Reversible NO motion in crystalline [Fe(Porph)(1-MeIm)(NO)] derivatives

The synthesis, characterization, and X-ray structures of three low-spin (nitrosyl)iron(II) tetraarylporphyrinates, [Fe(TpXPP)(NO)(1-MeIm)], where X = F (in a triclinic and a monoclinic form) and OCH(3) are reported. All three molecules, at 100 K, have a single orientation of NO. These structures are the first examples of ordered NO's in [Fe(Porph)(NO)(1-MeIm)] complexes. The three new derivatives have similar structural features including a previously unnoted "bowing" of the N(NO)-Fe-N(Im) angle caused by a concerted tilting of the axial Fe-N(NO) and Fe-N(Im) bonds. Structural features such as the displacement of Fe out of the mean porphyrin plane toward NO, tilting of the Fe-N(NO) bond off the heme normal, and the asymmetry of the Fe-N(por) bonds further strengthen and confirm observations from earlier studies. The [Fe(TpXPP)(NO)(1-MeIm)] complexes were also studied at temperatures between 125 and 350 K to investigate temperature-dependent variations and trends in the coordination group geometry. At varying temperatures (above 150 K), all three derivatives display a second orientation of the NO ligand. The population and depopulation of this second orientation are thermally driven, with no apparent hysteresis. Crystal packing appears to be the significant feature in defining the order/disorder of the NO ligand. The length of the bond trans to NO, Fe-N(Im), was also found to be sensitive to temperature variation. The Fe-N(Im) bond length increases with increased temperature, whereas no other bonds change appreciably. The temperature-dependent Fe-N(Im) bond length change and cell volume changes are consistent with a "soft" Fe-N(Im) bond. Variable-temperature measurements show that the N-O stretching frequency changes with the Fe-N(Im) bond length. Temperature-dependent changes in the Fe-NIm bond length and N-O stretching frequency were also found to be completely reversible with no apparent hysteresis.     

Spectroscopic and structural characterization of the [Fe(imidazole)(6)](2+) cation

Spectroscopic, structural, and magnetic data are presented for Fe(C(3)H(4)N(2))(6)(NO(3))(2), which facilitate a precise definition of the electronic and molecular structure of the [Fe(Im)(6)](2+) cation. The structure was determined at 120(1) K by X-ray diffraction methods. The salt crystallizes in the trigonal space group R3 with unit-cell parameters a = 12.4380(14) A, c = 14.5511(18) A, and Z = 3. All the imidazole ligands are equivalent with an Fe-N bond distance of 2.204(1) A. Variable-temperature inelastic neutron scattering (INS) measurements identify a cold magnetic transition at 19.4(2) cm(-1) and a hot transition at 75.7(6) cm(-1). The data are interpreted using a ligand field Hamiltonian acting in the weak-field (5)D basis, from which the diagonal trigonal field splitting of the (5)T(2g) (O(h)) term is estimated as approximately 485 cm(-1), with the (5)A(g) (S(6)) component lower lying. High-field multifrequency (HFMF) EPR data and measurements of the magnetic susceptibility are also reported and can be satisfactorily modeled using the energies and wave functions derived from analysis of the INS data. The electronic and molecular structures are related through angular overlap model calculations, treating the imidazole ligand as a weak pi-donor.     

Neutral {Fe(NO)2}9 dinitrosyliron complex (DNIC) [(SC6H4-o-NHCOPh)(Im)Fe(NO)2] (Im = imidazole): interconversion among the anionic/neutral {Fe(NO)2}9 DNICs and Roussin's red ester

A neutral {Fe(NO)2}9 dinitrosyliron complex (DNIC) [(SC6H4-o-NHCOPh)(Im)Fe(NO)2] (Im = imidazole; 2) was prepared by the reaction of [Fe(mu-SC6H4-o-NHCOPh)(NO)2]2 (1) and 2 equiv of imidazole. In the synthesis of the anionic {Fe(NO)2}9 DNIC [(SC6H4-o-NHCOPh)2Fe(NO)2]- (3), thiolate [SC6H4-o-NHCOPh]- triggers ligand substitution of DNIC 2 to yield DNIC 3. At 298 K, complexes 2 and 3 exhibit well-resolved nine- and five-line electron paramagnetic resonance (EPR) signals at g = 2.031 and 2.029, respectively, the characteristic g value of DNICs. The facile interconversions among the neutral {Fe(NO)2}9 complex 2, the anionic {Fe(NO)2}9 complex 3, and Roussin's red ester 1 were demonstrated. The EPR spectrum (the pattern of hyperfine splitting) in combination with the IR nu(NO) (the relative position of the nu(NO) stretching frequencies) spectrum may serve as an efficient tool for the discrimination of the anionic {Fe(NO)2}9 DNICs, the neutral {Fe(NO)2}9 DNICs, and Roussin's red ester.     

Low-spin ferriheme models of the cytochromes: correlation of molecular structure with EPR spectral type

The preparation and characterization of the following bis-imidazole and bis-pyridine complexes of octamethyltetraphenylporphyrinatoiron(III), Fe(III)OMTPP, octaethyltetraphenylporphyrinatoiron(III), Fe(III)OETPP, and tetra-beta,beta'-tetramethylenetetraphenylporphyrinatoiron(III), Fe(III)TC(6)TPP, are reported: paral-[FeOMTPP(1-MeIm)(2)]Cl, perp-[FeOMTPP(1-MeIm)(2)]Cl, [FeOETPP(1-MeIm)(2)]Cl, [FeTC(6)TPP(1-MeIm)(2)]Cl, [FeOMTPP(4-Me(2)NPy)(2)]Cl, and [FeOMTPP(2-MeHIm)(2)]Cl. Crystal structure analysis shows that paral-[FeOMTPP(1-MeIm)(2)]Cl has its axial ligands in close to parallel orientation (the actual dihedral angle between the planes of the imidazole ligands is 19.5 degrees ), while perp-[FeOMTPP(1-MeIm)(2)]Cl has the axial imidazole ligand planes oriented at 90 degrees to each other and 29 degrees away from the closest N(P)-Fe-N(P) axis. [FeOETPP(1-MeIm)(2)]Cl has its axial ligands close to perpendicular orientation (the actual dihedral angle between the planes of the imidazole ligands is 73.1 degrees ). In all three cases the porphyrin core adopts relatively purely saddled geometry. The [FeTC(6)TPP(1-MeIm)(2)]Cl complex is the most planar and has the highest contribution of a ruffled component in the overall saddled structure compared to all other complexes in this study. The estimated numerical contribution of saddled and ruffled components is 0.68:0.32, respectively. Axial ligand planes are perpendicular to each other and 15.3 degrees away from the closest N(P)-Fe-N(P) axis. The Fe-N(P) bond is the longest in the series of octaalkyltetraphenylporphyrinatoiron(III) complexes due to [FeTC(6)TPP(1-MeIm)(2)]Cl having the least distorted porphyrin core. In addition to these three complexes, two crystalline forms each of [FeOMTPP(4-Me(2)NPy)(2)]Cl and [FeOMTPP(2-MeHIm)(2)]Cl were obtained. In all four of these cases the axial planes are in nearly perpendicular planes in spite of quite different geometries of the porphyrin cores (from purely saddled to saddled with 30% ruffling). The EPR spectral type correlates with the geometry of the OMTPP, OETPP and TC(6)TPP complexes. For the paral-[FeOMTPP(1-MeIm)(2)]Cl, a rhombic signal with g(1) = 1.54, g(2) = 2.51, and g(3) = 2.71 is consistent with nearly parallel axial ligand orientation. For all other complexes of this study, "large g(max)" signals are observed (g(max) = 3.61 - 3.27), as are observed for nearly perpendicular ligand plane arrangement. On the basis of this and previous work, the change from "large g(max)" to normal rhombic EPR signal occurs between axial ligand plane dihedral angles of 70 degrees and 30 degrees.     

Conformational preferences of the axial ligands in some metalloporphyrins. A theoretical study

The conformational preferences of the axial ligands have been determined for several metalloporphyrins MPL and MPLL′ (M = Mo, Fe; P = porphine dianion; L and L′ being the axial ligands). For MoP(C₂H₂) a qualitative analysis indicates that the conformation with the acetylenic bond eclipsing two Mo-N bonds will be favored. Ab initio SCF calculations indicate that:

1. iron porphyrins with an axial imidazole ligand show a flat potential energy curve for the rotation of the imidazole ligand;
2. iron porphyrins with a dioxygen ligand prefer the staggered conformation with the O-O bond projecting along the bisectors of the Fe-N bonds;
3. in the cis-dinitrosyl molybdenum porphyrin, the nitrosyl ligands should be eclipsed with respect to the Mo-N_pyr bonds.

These theoretical predictions are compared with the experimental structures from the literature.     

Models for cytochromes c': spin states of mono(imidazole)-ligated (meso-tetramesitylporphyrinato)iron(III) complexes as studied by UV-Vis, 13C NMR, 1H NMR,and EPR spectroscopy

A number of mono(imidazole)-ligated complexes of perchloro(meso-tetramesitylporphyrinato)iron(III), [Fe(TMP)L]ClO(4), have been prepared, and their spin states have been examined by (1)H NMR, (13)C NMR, and EPR spectroscopy as well as solution magnetic moments. All the complexes examined have shown a quantum mechanical spin admixed state of high and intermediate-spin (S = 5/2 and 3/2) states though the contribution of the S = 3/2 state varies depending on the nature of axial ligands. While the complex with extremely bulky 2-tert-butylimidazole (2-(t)()BuIm) has exhibited an essentially pure S = 5/2 state, the complex with electron-deficient 4,5-dichloroimidazole (4,5-Cl(2)Im) adopts an S = 3/2 state with 30% of the S = 5/2 spin admixture. On the basis of the (1)H and (13)C NMR results, we have concluded that the S = 3/2 contribution at ambient temperature increases according to the following order: 2-(t)BuIm < 2-(1-EtPr)Im < 2-MeIm 相似文献   

Electronic configuration of five-coordinate high-spin pyrazole-ligated iron(II) porphyrinates

Pyrazole, a neutral nitrogen ligand and an isomer of imidazole, has been used as a fifth ligand to prepare two new species, [Fe(TPP)(Hdmpz)] and [Fe(Tp-OCH(3)PP)(Hdmpz)] (Hdmpz = 3,5-dimethylpyrazole), the first structurally characterized examples of five-coordinate iron(II) porphyrinates with a nonimidazole neutral ligand. Both complexes are characterized by X-ray crystallography, and structures show common features for five-coordinate iron(II) species, such as an expanded porphyrinato core, large equatorial Fe-N(p) bond distances, and a significant out-of-plane displacement of the iron(II) atom. The Fe-N(pyrazole) and Fe-N(p) bond distances are similar to those in imidazole-ligated species. These suggest that the coordination abilities to iron(II) for imidazole and pyrazole are very similar even though pyrazole is less basic than imidazole. Mo?ssbauer studies reveal that [Fe(TPP)(Hdmpz)] has the same behavior as those of imidazole-ligated species, such as negative quadrupole splitting values and relative large asymmetry parameters. Both the structures and the Mo?ssbauer spectra suggest pyrazole-ligated five-coordinate iron(II) porphyrinates have the same electronic configuration as imidazole-ligated species.     

NO-induced activation mechanism of the heme-regulated eIF2alpha kinase

The heme-regulated eukaryotic initiation factor 2alpha (eIF2alpha) kinase (HRI), which is found primarily in reticulocytes, contains an N-terminal heme-binding domain (NT-HBD). Binding of NO to the heme iron of the NT-HBD of HRI activates its eIF2alpha kinase activity, thus inhibiting the initiation of translation in reticulocyte lysate. The EPR spectrum of the NO-bound NT-HBD showed several derivative-shaped lines around g = 2.00, which is one of the well-documented signature patterns of a six-coordinate NO complex with histidine as the axial ligand. This is in sharp contrast to that of another prototypical NO-sensor protein, soluble guanylate cyclase (sGC), in which the NO binding to the heme iron disrupts the iron-histidyl bond forming a five-coordinate NO. The NO-mediated activation of HRI is, therefore, not triggered by the cleavage of the iron-histidyl bond. As evidenced by the resonance Raman spectra, two inactive forms of HRI, the ferrous ligand-unbound and the CO-bound states of the NT-HBD, contain a six-coordinate complex as found for the NO complex, indicating that the replacement of the sixth ligand of the heme iron is not sufficient to trigger the activation of HRI. Because the configuration of liganded NO is different from that of liganded CO, we propose that specific interactions between liganded NO and surrounding amino acid residues, which would not be formed in the CO complex, are responsible for the NO-induced activation of HRI.     

Hydrogen bonding effects on the electronic configuration of five-coordinate high-spin iron(II) porphyrinates

The characterization of a new five-coordinate derivative of (2-methylimidazole)(tetraphenylporphinato)iron(II) provides new and unique information about the effects of forming a hydrogen bond to the coordinated imidazole on the geometric and electronic structure of iron in these species. The complex studied has two crystallographically distinct iron sites; one site has an axial imidazole ligand modified by an external hydrogen bond, and the other site has an axial imidazole ligand with no external interactions. The iron atoms at the two sites have distinct geometric features, as revealed in their molecular structures, and distinct electronic structures, as shown by M?ssbauer spectroscopy, although both are high spin (S = 2). The molecule with the external hydrogen bond has longer equatorial Fe-N(p) bonds, a larger displacement of the iron atom out of the porphyrin plane, and a shorter axial bond compared to its counterpart with no hydrogen bonding. The M?ssbauer features are distinct for the two sites, with differing quadrupole splitting and isomer shift values and probably differing signs for the quadrupole splitting as shown by variable-temperature measurements in applied magnetic field. These features are consistent with a significant change in the nature of the doubly populated d orbital and are all in the direction of the dichotomy displayed by related imidazole and imidazolate species where deprotonation leads to major differences. The results points out the possible effects of strong hydrogen bonding in heme proteins.     

Electron distribution in iron octaethyloxophlorin complexes. Importance of the Fe(III) oxophlorin trianion form in the bis-pyridine and bis-imidazole complexes

The apportionment of electrons between iron and the porphyrinic macrocycle in complexes of octaethyloxophlorin (H3OEPO) has been a vexing problem. In particular, for (Py)2Fe(OEPO), which is an important intermediate in heme degradation, three resonance structures involving Fe(III), Fe(II), or Fe(I), respectively, have been considered. To clarify this matter, the electronic and geometric structures of (Py)2Fe(III)(OEPO), (Im)2Fe(III)(OEPO).2THF, and (Im)2Fe(III)(OEPO).1.6CHCl3 have been examined by single-crystal X-ray diffraction, measurement of magnetic moments as a function of temperature, and EPR and NMR spectral studies. The results clearly show that both complexes exist in the Fe(III)/oxophlorin trianion form rather than the Fe(II)/oxophlorin radical form previously established for (2,6-xylylNC)(2)Fe(II)(OEPO.). In the solid state from 10 to 300 K, (Py)2Fe(III)(OEPO) exists in the high-spin (S = 5/2) state with the axial ligands in parallel planes, a planar porphyrin, and long axial Fe-N distances. However, in solution it exists predominantly in a low-spin (S = 1/2) form. In contrast, the structures of (Im)2Fe(III)(OEPO).2THF and (Im)2Fe(III)(OEPO).1.6CHCl3 consist of porphyrins with a severe ruffled distortion, axial ligands in nearly perpendicular planes, and relatively short axial Fe-N distances. The crystallographic, magnetic, EPR, and NMR results all indicate that (Im)2Fe(III)(OEPO) exists in the low-spin Fe(III) form in both the solid state and in solution.     

Axial Imidazole Distortion Effects on the Catalytic and Binding Properties of Chelated Deuterohemin Complexes

The effect of strain in the axial coordination of imidazole to the heme has been studied in the chelate complexes deuterohemin-histidine (DH-His) and deuterohemin-alanylhistidine (DH-AlaHis). Molecular mechanics calculations indicate that three types of distortion of the axial ligand occur in DH-His, due to the relatively short length of the arm carrying the donor group: tilting off-axis, tipping, and inclination of the imidazole plane with respect to the axial Fe-N bond. The effects of tilting (Deltagamma approximately 10 degrees ) and inclination of the imidazole ring (Deltadelta approximately 17 degrees ) are dominant, while tipping is small and is probably of little importance here. By contrast, the axial imidazole coordination is normal in DH-AlaHis and other computed deuterohemin-dipeptide or -tripeptide complexes where histidine is the terminal residue, the only exception being DH-ProHis, where the rigidity of the proline ring reduces the flexibility of the chelating arm. The distortion in the axial iron-imidazole bond in DH-His has profound and negative influence on the binding and catalytic properties of this complex compared to DH-AlaHis. The former complex binds more weakly carbon monoxide, in its reduced form, and imidazole, in its oxidized form, than the latter. The catalytic efficiency in peroxidative oxidations is also reduced in DH-His with respect to DH-AlaHis. The activity of the latter complex is similar to that of microperoxidase-11, the peptide fragment incorporating the heme that results from hydrolytic cleavage of cytochrome c.     

Transformation and structural discrimination between the neutral {Fe(NO)2}10 dinitrosyliron complexes (DNICs) and the anionic/cationic {Fe(NO)2}9 DNICs

Reaction of Fe(CO)2(NO)2 and sparteine/tetramethylethylenediamine (TMEDA) in tetrahydrofuran afforded the electron paramagnetic resonance (EPR)-silent, neutral {Fe(NO)2}10 dinitrosyliron complexes (DNICs) [(sparteine)Fe(NO)2] (1) and [(TMEDA)Fe(NO)2] (2), respectively. The stable and isolable anionic {Fe(NO)2}9 DNIC [(S(CH2)3S)Fe(NO)2]- (4), with a bidentate alkylthiolate coordinated to a {Fe(NO)(2)} motif, was prepared by the reaction of [S(CH2)3S]2- and the cationic {Fe(NO)2}9 [(sparteine)Fe(NO)2]+ (3) obtained from the reaction of complex 1 and [NO][BF4] in CH(3)CN. Transformation from the neutral complex 1 to the anionic complex 4 was verified via the cationic complex 3. Here complex 3 acts as an {Fe(NO)2}-donor reagent in the presence of thiolates. The EPR spectra of complexes 3 and 4 exhibit an isotropic signal with g = 2.032 and 2.031 at 298 K, respectively, the characteristic g value of {Fe(NO)2}9 DNICs. On the basis of N-O/Fe-N(O) bond lengths of the single-crystal X-ray structures of the {Fe(NO)2}9/{Fe(NO)2}10 DNICs, the oxidation level of the {Fe(NO)2} core of DNICs can be unambiguously assigned. The mean N-O distances falling in the range of 1.214(6)-1.189(4) A and the Fe-N(O) bond distances in the range of 1.650(7)-1.638(3) A are assigned as the neutral {Fe(NO)(2)}(10) DNICs. In contrast, the mean N-O bond distances ranging from 1.178(3) to 1.160(6) A and the mean Fe-N(O) bond distances ranging from 1.695(3) to 1.661(4) A are assigned as the anionic/neutral/cationic {Fe(NO)2}9 DNICs. In addition, an EPR spectrum in combination with the IR nu(NO) (the relative position of the nu(NO) stretching frequencies and their difference Deltanu(NO)) spectrum may serve as an efficient tool for discrimination of the existence of the anionic/cationic/neutral {Fe(NO)2}9 DNICs and the neutral {Fe(NO)2}10 DNICs.     

The effects of axial ligands on electron distribution and spin states in iron complexes of octaethyloxophlorin, intermediates in heme degradation

The results presented here show that the nature of the axial ligand can alter the distribution of electrons between the metal and the porphyrin in complexes where there is an oxygen atom replacing one of the meso protons. The complexes (1-MeIm)(2)Fe(III)(OEPO) and (2,6-xylylNC)(2)Fe(II)(OEPO(*)) (where OEPO is the trianionic octaethyloxophlorin ligand and OEPO(*) is the dianionic octaethyloxophlorin radical) were prepared by addition of an excess of the appropriate axial ligand to a slurry of [Fe(III)(OEPO)](2) in chloroform under anaerobic conditions. The magnetic moment of (2,6-xylylNC)(2)Fe(II)(OEPO(*)) is temperature invariant and consistent with a simple S = (1)/(2) ground state. This complex with an EPR resonance at g = 2.004 may be considered as a model for the free-radical like EPR signal seen when the meso-hydroxylated heme/heme oxygenase complex is treated with carbon monoxide. In contrast, the magnetic moment of (1-MeIm)(2)Fe(III)(OEPO) drops with temperature and indicates a spin-state change from an S = (5)/(2) or an admixed S = (3)/(2),(5)/(2) state at high temperatures (near room temperature) to an S = (1)/(2) state at temperatures below 100 K. X-ray diffraction studies show that each complex crystallizes in centrosymmetric form with the expected six-coordinate geometry. The structure of (1-MeIm)(2)Fe(III)(OEPO) has been determined at 90, 129, and 296 K and shows a gradual and selective lengthening of the Fe-N(axial bond). This behavior is consistent with population of a higher spin state at elevated temperatures.     

Synthesis,X-ray crystal structure,and redox and electronic properties of iron(III)-polyimidazole complexes relevant to the metal sites of iron proteins

A new tripod N(3) ligand (L), containing three imidazole rings, was synthesized in good yield. At variance with usual aromatic ligands with N(2) or N(3) donor sets such as pyridine or pyrazole derivatives, L stabilizes the Fe(III) oxidation state. The corresponding iron(III) complexes [Fe(L)Cl(3)] (1) and [Fe(L)(2)](ClO(4))(3) (2) were prepared and characterized by X-ray structural analysis and spectroscopic methods. The coordination environment around all the Fe(III) centers has a distorted octahedral geometry. [Fe(L)Cl(3)] (1) belongs to the monoclinic system, space group P2(1)/n, a = 9.7406(5) A, b = 17.207(2) A, c = 14.615(2) A, beta = 104.448(9)(o) Z = 4, V = 2372.1(4) A(3); R = 0.044, R(w) = 0.055. [Fe(L)(2)](ClO(4))(3) (2) belongs to the monoclinic system, space group P2(1)/c, a = 16.1057(15) A, b = 11.1079(12) A, c = 26.283(2) A, beta = 102.062(10)(o), Z = 4, V = 4598.2(8) A(3); R = 0.0465, R(w) = 0.0902. The Fe-N((i)PrIm) bond lengths are systematically longer than the Fe-N(MeIm) ones. Compound 2 is a highly anisotropic low-spin Fe(III) complex displaying a rather unusual EPR spectrum with a sharp signal at g = 3.5 and a broad one at g approximately 1.6. The fitting of this EPR spectrum is discussed.     

Structural, electronic, and vibrational characterization of Fe-HNO porphyrinates by density functional theory

A recent report of the structural and vibrational properties of heme-bound HNO in myoglobin, MbHNO, revealed a long Fe-N(HNO) bond with the hydrogen atom bonded to the coordinated N atom. The Fe-N(H)-O moiety was reported to exhibit an unusually high Fe-N(HNO) stretching frequency relative to those of the corresponding [FeNO]6 and [FeNO]7 porphyrinates, despite the Fe-N(HNO) bond being longer than either of its Fe-N(NO) counterparts. Herein, we present results from density functional theory calculations of an active site model of MbHNO that support the previous assignment and clarify this seemingly contradictory result. The results are consistent with the experimental evidence for a ground-state Fe-N(H)-O structure having a long Fe-N(HNO) bond and a uniquely high nu(Fe)(-)(N(HNO)) frequency. This high frequency is the result of the correspondingly low reduced mass of the normal mode, which is largely attributable to significant motion of the N-bound hydrogen atom. Additionally, the calculations show the Fe-N(H)O bonding in this complex to be remarkably insensitive to whether the HNO and ImH ligand planes are parallel or perpendicular. This is attributed to insensitivities of the Fe-L(axial) characters of molecular orbitals to the relative HNO and ImH orientation in both the parallel and perpendicular conformers.     

Electronic, magnetic, and structural characterization of the five-coordinate, high-spin iron(II) nitrato complex [Fe(TpivPP)(NO3)]-

The preparation and characterization of the five-coordinate iron(II) porphyrinate derivative [Fe(TpivPP)(NO3)]- (TpivPP = picket-fence porphyrin) is described. Structural and magnetic susceptibility data support a high-spin state (S = 2) assignment for this species. The anionic axial nitrate ligand is O-bound, through a single O atom, with an Fe-O bond length of 2.069(4) A. The planar nitrate ligand bisects a N(p)-Fe-N(p) angle. The average Fe-N(p) bond length is 2.070(16) A. The Fe atom is located 0.49 A out of the 24-atom mean porphyrin plane toward the nitrate ligand. From solid-state M?ssbauer data, the isomer shift of 0.98 mm/s at 77 K is entirely consistent with high-spin iron(II). However the quadrupole splitting of 3.59 mm/s at 77 K is unusually high for iron(II), S = 2 systems but within the range of other five-coordinate high-spin ferrous complexes with a single anionic axial ligand. Crystal data for [K(222)][Fe(TpivPP)(NO3)] x C6H5Cl: a = 17.888 (5) A, b = 21.500 (10) A, c = 22.514 (11) A, beta = 100.32 (3) degrees, monoclinic, space group P2(1)/n, V = 8519 A3, Z = 4.     

Stereochemical aspects of axial ligation in ferrous iron-porphyrins probed by resonance Raman spectroscopy

We report the concentration-dependent resonance Raman (RR) studies of the FEN_Im stretching modes in the photo-reduced iron-octaethyl porphyrin (Fe^IIOEP) and iron-protoporphyrin-IX dimethylester (Fe^IIPPDME) complexes with 2-methyl imidazole (2-MeIm) and 1,2-dimethyl imidazole (1,2-Me₂Im) as axial ligands. The FeN_Im stretching modes in both iron complexes have revealed two components due to the co-existence of the upright and tilted configurations of the FEN_Im bond with respect to the normal to the heme plane. The frequencies of the two components in both the complexes shift to higher side with an increase in concentration of 2-MeIm as axial ligand. With the more sterically hindered 1,2-Me₂Im as axial ligand, the (1,2-Me₂Im)Fe^II-OEP complex exists mainly with the tilted configuration of the FEN_Im bond while the upright configuration is the dominant species in the (1,2-Me₂Im)Fe^IIPPDME complex. From the comparative study, we infer that the vinyl groups in the protoporphyrin complex play a dominant role in non-bonded interactions with the sterically hindered axial ligands in stabilizing the specific configurations.     

Ab initio molecular dynamics of heme in cytochrome c

Ab initio molecular dynamics (AIMD) calculations, based on the Car-Parrinello method, have been carried out for three models of heme c that is present in cytochrome c. Both the reduced (Fe(II)) and oxidized (Fe(III)) forms have been analyzed. The simplest models (1R and 1O, respectively) consist of a unsubstituted porphyrin (with no side chains) and two axially coordinated imidazole and ethylmethylthioether ligands. Density functional theory optimizations of these models confirm the basic electronic features and are the starting point for building more complex derivatives. AIMD simulations were performed after reaching the thermal stability at T = 300 K. The evolution of the Fe-L(ax) bond strengths is examined together with the relative rotations of the imidazole and methionine about the axial vector, which appear rather independent from each other. The next models (2R and 2O) contain side chains at the heme to better simulate the actual active site. It is observed that two adjacent propionate groups induce some important effects. The axial Fe-Sdelta bond is only weakened in 2R but is definitely cleaved in the oxidized species 2O. Also the mobility of the Im ligand seems to be reduced by the formation of a strong hydrogen bond that involves the Im Ndelta1-Hdelta1 bond and one carboxylate group. In 2O the interaction becomes so strong that a proton transfer occurs and the propionic acid is formed. Finally, the models 3 include a free N-methyl-acetamide molecule to mimic a portion of the protein backbone. This influences the orientation of carboxylate groups and limits the amount of their hydrogen bonding with the Im ligand. Residual electrostatic interactions are maintained, which are still able to modulate the dissociation of the methionine from the heme.     

A comparative study of O2, CO,and NO binding to iron–porphyrin

Minimum-energy structures of O₂, CO, and NO iron–porphyrin (FeP) complexes, computed with the Car–Parrinello molecular dynamics, agree well with the available experimental data for synthetic heme models. The diatomic molecule induces a 0.3–0.4 Å displacement of the Fe atom out of the porphyrin nitrogen (N_p) plane and a doming of the overall porphyrin ring. The energy of the iron–diatomic bond increases in the order Fe(SINGLE BOND)O₂ (9 kcal/mol) < Fe(SINGLE BOND)CO (26 kcal/mol) < Fe(SINGLE BOND)NO (35 kcal/mol). The presence of an imidazole axial ligand increases the strength of the Fe(SINGLE BOND)O₂ and Fe(SINGLE BOND)CO bonds (15 and 35 kcal/mol, respectively), with few structural changes with respect to the FeP(CO) and FeP(O₂) complexes. In contrast, the imidazole ligand does not affect the energy of the Fe(SINGLE BOND)NO bond, but induces significant structural changes with respect to the FeP(NO) complex. Similar variations in the iron–imidazole bond with respect to the addition of CO, O₂, and NO are also discussed. © 1998 John Wiley & Sons, Inc. Int J Quant Chem 69: 31–35, 1998