regardless of the kind and basicity of the axial ligand (L). In fact, low spin bis(pyridine) complex [Fe(TiPrP)(Py)2]+ gives a pyrrole signal at quite a low field, δ = +16.4 ppm at −87 °C, which is actually the lowest pyrrole signal ever reported for the low spin ferric porphyrin complexes. Correspondingly, the EPR spectrum taken at 77 K showed a clear axial type spectrum, g⊥ = 2.46 and g∥ = 1.59. In every case examined, (dxz,dyz)4(dxy)1 ground state is more or less stabilized by the addition of methanol as exemplified by the further downfield shift of the pyrrole proton and cyanide carbon signals together with the smaller EPR g⊥ values. The methanol effect is explained in terms of the stabilization of dxz and dyz relative to dxy due to the hydrogen bond formation between coordinated cyanide and methanol.

NMR studies of the electronic structure of low-spin ferric phosphonite complexes: Effect of axial p-acceptors

NMR studies of the electronic structure of low-spin ferric phosphonite complexes: Effect of axial p-acceptors

Reactions of reducing xenobiotics with oxymyoglobin. Formation of metmyoglobin, ferryl myoglobin and free radicals: An electron spin resonance and chemiluminescence study

The oxygen-haem centre of oxymyoglobin reacts with reducing xenobiotics such as hydroxylamines and phenols with the concomitant formation of metmyoglobin and oxidation of the respective xenobiotic. Metmyoglobin formation rates were measured by visible spectroscopy with xenobiotic concentrations ranging from 100 μM to 30 mM. Analogous to previous results obtained with oxyhaemoglobin, the first step in the reaction of hydroxylamines with oxymyoglobin leads to the formation of the one-electron oxidation product of hydroxylamine, a nitroxyl radical detectable by electron spin resonance. A variety of paramagnetic secondary products were also found. The terminal oxidation product of hydroxylamine and hydroxyurea was the myoglobin-nitric oxide complex, one showing similar spectral characteristics to the analogous haemoglobin-nitric oxide adduct found in our previous experiments. On the other hand, the amount of low-spin ferric complexes obtained from metmyoglobin and an excess of the respective hydroxylamine was considerably lower than the corresponding results with methaemoglobin. A second important reaction intermediate was the compound I-type ferryl haem-species detected by a recently-published chemiluminescence assay. Partial spectral resolution of the emitted light using a set of cut-off filters indicated that maximum light emission occurred above 600 nm, most probably involving excited porphyrin states. The intensity of oxymyoglobin-related light emission was considerably higher than that reported earlier with oxyhaemoglobin. This indicates a difference in the excitation mechanism which leads to the formation of the compound I-type ferry haem species.

Determination of the Geometric and Electronic Structure of Activated Bleomycin Using X-ray Absorption Spectroscopy

ADVERTISEMENT RETURN TO ISSUEPREVArticleNEXTDetermination of the Geometric and Electronic Structure of Activated Bleomycin Using X-ray Absorption SpectroscopyTami E. Westre, Kelly E. Loeb, Jeffrey M. Zaleski, Britt Hedman, Keith O. Hodgson, and Edward I. SolomonCite this: J. Am. Chem. Soc. 1995, 117, 4, 1309–1313Publication Date (Print):February 1, 1995Publication History Published online1 May 2002Published inissue 1 February 1995https://doi.org/10.1021/ja00109a014RIGHTS & PERMISSIONSArticle Views282Altmetric-Citations60LEARN ABOUT THESE METRICSArticle Views are the COUNTER-compliant sum of full text article downloads since November 2008 (both PDF and HTML) across all institutions and individuals. These metrics are regularly updated to reflect usage leading up to the last few days.Citations are the number of other articles citing this article, calculated by Crossref and updated daily. Find more information about Crossref citation counts.The Altmetric Attention Score is a quantitative measure of the attention that a research article has received online. Clicking on the donut icon will load a page at altmetric.com with additional details about the score and the social media presence for the given article. Find more information on the Altmetric Attention Score and how the score is calculated. Share Add toView InAdd Full Text with ReferenceAdd Description ExportRISCitationCitation and abstractCitation and referencesMore Options Share onFacebookTwitterWechatLinked InReddit PDF (602 KB) Get e-Alerts Get e-Alerts

Rotation and orientation of axially coordinated imidazoles in low-spin ferric porphyrin complexes

The variable temperature 1H NMR spectra of a series of low-spin bis(imidazole)tetrakis(2, 4, 6-trialkylphenyl)porphinatoiron(III) chlorides, (R-TPP)Fe(L) 2Cl, showed that the rotation of axially coordinated 2-alkyl- and benzimidazoles slowed down on the NMR time scale at low temperature to give four pyrrole signals. The 13C NMR spectra of the 13C enriched (Me-TPP)Fe(2- methylimidazole) 2Cl at the meso positions gave two meso signals with equal intensity at the temperature range below −25 °C. These results indicate that each of the axial ligands is perpendicularly aligned over a diagonal C mesoFeC meso axis. The shift range of the pyrrole protons reached as much as 12 ppm at −56 °C. The apparent unfavorable orientation of the ligands was explained in terms of the S 4 deformed structure of the porphinatoiron core in solution. The relatively small slopes in Curie plots of the pyrrole-H and the meso 13C signals were also ascribed to the deformed structure of the core. In the mixed ligand complexes having a hindered 2-isopropylimidazole and an unhindered 1-methylimidazole ligand, it was suggested from the 1H and 13C NMR splitting patterns that the rotation of only one of the ligands, 2-isopropylimidazole, was frozen. In these complexes the spread of the pyrrole signals increased to nearly 20 ppm. The activation free energies for rotation were determined by the dynamic NMR technique. They changed in the range of 11.3 to 13.6 kcal mol −1 depending upon the bulkiness of the axial ligands and o-alkyl substituents. Close examination of the dynamic process using the saturation transfer technique revealed that the dissociation of the axial ligands occurred concomitantly during the rotation process in the case of the bis(2-isopropylimidazole) complexes. In contrast, pure rotation process was observed in the complexes with 2-methyl-, 2-ethyl-, 1, 2-dimethyl- and benzimidazole. X band ESR spectra of these complexes were taken at 4.2 K in CH 2Cl 2 glass. Although complexes with perpendicularly aligned planar ligands tend to exhibit so called ‘strong g max’ type signals, the complexes studied here showed signals with smaller g z values.

Coordination structure and chemical reactivity of hemoprotein-butyl peroxide complex

ESR and optical spectra ascribed to be the hemoprotein-butylperoxide complex were detected for the frozen aqueous solution containing whale met-Mb and t- or n-butylhydroperoxide at pH 10. The observed ESR and optical parameters of the complex were characteristic to those of six coordinate ferric low-spin complexes, having the butylperoxide anion at the axial position of heme. pH dependent ESR measurements demonstrated the formation of the complex in the biological pH regions (7.0). Time dependent ESR and optical measurements indicated that the complex may be one of the intermediate species in the processes of heme degradation reaction induced by butylperoxide.