Comprehensive Fe-ligand vibration identification in {FeNO}6 hemes

Local Oxidation States in {FeNO}6-8 Porphyrins: Insights from DMRG/CASSCF-CASPT2 Calculations

A first DMRG/CASSCF-CASPT2 study of a series of paradigmatic {FeNO}6, {FeNO}7, and {FeNO}8 heme-nitrosyl complexes has led to substantial new insight as well as uncovered key shortcomings of the DFT approach. By virtue of its balanced treatment of static and dynamic correlation, the calculations have provided some of the most authoritative information available to date on the energetics of low- versus high-spin states of different classes of heme-nitrosyl complexes. Thus, the calculations indicate low doublet-quartet gaps of 1-4 kcal/mol for {FeNO}7 complexes and high singlet-triplet gaps of ≳20 kcal/mol for both {FeNO}6 and {FeNO}8 complexes. In contrast, DFT calculations yield widely divergent spin state gaps as a function of the exchange-correlation functional. DMRG-CASSCF calculations also help calibrate DFT spin densities for {FeNO}7 complexes, pointing to those obtained from classic pure functionals as the most accurate. The general picture appears to be that nearly all the spin density of Fe[P](NO) is localized on the Fe, while the axial ligand imidazole (ImH) in Fe[P](NO)(ImH) pushes a part of the spin density onto the NO moiety. An analysis of the DMRG-CASSCF wave function in terms of localized orbitals and of the resulting configuration state functions in terms of resonance forms with varying NO(π*) occupancies has allowed us to address the longstanding question of local oxidation states in heme-nitrosyl complexes. The analysis indicates NO(neutral) resonance forms [i.e., Fe(II)-NO0 and Fe(III)-NO0] as the major contributors to both {FeNO}6 and {FeNO}7 complexes. This finding is at variance with the common formulation of {FeNO}6 hemes as Fe(II)-NO+ species but is consonant with an Fe L-edge XAS analysis by Solomon and co-workers. For the {FeNO}8 complex {Fe[P](NO)}-, our analysis suggests a resonance hybrid description: Fe(I)-NO0 ↔ Fe(II)-NO-, in agreement with earlier DFT studies. Vibrational analyses of the compounds studied indicate an imperfect but fair correlation between the NO stretching frequency and NO(π*) occupancy, highlighting the usefulness of vibrational data as a preliminary indicator of the NO oxidation state.

Heme protonation affects iron-NO binding in the NO transport protein nitrophorin

The nitrophorins (NPs) comprise an unusual group of heme proteins with stable ferric heme iron nitric oxide (Fe-NO) complexes. They are found in the salivary glands of the blood-sucking kissing bug Rhodnius prolixus, which uses the NPs to transport the highly reactive signaling molecule NO. Nuclear resonance vibrational spectroscopy (NRVS) of both isoform NP2 and a mutant NP2(Leu132Val) show, after addition of NO, a strong structured vibrational band at around 600 cm^-1, which is due to modes with significant Fe-NO bending and stretching contribution. Based on a hybrid calculation method, which uses density functional theory and molecular mechanics, it is demonstrated that protonation of the heme carboxyl groups does influence both the vibrational properties of the Fe-NO entity and its electronic ground state. Moreover, heme protonation causes a significant increase of the gap between the highest occupied and lowest unoccupied molecular orbital by almost one order of magnitude leading to a stabilization of the Fe-NO bond.

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.

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.

Structure and reactivity of chlorite dismutase nitrosyls

Ferric nitrosyl ({FeNO}6) and ferrous nitrosyl ({FeNO}7) complexes of the chlorite dismutases (Cld) from Klebsiella pneumoniae and Dechloromonas aromatica have been characterized using UV-visible absorbance and Soret-excited resonance Raman spectroscopy. Both of these Clds form kinetically stable {FeNO}6 complexes and they occupy a unique region of ν(Fe-NO)/ν(N-O) correlation space for proximal histidine liganded heme proteins, characteristic of weak Fe-NO and N-O bonds. This location is attributed to admixed FeIII-NO character of the {FeNO}6 ground state. Cld {FeNO}6 complexes undergo slow reductive nitrosylation to yield {FeNO}7 complexes. The effects of proximal and distal environment on reductive nitroylsation rates for these dimeric and pentameric Clds are reported. The ν(Fe-NO) and ν(N-O) frequencies for Cld {FeNO}7 complexes reveal both six-coordinate (6c) and five-coordinate (5c) nitrosyl hemes. These 6c and 5c forms are in a pH dependent equilibrium. The 6c and 5c {FeNO}7 Cld frequencies provided positions of both Clds on their respective ν(Fe-NO) vs ν(N-O) correlation lines. The 6c {FeNO}7 complexes fall below (along the ν(Fe-NO) axis) the correlation line that reports hydrogen-bond donation to NNO, which is consistent with a relatively weak Fe-NO bond. Kinetic and spectroscopic evidence is consistent with the 5c {FeNO}7 Clds having NO coordinated on the proximal side of the heme, analogous to 5c {FeNO}7 hemes in proteins known to have NO sensing functions.

Non-heme High-Spin {FeNO}6-8 Complexes: One Ligand Platform Can Do It All

Heme and non-heme iron-nitrosyl complexes are important intermediates in biology. While there are numerous examples of low-spin heme iron-nitrosyl complexes in different oxidation states, much less is known about high-spin (hs) non-heme iron-nitrosyls in oxidation states other than the formally ferrous NO adducts ({FeNO}⁷ in the Enemark-Feltham notation). In this study, we present a complete series of hs-{FeNO}^6-8 complexes using the TMG₃tren coligand. Redox transformations from the hs-{FeNO}⁷ complex [Fe(TMG₃tren)(NO)]²⁺ to its {FeNO}⁶ and {FeNO}⁸ analogs do not alter the coordination environment of the iron center, allowing for detailed comparisons between these species. Here, we present new MCD, NRVS, XANES/EXAFS, and Mössbauer data, demonstrating that these redox transformations are metal based, which allows us to access hs-Fe(II)-NO^-, Fe(III)-NO^-, and Fe(IV)-NO^- complexes. Vibrational data, analyzed by NCA, directly quantify changes in Fe-NO bonding along this series. Optical data allow for the identification of a "spectator" charge-transfer transition that, together with Mössbauer and XAS data, directly monitors the electronic changes of the Fe center. Using EXAFS, we are also able to provide structural data for all complexes. The magnetic properties of the complexes are further analyzed (from magnetic Mössbauer). The properties of our hs-{FeNO}^6-8 complexes are then contrasted to corresponding, low-spin iron-nitrosyl complexes where redox transformations are generally NO centered. The hs-{FeNO}⁸ complex can further be protonated by weak acids, and the product of this reaction is characterized. Taken together, these results provide unprecedented insight into the properties of biologically relevant non-heme iron-nitrosyl complexes in three relevant oxidation states.

How Does a Heme Carbene Differ from Diatomic Ligated (NO, CO, and CN-) Analogues in the Axial Bond?

Compared to well studied diatomic ligands (NO, CN^-, CO), the axial bonds of carbene hemes is much less known although its significance in biological chemistry. The unusually large quadrupole splitting (Δ E_Q = +2.2 mm·s^-1) and asymmetric parameter (η = 0.9) of the five-coordinate heme carbene [Fe(TTP)(CCl₂)], which is the largest among all known low spin ferrohemes, has driven investigations by means of Mössbauer effect Nuclear Resonance Vibrational Spectroscopy (NRVS). Three distinct measurements on one single crystal (two in-plane and one out-of-plane) have demonstrated comprehensive vibrational structures including stretch (429) and bending modes (472 cm^-1) of the axial Fe-CCl₂, and revealed iron vibrational anisotropy in three orthogonal directions for the first time. Frontier orbital analysis especially comparisons with diatomic analogues (NO, CN^-, CO) suggest that CCl₂, similar to NO, has led to strong but anisotropic π bonding in a ligand-based "4C"-coordinate which induced the vibrational anisotropies and very large Mössbauer parameters. This is contrasted to CN^- and CO complexes which possess a porphyrin-based "4N"-coordinate electronic and vibrational structures due to inherent on-axis linear ligation.

What Can Be Learned from Nuclear Resonance Vibrational Spectroscopy: Vibrational Dynamics and Hemes

Nuclear resonance vibrational spectroscopy (NRVS; also known as nuclear inelastic scattering, NIS) is a synchrotron-based method that reveals the full spectrum of vibrational dynamics for Mössbauer nuclei. Another major advantage, in addition to its completeness (no arbitrary optical selection rules), is the unique selectivity of NRVS. The basics of this recently developed technique are first introduced with descriptions of the experimental requirements and data analysis including the details of mode assignments. We discuss the use of NRVS to probe ⁵⁷Fe at the center of heme and heme protein derivatives yielding the vibrational density of states for the iron. The application to derivatives with diatomic ligands (O₂, NO, CO, CN^–) shows the strong capabilities of identifying mode character. The availability of the complete vibrational spectrum of iron allows the identification of modes not available by other techniques. This permits the correlation of frequency with other physical properties. A significant example is the correlation we find between the Fe–Im stretch in six-coordinate Fe(XO) hemes and the trans Fe–N(Im) bond distance, not possible previously. NRVS also provides uniquely quantitative insight into the dynamics of the iron. For example, it provides a model-independent means of characterizing the strength of iron coordination. Prediction of the temperature-dependent mean-squared displacement from NRVS measurements yields a vibrational “baseline” for Fe dynamics that can be compared with results from techniques that probe longer time scales to yield quantitative insights into additional dynamical processes.

Ferric Heme-Nitrosyl Complexes: Kinetically Robust or Unstable Intermediates?

We have determined a convenient method for the bulk synthesis of high-purity ferric heme-nitrosyl complexes ({FeNO}⁶ in the Enemark-Feltham notation); this method is based on the chemical or electrochemical oxidation of corresponding {FeNO}⁷ precursors. We used this method to obtain the five- and six-coordinate complexes [Fe(TPP)(NO)]⁺ (TPP^2- = tetraphenylporphyrin dianion) and [Fe(TPP)(NO)(MI)]⁺ (MI = 1-methylimidazole) and demonstrate that these complexes are stable in solution in the absence of excess NO gas. This is in stark contrast to the often-cited instability of such {FeNO}⁶ model complexes in the literature, which is likely due to the common presence of halide impurities (although other impurities could certainly also play a role). This is avoided in our approach for the synthesis of {FeNO}⁶ complexes via oxidation of pure {FeNO}⁷ precursors. On the basis of these results, {FeNO}⁶ complexes in proteins do not show an increased stability toward NO loss compared to model complexes. We also prepared the halide-coordinated complexes [Fe(TPP)(NO)(X)] (X = Cl^-, Br^-), which correspond to the elusive, key reactive intermediate in the so-called autoreduction reaction, which is frequently used to prepare {FeNO}⁷ complexes from ferric precursors. All of the complexes were characterized using X-ray crystallography, UV-vis, IR, and nuclear resonance vibrational spectroscopy (NRVS). On the basis of the vibrational data, further insight into the electronic structure of these {FeNO}⁶ complexes, in particular with respect to the role of the axial ligand trans to NO, is obtained.

Electronic Configuration and Ligand Nature of Five-Coordinate Iron Porphyrin Carbene Complexes: An Experimental Study

The five-coordinate iron porphyrin carbene complexes [Fe(TPP) (CCl₂)] (TPP = tetraphenylporphyrin), [Fe(TTP) (CCl₂)] (TTP = tetratolylporphyrin) and [Fe(TFPP) (CPh₂)] (TFPP = tetra(pentafluorophenyl)porphyrin), utilizing two types of carbene ligands (CCl₂ and CPh₂), have been investigated by single crystal X-ray, XANES (X-ray absorption near edge spectroscopy), Mössbauer, NMR and UV-vis spectroscopies. The XANES suggested the iron(II) oxidation state of the complexes. The multitemperature and high magnetic field Mössbauer experiments, which show very large quadrupole splittings (QS, ΔE_Q), determined the S = 0 electronic configuration. More importantly, combined structural and Mössbauer studies, especially the comparison with the low spin iron(II) porphyrin complexes with strong diatomic ligands (CS, CO and CN^-) revealed the covalent bond nature of the carbene ligands. A correlation between the iron isomer shifts (IS, δ) and the axial bond distances is established for the first time for these donor carbon ligands (:C-R).

L-Edge X-ray Absorption Spectroscopic Investigation of {FeNO}6: Delocalization vs Antiferromagnetic Coupling

NO is a classic non-innocent ligand, and iron nitrosyls can have different electronic structure descriptions depending on their spin state and coordination environment. These highly covalent ligands are found in metalloproteins and are also used as models for Fe-O2 systems. This study utilizes iron L-edge X-ray absorption spectroscopy (XAS), interpreted using a valence bond configuration interaction multiplet model, to directly experimentally probe the electronic structure of the S = 0 {FeNO}6 compound [Fe(PaPy3)NO]2+ (PaPy3 = N,N-bis(2-pyridylmethyl)amine-N-ethyl-2-pyridine-2-carboxamide) and the S = 0 [Fe(PaPy3)CO]+ reference compound. This method allows separation of the σ-donation and π-acceptor interactions of the ligand through ligand-to-metal and metal-to-ligand charge-transfer mixing pathways. The analysis shows that the {FeNO}6 electronic structure is best described as FeIII-NO(neutral), with no localized electron in an NO π* orbital or electron hole in an Fe dπ orbital. This delocalization comes from the large energy gap between the Fe-NO π-bonding and antibonding molecular orbitals relative to the exchange interactions between electrons in these orbitals. This study demonstrates the utility of L-edge XAS in experimentally defining highly delocalized electronic structures.

3D Motions of Iron in Six-Coordinate {FeNO}(7) Hemes by Nuclear Resonance Vibration Spectroscopy

The vibrational spectrum of a six-coordinate nitrosyl iron porphyrinate, monoclinic [Fe(TpFPP)(1-MeIm)(NO)] (TpFPP=tetra-para-fluorophenylporphyrin; 1-MeIm=1-methylimidazole), has been studied by oriented single-crystal nuclear resonance vibrational spectroscopy (NRVS). The crystal was oriented to give spectra perpendicular to the porphyrin plane and two in-plane spectra perpendicular or parallel to the projection of the FeNO plane. These enable assignment of the FeNO bending and stretching modes. The measurements reveal that the two in-plane spectra have substantial differences that result from the strongly bonded axial NO ligand. The direction of the in-plane iron motion is found to be largely parallel and perpendicular to the projection of the bent FeNO on the porphyrin plane. The out-of-plane Fe-N-O stretching and bending modes are strongly mixed with each other, as well as with porphyrin ligand modes. The stretch is mixed with v50 as was also observed for dioxygen complexes. The frequency of the assigned stretching mode of eight Fe-X-O (X=N, C, and O) complexes is correlated with the Fe-XO bond lengths. The nature of highest frequency band at ≈560 cm(-1) has also been examined in two additional new derivatives. Previously assigned as the Fe-NO stretch (by resonance Raman), it is better described as the bend, as the motion of the central nitrogen atom of the FeNO group is very large. There is significant mixing of this mode. The results emphasize the importance of mode mixing; the extent of mixing must be related to the peripheral phenyl substituents.

Hydride bridge in [NiFe]-hydrogenase observed by nuclear resonance vibrational spectroscopy

The metabolism of many anaerobes relies on [NiFe]-hydrogenases, whose characterization when bound to substrates has proven non-trivial. Presented here is direct evidence for a hydride bridge in the active site of the ⁵⁷Fe-labelled fully reduced Ni-R form of Desulfovibrio vulgaris Miyazaki F [NiFe]-hydrogenase. A unique ‘wagging' mode involving H⁻ motion perpendicular to the Ni(μ-H)⁵⁷Fe plane was studied using ⁵⁷Fe-specific nuclear resonance vibrational spectroscopy and density functional theory (DFT) calculations. On Ni(μ-D)⁵⁷Fe deuteride substitution, this wagging causes a characteristic perturbation of Fe–CO/CN bands. Spectra have been interpreted by comparison with Ni(μ-H/D)⁵⁷Fe enzyme mimics [(dppe)Ni(μ-pdt)(μ-H/D)⁵⁷Fe(CO)₃]⁺ and DFT calculations, which collectively indicate a low-spin Ni(II)(μ-H)Fe(II) core for Ni-R, with H⁻ binding Ni more tightly than Fe. The present methodology is also relevant to characterizing Fe–H moieties in other important natural and synthetic catalysts.

Understanding the catalytic mechanism of redox-active hydrogenases is a key to efficient hydrogen production and consumption. Here, the authors use nuclear resonance vibrational spectroscopy to study [NiFe]-hydrogenase, and observe a bridging hydride structure in an EPR silent intermediate.