Iron-ligand Bonds Research Articles

Influence of the Fe(II) Spin State on Iron-Ligand Bonds in Heme Model Iron-Porphyrin Complexes with 4, 5 and 6 Ligands.

In this work heme models with four [Fe(II)(P)], five [Fe(II)(P)Im], [Fe(II)(P)O2] and six ligands [Fe(II)(P)(Im)O2], where P=porphyrin, with different spin states (ms=5, 3 and 1) of the iron atom were investigated using relativistic-corrected quantum chemistry methods (PW6B95-D3-DKH/jorge-TZP-DKH). Dependence of the iron-ligand bond properties on (i) spin state and (ii) number of ligands were analyzed using natural bond orbital analysis, electron density topology, electrostatic potential and electron localization function. It is shown that reversible binding of O2 is possible in case of formation of semicoordination bond between Fe(II) and imidazole. Binding of the fifth and sixth ligand from the energetic and orbital points of view is more favorable for the triplet Fe(II) state. At the same time for the six-coordinated complex [Fe(II)(P)(Im)O2] interconversion of Fe(II) electrons of valent 3d orbital from quintet to triplet and vice versa is possible under thermal fluctuations (energy barriers less than 2 kcal/mol).

Temperature Dependence of Structural Dynamics at the Catalytic Cofactor of [FeFe]-hydrogenase.

[FeFe]-hydrogenases are nature's blueprint for efficient hydrogen turnover. Understanding their enzymatic mechanism may improve technological H2 fuel generation. The active-site cofactor (H-cluster) consists of a [4Fe-4S] cluster ([4Fe]H), cysteine-linked to a diiron site ([2Fe]H) carrying an azadithiolate (adt) group, terminal cyanide and carbon monoxide ligands, and a bridging carbon monoxide (μCO) in the oxidized protein (Hox). Recently, the debate on the structure of reduced H-cluster states was intensified by the assignment of new species under cryogenic conditions. We investigated temperature effects (4-280 K) in infrared (IR) and X-ray absorption spectroscopy (XAS) data of [FeFe]-hydrogenases using fit analyses and quantum-chemical calculations. IR data from our laboratory and literature sources were evaluated. At ambient temperatures, reduced H-cluster states with a bridging hydride (μH-, in Hred and Hsred) or with an additional proton at [4Fe]H (Hred') or at the distal iron of [2Fe]H (Hhyd) prevail. At cryogenic temperatures, these species are largely replaced by states that hold a μCO, lack [4Fe]H protonation, and bind an additional proton at the adt nitrogen (HredH+ and HsredH+). XAS revealed the atomic coordinate dispersion (i.e., the Debye-Waller parameter, 2σ2) of the iron-ligand bonds and Fe-Fe distances in the oxidized and reduced H-cluster. 2σ2 showed a temperature dependence typical for the so-called protein-glass transition, with small changes below ∼200 K and a pronounced increase above this "breakpoint". This behavior is attributed to the freezing-out of larger-scale anharmonic motions of amino acid side chains and water species. We propose that protonation at [4Fe]H as well as ligand rearrangement and μH- binding at [2Fe]H are impaired because of restricted molecular mobility at cryogenic temperatures so that protonation can be biased toward adt. We conclude that a H-cluster with a μCO, selective [4Fe]H or [2Fe]H protonation, and catalytic proton transfer via adt facilitates efficient H2 conversion in [FeFe]-hydrogenase.

Single-Molecule Force Spectroscopy Reveals that Iron-Ligand Bonds Modulate Proteins in Different Modes.

The iron-amino acid interactions Fe-O(Glu/Asp), Fe-N(His), and Fe-S(Cys) are the three major iron-ligand bonds in proteins. To compare their properties in proteins, we used atomic force microscopy (AFM)-based single-molecule force spectroscopy to investigate a superoxide reductase (Fe(III)-SOR) with all three types of bonds forming an Fe(His)4CysGlu center. We first found that Apo-SOR without bound iron showed multiple unfolding pathways only from the β-barrel core. Then, using Holo-SOR with a ferric ion, we found that a single Fe-O(Glu) bond can tightly connect the flexible N-terminal fragment to the β-barrel and stabilize the whole protein, showing a complete protein unfolding scenario, while the single Fe-N(His) bond was weak and unable to provide such a stabilization. Moreover, when multiple Fe-N bonds are present, a similar stabilization effect can be achieved. Our results showed that the iron-ligand bond modulates protein structure and stability in different modes at the single-bond level.

A highly stretchable autonomous self-healing elastomer

It is a challenge to synthesize materials that possess the properties of biological muscles-strong, elastic and capable of self-healing. Herein we report a network of poly(dimethylsiloxane) polymer chains crosslinked by coordination complexes that combines high stretchability, high dielectric strength, autonomous self-healing and mechanical actuation. The healing process can take place at a temperature as low as -20 °C and is not significantly affected by surface ageing and moisture. The crosslinking complexes used consist of 2,6-pyridinedicarboxamide ligands that coordinate to Fe(III) centres through three different interactions: a strong pyridyl-iron one, and two weaker carboxamido-iron ones through both the nitrogen and oxygen atoms of the carboxamide groups. As a result, the iron-ligand bonds can readily break and re-form while the iron centres still remain attached to the ligands through the stronger interaction with the pyridyl ring, which enables reversible unfolding and refolding of the chains. We hypothesize that this behaviour supports the high stretchability and self-healing capability of the material.

Electromerism and linkage isomerism in biologically-relevant Fe[sbnd]SO complexes

Sulfur monoxide, SO, is a relatively unstable molecule whose metal-coordinating properties have received little attention in bioinorganic chemistry. Reported here is a density functional theory (DFT) examination of the four possible oxidation states for a heme-SO/OS adduct previously proposed to be a part of the catalytic cycle of sulfite reductases. The FeOS and FeSO isomers are found to be degenerate in energy in most cases, suggesting that they both may be observable; the FeSO isomers would be the ones more likely to occur during the catalytic cycle of sulfite reductases — a cycle which indeed is initiated with the sulfite bound to iron via the sulfur, not via the oxygen. More importantly, higher spin states tend to be favored especially in the more oxidized models — which are the states occurring earlier in the proposed catalytic cycle. This implies weaker iron–ligand bonds — and, in fact, in several cases, essentially broken bonds. The sulfite reductase active site features an iron–sulfur cluster sharing one of its sulfur thiolates with the heme as the axial ligand. This uncommon proximity has as unavoidable effect an increase in the efficiency of delivery of electrons to the heme once SO has been generated at the active site. This would then allow the catalytic cycle to proceed to the next step — exothermic protonation of the SO.

Site-Selective X-ray Spectroscopy on an Asymmetric Model Complex of the [FeFe] Hydrogenase Active Site

The active site for hydrogen production in [FeFe] hydrogenase comprises a diiron unit. Bioinorganic chemistry has modeled important features of this center, aiming at mechanistic understanding and the development of novel catalysts. However, new assays are required for analyzing the effects of ligand variations at the metal ions. By high-resolution X-ray absorption spectroscopy with narrow-band X-ray emission detection (XAS/XES = XAES) and density functional theory (DFT), we studied an asymmetrically coordinated [FeFe] model complex, [(CO)(3)Fe(I)1-(bdtCl(2))-Fe(I)2(CO)(Ph(2)P-CH(2)-NCH(3)-CH(2)-PPh(2))] (1, bdt = benzene-1,2-dithiolate), in comparison to iron-carbonyl references. Kβ emission spectra (Kβ(1,3), Kβ') revealed the absence of unpaired spins and the low-spin character for both Fe ions in 1. In a series of low-spin iron compounds, the Kβ(1,3) energy did not reflect the formal iron oxidation state, but it decreases with increasing ligand field strength due to shorter iron-ligand bonds, following the spectrochemical series. The intensity of the valence-to-core transitions (Kβ(2,5)) decreases for increasing Fe-ligand bond length, certain emission peaks allow counting of Fe-CO bonds, and even molecular orbitals (MOs) located on the metal-bridging bdt group of 1 contribute to the spectra. As deduced from 3d → 1s emission and 1s → 3d absorption spectra and supported by DFT, the HOMO-LUMO gap of 1 is about 2.8 eV. Kβ-detected XANES spectra in agreement with DFT revealed considerable electronic asymmetry in 1; the energies and occupancies of Fe-d dominated MOs resemble a square-pyramidal Fe(0) for Fe1 and an octahedral Fe(II) for Fe2. EXAFS spectra for various Kβ emission energies showed considerable site-selectivity; approximate structural parameters similar to the crystal structure could be determined for the two individual iron atoms of 1 in powder samples. These results suggest that metal site- and spin-selective XAES on [FeFe] hydrogenase protein and active site models may provide a powerful tool to study intermediates under reaction conditions.

Assessing the impact of inductive electronic effects on the metrical parameters and reactivity of a series of ferrous complexes

Reported are four iron(II) complexes with N-benzyl- N, N′-bis(2-pyridylmethyl)-1,2-ethanediamine (L H) and three electronically modified derivatives: N-(4-methoxy)benzyl- N, N′-bis(2-pyridylmethyl)-1,2-ethanediamine (L OMe), N-(4-chloro)benzyl- N, N′-bis(2-pyridylmethyl)-1,2-ethanediamine (L Cl), and N-(4-nitro)benzyl- N, N′-bis(2-pyridylmethyl)-1,2-ethanediamine ( L NO 2 ). The four ligands react with FeCl 2 to form a series of mononuclear species with the general formula [Fe(L R)Cl 2]. The cis-α conformation of the ligand places the amine N-donors trans to the Fe–Cl bonds. The identity of the 4-benzyl substituent has profound influences on the lengths of the iron-ligand bonds, the optical spectra, and the redox activities of the [Fe(L R)Cl 2] compounds.

Electronic structure of Fe-bearing lazulites

The Fe end-members scorzalite [Fe2+Al23+(PO4)(2)(OH)(2)] and barbosalite [Fe2+Fe23+(PO4)(2)(OH)(2)] of the lazulite series have been investigated by Mossbauer and diffuse reflectance spectroscopy, and by electronic structure calculations in the local spin density approximation. The measured quadrupole splitting (DeltaE(Q) = -3.99 mm/s) in scorzalite is in quantitative agreement with the calculated value (DeltaE(Q) = -3.90 mm/s), as well as its temperature dependence. The optical spectrum of barbosalite can be resolved into three peaks at 8985 cm(-1), 10980 cm(-1), and 14110 cm-1. These positions correlate well with the two calculated spin-allowed d-d transitions at 8824 cm-1 and 11477 cm-1, and with an intervalence charge transfer transition at about 14200 cm-1. The calculated low-temperature magnetic structure of barbosalite is characterized by a strong antiferromagnetic coupling (J = -84.6 cm-1) within the octahedral Fe3+-chains, whereas a weak antiferromagnetic coupling within the trioctabedral subunit cannot be considered as conclusive. The analysis of the charge and spin densities reveals that more than 90% of the covalent part of the iron-ligand bonds arises from the Fe(4s,4p)-electrons. Clusters of at least 95 atoms are required to reproduce the available experimental data with quantitative accuracy.

Correlations among parameters that describe low-spin ferric heme complexes

The g values from low-spin ferric hemes can be related through the t 2 g hole model to rhombic ( V λ ) and tetragonal (Δ/λ) ligand field components and to the lowest Kramer's doublet energy ( E λ ). The latter is also a measure of unpaired electron sharing among the iron 3 d ( t 2 g ) orbitals. For a series of ligands (X), there is a monotonie increase in myoglobin complex (Mb · X) ¦ E λ ¦values with nonheme hexacoordinate metal complex (M · X 6) ¦ e g − t 2 g ¦orbital separations. As the aqueous solution p K a values of the sulfurous or nitrogenous ligands in model heme complexes increase, values of V λ and Δ/λ increase linearly, but those of ¦ E λ ¦decrease linearly. The greater the electron-acceptor ability of the ligand, as suggested by its position in the spectrochemical series or its p K a , the more the unpaired electron sharing among the heme t 2 g orbitals increases. The rate of change of ¦ E λ ¦with V λ and the p K a is different with sulfurous and nitrogenous ligands and the magnitude of both rates increases with two sulfurs < sulfur and nitrogen < two nitrogens bound to the heme. The maximum magnitude of this rate with V λ for cytochrome P-450 is four times less than that for myoglobin, which may explain, in part, the differences in ligand binding between these two hemeproteins. The perturbation of ¦ E λ ¦, V λ and Δ/λ induced by strain of iron-ligand bonds is quantitated for several hemeproteins and heme models. In addition, energy level comparisons suggest that the largest-magnitude g value falls approximately along the iron-chlorin ring normal. This suggestion implies that the electron distribution of the iron at the catalytic sites of cytochrome P-450 and certain chlorin-containing enzymes is in some way similar, but distinct from that at the transport site of myoglobin.

Mössbauer spectra of square-planar organometallic compounds of iron(II)

Mössbauer spectra of [Fe(C 6Cl 5) 2L 2], L = PEt 3 and PEt 2Ph have been recorded at different temperatures. The low δ values show a high degree of covalency in the ironligand bonds, and the large Δ values are consistent with a planar geometry. The Mössbauer emission spectra of some [ 57Co(C 6Cl 5) 2L 2] (L = PEt 3, PEt 2Ph or PEtPh 2) show no Mössbauer effect, suggesting the formation of a new iron specimen during the 57Co(EC) 57Fe reaction, with an extremely small f-factor. [Fe(C 6Cl 5) 2L 2] compounds have been prepared either from the reaction of C 6Cl 5MgCl and [FeCl 2L 2] or by the addition of the phosphine to a solution of the ‘Fe(C 6Cl 5) 2’ complex. The stability of these complexes has been studied in different solvents and in the presence of bromine.

Structure of milk lactoperoxidase. A study using circular dichroism and difference absorption spectroscopy

Circular dichroism spectra of milk lactoperoxidase, its fluoride and cyanide derivatives, and those of ferrous lactoperoxidase and its carbonyl and cyanide compounds, were recorded in the wavelength region 200–670 nm. All derivatives have split ellipticity bands, suggesting that lactoperoxidase has a narrow heme pocket that prevents ligands forming linear iron-ligand bonds. Difference absorption spectroscopy of the enzyme in the far-ultraviolet region supports the previously held view that the fifth ligand of the heme iron is histidine. The secondary structure of lactoperoxidase, calculated from the far-ultraviolet circular dichroism spectrum, contains 65% β-structure, 23% α-helix and 12% unorderded structure. Reduction of lactoperoxidase with dithionite gives two forms, indicating that after reduction some compound arising from dithionite binds in the vicinity of the heme iron.

Thermochemistry of η 3-Allyltricarbonyliodoiron, Tricarbonyl (η 4-cyclooctatetraene)iron and Tetracarbonyldiiodoiron. The Enthalpy Contributions of Allyliron and Cyclooctatetraene-iron Bonds

Microcalorimetric measurements at elevated temperatures of the heats of thermal decomposition and iodination have led to values for the standard enthalpies of formation of the following crystalline compounds (values given in kJ mol −1) at 298K : [(η 3-C 3H 5) Fe(CO) 3I] = −(428±10); [(η 4-C 8H 8)Fe(CO) 3] = −(237±12); [Fe(CO) 4I 2] =−(722±8). Separate measurements by the vacuum-sublimation microcalorimetric technique gave the following values for the enthalpy of sublimation at 298K (kJ mol −1: [(η 3-C 3H 5)Fe(CO) 3I] = (84.5±4); [(η 4-C 8H 8)Fe(CO) 3] = (87±4); [Fe(CO) 4I 2] = (86.0±4). From these and other data, the bond enthalpy contributions of the various metal-ligand bonds in the gaseous metal complexes were evaluated as follows: [(η 3-C 3H 5)-Fe] 176; [(η 4-C 8H 8)-Fe] 180; Fe-I 177 kJ mol −1. The question of the transferability of the enthalpy contributions of iron-ligand bonds between organoiron complexes is discussed with the aid of structural and thermochemical data. It is concluded that the transfer of bond enthalpy contributions is valid within the scheme which relates iron-carbon bond lengths to iron-carbon bond enthalpies.

Electronic Structure of Some Iron Compounds

The electronic structure of iron-ligand bonds is investigated using X-ray emission and X-ray photoelectron spectroscopies for sodium ferrocyanide, ferrous fluoride and ferric fluoride. The utility of X-ray emission spectra in identifying X-ray photoelectron peaks is also demonstrated.