Chemical properties of water-soluble porphyrins. 4. The reaction of a 'picket-fence-like' iron (III) complex with the superoxide oxygen couple

Solution properties of the iron-(III) 'picket-fence-like' porphyrin, Fe(III)-alpha,alpha,alpha, beta-tetra-ortho (N-methyl-isonicotinamidophenyl) porphyrin, (Fe(III)PFP) were investigated. These were acid/base properties of the aquo complex with pKa of 3.9 and its aggregation (formation of dimer with K = 1 X 10(-10) dm3 mol-1), complex formation with cyanide ions and 1-methyl imidazole (1-MeIm), spectral properties of the three iron complexes in their ferric and ferrous form and the one-electron reduction potential of these complexes. Knowing these properties, the reaction of the ferric complexes, aquo, dicyano and bis (1-MeIm), with the superoxide radical and other reducing radicals were studied using the pulse radiolysis technique. The second-order reaction rate constant of O2- with the iron (III) aquo complex which governs the catalytic efficiency of the metalloporphyrin upon the disproportionation of the superoxide radical was 7.6 X 10(7) dm3 mol-1 s-1, two orders of magnitude faster when compared to the reaction of each of the other complexes. The reduction by other radicals with all iron (III) complexes had similar second-order rate constants (10(9) to 10(10) dm3 mol-1 s-1). The reduction reaction in all cases produced Fe(II)PEP and no intermediate was found. The oxidation reaction of Fe(II)PEP by O2- was one order of magnitude faster when compared to the reduction of Fe(III)PFP by the same radical. Since the reactivity of O2- toward the three iron (III) porphyrin complexes follows their reduction potentials, it is suggesting the formation of a peroxo Fe(II) porphyrin as an intermediate. The reactions of the Fe(II)PFP complexes with dioxygen were also studied. The aquo complex was found to be first order in O2 and second order in Fe(II)PFP, suggesting the formation of a peroxo Fe(II) porphyrin as an intermediate. The intermediate formation was corroborated by evidence of the rapid CO binding reaction to the aquo complex of Fe(II)PFP. The two other complexes reacted very slowly with O2 as well as with CO.     

Transient intermediates in the reduction of Fe(III) myoglobin-ligand complexes by electrons at low temperature

1. The reductions of a number of sperm-whale Fe(III) myoglobin-ligand complexes by electrons generated by gamma-irradiation in ethylene glycol/water glass, have been investigated by using low-temperature spectrophotometry. The ligands are azide, fluoride, imidazole and water. 2. The reduction of the Fe(III) myoglobin-ligand complexes at 77 K leads to the formation of low-spin liganded Fe(II) myoglobin, in the case of the azide, imidazole and water derivatives, while the reduction of the fluoride derivative proceeds both by a pathway involving prior dissociation of the ligand and with the ligand in position. 3. Investigation of the effect of temperature on the stability of the Fe(II) myoglobin-ligand complexes indicates that more than one bound states exists in dissociation of the ligand molecule from the ferrous heme iron of the reduced azide and imidazole derivatives. 4. The results are discussed in terms of the possible structure of the Fe(II) myoglobin complexes and it is suggested that the low-spin state is created by a strained configuration of the heme center with the iron atom in an intermediate position relative to the heme plane.     

Spin, oxidation, and ligand states of p-nitrothiophenolatoiron(III) complex of protoporphyrin-IX-dimethylester in the presence of 1-MeIm: magnetic circular dichroism and 1H nuclear magnetic resonance spectroscopic studies

Complex formation of 5-coordinated iron(III) heme containing thiolate anion (p-nitrothiophenol) with imidazole (1-methylimidazole) showed very interesting features depending on the nature of the solvent and the ratio of the ligand to heme. The complexes formed under different conditions were not only low spin iron(III) complexes with a thiolate anion and an imidazole or with two imidazoles, but also reduced (iron(II] complexes with a thiolate and an imidazole or with two imidazoles. Absorption, magnetic circular dichroism, and 1H NMR spectroscopies could identify the complex formed when they were used concurrently. The dependence of polarity of the solvents used on the resultant chemical species was ascribed to the stability of Fe(III) or Fe(II) complex in the different solvents. The iron(III) complex with a thiolate anion and an imidazole was found to be reduced automatically to the iron(II) complex with a thiolate and an imidazole which exchanged ligand to the iron(II) bisimidazoles in the presence of excess imidazole. This study showed that the ligands of heme are easily exchanged and that the heme iron(III) is automatically reduced in several conditions. Possible significance with respect to biological systems containing a sulfur ligand is discussed.     

Ligational effects on reduction of myoglobin and horseradish peroxidase by inorganic reagents

Previous studies of the reduction of metmyoglobin and adducts by dithionite have been extended to horseradish peroxidase and its complexes. In addition, the reduction of metmyoglobin, horseradish peroxidase and adducts by a much bulkier reactant, cobalt(II) sepulchrate has been studied. Similar patterns of kinetic behavior were observed, namely, direct reduction of cyanide and imidazole adducts of the iron(III) proteins and indirect (via dissociation) reduction of the fluoride adduct. In the reduction of horseradish ferriperoxidase by cobalt(II) sepulchrate, three steps are observed and the spectral properties of the intermediate(s) and their kinetic behavior delineated. The final product is ferroperoxidase confirmed by spectral properties and its behavior on oxygenation. Reduction of cytochrome c(III) and Hipip by cobalt(II) sepulchrate appears to be a uniphasic reaction and second-order rate constants have been determined.     

Evidence for a μ-oxi-diiron(III) species from Mössbauer spectroscopy

Mössbauer spectra of frozen aqueous solutions that were formed by dissolving pyridinium μ-oxo- bis(trichloroferrate(III))pyridine in watet provide direct evidence for the existence of a μ-oxo-bis- (ferrate(III)) species. Thus the μ-oxo-bis(ferrate(III)) bridge remains intact in aqueous solution. This species should be of use in modelling two iron proteins such as haemerythrin.     

Formation of mu-oxo dimer by the reaction of (meso-tetraphenylporphyrinato)iron(III) with various imidazoles

Reactions between (meso-tetraphenylporphyrinato)iron(III) perchlorate [Fe(tpp)]ClO4 and various imidazoles have been examined in CD2Cl2 solutions. 1H NMR analysis revealed the formation of three kinds of complex; mu-oxo dimer, mono-imidazole adduct, and bis-imidazole adduct. The product ratios changed to a great extent depending on the amount and nature of imidazoles. In general, addition of less than 1.0 equiv of imidazole relative to [Fe(tpp)]ClO4 led to the formation of both mu-oxo dimer and mono-imidazole adduct. However, by the addition of excess amount of imidazole, either the mu-oxo dimer or bis-imidazole adduct was formed exclusively depending on the bulkiness of the imidazole used. In the case of bulky imidazole such as 2-methylbenzimidazole or 2-isopropyl-1-methylimidazole, the mu-oxo dimer was formed quantitatively. In the case of less bulky imidazole such as parent imidazole or 1-methylimidazole, bis-imidazole adduct became the sole product. The results have been explained in terms of the difference in steric interactions between the axial ligands and porphyrin core; the severe steric repulsion prohibits the formation of bis-adduct in the case of bulky imidazoles. As a result, bulky imidazoles prefer to behave as a base; they abstract a proton from coordinated water, and lead to the formation of mu-oxo dimer. Thus, the role of bulky imidazoles in these reactions has some relevance to that of distal histidine in hemoglobin and peroxidase.     

Noninnocent effect of axial ligand on the heme degradation process: a theoretical approach to hydrolysis pathway of verdoheme to biliverdin

Conversion of iron(II) verdoheme to iron(II) biliverdin in the presence of hydroxyl ion as a nucleophile and imidazole, pyridine, water, hydroxyl, cyanide, phenolate, chloride, thiolate and imidazolate as axial ligands was investigated using the B3LYP method and the 6-31G basis set. In the five-coordinated pathway the reactants and products are in the ground triplet state. In this path, hydroxyl ion directly attacks the macrocycle. The exothermic energy for addition of hydroxyl ion to iron(II) verdoheme with various ligands is 169.55, 166.34 and 164 kcal mol⁻¹ for water, pyridine and imidazole, energies which are around 30–60 kcal mol⁻¹ more exothermic than those for the other axial ligands used in this study. Therefore, imidazole, water and pyridine axial ligands can facilitate hydrolytic cleavage of iron(II) verdoheme to form open-chained helical iron(II) biliverdin complexes. The activation barrier for the conversion of iron(II) verdoheme hydroxyl species to the iron(II) biliverdin complex is estimated to be 5.2, 4.2, 4.35, 13.76 and 14.05 kcal mol⁻¹ for imidazole, water, cyanide, thiolate and imidazolate, respectively. Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Role of the low-affinity binding site in electron transfer from cytochrome C to cytochrome C peroxidase

The interaction of yeast iso-1-cytochrome c (yCc) with the high- and low-affinity binding sites on cytochrome c peroxidase compound I (CMPI) was studied by stopped-flow spectroscopy. When 3 microM reduced yCc(II) was mixed with 0.5 microM CMPI at 10 mM ionic strength, the Trp-191 radical cation was reduced from the high-affinity site with an apparent rate constant >3000 s(-1), followed by slow reduction of the oxyferryl heme with a rate constant of only 10 s(-1). In contrast, mixing 3 microM reduced yCc(II) with 0.5 microM preformed CMPI *yCc(III) complex led to reduction of the radical cation with a rate constant of 10 s(-1), followed by reduction of the oxyferryl heme in compound II with the same rate constant. The rate constants for reduction of the radical cation and the oxyferryl heme both increased with increasing concentrations of yCc(II) and remained equal to each other. These results are consistent with a mechanism in which both the Trp-191 radical cation and the oxyferryl heme are reduced by yCc(II) in the high-affinity binding site, and the reaction is rate-limited by product dissociation of yCc(III) from the high-affinity site with apparent rate constant k(d). Binding yCc(II) to the low-affinity site is proposed to increase the rate constant for dissociation of yCc(III) from the high-affinity site in a substrate-assisted product dissociation mechanism. The value of k(d) is <5 s(-1) for the 1:1 complex and >2000 s(-1) for the 2:1 complex at 10 mM ionic strength. The reaction of horse Cc(II) with CMPI was greatly inhibited by binding 1 equiv of yCc(III) to the high-affinity site, providing evidence that reduction of the oxyferryl heme involves electron transfer from the high-affinity binding site rather than the low-affinity site. The effects of CcP surface mutations on the dissociation rate constant indicate that the high-affinity binding site used for the reaction in solution is the same as the one identified in the yCc*CcP crystal structure.     

Bile pigment--protein interactions. Coupled oxidation of cytochrome c

A new methodology is described for the chemical modification of the heme prosthetic group of horse heart cytochrome c. The selective modification of the heme moiety of cytochrome c is facilitated by utilizing coupling oxidation conditions. Comparison of the absorption spectra of this chemically modified cytochrome c species in two different solvents (aqueous pyridine and carbon monoxide saturated 6 M guanidinium chloride) with those of two model compounds [bis(pyridine)(2,3,7,8,12,13,17,18-octaethyl-5-oxaporphyrinato)iron(II) tetrafluoroborate salt and (pyridine)carbonyl-(2,3,7,8,12,13,17,18-octaethyl-5-oxaporphyrinato)iron(II) tetrafluoroborate salt] shows that coupled oxidation of cytochrome c affords a new protein with a covalently bound iron(II) oxaporphyrin prosthetic group. Amino acid analysis of this protein-bound iron(II) oxaporphyrin species reveals that only limited modification of the primary structure of the apoprotein occurs during coupled oxidation of cytochrome c. This protein-bound iron(II) oxaporphyrin species is also interconvertible to a protein-bound bilatriene species under hydrolytic conditions. The synthetic utility of the coupled oxidation of cytochrome c for the preparation of chromoproteins which possess covalently bound iron(II) oxaporphyrin and bilatriene prosthetic groups is considered.     

First success of catalytic epoxidation of olefins by an electron-rich iron(III) porphyrin complex and H2O2: imidazole effect on the activation of H2O2 by iron porphyrin complexes in aprotic solvent

An electron-rich iron(III) porphyrin complex (meso-tetramesitylporphinato)iron(III) chloride [Fe(TMP)Cl], was found to catalyze the epoxidation of olefins by aqueous 30% H₂O₂ when the reaction was carried out in the presence of 5-chloro-1-methylimidazole (5-Cl-1-MeIm) in aprotic solvent. Epoxides were the predominant products with trace amounts of allylic oxidation products, indicating that Fenton-type oxidation reactions were not involved in the olefin epoxidation reactions. cis-Stilbene was stereospecifically oxidized to cis-stilbene oxide without giving isomerized trans-stilbene oxide product, demonstrating that neither hydroperoxy radical (HOO·) nor oxoiron(IV) porphyrin [(TMP)Fe^IV=O] was responsible for the olefin epoxidations. We also found that the reactivities of other iron(III) porphyrin complexes such as (meso-tetrakis(2,6-dichlorophenyl)porphinato)iron(III) chloride [Fe(TDCPP)Cl], (meso-tetrakis(2,6-difluorophenyl)porphinato)iron(III) chloride [Fe(TDFPP)Cl], and (meso-tetrakis(pentafluorophenyl)porphinato)iron(III) chloride [Fe(TPFPP)Cl] were significantly affected by the presence of the imidazole in the epoxidation of olefins by H₂O₂. These iron porphyrin complexes did not yield cyclohexene oxide in the epoxidation of cyclohexene by H₂O₂ in the absence of 5-Cl-1-MeIm in aprotic solvent; however, addition of 5-Cl-1-MeIm to the reaction solutions gave high yields of cyclohexene oxide with the formation of trace amounts of allylic oxidation products. We proposed, on the basis of the results of mechanistic studies, that the role of the imidazole is to decelerate the O–O bond cleavage of an iron(III) hydroperoxide porphyrin (or H₂O₂–iron(III) porphyrin adduct) and that the intermediate transfers its oxygen to olefins prior to the O–O bond cleavage.     

The reactions of octaethylporphyrin and its iron (II) and iron (III) complexes with free radicals

The reactions of dilute solutions of octaethylporphyrin and its iron (II) and iron (III) complexes with methyl, 2-cyanopropyl, t-butoxy, and benzoyloxy radicals are described. The results are summarized: (i) The reactivity of the porphyrin and its high-spin iron (II) and iron (III) complexes toward alkyl and t-butoxy radicals stands in the order: Fe^II > Fe^III ? free porphyrin. For benzoyloxy radicals the order is Fe^II > Porp > Fe^III. (ii) The exclusive path of reaction of high-spin iron (II) porphyrin with radicals is the rapid reduction of the radical and generation of an iron (III) porphyrin. The dominant path of reaction of high-spin iron (III) porphyrin with alkyl and (presumably) t-butoxy radicals is a rapid axial inner sphere reduction of the porphyrin. An axial ligand of iron is transferred to the radical. (iv) The reaction of benzoyloxy radicals with high or low-spin iron (III) porphyrins occurs primarily at the meso position. With the low-spin dipyridyl complex in pyridine the attendant reduction to iron (II) can be observed spectrally. Methyl radicals also reduce this complex by adding to the meso position. (v) The reaction of a radical with either an iron (II) or an iron (III) porphyrin results in the generation of the other valence state of iron and consequently oxidation and reduction products emanating from both iron species are obtained. (vi) No evidence for an iron (IV) is intermediate is apparent. (vii) Iron (II) porphyrins in solvents that impart either spin state are easily oxidized by diacyl peroxides. The occurrence of both axial and peripheral redox reactions with the iron complexes supports an underlying premise of a recent theory of hemeprotein reactivity. The relevance of the work to bioelectron transfer and heme catabolism is noted.     

Electron spin echo envelope modulation spectroscopic study of iron-nitrogen interactions in myoglobin hydroxide and Fe(III) tetraphenylporphyrin models.

The electron-nuclear coupling in low-spin iron complexes including myoglobin hydroxide (MbOH) and two related model compounds, Fe(III) tetraphenylporphyrin(pyridine)(OR-) (R = H or CH3) and Fe(III) tetraphenylporphyrin(butylamine)(OR-) was investigated using electron spin echo envelope modulation (ESEEM) spectroscopy. The assignment of frequency components in ESEEM spectra was accomplished through the use of nitrogen isotopic substitution wherever necessary. For example, the proximal imidazole coupling in MbOH was investigated without interference from the contributions of porphyrin 14N nuclei after substitution of the heme in native Mb with 15N-labeled heme. Computer simulation of spectra using angle selected techniques enabled the assignment of parameters describing the hyperfine and quadrupole interactions for axially bound nitrogen of imidazole in MbOH, of axial pyridine and butylamine in the models, and for the porphyrin nitrogens of the heme in native MbOH. The isotropic component of axial nitrogen hyperfine interactions exhibits a trend from 5 to 4 MHz, with imidazole (MbOH) greater than pyridine greater than amine. The nuclear quadrupole interaction coupling constant e2Qq was near 2 MHz for all nitrogens in these complexes. The Qzz axis of the nuclear quadrupole interaction tensor for the proximal imidazole nitrogen in MbOH was found to be aligned near gz (gmax) in MbOH, suggesting that gz is near the heme normal. A crystal field analysis, that allows a calculation of rhombic and axial splittings for the d orbitals of the t2g set in a low-spin heme complex, based on the g tensor assignment gz greater than gy greater than gx, yielded results that are consistent with the poor pi-acceptor properties expected for the closed shell oxygen atom of the hydroxide ligand in MbOH. A discussion is presented of the unusual results reported in a linear electric field effect in EPR (LEFE) study of MbOH published previously [Mims, W. B., & Peisach, J. (1976) J. Chem. Phys. 64, 1074-1091].     

Oxidative dehydrogenation of an iron-tetraazacyclotetradecatriene complex

The oxidative dehydrogenation of the bis(N- methyl imidazole)(meso-5,5,7,12,12,14-hexamethyl- 1 4,8 11-tetraazacyclotetradeca-1,3,8-triene)iron(II) complex, forming the corresponding 1 3,8,10-tetraene product was investigated by cyclic voltammetry, spectroelectrochemistry and stopped-flow kinetics with [Fe(CN)₆]³⁻, at 25 °C, I = 0.50 M and pH 7–10. The results led to a mechanism consistent with a reversible one electron transfer process generating iron(III) species which lose a proton (pK_a = 9.77) and undergo induced electron transfer in the presence of the hexacyanoferrate(III) ion (k = 4.2 × 10⁵ M⁻¹ s⁻¹). The intermediate precursor complex (λ_max = 665 nm) formed at this step, converted to the tetraene product according to a first order kinetics, with k = 0.12 s⁻¹.     

Preparation,characterization and crystal structures of manganese(II), iron(III) and copper(II) complexes of the bis[di-1,1-(2-pyridyl)ethyl]amine (BDPEA) ligand; evaluation of their DNA cleavage activities

The synthesis of a new tetrapyridyl ligand, bis[di-1,1-(2-pyridyl)ethyl]amine (BDPEA), is described. Complexation of this ligand with manganese(II), iron(III) or copper(II) chlorides afforded mononuclear complexes: Mn(BDPEA)Cl2 (1) [Fe (BDPEA)Cl2]Cl (2) and [Cu(BDPEA)Cl]Cl (3). In all cases, BDPEA is coordinated to the metal center by three pyridine nitrogen atoms and the secondary amine. The geometrical environments around the metals in Mn(BDPEA)Cl2 and [Fe(BDPEA)Cl2]Cl are best described as distorted octahedrals and in [Cu (BDPEA)Cl]Cl as a slightly distorted square pyramid. The DNA cleavage activities of manganese(II), iron (III) or copper(II) complexes of both BDPEA and another tetrapyridyl ligand, bis[di(2-pyridyl) methyl]amine (BDPMA), in the presence of an oxidant (H2O2) or a reducing agent (ascorbate) with air, are reported. The iron(III) complexes exhibited significantly enhanced efficiencies, compared to copper(II) complexes. [Fe(BDPEA)Cl2]Cl is found to be the most active DNA cleaver, in agreement with a better stability of BDPEA in oxidizing conditions.     

EPR study of dimerization and conformational change of Cu(II)-cytochrome c and its undeca- and octapeptide

The EPR study of cytochrome c in which FE(III) ion is replaced with Cu(II) shows that there are two types of monomer (a: 4 less than pH less than 6, and b: 6 less than pH less than 11.5) and two types of dimer (A: pH less than 4 and B: pH less than 11.5) formed depending upon the pH value of the solution. Computer simulation of the EPR spectra of the dimers indicates that the structure of the dimer A has a larger lateral shift than in the dimer B. It is also shown that in monomer a, the imidazole nitrogen of 18-His is not bound to Cu(II), while it is bound in the monomer b. In the undeca- and octapeptide of Cu(II)-cytochrome c, polymers are formed in acidic solutions. As the pH is raised, depolymerization proceeds to yield the monomer and the dimer. The structure of the dimer in both peptides is found to be similar to that of the dimer B of Cu(II)-cytochrome c. In the monomer of the peptides, neither the imidazole of 18-His nor the imidazole added in excess is bound to Cu(II) in the entire pH range. It is also concluded that the dimerization in Cu(II)-porphyrins interferes with the apical coordination of basic ligand, or vice versa.     

First success of catalytic epoxidation of olefins by an electron-rich iron(III) porphyrin complex and H2O2: imidazole effect on the activation of H2O2 by iron porphyrin complexes in aprotic solvent

An electron-rich iron(III) porphyrin complex (meso-tetramesitylporphinato)iron(III) chloride [Fe(TMP)Cl], was found to catalyze the epoxidation of olefins by aqueous 30% H₂O₂ when the reaction was carried out in the presence of 5-chloro-1-methylimidazole (5-Cl-1-MeIm) in aprotic solvent. Epoxides were the predominant products with trace amounts of allylic oxidation products, indicating that Fenton-type oxidation reactions were not involved in the olefin epoxidation reactions. cis-Stilbene was stereospecifically oxidized to cis-stilbene oxide without giving isomerized trans-stilbene oxide product, demonstrating that neither hydroperoxy radical (HOO·) nor oxoiron(IV) porphyrin [(TMP)Fe^IV=O] was responsible for the olefin epoxidations. We also found that the reactivities of other iron(III) porphyrin complexes such as (meso-tetrakis(2,6-dichlorophenyl)porphinato)iron(III) chloride [Fe(TDCPP)Cl], (meso-tetrakis(2,6-difluorophenyl)porphinato)iron(III) chloride [Fe(TDFPP)Cl], and (meso-tetrakis(pentafluorophenyl)porphinato)iron(III) chloride [Fe(TPFPP)Cl] were significantly affected by the presence of the imidazole in the epoxidation of olefins by H₂O₂. These iron porphyrin complexes did not yield cyclohexene oxide in the epoxidation of cyclohexene by H₂O₂ in the absence of 5-Cl-1-MeIm in aprotic solvent; however, addition of 5-Cl-1-MeIm to the reaction solutions gave high yields of cyclohexene oxide with the formation of trace amounts of allylic oxidation products. We proposed, on the basis of the results of mechanistic studies, that the role of the imidazole is to decelerate the O–O bond cleavage of an iron(III) hydroperoxide porphyrin (or H₂O₂–iron(III) porphyrin adduct) and that the intermediate transfers its oxygen to olefins prior to the O–O bond cleavage.     

Hydroxyl radical and singlet oxygen production and DNA damage induced by carcinogenic metal compounds and hydrogen peroxide

Carcinogenic chromium(VI), iron(III) nitrilotriacetate, cobalt(II), and nickel(II) react with hydrogen peroxide leading to the production of active species including hydroxyl radical and singlet oxygen, which cause DNA damage.     

Iron-bleomycin interactions with oxygen and oxygen analogues. Effects on spectra and drug activity.

Despite extensive structural dissimilarities, iron . bleomycin complexes and heme-containing oxygenases display remarkable similarities in binding oxygen antagonists and in spectral properties deriving from bound iron. Fe(II)-bleomycin reversibly forms a complex with either CO or isocyanide (lambda max = 384 and 497 nm, respectively), either of which interfere with its oxygen-dependent cleavage of DNA. A similar but paramagnetic complex forms with NO (lambda max = 470 nm; AN = 24 G). In contrast, cyanide enhances bleomycin activity against DNA. Complexes of bleomycin and FE(III), formed either by direct association or by autoxidation of the Fe(II) . bleomycin complex, exhibit indistinguishable EPR and visible spectra, which change characteristically with pH. At neutral pH, Fe(III) . bleomycin is a low spin complex (g = 2.45, 2.18, 1.89; lambda max = 365, 384 nm) and, at low pH, it is a high spin rhombic complex (geff = 9.4, 4.3; lambda max = 430 nm). These complexes are interconvertible (pK 4.3). Fe(II) . bleomycin oxidation, although reversible by spectral criteria, is accompanied by drug inactivation unless DNA is present.     

Coordination chemistry of the heme in cystathionine beta-synthase: formation of iron(II)-isonitrile complexes

Interaction of rat and human cystathionine-beta-synthase (CBS) with various potential ligands has been studied by visible and EPR spectroscopy in order to explore the coordination chemistry of this atypical hemeprotein. Ferric CBS did not react with any classical hemeprotein ligands, such as various imidazole and pyridine derivatives, N(-)(3) and isonitriles RNC. Ferrous CBS also failed to bind these nitrogenous ligands or nitrosoalkanes. However, it reacts with various isonitriles RNC, leading to complexes characterized by a Soret peak at 433 +/- 2 nm. Binding of isonitriles to ferrous CBS is a relatively slow process; its rate markedly depends on the nature of R. It thus seems that the only exogenous ligands able to bind CBS iron are carbon-centered, very strong heme-Fe(II) ligands such as CNR, CO, and CN(-), presumably after dissociation of the CBS-iron(II)-cysteinate bond. Isonitriles appear as interesting tools for further studies on the topology of CBS active site.     

The reaction of Fe(III) myoglobin fluoride with reducing agents.

The reaction of reducing agents with Fe(III) myoglobin fluoride from sperm whale was studied at alkaline pH values. The rate of reduction by dithionite was indistinguishable from the rate of ligand dissociation even when the values of the rate constants for both were only 1.0 X 10(-3)S-1 (at pH 10.6). Reduction by the reduced Methyl Viologen radical ion and reduced Safranine was faster than the rate of dissociation, providing evidence that these reductants can donate electrons to the iron centre via a pathway involving an (undetectable) liganded Fe(II) intermediate.