Functional models of [Fe—S] nitrosyl proteins

Exchange of cysteamine, thiol ligand in binuclear cationic tetranitrosyl iron complex, for glutathione

This paper describes the comparative study of the decomposition of two iron nitrosyl complexes (NICs) with a cysteamine thiolate ligand {Fe₂[S(CH₂)₂NH₃]₂(NO)₄}SO₄·2.5H₂O (I) and a glutathione (GSH)-ligand, [Fe₂(SC₁₀H₁₇N₃O₆)₂(NO)₄]SO₄·2H₂O (II), which spontaneously evolve NO in aqueous medium.

The Preparation, Structural Characteristics, and Physical Chemical Properties of Metal-Nitrosyl Complexes

The preparation and characterization of a representative group of novel non-heme metal nitrosyl complexes that have been synthesized over the last decade are discussed here. Their structures are examined and classified based on metal type, the number of metal centers present, and the type of ligand that is coordinated with the metal. The ligands can be phosphorus, nitrogen, or sulfur based (with a few exceptions) and can vary depending on the presence of chelation, intermolecular forces, or the presence of other ligands. Structural and bonding characteristics are summarized and examples of reactivity regarding nitrosyl ligands are given. Some of the relevant physical chemical properties of these complexes, including IR, EPR, NMR, UV-vis, cyclic voltammetry, and X-ray crystallography are examined.

Calibration of DFT Functionals for the Prediction of Fe Mössbauer Spectral Parameters in Iron-Nitrosyl and Iron-Sulfur Complexes: Accurate Geometries Prove Essential

Six popular density functionals in conjunction with the conductor-like screening (COSMO) solvation model have been used to obtain linear Mössbauer isomer shift (IS) and quadrupole splitting (QS) parameters for a test set of 20 complexes (with 24 sites) comprised of nonheme nitrosyls (Fe-NO) and non-nitrosyl (Fe-S) complexes. For the first time in an IS analysis, the Fe electron density was calculated both directly at the nucleus, ρ(0)(N), which is the typical procedure, and on a small sphere surrounding the nucleus, ρ(0)(S), which is the new standard algorithm implemented in the ADF software package. We find that both methods yield (near) identical slopes from each linear regression analysis but are shifted with respect to ρ(0) along the x-axis. Therefore, the calculation of the Fe electron density with either method gives calibration fits with equal predictive value. Calibration parameters obtained from the complete test set for OLYP, OPBE, PW91, and BP86 yield correlation coefficients (r(2)) of approximately 0.90, indicating that the calibration fit is of good quality. However, fits obtained from B3LYP and B3LYP* with both Slater-type and Gaussian-type orbitals are generally found to be of poorer quality. For several of the complexes examined in this study, we find that B3LYP and B3LYP* give geometries that possess significantly larger deviations from the experimental structures than OLYP, OPBE, PW91 or BP86. This phenomenon is particularly true for the di- and tetranuclear Fe complexes examined in this study. Previous Mössbauer calibration fit studies using these functionals have usually included mononuclear Fe complexes alone, where these discrepancies are less pronounced. An examination of spin expectation values reveals B3LYP and B3LYP* approach the weak-coupling limit more closely than the GGA exchange-correlation functionals. The high degree of variability in our calculated S(2) values for the Fe-NO complexes highlights their challenging electronic structure. Significant improvements to the isomer shift calibrations are obtained for B3LYP and B3LYP* when geometries obtained with the OLYP functional are used. In addition, greatly improved performance of these functionals is found if the complete test set is grouped separately into Fe-NO and Fe-S complexes. Calibration fits including only Fe-NO complexes are found to be excellent, while those containing the non-nitrosyl Fe-S complexes alone are found to demonstrate less accurate correlations. Similar trends are also found with OLYP, OPBE, PW91, and BP86. Correlations between experimental and calculated QSs were also investigated. Generally, universal and separate Fe-NO and Fe-S fit parameters obtained to determine QSs are found to be of good to excellent quality for every density functional examined, especially if [Fe(4)(NO)(4)(μ(3)-S)(4)](-) is removed from the test set.

Spin coupling in Roussin's red and black salts

Although DFT calculations have provided a first-order electronic-structural description for Roussin's red and black salts, a detailed study of spin coupling in these species has yet to be reported. Such an analysis is presented here for the first time, based on broken-symmetry density functional theory (DFT, chiefly OLYP/STO-TZP) calculations. Both the Noodleman and Yamaguchi formulas were used to evaluate the Heisenberg coupling constants (J). Three nitrosylated binuclear clusters were studied: [Fe(2)(NO)(2)(Et-HPTB)(O(2)CPh)](2+) (1; Et-HPTB=N,N,N',N'-tetrakis-(N-ethyl-2-benzimidazolylmethyl)-2-hydroxy-1,3-diaminopropane), [Fe(NO)(2){Fe(NO)(NS(3))}-S,S'] (2), and Roussin's red salt anion [Fe(2)(NO)(4)(μ-S)(2)](2-) (3). Although the Heisenberg J for 1 is small (≈10(2) cm(-1)), 2 and 3 exhibit J values that are at least an order of magnitude higher (≈10(3) cm(-1)), where the J values refer to the following Heisenberg spin Hamiltonian: ℋ=JS(A)⋅S(B). For Roussin's black salt anion, [Fe(4)(NO)(7)(μ(3)-S)(3)](-) (4), the Heisenberg spin Hamiltonian describing spin coupling between the {FeNO}(7) unit (S(A)=3/2) and the three {Fe(NO)(2)}(9) units (S(B)=S(C)=S(D)=1/2) in [Fe(4)(NO)(7)(μ(3)-S)(3)](-) was assumed to have the form: ℋ=J(12)(S(A)⋅S(B)+S(A)⋅S(C)+S(A)⋅S(D))+J(22)(S(B)⋅S(C)+S(B)⋅S(D)+S(C)⋅S(D)), in which J(12) corresponds to the interaction between the apical iron and a basal iron, and J(22) refers to that between any two basal iron centers. Although the basal-basal coupling constant J(22) was found to be small (≈10(2) cm(-1)), the apical-basal coupling constant J(12) is some forty times higher (≈4000 cm(-1)). Thus, the nitrosylated iron-sulfur clusters feature some exceptionally high J values relative to the non-nitrosylated {2Fe2S} and {4Fe4S} clusters. An analysis of spin-dependent bonding energies shed light on this curious feature. In essence, the energy difference between the high-spin (i.e., ferromagnetically coupled iron sites) and low-spin (i.e., maximum spin coupling) states of Roussin's salts are indeed rather similar to those of analogous non-nitrosylated iron-sulfur clusters. However, the individual Fe(NO)(x) (x=1, 2) site spins are lower in the nitrosylated systems, resulting in a smaller denominator in both the Noodleman and Yamaguchi formulas for J, which in turn translates into the very high J values.

Density functional theory calculations on Mössbauer parameters of nonheme iron nitrosyls

Density Functional Theory (DFT) calculations on transition metal nitrosyls often reveal unusual spin density profiles, involving substantial spatial separation of majority and minority spin densities. Against this context, there is a significant lack of studies where DFT calculations have been quantitatively calibrated against experimental spectroscopic properties. Reported herein are DFT calculations of Mössbauer isomer shifts and quadrupole splittings for 21 nonheme iron complexes (26 distinct iron sites) including 9 iron nitrosyls. Low- (S = 1/2) and high-spin (S = 3/2) {FeNO}(7) complexes, S = 1/2 {Fe(NO)(2)}(9) species, and polynuclear iron nitrosyls are all represented within the set of compounds examined. The general conclusion with respect to isomer shifts is that DFT (OLYP/STO-TZP) performs comparably well for iron nitrosyls and for iron complexes in general. However, quadrupole splittings are less accurately reproduced for nitrosyl complexes.

Dinitrosyl iron complexes relevant to Rieske cluster nitrosylation

Reaction of the Rieske cluster model complex (Et(4)N)(2)[(N(2)CHPh)Fe(2)S(2)(S(2)-o-xyl)] (N(2)CHPh = dianion of 2,2'-(phenylmethylene)bis(3-methylindole); S(2)-o-xyl = dianion of 1,2-phenylenedimethanethiol) with nitric oxide results in disassembly of the iron-sulfur core and formation of {Fe(NO)(2)}(9) dinitrosyliron complexes (DNICs). Isolation and characterization of these DNICs, including the new compound, (Et(4)N)[(N(2)CHPh)Fe(NO)(2)], demonstrates a homology between the synthetic Riekse cluster and purely thiolate-bound Fe(2)S(2) clusters in reactions involving NO. To model the nitrogen-rich environment of Rieske cluster-derived dinitroysliron species, a new type of neutral {Fe(NO)(2)}(9) DNIC was prepared containing a beta-diketiminate ligand. One-electron reduction of this compound affords the isolable {Fe(NO)(2)}(10) DNIC. These compounds represent a rare example of structurally analogous DNIC redox partners.

The role of photoinduced electron transfer processes in photodegradation of the [Fe4(μ3-S)3(NO)7]− cluster

Spectroscopic and electrochemical study of the [Fe(4)(mu(3)-S)(3)(NO)(7)](-) photochemical reaction and thermodynamic calculations of relevant systems demonstrate the redox character of this process. The photoinduced electron transfer between substrate clusters in excited and ground state (probably via exciplex formation) results in dismutation yielding unstable [Fe(4)(mu(3)-S)(3)(NO)(7)](2-) and [Fe(4)(mu(3)-S)(3)(NO)(7)](0). Back electron transfer between the primary products is responsible for fast reversibility of the photochemical reaction in deoxygenated solutions. In the presence of an electron acceptor (such as O(2), MV(2+) or NO) an oxidative quenching of the (*)[Fe(4)(mu(3)-S)(3)(NO)(7)](-) is anticipated, although NO seems to participate as well in the reductive quenching. The electron acceptors can also regenerate the substrate from its reduced form ([Fe(4)(mu(3)-S)(3)(NO)(7)](2-)), whereas the other primary product ([Fe(4)(mu(3)-S)(3)(NO)(7)](0)) decomposes to the final products. The suggested mechanism fits well to all experimental observations and shows the thermodynamically favored pathways and explains formation of all major (Fe(2+), S(2-), NO) and minor products (N(2)O, Fe(3+)). The photodissociation of nitrosyl ligands suggested earlier as the primary photochemical step cannot be, however, definitely excluded and may constitute a parallel pathway of [Fe(4)(mu(3)-S)(3)(NO)(7)](-) photolysis.

Photochemistry of the [Fe4(μ3-S)3(NO)7]− complex in the presence of S-nucleophiles: A spectroscopic study

Biological systems usually contain cysteine, glutathione or other sulfur-containing biomolecules. These S-nucleophiles were found to affect drastically the [Fe(4)(mu(3)-S)(3)(NO)(7)](-) photolysis pathway generating products completely different from that of the neat cluster, which produces Fe(II) and NO and S(2-). The effect is interpreted in terms of formation of a pseudo-cubane adduct, [Fe(4)(mu(3)-S)(3)(mu(3)-SR)(NO)(7)](2-), whose existence in equilibrium with the parent complex has no detectable influence on the spectral properties, whereas shifts the redox potential and induces photoconversion leading to the Fe(III) species and N(2)O. Characteristic bond lengths, bond angles and atomic Mulliken charges were calculated using semi-empirical quantum chemical methods for the RBS anion and a series of pseudo-cubane complexes with S-donor or N-donor ligands. The results justify the hypothesis of the adduct formation and show that only in case of S-ligands the higher contribution of the Fe(III)-NO(-) components in adduct than in RBS is observed, which on excitation can undergo heterolytic cleavage yielding Fe(III) and NO(-), converted rapidly into N(2)O. These results are crucial in understanding the physiological activity of RBS. Fe(III) formation can be detected only when the S-ligand enables formation of a stable Fe(III) compound; the effect was recorded in the presence of sulfide, thioglycolate, 2-mercaptopropionate, mercaptosuccinate, penicillamine, 2,3-dimercaptosuccinate, 2,3-dimercaptopropanol, and thiocyanate. For all these S-ligands the Fe(III) photoproducts were identified and characterised. In the case of other thiolates, their excess results in fast reduction of Fe(III) to Fe(II), whereas N(2)O can be still detected. Quantum yields of Fe(III) formation in the presence of the S-ligands are considerably higher than that of the Fe(II) photoproduction from neat [Fe(4)(mu(3)-S)(3)(NO)(7)](-).