Five- to six-coordination in (nitrosyl)iron(II) porphyrinates: effects of binding the sixth ligand

Dynamics of NO motion in solid-state [Co(tetraphenylporphinato)(NO)]

The temperature dependence of the crystalline phase of (nitrosyl)(tetraphenylporphinato)cobalt(II), [Co(TPP)(NO)], has been explored over the temperature range of 100-250 K by X-ray diffraction experiments. The crystalline complex is found in the tetragonal crystal system at higher temperatures and in the triclinic crystal system at lower temperatures. In the tetragonal system, the axial ligand is strongly disordered, with the molecule having crystallographically required 4/m symmetry, leading to eight distinct positions of the single nitrosyl oxygen atom. The phase transition to the triclinic crystal system leads to a partial ordering with the molecule now having inversion symmetry and disorder of the axial nitrosyl ligand over only two positions. At an intermediate temperature near the transition point, a transition structure in which the ordering observed at lower temperatures is only partially complete has been characterized. The increase in ordering allows subtle molecular geometry features to be observed. The transition of the reversible phase change begins at about 195 K. This transition has been confirmed by both X-ray diffraction studies and a differential scanning calorimetry study.

Electronic structure and dynamics of nitrosyl porphyrins

Nitric oxide (NO) is a signaling molecule employed to regulate essential physiological processes. Thus, there is great interest in understanding the interaction of NO with heme, which is found at the active site of many proteins that recognize NO, as well as those involved in its creation and elimination. We summarize what we have learned from investigations of the structure, vibrational properties, and conformational dynamics of NO complexes with ferrous porphyrins, as well as computational investigations in support of these experimental studies. Multitemperature crystallographic data reveal variations in the orientational disorder of the nitrosyl ligand. In some cases, equilibria among NO orientations can be analyzed using the van't Hoff relationship and the free energy and enthalpy of the solid-state transitions evaluated experimentally. Density functional theory (DFT) calculations predict that intrinsic barriers to torsional rotation are smaller than thermal energies at physiological temperatures, and the coincidence of observed NO orientations with minima in molecular mechanics potentials indicates that nonbonded interactions with other chemical groups control the conformational freedom of the bound NO. In favorable cases, reduced disorder at low temperatures exposes subtle structural features including off-axis tilting of the Fe-NO bond and anisotropy of the equatorial Fe-N bonds. We also present the results of nuclear resonance vibrational spectroscopy measurements on oriented single crystals of [Fe(TPP)(NO)] and [Fe(TPP)(1-MeIm)(NO)]. These describe the anisotropic vibrational motion of iron in five- and six-coordinate heme-NO complexes and reveal vibrations of all Fe-ligand bonds as well as low-frequency molecular distortions associated with the doming of the heme upon ligand binding. A quantitative comparison with predicted frequencies, amplitudes, and directions facilitates identification of the vibrational modes but also suggests that commonly used DFT functionals are not fully successful at capturing the trans interaction between the axial NO and imidazole ligands. This supports previous conclusions that heme-NO complexes exhibit an unusual degree of variability with respect to the computational method, and we speculate that this variability hints at a genuine electronic instability that a protein can exploit to tune its reactivity. We anticipate that ongoing characterization of heme-NO complexes will deepen our understanding of their structure, dynamics, and reactivity.

Nuclear resonance vibrational spectroscopy applied to [Fe(OEP)(NO)]: the vibrational assignments of five-coordinate ferrous heme-nitrosyls and implications for electronic structure

This study presents Nuclear Resonance Vibrational Spectroscopy (NRVS) data on the five-coordinate (5C) ferrous heme-nitrosyl complex [Fe(OEP)(NO)] (1, OEP(2-) = octaethylporphyrinato dianion) and the corresponding (15)N(18)O labeled complex. The obtained spectra identify two isotope sensitive features at 522 and 388 cm(-1), which shift to 508 and 381 cm(-1), respectively, upon isotope labeling. These features are assigned to the Fe-NO stretch nu(Fe-NO) and the in-plane Fe-N-O bending mode delta(ip)(Fe-N-O), the latter has been unambiguously assigned for the first time for 1. The obtained NRVS data were simulated using our quantum chemistry centered normal coordinate analysis (QCC-NCA). Since complex 1 can potentially exist in 12 different conformations involving the FeNO and peripheral ethyl orientations, extended density functional theory (DFT) calculations and QCC-NCA simulations were performed to determine how these conformations affect the NRVS properties of [Fe(OEP)NO]. These results show that the properties and force constants of the FeNO unit are hardly affected by the conformational changes involving the ethyl substituents. On the other hand, the NRVS-active porphyrin-based vibrations around 340-360, 300-320, and 250-270 cm(-1) are sensitive to the conformational changes. The spectroscopic changes observed in these regions are due to selective mechanical couplings of one component of E(u)-type (in ideal D(4h) symmetry) porphyrin-based vibrations with the in-plane Fe-N-O bending mode. This leads to the observed variations in Fe(OEP) core mode energies and NRVS intensities without affecting the properties of the FeNO unit. The QCC-NCA simulated NRVS spectra of 1 show excellent agreement with experiment, and indicate that conformer F is likely present in the samples of this complex investigated here. The observed porphyrin-based vibrations in the NRVS spectra of 1 are also assigned based on the QCC-NCA results. The obtained force constants of the Fe-NO and N-O bonds are 2.83-2.94 (based on the DFT functional applied) and about 12.15 mdyn/A, respectively. The electronic structures of 5C ferrous heme-nitrosyls in different model complexes are then analyzed, and variations in their properties based on different porphyrin substituents are explained. Finally, the shortcomings of different DFT functionals in describing the axial FeNO subunit in heme-nitrosyls are elucidated.

Revisiting the nitrosyl complex of myoglobin by high-field pulse EPR spectroscopy and quantum mechanical calculations

The binding of NO to reduced myoglobin in solution results in the formation of two paramagnetic nitrosyl myoglobin (MbNO) complexes: one with a rhombic g-factor and the other with an axial one, referred to as the R- and A-forms. In spite of past extensive studies of MbNO by crystallography, spectroscopy and quantum chemical calculations it is still not clear what factors determine the appearance of the two forms. In this work we applied a combination of state of the art quantum chemical calculations and high field pulsed EPR spectroscopy (W-band, 3.4 T/95 GHz) to further characterize the two forms. Specifically, we have used (1)H and (2)H electron-nuclear double resonance (ENDOR) spectroscopy to identify and characterize the H-bond to the NO, and hyperfine sub-level correlation (HYSCORE) spectroscopy to determine the hyperfine and quadrupole interactions of the Fe(ii) coordinated (14)N of the proximal histidine (14)N(His93). The calculations employed quantum mechanics (QM), particularly density functional theory (DFT) methods in combination with molecular mechanics (MM) force-field to model the protein environment. Through QM/MM calculations of the EPR parameters we have explored their dependence on several geometrical factors of the Fe-NO bond and found those that reproduce the best experimental results. The spread of the W-band EPR spectrum of MbNO due to the g-anisotropy is large and there is a significant part of the spectrum where the R-form is the sole contributor. This allowed us to resolve some new characteristics of the R-form: (i) a NO-H hydrogen bond has been detected and characterized and through QM/MM calculations has been unambiguously assigned to (epsilon2)H(His64). (ii) The complete hyperfine and quadrupole interactions of (14)N(His93) have been determined and correlated with structural parameters again using QM/MM calculations. The agreement between the experimental results and calculations varied between excellent and good, depending on the EPR parameter in question. As for the more elusive A-form, the results only suggest that it does have a (14)N(His93) ligand with a hyperfine comparable to that of the R-form and it has less hydrogen bonding interaction with His(64). The calculations also established the orientation of the principal g-values, finding that they are closely related to the orientation of the NO bond. This information is essential for deriving structural information from the experimental orientation selective HYSCORE and ENDOR data.

NMR, IR/Raman, and structural properties in HNO and RNO (R = alkyl and aryl) metalloporphyrins with implication for the HNO-myoglobin complex

Structural and functional details of heme protein complexes with HNO and the isoelectronic RNO (R = alkyl and aryl) molecules (metabolic intermediates) are largely unknown. We report a quantum chemical investigation of three characteristic spectroscopic properties, (1)H and (15)N NMR chemical shifts and NO vibrational frequencies in synthetic HNO and RNO heme complexes, with theory-versus-experiment correlation coefficients R(2) = 0.990-0.998. A new density functional theory (DFT) method was found to yield excellent predictions of experimental structures of HNO, RNO, and NO heme systems. Interestingly, this method also helps the identification of an excellent linear quantitative structure observable relationship between NO vibrational frequencies and bond lengths in all of these NO-containing systems. This suggests that NO vibrations are largely local effects of the NO bonds in these complexes and may help deduce the NO bond lengths from using experimental vibrational data in these systems. The NO vibrational frequencies in HNO, RNO, and NO metalloporphyrins were found to follow a general trend of NO > RNO > HNO complexes, as a result of the electron populations in the antibonding NO orbitals of NO < RNO < HNO complexes. Investigations of the NMR and IR/Raman spectroscopic data in HNO metal complexes show that HNO is a strong pi-acid. In addition, we performed the first quantum chemical investigation of the hydrogen-bond effect on HNO in MbHNO (Mb = myoglobin) models. On the basis of comparisons with experimental (1)H and (15)N NMR results and NO vibrational frequency in MbHNO, a dual hydrogen-bond mode for HNO in MbHNO was proposed. The enhanced stability from this dual hydrogen bonding may provide a basis for the unusual stability of MbHNO observed experimentally. These results should facilitate spectroscopic characterizations and structural investigations of HNO and RNO heme proteins and models.

Iron-porphyrin NO complexes with covalently attached N-donor ligands: formation of a stable six-coordinate species in solution

A series of substituted tetraphenylporphyrin type macrocycles (TMP or To-F(2)PP) with covalently attached N-donor ligands (pyridine or imidazole linker) have been synthesized. Linkers with varying chain lengths and designs have been applied to systematically investigate the effect of chain length and rigidity on the binding affinity of the linker to the corresponding Fe(II)-NO heme complexes. The binding of the linker is monitored in solution using a variety of spectroscopic methods including UV-vis absorption, EPR, and IR spectroscopy. Both the N-O stretching frequency and the imidazole (14)N hyperfine coupling constants show a good correlation with the Fe-(N-donor) bond strength in these systems. The complexes with covalently attached pyridyl and alkyl imidazole ligands only exhibit weak interactions of the linker with iron(II). However, the stable six-coordinate complex [Fe(To-F(2)PP-BzIM)(NO)] (4) is obtained when a rigid benzyl linker is applied. This complex exhibits typical properties of six-coordinate ferrous heme-nitrosyls in which an N-donor ligand is bound trans to NO, including the Soret band at 427 nm and the typical nine line (14)N hyperfine splitting in the EPR spectrum. A crystal structure has been obtained for the corresponding zinc complex. Here, we report the first systematic study on the requirements for the formation of stable six-coordinate ferrous heme nitrosyl complexes in solution at room temperature in the absence of excess axial N-donor ligand.

Mapping NO movements in crystalline [Fe(Porph)(NO)(1-MeIm)]

Orientational disorder of the distal nitrosyl (NO) ligand in iron porphyrinates is a common phenomenon. We present an analysis of multitemperature crystallographic data for the order/disorder phenomenon. The observed temperature-dependent order/disorder and variable rotational orientations of nitrosyl ligands for six different six-coordinate iron porphyrinates have been examined in terms of the nonbonded contacts found in the solid state. Favorable orientations for NO can be identified either by calculation of the close nonbonded contacts or by evaluation of the geometry-dependent potential energy using semiempirical nonbonded potential functions. The nonbonded contacts display temperature-dependent differences consistent with observed structural differences. The motion of NO appears to be controlled by intermolecular interactions that allow a limited set of orientations, and under some conditions, only a single NO orientation is allowed. In some cases, the equilibria involving the orientations of NO can be analyzed using the van't Hoff relationship, and the free energy and enthalpy of the solid-state transitions can be evaluated. The intrinsic barriers to rotation of the NO were examined using a fine-meshed series of DFT calculations. The calculations also showed the detailed effects of the variation of the NO orientation on the equatorial bond distances.

The power of using continuous-wave and pulsed electron paramagnetic resonance methods for the structure analysis of ferric forms and nitric oxide-ligated ferrous forms of globins

For several decades now, electron paramagnetic resonance (EPR) has been a valuable spectroscopic tool for the characterization of globin proteins. In the early years, the majority of EPR studies were performed using standard continuous-wave EPR techniques at conventional microwave frequencies. In the last years, the field of EPR has known tremendous technological developments, including the introduction of advanced pulsed EPR and high-frequency EPR techniques. After a short overview of the basics of EPR and recent advances in the field, we will illustrate how these different EPR methods can provide information about the dynamics and geometric and electronic structures of heme proteins. Although the main focus of this chapter lies on the EPR analysis of nitric oxide-ligated ferrous heme proteins and ferric heme systems, we also briefly outline the possibility of site-directed spin labeling of heme proteins. The last section highlights the future potential and challenges in using this magnetic resonance technique in globin research.

Crystal structures of manganese- and cobalt-substituted myoglobin in complex with NO and nitrite reveal unusual ligand conformations

Nitrite is now recognized as a storage pool of bioactive nitric oxide (NO). Hemoglobin (Hb) and myoglobin (Mb) convert, under certain conditions, nitrite to NO. This newly discovered nitrite reductase activity of Hb and Mb provides an attractive alternative to mammalian NO synthesis from the NO synthase pathway that requires dioxygen. We recently reported the X-ray crystal structure of the nitrite adduct of ferric horse heart Mb, and showed that the nitrite ligand binds in an unprecedented O-binding (nitrito) mode to the d(5) ferric center in Mb(III)(ONO) [D.M. Copeland, A. Soares, A.H. West, G.B. Richter-Addo, J. Inorg. Biochem. 100 (2006) 1413-1425]. We also showed that the distal pocket in Mb allows for different conformations of the NO ligand (120 degrees and 144 degrees ) in Mb(II)NO depending on the mode of preparation of the compound. In this article, we report the crystal structures of the nitrite and NO adducts of manganese-substituted hh Mb (a d(4) system) and of the nitrite adduct of cobalt-substituted hh Mb (a d(6) system). We show that the distal His64 residue directs the nitrite ligand towards the rare nitrito O-binding mode in Mn(III)Mb and Co(III)Mb. We also report that the distal pocket residues allow a stabilization of an unprecendented bent MnNO moiety in Mn(II)MbNO. These crystal structural data, when combined with the data for the aquo, methanol, and azide MnMb derivatives, provide information on the role of distal pocket residues in the observed binding modes of nitrite and NO ligands to wild-type and metal-substituted Mb.

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.

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.

Metalloporphyrin-Based Inclusion Materials: Exploiting Ligating Topologies and Hydrogen-Bonding Backbones in Generating New Supramolecular Architectures

By coordination of the metal center of tetraphenylmetalloporphyrins (TPMP) [metal center=Zn(II) or Mn(II)] with pyridyl-based bidentate ligands, namely, N,N'-bis(4-pyridyl)urea (4BPU), N,N'-bis(3-pyridyl)urea (3BPU), and N-(4-pyridyl)isonicotinamide (4PIN), various axially modified tetraarylmetalloporphyrins (AMTAMPs) have been crystallographically characterized in their corresponding lattice inclusion complexes. Nine such inclusion crystals are prepared by crystallizing TPMP and the corresponding bidentate ligands in 1:2 molar ratio from suitable solvent systems. While the metal center Zn(II) of TPMP leads to the formation of both dimeric and monomeric AMTAMPs due to its preference for pentacoordinated geometry, the Mn(II) metal center of TPMP forms both polymeric and discrete hexacoordinated AMTAMPs due to its preference for hexacoordinated geometry. However, there seem to be no control on the formation of a particular AMTAMP. Structural analyses suggest that most of the AMTAMPs display new types of packing in the corresponding inclusion crystals.

Spectroscopic characterization of heme iron-nitrosyl species and their role in NO reductase mechanisms in diiron proteins

Nitric oxide (NO) plays an important role in cell signalling and in the mammalian immune response to infection. On its own, NO is a relatively inert radical, and when it is used as a signalling molecule, its concentration remains within the picomolar range. However, at infection sites, the NO concentration can reach the micromolar range, and reactions with other radical species and transition metals lead to a broad toxicity. Under aerobic conditions, microorganisms cope with this nitrosative stress by oxidizing NO to nitrate (NO3−). Microbial hemoglobins play an essential role in this NO-detoxifying process. Under anaerobic conditions, detoxification occurs via a 2-electron reduction of two NO molecules to N2O. In many bacteria and archaea, this NO-reductase reaction is catalyzed by diiron proteins. Despite the importance of this reaction in providing microorganisms with a resistance to the mammalian immune response, its mechanism remains ill-defined. Because NO is an obligatory intermediate of the denitrification pathway, respiratory NO reductases also provide resistance to toxic concentrations of NO. This family of enzymes is the focus of this review. Respiratory NO reductases are integral membrane protein complexes that contain a norB subunit evolutionarily related to subunit I of cytochrome c oxidase (Cc O). NorB anchors one high-spin heme b3 and one non-heme iron known as FeB, i.e ., analogous to CuB in Cc O. A second group of diiron proteins with NO-reductase activity is comprised of the large family of soluble flavoprotein A found in strict and facultative anaerobic bacteria and archaea. These soluble detoxifying NO reductases contain a non-heme diiron cluster with a Fe–Fe distance of 3.4 Å and are only briefly mentioned here as a promising field of research. This article describes possible mechanisms of NO reduction to N2O in denitrifying NO-reductase (NOR) proteins and critically reviews recent experimental results. Relevant theoretical model calculations and spectroscopic studies of the NO-reductase reaction in heme/copper terminal oxidases are also overviewed.

S-nitrosylation-induced conformational change in blackfin tuna myoglobin

S-nitrosylation is a post-translational protein modification that can alter the function of a variety of proteins. Despite the growing wealth of information that this modification may have important functional consequences, little is known about the structure of the moiety or its effect on protein tertiary structure. Here we report high-resolution x-ray crystal structures of S-nitrosylated and unmodified blackfin tuna myoglobin, which demonstrate that in vitro S-nitrosylation of this protein at the surface-exposed Cys-10 directly causes a reversible conformational change by "wedging" apart a helix and loop. Furthermore, we have demonstrated in solution and in a single crystal that reduction of the S-nitrosylated myoglobin with dithionite results in NO cleavage from the sulfur of Cys-10 and rebinding to the reduced heme iron, showing the reversibility of both the modification and the conformational changes. Finally, we report the 0.95-A structure of ferrous nitrosyl myoglobin, which provides an accurate structural view of the NO coordination geometry in the context of a globin heme pocket.

Differential sensing of protein influences by NO and CO vibrations in heme adducts

Heme proteins bind the gaseous ligands XO (X = C, N, O) via backbonding from Fe d(pi) electrons. Backbonding is modulated by distal interactions of the bound ligand with the surrounding protein and by variations in the strength of the trans proximal ligand. Vibrational modes associated with FeX and XO bond stretching coordinates report on these interactions, but the interpretive framework developed for CO adducts, involving anticorrelations of nuFeC and nuCO, has seemed not to apply to NO adducts. We have now obtained an excellent anticorrelation of nuFeN and nuNO, via resonance Raman spectroscopy on (N-methylimidazole)Fe(II)TPP-Y(NO), where TPP-Y is tetraphenylporphine with electron-donating or -withdrawing substituents, Y, that modulate the backbonding; the problem of laser-induced dissociation of the axial base was circumvented by using frozen solutions. New data are also reported for CO adducts. The anticorrelations are supported by DFT calculations of structures and spectra. When protein data are examined, the NO adducts show large deviations from the modeled anticorrelation when there are distal H-bonds or positive charges. These deviations are proposed to result from closing of the FeNO angle due to a shift in the valence isomer equilibrium toward the Fe(III)(NO-) form, an effect that is absent in CO adducts. The differing vibrational patterns of CO and NO adducts provide complementary information with respect to protein interactions, which may help to elucidate the mechanisms of ligand discrimination and signaling in heme sensor proteins.

Coordination of diatomic ligands to heme: simply CO

The synthesis and molecular structures of three iron(II) porphyrinates with only CO as the axial ligand(s) are reported. Two five-coordinate [Fe(OEP)(CO)] derivatives have Fe-C = 1.7077(13) and 1.7140(10) A, much shorter than those of six-coordinate [Fe(OEP)(Im)(CO)], although nu(C-O) is 1944-1948 cm(-1). The six-coordinate species [Fe(OEP)(CO)2] has also been studied. The competition for pi-back-bonding of two CO ligands leads to Fe-C distance of 1.8558(10) A and nu(C-O) being increased to 2021 cm(-1). The Mössbauer spectrum has a quadrupole splitting constant of 0 mm/s at 4.2 K, indicating high electronic symmetry.

Electronic structure of iron(II)-porphyrin nitroxyl complexes: Molecular mechanism of fungal nitric oxide reductase (P450nor)

Density functional calculations are employed to investigate key intermediates of the catalytic cycle of fungal nitric oxide reductase (P450nor). The formal Fe(II)-nitroxyl species Fe(II)--NO/(-) can principally exist in the two spin-states S = 0 and S = 1. In the S = 0 case, a very covalent Fe--NO sigma bond is present, which leads to an electronic structure description that is actually intermediate between Fe(I)--NO and Fe(II)--NO(-). In contrast, the S = 1 case shows a ferrous Fe(II)--NO complex with the extra electron being stored in the pi system of the porphyrin ligand. Importantly, the Fe(II)--NO/(-) species are very basic. The electronic structures and spectroscopic properties of the corresponding N- and O-protonated forms are very different, and unequivocally show that the Mb-HNO adduct (Mb-Myoglobin) prepared by farmer and coworkers is in fact N-protonated. The presence of an axial thiolate ligand enables a second protonation leading to the corresponding Fe(IV)--NHOH- species, which is identified with the catalytically active intermediate I of P450nor. This species reacts with a second molecule of NO by initial electron transfer from NO to Fe(IV) followed by addition of NO+ forming an N--N bond. This is accompanied by an energetically very favorable intramolecular proton transfer leading to the generation of a quite stable Fe(III)--N(OH)(NOH) complex. This way, the enzyme is able to produce dimerized HNO under very controlled conditions and to prevent loss of this ligand from Fe(III). The energetically disfavoured tautomer Fe(III)--N(OH2)(NO) is the catalytically productive species that spontaneously cleaves the N--OH2 bond forming N2O and H2O in a highly exergonic reaction.

Crystal structures of the nitrite and nitric oxide complexes of horse heart myoglobin

Nitrite is an important species in the global nitrogen cycle, and the nitrite reductase enzymes convert nitrite to nitric oxide (NO). Recently, it has been shown that hemoglobin and myoglobin catalyze the reduction of nitrite to NO under hypoxic conditions. We have determined the 1.20 A resolution crystal structure of the nitrite adduct of ferric horse heart myoglobin (hh Mb). The ligand is bound to iron in the nitrito form, and the complex is formulated as MbIII(ONO-). The Fe-ONO bond length is 1.94 A, and the O-N-O angle is 113 degrees . In addition, the nitrite ligand is stabilized by hydrogen bonding with the distal His64 residue. We have also determined the 1.30 A resolution crystal structures of hh MbIINO. When hh MbIINO is prepared from the reaction of metMbIII with nitrite/dithionite, the FeNO angle is 144 degrees with a Fe-NO bond length of 1.87 A. However, when prepared from the reaction of NO with reduced MbII, the FeNO angle is 120 degrees with a Fe-NO bond length of 2.13 A. This difference in FeNO conformations as a function of preparative method is reproducible, and suggests a role of the distal pocket in hh MbIINO in stabilizing local FeNO conformational minima.

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 Mössbauer 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.

Reductive nitrosylation of water-soluble iron porphyrins by S-nitroso-N-acetylpenicillamine: rate constants and EPR characterization

Reductive nitrosylation of the water-soluble iron derivatives of the cationic Fe(III)(TMPyP) and anionic Fe(III)(TPPS) porphyrins [where TMPyP=tetra-meso-(4-N-methylpyridiniumyl)porphinate and TPPS=tetra-meso-(4-sulfonatophenyl)porphinate] by the nitric oxide donor S-nitroso-N-acetylpenicillamine (SNAP) was studied using optical absorption spectroscopy and electron paramagnetic resonance. Nitrosylation rates were obtained, the reaction was found to be first order in the SNAP concentration and the stoichiometry of the reaction was one to one. The similarity between the obtained second-order rate constants for both porphyrins, k(TMPyP)=0.84 x 10(3)M(-1)s(-1) and k(TPPS)=0.97 x 10(3)M(-1)s(-1), suggested that the reaction mechanism is approximately independent of the nature of the porphyrin meso-substituents. A mechanism was proposed involving the hydrolysis of SNAP by an out of plane liganded H(2)O yielding the sulfenic acid of N-acetylpenicillamine and the transfer of NO(-) to Fe(III). The EPR (electron paramagnetic resonance) spectra of the SNAP- and gaseous NO-treated porphyrins were obtained and compared. The difference between the spectra of the cationic and anionic porphyrins indicates different local symmetry and Fe-N-O bond angle. SNAP-treatment produced much more resolved hyperfine structures than gaseous NO-treatment.

The first structurally characterized nitrosyl heme thiolate model complex

The NO ligand in the formally {FeNO}6 compound [Fe(oep)(NO)(thiolate)] is bent, and does not impart a significant structural trans effect to the Fe-S bond.

Role of Heme Iron Coordination and Protein Structure in the Dynamics and Geminate Rebinding of Nitric Oxide to the H93G Myoglobin Mutant

The influence of the heme iron coordination on nitric oxide binding dynamics was investigated for the myoglobin mutant H93G (H93G-Mb) by picosecond absorption and resonance Raman time-resolved spectroscopies. In the H93G-Mb, the glycine replacing the proximal histidine does not interact with the heme iron so that exogenous substituents like imidazole may coordinate to the iron at the proximal position. Nitrosylation of H93G-Mb leads to either 6- or 5-coordinate species depending on the imidazole concentration. At high concentrations, (imidazole)-(NO)-6-coordinate heme is formed, and the photoinduced rebinding kinetics reveal two exponential picosecond phases ( approximately 10 and approximately 100 ps) similar to those of wild type myoglobin. At low concentrations, imidazole is displaced by the trans effect leading to a (NO)-5-coordinate heme, becoming 4-coordinate immediately after photolysis as revealed from the transient Raman spectrum. In this case, NO rebinding kinetics remain bi-exponential with no change in time constant of the fast component whose amplitude increases with respect to the 6-coordinate species. Bi-exponential NO geminate rebinding in 5-coordinate H93G-Mb is in contrast with the single-exponential process reported for nitrosylated soluble guanylate cyclase (Negrerie, M., Bouzhir, L., Martin, J. L., and Liebl, U. (2001) J. Biol. Chem. 276, 46815-46821). Thus, our data show that the iron coordination state or the heme iron out-of-plane motion are not at the origin of the bi-exponential kinetics, which depends upon the protein structure, and that the 4-coordinate state favors the fast phase of NO geminate rebinding. Consequently, the heme coordination state together with the energy barriers provided by the protein structure control the dynamics and affinity for NO-binding enzymes.

Direct probe of iron vibrations elucidates NO activation of heme proteins

We use nuclear resonance vibrational spectroscopy (NRVS) to identify the Fe-NO stretching frequency in the NO adduct of myoglobin (MbNO) and in the related six-coordinate porphyrin Fe(TPP)(1-MeIm)(NO). Frequency shifts observed in MbNO Raman spectra upon isotopic substitution of Fe or the nitrosyl nitrogen confirm and extend the NRVS results. In contrast with previous assignments, the Fe-NO frequency of these six-coordinate complexes lies 70-100 cm-1 lower than in the analogous five-coordinate nitrosyl complexes, indicating a significant weakening of the Fe-NO bond in the presence of a trans imidazole ligand. This result supports proposed mechanisms for NO activation of heme proteins and underscores the value of NRVS as a direct probe of metal reactivity in complex biomolecules.

Heme carbonyls: environmental effects on nu(C-O) and Fe-C/C-O bond length correlations

The synthesis and characterization of four low-spin (carbonyl)iron(II) tetraphenylporphyrinates, [Fe(TPP)(CO)(L)], where L = 1-methylimidazole, 2-methylimidazole, 1,2-dimethylimidazole (unsolvated), and 1,2-dimethylimidazole (toluene solvate) are reported. The complexes show nearly the same value of nu(C-O) in toluene solution (1969-72 cm(-1)) but a large range of CO stretching frequencies in the solid-state (1926-1968 cm(-1)). The large solid-state variation results from CO interactions in the solid state, as shown by an examination of the crystal structures of the four complexes. The high precision of the four structures obtained allows us to make a number of structural and spectroscopic correlations that describe the Fe-C-O and N(Im)-Fe-CO units. The values of nu(C-O) and the Fe-C and C-O bond distances are strongly correlated and provide a structural, as well as a spectroscopic, correlation of the pi back-bonding model. The interactions of CO described are closely related to the large range of CO stretching frequencies observed in heme proteins and specific interactions observed in carbonylmyoglobin (MbCO).

Potential Energy Surface and Molecular Dynamics of MbNO: Existence of an Unsuspected FeON Minimum

Ligands such as CO, O(2), or NO are involved in the biological function of myoglobin. Here we investigate the energetics and dynamics of NO interacting with the Fe(II) heme group in native myoglobin using ab initio and molecular dynamics simulations. At the global minimum of the ab initio potential energy surface (PES), the binding energy of 23.4 kcal/mol and the Fe-NO structure compare well with the experimental results. Interestingly, the PES is found to exhibit two minima: There exists a metastable, linear Fe-O-N minimum in addition to the known, bent Fe-N-O global minimum conformation. Moreover, the T-shaped configuration is found to be a saddle point, in contrast to the corresponding minimum for NO interacting with Fe(III). To use the ab initio results for finite temperature molecular dynamics simulations, an analytical function was fitted to represent the Fe-NO interaction. The simulations show that the secondary minimum is dynamically stable up to 250 K and has a lifetime of several hundred picoseconds at 300 K. The difference in the topology of the heme-NO PES from that assumed previously (one deep, single Fe-NO minimum) suggests that it is important to use the full PES for a quantitative understanding of this system. Why the metastable state has not been observed in the many spectroscopic studies of myoglobin interacting with NO is discussed, and possible approaches to finding it are outlined.

Thiolate coordination to Fe(II)–porphyrin NO centers

The interaction of the Fe(II)-porphyrin NO model complex [Fe(TPP)(NO)] (1, TPP=tetraphenylporphyrin) with thiophenolate ligands and tetrahydrothiophene is explored both computationally and experimentally. Complex 1 is reacted with substituted thiophenolates and the obtained six-coordinate adducts of type [Fe(TPP)(SR)(NO)](-) are investigated in solution using electron paramagnetic resonance (EPR) spectroscopy. From the obtained g values and (14)N hyperfine pattern of the NO ligand it is concluded that the interaction of the thiophenolates with the Fe(II) center is weak in comparison to the corresponding 1-methylimidazole adduct. The strength of the Fe-S bond is increased when alkylthiolates are used as evidenced by comparison with the published EPR spectra of ferrous NO adducts in cytochromes P450 and P450nor, which have an axial cysteinate ligand. These results are further evaluated by density functional (DFT) calculations. The six-coordinate model complex [Fe(P)(SMe)(NO)](-) (1-SMe; P=porphine ligand used for the calculations) has an interesting electronic structure where NO acts as a medium strong sigma donor and pi acceptor ligand. Compared to the N-donor adducts with 1-methylimidazole (1-MeIm), etc., donation from the pi(h)( *) orbital of NO to Fe(II) is reduced due to the stronger trans effect of the alkylthiolate ligand. This is reflected by the predicted longer Fe-NO bond length and smaller Fe-NO force constant for 1-SMe compared to the 1-MeIm adduct. Therefore, the Fe(II)-porphyrin NO adducts with trans alkylthiolate coordination have to be described as Fe(II)-NO(radical) systems. The N-O stretching frequency of these complexes is predicted below 1600cm(-1) in agreement with the available experimental data. In addition, 1-SMe has a unique spin density distribution where Fe has a negative spin density of -0.26 from the calculations. The implications of this unusual electronic structure for the reactivity of the Fe(II)-NO alkylthiolate adducts as they occur in cytochrome P450nor are discussed.

Spin Density Distribution in Five- and Six-Coordinate Iron(II)−Porphyrin NO Complexes Evidenced by Magnetic Circular Dichroism Spectroscopy

Using magnetic circular dichroism (MCD) spectroscopy together with DFT calculations, the spin density distributions in five-coordinate [Fe(TPP)(NO)] (I) and six-coordinate [Fe(TPP)(MI)(NO)] (II, MI = 1-methylimidazole) are defined. In the five-coordinate complex, a strong Fe-NO sigma bond between pi(*)(h) and d(z)(2) is present that leads to a large transfer of spin density from the NO ligand to Fe(II) corresponding to an electronic structure with noticeable Fe(I)-NO(+) character. Consequently, the MCD spectrum is dominated by paramagnetic C-term contributions. On coordination of the sixth ligand, the spin density is pushed back from the iron toward the NO ligand, resulting in an Fe(II)-NO(radical) type of electronic structure. This is reflected by the fact that the MCD spectrum is dominated by diamagnetic contributions.

Temperature dependence of NO binding modes in human neuroglobin

Both the ferrous and ferric forms of wild-type neuroglobin are found to be hexacoordinated with axial ligation of the F8-His and E7-His. Rapidly growing Escherichia coli cell cultures with low O2 concentration generate nitric oxide (NO). Combined electron paramagnetic resonance (EPR) and optical measurements show that wild-type human recombinant neuroglobin, overexpressed in such E. coli cells, still favors the F8His-Fe2+ -E7His conformation, whereby only a small fraction of the protein binds NO. Upon mutation of the E7-His to Leu and Gln, the competition with the distal histidine disappears and the nitrosyl ferrous form is readily observed. At low temperature, the EPR spectra of the NO-ligated Ngb proteins consist of contributions from two geometrically different NO-heme conformations. In combination with EPR data of vertebrate hemoglobins and myoglobins, the temperature dependence of the EPR spectra of the NO adducts of ferrous hNgb and its E7-mutants proves a strong stabilization of one isomer by the E7-histidine in wt hNgb. It is shown that this is not related to the polarity of histidine, but to its specific binding characteristics.

Single- and double-linkage isomerism in a six-coordinate iron porphyrin containing nitrosyl and nitro ligands

Density Functional theoretical calculations confirm the experimental observation that the low-temperature photolysis of (TPP)Fe(NO)(NO2) (as a KBr pellet) results in the generation of linkage isomers involving the axial NO and NO2 groups and suggest the possible formation of the double linkage isomer (TPP)Fe(ON)(ONO). The energy difference between the ground state (porphine)Fe(NO)(NO2) and the double-linkage isomer (porphine)Fe(ON)(ONO) is 1.57 eV, which is comparable to the 1.59 eV calculated previously for the nitrosyl-to-isonitrosyl linkage isomerism in the five-coordinate (porphine)Fe(NO) analogue.

Variable pi-bonding in iron(II) porphyrinates with nitrite, CO, and tert-butyl isocyanide: characterization of [Fe(TpivPP)(NO2)(CO)]-

The addition of the strongly pi-bonding ligands CO or tert-butyl isocyanide to the low-spin five-coordinate iron(II) nitrite species [Fe(TpivPP)(NO2)]- (TpivPP = picket fence porphyrin) gives two new six-coordinate species [Fe(TpivPP)(NO2)(CO)]- and [Fe(TpivPP)(NO2)(t-BuNC)]-. These species have been characterized by single-crystal structure determinations and by UV-vis, IR, and Mössbauer spectroscopies. All evidence shows that in the mixed-ligand iron(II) porphyrin species, [Fe(TpivPP)(NO2)(CO)]-, the two trans, pi-accepting ligands CO and nitrite compete for pi density. The CO ligand however dominates the bonding. The Fe-N(NO2) bond lengths for the two independent anions in the unit cell at 2.006(4) and 2.009(4) A are lengthened compared to other nitrite species with either no trans ligands or non-pi-accepting trans ligands to nitrite. The Fe-C(CO) bond lengths are 1.782(4) A and 1.789(5) A for the two anions. The two Fe-C-O angles at 175.5(4) and 177.5(4) degrees are essentially linear in both anions. The quadrupole splitting for [Fe(TpivPP)(NO2)(CO)]- was determined to be 0.32 mm/s, and the isomer shift was 0.18 mm/s at room temperature in zero applied field. Both of the Mössbauer parameters are much smaller than those found for six-coordinate low-spin iron(II) porphyrinates with neutral nitrogen-donating ligands as well as iron(II) nitro complexes. However, the Mössbauer parameters are typical of other six-coordinate CO porphyrinates signifying that CO is the more dominant ligand. The CO stretching frequency of 1974 cm(-1) is shifted only slightly to higher energy compared to six-coordinate CO complexes with neutral nitrogen-donor ligands trans to CO. Crystal data for [K(222)][Fe(TpivPP)(NO2)(CO)].1/2C6H5Cl: monoclinic, space group P2(1)/c, Z = 8, a = 33.548(6) A, b = 18.8172(15) A, c = 27.187(2) A, beta = 95.240(7) degrees, V = 17091(4) A3.