Definitive assignment of the g tensor of [Fe(OEP)(NO)] by single-crystal EPR

The Biologically Relevant Coordination Chemistry of Iron and Nitric Oxide: Electronic Structure and Reactivity

Nitric oxide (NO) is an important signaling molecule that is involved in a wide range of physiological and pathological events in biology. Metal coordination chemistry, especially with iron, is at the heart of many biological transformations involving NO. A series of heme proteins, nitric oxide synthases (NOS), soluble guanylate cyclase (sGC), and nitrophorins, are responsible for the biosynthesis, sensing, and transport of NO. Alternatively, NO can be generated from nitrite by heme- and copper-containing nitrite reductases (NIRs). The NO-bearing small molecules such as nitrosothiols and dinitrosyl iron complexes (DNICs) can serve as an alternative vehicle for NO storage and transport. Once NO is formed, the rich reaction chemistry of NO leads to a wide variety of biological activities including reduction of NO by heme or non-heme iron-containing NO reductases and protein post-translational modifications by DNICs. Much of our understanding of the reactivity of metal sites in biology with NO and the mechanisms of these transformations has come from the elucidation of the geometric and electronic structures and chemical reactivity of synthetic model systems, in synergy with biochemical and biophysical studies on the relevant proteins themselves. This review focuses on recent advancements from studies on proteins and model complexes that not only have improved our understanding of the biological roles of NO but also have provided foundations for biomedical research and for bio-inspired catalyst design in energy science.

Synthesis, X-ray Structures, Electronic Properties, and O2/NO Reactivities of Thiol Dioxygenase Active-Site Models

Mononuclear non-heme iron complexes that serve as structural and functional mimics of the thiol dioxygenases (TDOs), cysteine dioxygenase (CDO) and cysteamine dioxygenase (ADO), have been prepared and characterized with crystallographic, spectroscopic, kinetic, and computational methods. The high-spin Fe(II) complexes feature the facially coordinating tris(4,5-diphenyl-1-methylimidazol-2-yl)phosphine (^Ph2TIP) ligand that replicates the three histidine (3His) triad of the TDO active sites. Further coordination with bidentate l-cysteine ethyl ester (CysOEt) or cysteamine (CysAm) anions yielded five-coordinate (5C) complexes that resemble the substrate-bound forms of CDO and ADO, respectively. Detailed electronic-structure descriptions of the [Fe(^Ph2TIP)(L_S,N)]BPh₄ complexes, where L_S,N = CysOEt (1) or CysAm (2), were generated through a combination of spectroscopic techniques [electronic absorption, magnetic circular dichroism (MCD)] and density functional theory (DFT). Complexes 1 and 2 decompose in the presence of O₂ to yield the corresponding sulfinic acid (RSO₂H) products, thereby emulating the reactivity of the TDO enzymes and related complexes. Rate constants and activation parameters for the dioxygenation reactions were measured and interpreted with the aid of DFT calculations for O₂-bound intermediates. Treatment of the TDO models with nitric oxide (NO)-a well-established surrogate of O₂-led to a mixture of high-spin and low-spin {FeNO}⁷ species at low temperature (-70 °C), as indicated by electron paramagnetic resonance (EPR) spectroscopy. At room temperature, these Fe/NO adducts convert to a common species with EPR and infrared (IR) features typical of cationic dinitrosyl iron complexes (DNICs). To complement these results, parallel spectroscopic, computational, and O₂/NO reactivity studies were carried out using previously reported TDO models that feature an anionic hydrotris(3-phenyl-5-methyl-pyrazolyl)borate (^Ph,MeTp^-) ligand. Though the O₂ reactivities of the ^Ph2TIP- and ^Ph,MeTp-based complexes are quite similar, the supporting ligand perturbs the energies of Fe 3d-based molecular orbitals and modulates Fe-S bond covalency, suggesting possible rationales for the presence of neutral 3His coordination in CDO and ADO.

Isocyanide or nitrosyl complexation to hemes with varying tethered axial base ligand donors: synthesis and characterization

A series of ferrous-heme 2,6-dimethylphenyl isocyanide (DIMPI) and ferrous-heme mononitrosyl complexes have been synthesized and characterized. The heme portion of the complexes studied is varied with respect to the nature of the axial ligand, including complexes, where it is covalently tethered to the porphyrinate periphery. Reduced heme complexes, [(F8)Fe(II)], [(P(Py))Fe(II)], [(P(Im))Fe(II)], and [(P(ImH))Fe(II)], where F8 = tetrakis(2,6-difluorophenyl)-porphyrinate and P(Py), P(Im), and P(ImH) are partially fluorinated tetraaryl porphyrinates with covalently appended axial base pyridyl/imidazolyl or histamine moieties, were employed; P(ImH) is a new construct. Room temperature addition of DIMPI to these iron(II) complexes affords the bis-isocyanide species [(F8)Fe(II)-(DIMPI)2] in the case of [(F8)Fe(II)], while for the other hemes, mono-DIMPI compounds are obtained, [(P(Py))Fe(II)-(DIMPI)] [(2)-DIMPI], [(P(Im))Fe(II)-(DIMPI)] [(3)-DIMPI], and [(P(ImH))Fe(II)-(DIMPI)] [(4)-DIMPI]. The structures of complexes (3)-DIMPI and (4)-DIMPI have been determined by single crystal X-ray crystallography, where interesting H…F(porphryinate aryl group) interactions are observed. (19)F-NMR spectra determined for these complexes suggest that H…F(porphyrinate aryl groups) attractions also occur in solution, the H atom coming either from the DIMPI methyl groups or from a porphyinate axial base imidazole or porphyrinate pyrrole. Similarly, we have used nitrogen monoxide to generate ferrous-nitrosyl complexes, a five-coordinate species for F8, [(F8)Fe(II)-(NO)], or low-spin six-coordinate compounds [(P(Py))Fe(II)-(NO)], [(P(Im))Fe(II)-(NO)], and [(P(ImH))Fe(II)-(NO)]. The DIMPI and mononitrosyl complexes have also been characterized using UV-Vis, IR, (1)H-NMR, and EPR spectroscopies.

Iron nitrosyl "natural" porphyrinates: does the porphyrin matter?

The synthesis and spectroscopic characterization of three five-coordinate nitrosyliron(II) complexes, [Fe(Porph)(NO)], are reported. These three nitrosyl derivatives, where Porph represents protoporphyrin IX dimethyl ester, mesoporphyrin IX dimethyl ester, or deuteroporphyrin IX dimethyl ester, display notable differences in their properties relative to the symmetrical synthetic porphyrins such as OEP and TPP. The N–O stretching frequencies are in the range of 1651–1660 cm^–1, frequencies that are lower than those of synthetic porphyrin derivatives. Mössbauer spectra obtained in both zero and applied magnetic field show that the quadrupole splitting values are slightly larger than those of known synthetic porphyrins. The electronic structures of these naturally occurring porphyrin derivatives are thus seen to be consistently different from those of the synthetic derivatives, the presumed consequence of the asymmetric peripheral substituent pattern. The molecular structure of [Fe(PPIX-DME)(NO)] has been determined by X-ray crystallography. Although disorder of the axial nitrosyl ligand limits the structural quality, this derivative appears to show the same subtle structural features as previously characterized five-coordinate nitrosyls.

The synthesis and characterization of three five-coordination {FeNO}₇ porphyrin derivatives based on natural porphyrin substitution patterns show that there are systematic differences compared to synthetic porphyrin derivatives with more symmetric substitution patterns. Characterization includes high-field Mössbauer spectroscopy and a crystal structure of the protoporphyrin IX dimethyl ester derivative.

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.

EPR and Mössbauer spectroscopy show inequivalent hemes in tryptophan dioxygenase

Tryptophan 2,3-dioxygenase (TDO) is an essential enzyme in the pathway of NAD biosynthesis and important for all living organisms. TDO catalyzes oxidative cleavage of the indole ring of L-tryptophan (L-Trp), converting it to N-formylkynurenine (NFK). The crystal structure of TDO shows a dimer of dimer quaternary structure of the homotetrameric protein. The four catalytic sites of the protein, one per subunit, contain a heme that catalyzes the activation and insertion of dioxygen into L-Trp. Because of the alpha(4) structure and because only one type of heme center has been identified in previous spectroscopic studies, the four hemes sites have been presumed to be equivalent. The present work demonstrates that the heme sites of TDO are not equivalent. Quantitative interpretation of EPR and Mössbauer spectroscopic data indicates the presence of two dominant inequivalent heme species in reduced and oxidized states of the enzyme, which is consistent with a dimer of dimer protein quaternary structure that now extends to the electronic properties of the hemes. The electronic properties of the hemes in the reduced state of TDO change significantly upon L-Trp addition, which is attributed to a change in the protonation state of the proximal histidine to the hemes. The binding of O(2) surrogates NO or CO shows two inequivalent heme sites. The heme-NO complexes are 5- and 6-coordinate without L-Trp, and both 6-coordinate with L-Trp. NO can be selectively photodissociated from only one of the heme-NO sites and only in the presence of L-Trp. Cryoreduction of TDO produces a novel diamagnetic heme species, tentatively assigned as a reduced heme-OH complex. This work presents a new description of the heme interactions with the protein, and with the proximal His, which must be considered during the general interpretation of physical data as it relates to kinetics, mechanism, and function of TDO.

Electronic structure of selected FeNO7 complexes in heme and non-heme architectures: a density functional and multireference ab initio study

The multiconfigurational CASSCF/CASPT2 approach, along with various functionals of density functional theory, is applied to selected iron(II)-nitrosyl ({FeNO}(7)) complexes, both with heme and nonheme groups. The energetics of the lowest doublet and quartet spin states at the correlated ab initio (CASPT2) level is presented for the first time. Comparison of the CASSCF and (unrestricted) DFT spin densities indicates that the nonhybrid functionals yield the spin densities most closely to the ab initio ones. The analysis of the multiconfigurational CASSCF wave function in terms of the localized active orbitals allows one to resolve the nature of Fe-NO bonding as a mixture of Fe(II)-NO(0) and Fe(III)-NO(-) resonance structures (in comparable contributions) for both spin states and various ligands.

The role of arginine-127 at the proximal NO-binding site in determining the electronic structure and function of 5-coordinate NO-heme in cytochrome c' of Rhodobacter sphaeroides

Cytochrome c' is a heme protein from a denitrifying variant of Rhodobacter sphaeroides which may serve to store and transport metabolic NO while protecting against NO toxicity. Its heme site bears resemblance through its 5-coordinate NO-binding capability to the regulatory site in soluble guanylate cyclase. A conserved arginine (Arg-127) abuts the 5-coordinate NO-heme binding site, and the alanine mutant R127A provided insight into the role of the Arg-127 in establishing the electronic structure of the heme-NO complex and in modifying the heme-centered redox potential and NO-binding affinity. By comparison to R127A, the wild-type Arg-127 was determined to increase the heme redox potential, diminish the NO-binding affinity, perturb and diminish the 14NO hyperfine coupling determined by ENDOR (electron nuclear double resonance), and increase the maximal electronic g-value. The larger isotropic NO hyperfine and the smaller maximal g-value of the R127A mutant together predicted that the Fe-N-O bond angle in the mutant is larger than that of the Arg-127-containing wild-type protein. Deuterium ENDOR provided evidence for exchangeable H/D consistent with hydrogen bonding of Arg-127, but not Ala-127, to the O of the NO. Proton ENDOR features previously assigned to Phe-14 on the distal side of the heme were unperturbed by the proximal side R127A mutation, implying the localized nature of that mutational perturbation at the proximal, NO-binding side of the heme. From this work two functions of positively charged Arg-127 emerged: the first was to maintain the KD of the cytochrome c' in the 1 microM range, and the second was to provide a redox potential that enhances the stability of the ferrous heme.

Spectroscopic Properties and Electronic Structure of Five- and Six-Coordinate Iron(II) Porphyrin NO Complexes: Effect of the Axial N-Donor Ligand

In this paper, the differences in the spectroscopic properties and electronic structures of five- and six-coordinate iron(II) porphyrin NO complexes are explored using [Fe(TPP)(NO)] (1; TPP = tetraphenylporphyrin) and [Fe(TPP)(MI)(NO)] (2; MI = 1-methylimidazole) type systems. Binding of N-donor ligands in axial position trans to NO to five-coordinate complexes of type 1 is investigated using UV-vis absorption and 1H NMR spectroscopies. This way, the corresponding binding constants Keq are determined and the 1H NMR spectra of 1 and 2 are assigned for the first time. In addition, 1H NMR allows for the determination of the degree of denitrosylation in solutions of 1 with excess base. The influence of the axial ligand on the properties of the coordinated NO is then investigated. Vibrational spectra (IR and Raman) of 1 and 2 are presented and assigned using isotope substitution and normal-coordinate analysis. Obtained force constants are 12.53 (N-O) and 2.98 mdyn/A (Fe-NO) for 1 compared to 11.55 (N-O) and 2.55 mdyn/A (Fe-NO) for 2. Together with the NMR results, this provides experimental evidence that binding of the trans ligand weakens the Fe-NO bond. The principal bonding schemes of 1 and 2 are very similar. In both cases, the Fe-N-O subunit is strongly bent. Donation from the singly occupied pi* orbital of NO into d(z2) of iron(II) leads to the formation of an Fe-NO sigma bond. In addition, a medium-strong pi back-bond is present in these complexes. The most important difference in the electronic structures of 1 and 2 occurs for the Fe-NO sigma bond, which is distinctively stronger for 1 in agreement with the experimental force constants. The increased sigma donation from NO in 1 also leads to a significant transfer of spin density from NO to iron, as has been shown by magnetic circular dichroism (MCD) spectroscopy in a preceding Communication (Praneeth, V. K. K.; Neese, F.; Lehnert, N. Inorg. Chem. 2005, 44, 2570-2572). This is confirmed by the 1H NMR results presented here. Hence, further experimental and computational evidence is provided that complex 1 has noticeable Fe(I)NO+ character relative to 2, which is an Fe(II)NO(radical) complex. Finally, using MCD theory and quantum chemical calculations, the absorption and MCD C-term spectra of 1 and 2 are assigned for the first time.

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.

Quantitative vibrational dynamics of iron in nitrosyl porphyrins

We use quantitative experimental and theoretical approaches to characterize the vibrational dynamics of the Fe atom in porphyrins designed to model heme protein active sites. Nuclear resonance vibrational spectroscopy (NRVS) yields frequencies, amplitudes, and directions for 57Fe vibrations in a series of ferrous nitrosyl porphyrins, which provide a benchmark for evaluation of quantum chemical vibrational calculations. Detailed normal mode predictions result from DFT calculations on ferrous nitrosyl tetraphenylporphyrin Fe(TPP)(NO), its cation [Fe(TPP)(NO)]+, and ferrous nitrosyl porphine Fe(P)(NO). Differing functionals lead to significant variability in the predicted Fe-NO bond length and frequency for Fe(TPP)(NO). Otherwise, quantitative comparison of calculated and measured Fe dynamics on an absolute scale reveals good overall agreement, suggesting that DFT calculations provide a reliable guide to the character of observed Fe vibrational modes. These include a series of modes involving Fe motion in the plane of the porphyrin, which are rarely identified using infrared and Raman spectroscopies. The NO binding geometry breaks the four-fold symmetry of the Fe environment, and the resulting frequency splittings of the in-plane modes predicted for Fe(TPP)(NO) agree with observations. In contrast to expectations of a simple three-body model, mode energy remains localized on the FeNO fragment for only two modes, an N-O stretch and a mode with mixed Fe-NO stretch and FeNO bend character. Bending of the FeNO unit also contributes to several of the in-plane modes, but no primary FeNO bending mode is identified for Fe(TPP)(NO). Vibrations associated with hindered rotation of the NO and heme doming are predicted at low frequencies, where Fe motion perpendicular to the heme is identified experimentally at 73 and 128 cm-1. Identification of the latter two modes is a crucial first step toward quantifying the reactive energetics of Fe porphyrins and heme proteins.

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

We report structural and spectroscopic data for a series of six-coordinate (nitrosyl)iron(II) porphyrinates. The structures of three tetraphenylporphyrin complexes [Fe(TPP)(NO)(L)], where L = 4-(dimethylamino)pyridine, 1-methylimidazole, 4-methylpiperidine, are reported here to a high degree of precision and allow observation of several previously unobserved structural features. The tight range of bonding parameters for the [FeNO] moiety for these three complexes suggests a canonical representation for six-coordinate systems (Fe-N(p) = 2.007 A, Fe-N(NO) = 1.753 A, angle FeNO = 138.5 degrees ). Comparison of these data with those obtained previously for five-coordinate systems allows the precise determination of the structural effects of binding a sixth ligand. These include lengthening of the Fe-N(NO) bond and a decrease in the Fe-N-O angle. Several other aspects of the geometry of these systems are also discussed, including the first examples of off-axis tilting of a nitrosyl ligand in a six-coordinate [FeNO](7) heme system. We also report the first examples of Mössbauer studies for these complexes. Measurements have been made in several applied magnetic fields as well as in zero field. The spectra differ from those of their five-coordinate analogues. To obtain reasonable fits to applied magnetic field data, rotation of the electrical field gradient is required, consistent with differing g-tensor orientations in the five- vs six-coordinate species.

Structural origin of two paramagnetic species in six-coordinated nitrosoiron(II) porphyrins revealed by density functional theory analysis of the g tensors

Potential energy and electron paramagnetic resonance (EPR) g tensor surfaces of model five- and six-coordinated porphyrins were examined. For both types of complexes, the NO ligand is preferably coordinated end-on, with a Fe-N-O bond angle of approximately 140 degrees. In the free five-coordinated structure, NO undergoes free rotation around the axial Fe-N(NO) bond. This motion is strongly coupled to the saddle-type distortion of the porphyrin ligand. Coordination by the second axial ligand (imidazole) raises the calculated barrier for NO rotation to about 1 kcal/mol, which is further increased by displacements of imidazole from the ideal axial position. The potential energy surface for the dissociation of the weakly coordinated imidazole ligand is exceptionally flat, with variation of the Fe-N(Im) bond length between 2.1 and 2.5 A changing the energy by less than 1 kcal/mol. Experimental orientations of both axial ligands, as well as the Fe-N(Im) bond length, are therefore likely to be determined by the environment of the complex. In contrast to the total energy, calculated EPR g-tensors are sensitive to the orientation of the NO ligand and to the Fe-N(Im) bond length. Contrary to a common assumption, the g tensor component closest to the free-electron value does not coincide with the direction of the Fe-N(NO) bond. From comparison of the calculated and experimental g-tensor components for a range of structures, the rhombic ("type I") EPR signal is assigned to a static structure with NO oriented toward the meso-C atom of the prophyrin ring, and RFe-N(Im) approximately 2.1 A (calcd g1 = 1.95, g2 = 2.00, g3 = 2.04; exptl g1 = 1.96-1.98, g2 = 2.00, g3 = 2.06-2.08). The axial ("type II") EPR signal cannot correspond to any of the static structures studied presently. It is tentatively assigned to a partially dissociated six-coordinated complex (RFe-N(Im) > 2.5 A), with a freely rotating NO ligand (calcd g parallel = 2.00, g perpendicular = 2.03; exptl g parallel = 1.99-2.00, g perpendicular = 2.02-2.03).