Models of the membrane-bound cytochromes: mössbauer spectra of crystalline low-spin ferriheme complexes having axial ligand plane dihedral angles ranging from 0 degree to 90 degrees

Perspectives on Ligand Properties of N-Heterocyclic Carbenes in Iron Porphyrin Complexes

There has been considerable research interest in the ligand nature of N-heterocyclic carbenes (NHCs). In this work, two six-coordinate NHC iron porphyrin complexes [Fe^II(TTP)(1,3-Me₂Imd)₂] (TTP = tetratolylporphyrin, 1,3-Me₂Imd = 1,3-dimethylimidazol-2-ylidene) and [Fe^III(TDCPP)(1,3-Me₂Imd)₂]ClO₄ (TDCPP = 5,10,15,20-tetrakis(2,6-dichlorophenyl)porphyrin) are reported. Single-crystal X-ray characterizations demonstrate that both complexes have strongly ruffled conformations and relatively perpendicular ligand orientations which are forced by the sterically bulky 1,3-Me₂Imd NHC ligands. Multitemperature (4.2-300 K) and high magnetic field (0-9 T) Mössbauer and low-temperature (4.0 K) EPR spectroscopies definitely confirmed the low-spin states of [Fe^II(TTP)(1,3-Me₂Imd)₂] (S = 0) and [Fe^III(TDCPP)(1,3-Me₂Imd)₂]ClO₄ (S = 1/2). The similarity of 1,3-Me₂Imd and imidazole, as well as the well-established correlations between the ligand nature and spectroscopic characteristics of [Fe^II,III(Porph)(L)₂]^0,+ (Porph: porphyrin; L: planar base ligand) species, allowed direct comparisons between the pair of ligands which revealed for the first time that NHC has a stronger π-acceptor ability than imidazoles, in addition to its very strong σ-donation.

Heme-Heme Interactions in Diheme Cytochromes: Effect of Mixed-Axial Ligation on the Electronic Structure and Electrochemical Properties

Diheme cytochromes, the simplest members in the multiheme family, play substantial biochemical roles in enzymatic catalysis as well as in electron transfer. A series of diiron(III) porphyrin dimers have been synthesized as active site analogues of diheme cytochromes. The complexes contain six-coordinated iron(III) having thiophenol and imidazole at the fifth and sixth coordination sites, respectively. The iron centers in the complexes have been found to be in a low-spin state, as confirmed through solid-state Mössbauer and electron paramagnetic resonance (EPR) spectroscopic investigations. Mössbauer quadrupole splitting of complexes having mixed ligands is substantially larger than that observed when both axial ligands are the same. Rhombic types of EPR spectra with narrow separation between g_x, g_y, and g_z clearly distinguish heme thiolate coordination compared to bis(imidazole)-ligated low-spin heme centers. The redox potential in diheme cytochromes has been found to be tuned by interheme interactions along with the nature of axial ligands. The effect of mixed-axial ligation within the diiron(III) porphyrin dimers is demonstrated by a positive shift in the Fe(III)/Fe(II) redox couple upon thiophenolate coordination compared to their bis(imidazole) analogues. The pK_a of the imidazole also decides the extent of the shift for the Fe(III)/Fe(II) couple, while the potential of the mixed-ligated diiron(III) porphyrin dimer is more positive compared to their monomeric analogue. A variation of around 1.1 V for the Fe(III)/Fe(II) redox potential in the diiron(III) porphyrin dimer can be achieved with the combined effect of axial ligation and a metal spin state, while such a large variation in the redox potential, compared to their monomeric analogues, is attributed to the heme-heme interactions observed in dihemes. Moreover, theoretical calculations also support the experimental shifts in the redox potential values.

Fe(TpivPP)(NO2)2]−: What is the nitrite orientation?

We have prepared a new crystalline derivative of [Fe(TpivPP)(NO2)2][Formula: see text] with the [K(222)][Formula: see text] cation. The newly determined structure of the anion resolves an ambiguity about the relative orientation of the two axial nitrite ligands which were found in a previous structure determination with a different cation. The two NO[Formula: see text] ligands are found to have a relative perpendicular orientation with Fe[Formula: see text]–N(NO2) distances of 1.974 (6) Åand 1.994 (6) Å. The average Fe[Formula: see text]–N[Formula: see text] distance in the low-spin six-coordinate iron(III) porphyrinate derivative is 1.987 (3) Å.

Probing heme vibrational anisotropy: an imidazole orientation effect?

The complete iron vibrational spectrum of the five-coordinate high-spin complex [Fe(OEP)(2-MeHIm)], where OEP = octaethylporphyrinato and 2-MeHIm = 2-methylimidazole, has been obtained by oriented single-crystal nuclear resonance vibrational spectroscopy (NRVS) data. Measurements have been made in three orthogonal directions, which provides quantitative information for all iron motion. These experimental data, buttressed by density functional theory (DFT) calculations, have been used to define the effects of the axial ligand orientation. Although the axial imidazole removes the degeneracy in the in-plane vibrations, the imidazole orientation does not appear to control the direction of the in-plane iron motion. This is in contrast to the effect of the imidazolate ligand, as defined by DFT calculations, which does have substantial effects on the direction of the in-plane iron motion. The axial NO ligand has been found to have the strongest orientational effect (Angew. Chem., Int. Ed., 2010, 49, 4400). Thus the strength of the directional properties are in the order NO > imidazolate > imidazole, consistent with the varying strength of the Fe-ligand bond.

Structural characterization of nitrosomonas europaea cytochrome c-552 variants with marked differences in electronic structure

Nitrosomonas europaea cytochrome c-552 (Ne c-552) variants with the same His/Met axial ligand set but with different EPR spectra have been characterized structurally, to aid understanding of how molecular structure determines heme electronic structure. Visible light absorption, Raman, and resonance Raman spectroscopy of the protein crystals was performed along with structure determination. The structures solved are those of Ne c-552, which displays a "HALS" (or highly anisotropic low-spin) EPR spectrum, and of the deletion mutant Ne N64Δ, which has a rhombic EPR spectrum. Two X-ray crystal structures of wild-type Ne c-552 are reported; one is of the protein isolated from N. europaea cells (Ne c-552n, 2.35 Å resolution), and the other is of recombinant protein expressed in Escherichia coli (Ne c-552r, 1.63 Å resolution). Ne N64Δ crystallized in two different space groups, and two structures are reported [monoclinic (2.1 Å resolution) and hexagonal (2.3 Å resolution)]. Comparison of the structures of the wild-type and mutant proteins reveals that heme ruffling is increased in the mutant; increased ruffling is predicted to yield a more rhombic EPR spectrum. The 2.35 Å Ne c-552n structure shows 18 molecules in the asymmetric unit; analysis of the structure is consistent with population of more than one axial Met configuration, as seen previously by NMR. Finally, the mutation was shown to yield a more hydrophobic heme pocket and to expel water molecules from near the axial Met. These structures reveal that heme pocket residue 64 plays multiple roles in regulating the axial ligand orientation and the interaction of water with the heme. These results support the hypothesis that more ruffled hemes lead to more rhombic EPR signals in cytochromes c with His/Met axial ligation.

Determination of the principal g-values of Type I or highly-anisotropic low spin (HALS) ferriheme centers in frozen solutions

Continuous wave (CW) electron paramagnetic resonance (EPR) spectroscopy of highly-anisotropic low spin (HALS) ferric heme centers in frozen solutions is not a very informative approach because usually only one feature is reliably observed in the spectra, that at the maximal principal g-value of, typically, 3.3-3.79. The other two EPR turning points are severely broadened by g-strain and are not easily observed in the first-derivative CW EPR spectra. In this work, we have explored the potential of alternative EPR techniques, the electron spin echo (ESE) field sweep and electron spin transient nutation (TN), for obtaining information about the g-tensors of such systems, using as an example a typical HALS ferric heme center, [Fe(III)((15)N-coproporphyrin)(CN)2]. The analysis of the experimental g-tensor of [Fe(III)((15)N-coproporphyrin)(CN)2](-) has shown that the widths of the underlying energy distributions for this HALS center are comparable to those found for the rhombic bis-imidazole complex. The greater effect on the g-value distributions for HALS centers is determined by near degeneracy of two of the three lower-energy d-orbitals, d(yz) and d(xz), which contain the unpaired electron.

Generation of a high-valent iron imido corrolazine complex and NR group transfer reactivity

The generation of a new high-valent iron terminal imido complex prepared with a corrolazine macrocycle is reported. The reaction of [Fe(III)(TBP8Cz)] (TBP8Cz = octakis(4-tert-butylphenyl)corrolazinato) with the commercially available chloramine-T (Na(+)TsNCl(-)) leads to oxidative N-tosyl transfer to afford [Fe(IV)(TBP8Cz(+•))(NTs)] in dichloromethane/acetonitrile at room temperature. This complex was characterized by UV-vis, Mössbauer (δ = -0.05 mm s(-1), ΔE(Q) = 2.94 mm s(-1)), and EPR (X-band (15 K), g = 2.10, 2.00) spectroscopies, and together with reactivity patterns and DFT calculations has been established as an iron(IV) species antiferromagnetically coupled with a Cz-π-cation-radical (S(total) = 1/2 ground state). Reactivity studies with triphenylphosphine as substrate show that [Fe(IV)(TBP8Cz(+•))(NTs)] is an efficient NTs transfer agent, affording the phospharane product Ph3P═NTs under both stoichiometric and catalytic conditions. Kinetic analysis of this reaction supports a bimolecular NTs transfer mechanism with rate constant of 70(15) M(-1) s(-1). These data indicate that [Fe(IV)(TBP8Cz(+•))(NTs)] reacts about 100 times faster than analogous Mn terminal arylimido corrole analogues. It was found that two products crystallize from the same reaction mixture of Fe(III)(TBP8Cz) + chloramine-T + PPh3, [Fe(IV)(TBP8Cz)(NPPh3)] and [Fe(III)(TBP8Cz)(OPPh3)], which were definitively characterized by X-ray crystallography. The sequential production of Ph3P═NTs, Ph3P═NH, and Ph3P═O was observed by (31)P NMR spectroscopy and led to a proposed mechanism that accounts for all of the observed products. The latter Fe(III) complex was then rationally synthesized and structurally characterized from Fe(III)(TBP8Cz) and OPPh3, providing an important benchmark compound for spectroscopic studies. A combination of Mössbauer and EPR spectroscopies led to the characterization of both intermediate spin (S = 3/2 and low spin (S = 1/2) Fe(III) corrolazines, as well as a formally Fe(IV) corrolazine which may also be described by its valence tautomer Fe(III)(Cz(+•)).

Role of calcium in metalloenzymes: effects of calcium removal on the axial ligation geometry and magnetic properties of the catalytic diheme center in MauG

MauG is a diheme enzyme possessing a five-coordinate high-spin heme with an axial His ligand and a six-coordinate low-spin heme with His-Tyr axial ligation. A Ca(2+) ion is linked to the two hemes via hydrogen bond networks, and the enzyme activity depends on its presence. Removal of Ca(2+) altered the electron paramagnetic resonance (EPR) signals of each ferric heme such that the intensity of the high-spin heme was decreased and the low-spin heme was significantly broadened. Addition of Ca(2+) back to the sample restored the original EPR signals and enzyme activity. The molecular basis for this Ca(2+)-dependent behavior was studied by magnetic resonance and Mössbauer spectroscopy. The results show that in the Ca(2+)-depleted MauG the high-spin heme was converted to a low-spin heme and the original low-spin heme exhibited a change in the relative orientations of its two axial ligands. The properties of these two hemes are each different than those of the heme in native MauG and are now similar to each other. The EPR spectrum of Ca(2+)-free MauG appears to describe one set of low-spin ferric heme signals with a large g(max) and g anisotropy and a greatly altered spin relaxation property. Both EPR and Mössbauer spectroscopic results show that the two hemes are present as unusual highly rhombic low-spin hemes in Ca(2+)-depleted MauG, with a smaller orientation angle between the two axial ligand planes. These findings provide insight into the correlation of enzyme activity with the orientation of axial heme ligands and describe a role for the calcium ion in maintaining this structural orientation that is required for activity.

Biochemical and biophysical characterization of succinate: quinone reductase from Thermus thermophilus

Enzymes serving as respiratory complex II belong to the succinate:quinone oxidoreductases superfamily that comprises succinate:quinone reductases (SQRs) and quinol:fumarate reductases. The SQR from the extreme thermophile Thermus thermophilus has been isolated, identified and purified to homogeneity. It consists of four polypeptides with apparent molecular masses of 64, 27, 14 and 15kDa, corresponding to SdhA (flavoprotein), SdhB (iron-sulfur protein), SdhC and SdhD (membrane anchor proteins), respectively. The existence of [2Fe-2S], [4Fe-4S] and [3Fe-4S] iron-sulfur clusters within the purified protein was confirmed by electron paramagnetic resonance spectroscopy which also revealed a previously unnoticed influence of the substrate on the signal corresponding to the [2Fe-2S] cluster. The enzyme contains two heme b cofactors of reduction midpoint potentials of -20mV and -160mV for b(H) and b(L), respectively. Circular dichroism and blue-native polyacrylamide gel electrophoresis revealed that the enzyme forms a trimer with a predominantly helical fold. The optimum temperature for succinate dehydrogenase activity is 70°C, which is in agreement with the optimum growth temperature of T. thermophilus. Inhibition studies confirmed sensitivity of the enzyme to the classical inhibitors of the active site, as there are sodium malonate, sodium diethyl oxaloacetate and 3-nitropropionic acid. Activity measurements in the presence of the semiquinone analog, nonyl-4-hydroxyquinoline-N-oxide (NQNO) showed that the membrane part of the enzyme is functionally connected to the active site. Steady-state kinetic measurements showed that the enzyme displays standard Michaelis-Menten kinetics at a low temperature (30°C) with a K(M) for succinate of 0.21mM but exhibits deviation from it at a higher temperature (70°C). This is the first example of complex II with such a kinetic behavior suggesting positive cooperativity with k' of 0.39mM and Hill coefficient of 2.105. While the crystal structures of several SQORs are already available, no crystal structure of type A SQOR has been elucidated to date. Here we present for the first time a detailed biophysical and biochemical study of type A SQOR-a significant step towards understanding its structure-function relationship.

Redox effects on the coordination geometry and heme conformation of bis(N-methylimidazole) complexes of superstructured Fe-porphyrins. A spectroscopic study

Electronic absorption, electron paramagnetic resonance (EPR), and Soret-excited resonance Raman (RR) spectra are reported for bis(N-alkylimidazole) complexes of various iron(III)-"basket-handle" (Fe(III)BHP(+)) and "picket-fence" (Fe(III)PFP(+)) porphyrins in methylene chloride. The Fe(III)BHP(+) derivatives consist of four cross-trans (CT) and two adjacent-cis (AC) -linked in which the composition and the length of the handles are variable (CT Fe(III)[(C(11)Im)(2)(+)], CT and AC Fe(III)[((C(4))(2)phi)(2)](+), CT Fe(III)[((C(3))(2)phi)(C(12))](+), CT and AC Fe(III)[((C(3))(2)phi)(2)](+)). The meso-alphaalpha betabeta and meso-alphabeta alphabeta atropisomers of Fe(III)-tetrakis(o-pivalamidophenyl)-porphyrins represents the Fe(III)PFP(+) derivatives (Fe(III)alphaalpha betabeta-T(piv)PP(+) and Fe(III)alphabeta alphabeta-T(piv)PP(+), respectively). The absorption and RR data obtained for these ferric compounds were compared to those previously published for the homologous ferrous complexes (Picaud, T., Le Moigne, C., Loock, B., Momenteau, M. and Desbois, A. J. Am. Chem. Soc. 2003, 125, 11616 and Le Moigne, C., Picaud, T., Boussac, A., Loock, B., Momenteau, M. and Desbois, A. Inorg. Chem. 2003, 42, 6081). The Soret band position of the eight investigated ferric compounds is observed between 417 and 424 nm, indicating that none of the complexes possesses a planar heme. The EPR spectra show that most of the Fe(III)BHP(+) complexes and all the Fe(III)PFP(+) complexes are rhombic B-type hemichromes (g(max) = 2.86-2.96). Notable exceptions concern the bis(N-methylimidazole) complexes of two CT Fe(III)BHP(+). The Fe(III)BHP(+) with the shortest handles (Fe(III)[((C(3))(2)phi)(2)](+)) exhibits a g value at 2.80. When the handles are lengthened by two methylene units (Fe(III)[((C(3))(2)phi)(2)](+)), the EPR spectrum corresponds to a mixture of two "highly anisotropic low-spin " or "large g(max)" type I EPR signals, a major species at g = 3.17 and a minor species at g = 3.77. All these EPR data were converted in terms of dihedral angle formed by the rings of the axial ligands. The RR spectra of the Fe(III)BHP(+) and Fe(III)PFP(+) complexes exhibited variable frequencies for the structure-sensitive nu(2) and nu(8) lines (1558-1563 cm(-1) and 386-401 cm(-1), respectively). In considering the ability of the different superstructures to stabilize particular out-of-plane distortions, this vibrational information was analyzed in terms of heme structure through changes in core size and Fe-N(pyrrole) bond length, in relation to changes in coordination geometry. The bis(N-methylimidazole) complex of Fe(III)[((C(3))(2)phi)(2)](+) was found to be the most distorted with a strongly ruffled tetrapyrrole. Because of a handle asymmetry, the heme conformation of the bis(N-methylimidazole) complex of Fe(III)[((C(3))(2)phi)(C(12))](+) was deduced to be a composition of ruffled and domed structures. The heme structure of the other complexes is a mixture of ruffled and saddled or ruffled and waved conformations. Taking into account our previous data on the ferrous series, this investigation provides information about the reorganization of the heme structure upon iron oxidation. The general trend is a decrease of either the core-size, or the Fe-N(pyrrole) bond length, or both. However, we demonstrated that the heme superstructures precisely control the nature and the extent of the tetrapyrrole reshaping. These results point out similar possible effect in the heme proteins, considering both an analogy between porphyrin superstructures and amino acids forming the heme sites and the diversity of the heme environments in the proteins.

Review: studies of ferric heme proteins with highly anisotropic/highly axial low spin (S = 1/2) electron paramagnetic resonance signals with bis-histidine and histidine-methionine axial iron coordination

Six-coordinated heme groups are involved in a large variety of electron transfer reactions because of their ability to exist in both the ferrous (Fe(2+)) and ferric (Fe(3+)) state without any large differences in structure. Our studies on hemes coordinated by two histidines (bis-His) and hemes coordinated by histidine and methionine (His-Met) will be reviewed. In both of these coordination environments, the heme core can exhibit ferric low spin (electron paramagnetic resonance EPR) signals with large g(max) values (also called Type I, highly anisotropic low spin, or highly axial low spin, HALS species) as well as rhombic EPR (Type II) signals. In bis-His coordinated hemes rhombic and HALS envelopes are related to the orientation of the His groups with respect to each other such that (i) parallel His planes results in a rhombic signal and (ii) perpendicular His planes results in a HALS signal. Correlation between the structure of the heme and its ligands for heme with His-Met axial ligation and ligand-field parameters, as derived from a large series of cytochrome c variants, show, however, that for such a combination of axial ligands there is no clear-cut difference between the large g(max) and the "small g-anisotropy" cases as a result of the relative Met-His arrangements. Nonetheless, a new linear correlation links the average shift delta of the heme methyl groups with the g(max) values.

Modulation of the ligand-field anisotropy in a series of ferric low-spin cytochrome c mutants derived from Pseudomonas aeruginosa cytochrome c-551 and Nitrosomonas europaea cytochrome c-552: a nuclear magnetic resonance and electron paramagnetic resonance study

Cytochromes of the c type with histidine-methionine (His-Met) heme axial ligation play important roles in electron-transfer reactions and in enzymes. In this work, two series of cytochrome c mutants derived from Pseudomonas aeruginosa (Pa c-551) and from the ammonia-oxidizing bacterium Nitrosomonas europaea (Ne c-552) were engineered and overexpressed. In these proteins, point mutations were induced in a key residue (Asn64) near the Met axial ligand; these mutations have a considerable impact both on heme ligand-field strength and on the Met orientation and dynamics (fluxionality), as judged by low-temperature electron paramagnetic resonance (EPR) and nuclear magnetic resonance (NMR) spectra. Ne c-552 has a ferric low-spin (S = 1/2) EPR signal characterized by large g anisotropy with g(max) resonance at 3.34; a similar large g(max) value EPR signal is found in the mitochondrial complex III cytochrome c1. In Ne c-552, deletion of Asn64 (NeN64Delta) changes the heme ligand field from more axial to rhombic (small g anisotropy and g(max) at 3.13) and furthermore hinders the Met fluxionality present in the wild-type protein. In Pa c-551 (g(max) at 3.20), replacement of Asn64 with valine (PaN64V) induces a decrease in the axial strain (g(max) at 3.05) and changes the Met configuration. Another set of mutants prepared by insertion (ins) and/or deletion (Delta) of a valine residue adjacent to Asn64, resulting in modifications in the length of the axial Met-donating loop (NeV65Delta, NeG50N/V65Delta, PaN50G/V65ins), did not result in appreciable alterations of the originally weak (Ne c-552) or very weak (Pa c-551) axial field but had an impact on Met orientation, fluxionality, and relaxation dynamics. Comparison of the electronic fingerprints in the overexpressed proteins and their mutants reveals a linear relationship between axial strain and average paramagnetic heme methyl shifts, irrespective of Met orientation or dynamics. Thus, for these His-Met axially coordinated Fe(III), the large g(max) value EPR signal does not represent a special case as is observed for bis-His axially coordinated Fe(III) with the two His planes perpendicular to each other.

Explorations in metalloporphyrin stereochemistry, physical properties and beyond

A review of selected portions of our work in the area of porphyrin structure and physical characterization is presented. Topics covered include early work on periodic trends in first row transtion metalloporphyrins, a survey of electronic structure of iron derivatives including spin-state trends, ligand orientation effects and the elucidtion of unusual low-spin states for iron(II). A discussion of the different tlypes of high-spin iron(II) complexes and the effects of hydrogen bonding is given. A survey of nitric oxide (NO) derivatives is presented as well as a brief introduction into the use of nuclear resonance vibrational spectroscopy for the study of iron porphyrins and heme proteins.

Models of the cytochromes: crystal structures and EPR spectral characterization of low-spin bis-imidazole complexes of (OETPP)Fe(III) having intermediate ligand plane dihedral angles

The preparation, EPR spectra, and crystal structures of octaethyltetraphenylporphyrinatoiron(III) having two imidazole, N-benzylimidazole, and N-methylimidazole axial ligands are reported, [(OETPP)Fe(HIm)2]Cl, [(OETPP)Fe(N-BzIm)2]Cl, and [(OETPP)Fe(N-MeIm)2]Cl. Despite large variation in axial ligand size, the unit cell parameters for all complexes are very similar; each structure has the same basic motif, with large voids formed by the extended porphyrin framework (filled by ordered or disordered axial ligands and disordered solvent), which allows differently sized ligands to fit within the same cell dimensions. Each porphyrin core adopts a saddled conformation with absolute value(deltaC(beta)) = 1.13-1.15 A. The dihedral angles between axial ligand planes, delta phi, are far from being either ideal parallel or perpendicular: 30.1 degrees, 57.2 degrees for [(OETPP)Fe(HIm)2]Cl (molecules 1 and 2), 56.8 degrees for [(OETPP)Fe(N-BzIm)(2)]Cl, and 16.0 degrees, 44.6 degrees, 59.6 degrees, and 88.1 degrees for [(OETPP)Fe(N-MeIm)2]Cl, which has disordered axial ligands. Among the complexes of this study, an axial ligand delta phi of 56.8 degrees is found to be the largest "parallel" angle (as defined by the observation of a normal rhombic or Type II EPR signal (N-BzIm, g = 3.08, 2.19, 1.31)), while 57.2 degrees is found to be the smallest "perpendicular" delta phi (as defined by the observation of a "large gmax" or Type I EPR signal (HIm, gmax = 3.24)). From the results of this study, it is clear that the size of the largest g for Types I and II complexes varies continuously, with no break between the two. While the switch in EPR signal type, from Type II to Type I, appears to be very sharp in this study, this may be somewhat artificial based upon limited numbers of examples and the required saddle distortion of the (OETPP)Fe(III) complexes. However, in comparison to several proteins with dihedral angles near 60 degrees and Type II EPR spectra, we may conclude that the switch in EPR signal type occurs near 57 degrees +/- 3-5 degrees.

Low-spin bis(2-methylimidazole)(octaethylporphyrinato)iron(III) chloride (perp-[Fe(OEP)(2-MeHIm)(2)]Cl): a consequence of hydrogen bonding?

The synthesis and characterization of low-spin bis(2-methylimidazole)(octaethylporphyrinato)iron(III) chloride (perp[Fe(OEP)(2-MeHIm)2]Cl) is reported. The structure shows that the cation is a low-spin species with two imidazole ligands having a relative perpendicular orientation. The porphyrin core is very ruffled, which leads to shortened equatorial bonds of 1.974(4) A and slightly elongated axial Fe-N bond lengths of 2.005(10) A that are about 0.02 A shorter and 0.03 A longer, respectively, in comparison to bis-imidazole ligated iron(III) species with parallel oriented axial ligands. A one-dimensional hydrogen-bond chain is formed between chloride anions and uncoordinated imidazole nitrogen atoms. Compared with paral-[Fe(OEP)(2-MeHIm)2]ClO4, hydrogen bonding may play an important role in the differences in the two structures. Mössbauer spectra show broadened quadrupole doublets with quadrupole splittings of 1.81 mm/s at RT and 1.94 mm/s at 20 K. The isomer shift ranges from 0.26 to 0.36 mm/s. These confirm that the title complex is a low-spin iron(III) species with the ground state (dxy)2(dxz,dyz)3. Crystal data: monoclinic, space group P2(1)/c, a = 14.066(3) A, b, 20.883(4) A, c = 19.245(4) A, beta = 109.67 degrees , and Z = 4.

The heme environment of mouse neuroglobin: histidine imidazole plane orientations obtained from solution NMR and EPR spectroscopy as compared with X-ray crystallography

The 1H NMR chemical shifts of the heme methyl groups of the ferriheme complex of metneuroglobin (Du et al. in J. Am. Chem. Soc. 125:8080-8081, 2003) predict orientations of the axial histidine ligands (Shokhirev and Walker in J. Biol. Inorg. Chem. 3:581-594, 1998) that are not consistent with the X-ray data (Vallone et al. in Proteins Struct. Funct. Bioinf. 56:85-94, 2004), and the EPR spectrum (Vinck et al. in J. Am. Chem. Soc. 126:4516-4517, 2004) is only marginally consistent with these data. The reasons for these inconsistencies appear to be rooted in the high degree of aqueous solution exposure of the heme group and the fact that there are no strong hydrogen-bond acceptors for the histidine imidazole N-H protons provided by the protein. Similar inconsistencies may exist for other water-soluble heme proteins, and 1H NMR spectroscopy provides a simple means to verify whether the solution structure of the heme center is the same as or different from that in the crystalline state.