Antibacterial activity of [1Fe-2S]- and [2Fe-2S]-nitrosyl complexes as nitric oxide donors

Russian Chemical Bulletin - The biological activity of a series of sulfur-nitrosyl iron complexes (NICs) depends on the structure of the ligands and the position of the functional groups in the...     

Secondary bonding interactions in biomimetic [2Fe-2S] clusters

A series of synthetic [2Fe-2S] complexes with terminal thiophenolate ligands and tethered ether or thioether moieties has been prepared and investigated in order to provide models for the potential interaction of additional donor atoms with the Fe atoms in biological [2Fe-2S] clusters. X-ray crystal structures have been determined for six new complexes that feature appended Et (1(C)), OMe (1(O)), or SMe (1(S)) groups, or with a methylene group (2(C) ), an ether-O (2(O)), or an thioether-S (2(S)) linking two aryl groups. The latter two systems provide a constrained chelate arrangement that induces secondary bonding interactions with the ether-O and thioether-S, which is confirmed by density functional theory (DFT) calculations that also reveal significant spin density on those fifth donor atoms. Structural consequences of the secondary bonding interactions are analyzed in detail, and effects on the spectroscopic and electronic properties are probed by UV-vis, M?ssbauer, and (1)H NMR spectroscopy, as well by SQUID measurements and cyclic voltammetry. The potential relevance of the findings for biological [2Fe-2S] sites is considered.     

Mechanism of glutaredoxin-ISU [2Fe-2S] cluster exchange

Exchange of [2Fe-2S] centers between Grx2 and the cluster scaffold protein ISU, and characterization of two mutually exclusive Grx2 binding sites on ISU by isothermal titration calorimetry supports a direct link for Grx and glutathione involvement in ISU promoted Fe-S cluster biosynthesis.     

Protonation and concerted proton-electron transfer reactivity of a bis-benzimidazolate ligated [2Fe-2S] model for Rieske clusters

A model system for biological Rieske clusters that incorporates bis-benzimidazolate ligands ((Pr)bbim)(2-) has been developed ((Pr)bbimH(2) = 4,4-bis(benzimidazol-2-yl)heptane). The diferric and mixed-valence clusters have been prepared and characterized in both their protonated and deprotonated states. The thermochemistry of interconversions of these species has been measured, and the effect of protonation on the reduction potential is in good agreement to that observed in the biological systems. The mixed-valence and protonated congener [Fe(2)S(2)((Pr)bbim)((Pr)bbimH)](Et(4)N)(2) (4) reacts rapidly with TEMPO or p-benzoquinones to generate diferric and deprotonated [Fe(2)S(2)((Pr)bbim)(2)](Et(4)N)(2) (1) and 1 equiv of TEMPOH or 0.5 equiv of p-benzohydroquinones, respectively. The reaction with TEMPO is the first well-defined example of concerted proton-electron transfer (CPET) at a synthetic ferric/ferrous [Fe-S] cluster.     

Probing the electronic structure of [2Fe-2S] clusters with three coordinate iron sites by use of photoelectron spectroscopy

Five series of [2Fe-2S] complexes, [Fe(2)S(2)Cl(2)(-)(x)(CN)(x)](-), [Fe(2)S(2)(SEt)(2)(-)(x)Cl(x)](-), [Fe(2)S(2)(SEt)(2)(-)(x)(CN)(x)](-), [Fe(2)S(2)Cl(2)(-)(x)(OAc)(x)](-) (OAc = acetate), and [Fe(2)S(2)(SEt)(2)(-)(x)(OPr)(x)](-) (OPr = propionate) (x = 0-2), were produced by collision-induced dissociation of the corresponding [4Fe-4S] complexes, and their electronic structures were studied by photoelectron spectroscopy. All the [2Fe-2S] complexes contain a [Fe(2)S(2)](+) core similar to that in reduced [2Fe] ferredoxins but with different coordination geometries. For the first three series, which only involve tricoordinated Fe sites, a linear relationship between the measured binding energies and the substitution number (x) was observed, revealing the independent ligand contributions to the total electron binding energies. The effect of the ligand increases in the order SEt --> Cl --> CN, conforming to their electron-withdrawing ability in the same order. The carboxylate ligands in the [Fe(2)S(2)Cl(2)(-)(x)(OAc)(x)](-) and [Fe(2)S(2)(SEt)(2)(-)(x)(OPr)(x)](-) complexes were observed to act as bidentate ligands, giving rise to tetracoordinated iron sites. This is different from their monodentate coordination behavior in the [4Fe-4S] cubane complexes, reflecting the high reactivity of the unsatisfied three-coordinate iron site in the [2Fe-2S] complexes. The [2Fe-2S] complexes with tetracoordinated iron sites exhibit lower electron binding energies, that is, higher reductive activity than the all tricoordinate planar clusters. The electronic structures of all the [2Fe-2S] complexes were shown to conform to the "inverted energy level scheme".     

[MoFe3S4]3+ and [MoFe3S4]2+ cubane clusters containing a pentamethylcyclopentadienyl molybdenum moiety

A series of organometallic molybdenum/iron/sulfur clusters of the general formula [Cp¹MoFe₃S₄L_n]^m (Cp¹ = η⁵-C₅Me₅; L = S^tBu, SPh, Cl, I, n = 3, m = 1−; L_n = I₂(P^tBu₃), m = 0; L = 2,6-diisopropylphenylisocyanide (ArNC), n = 7, m = 1+) have been synthesized. A cubane cluster (PPh₄)[Cp¹MoFe₃S₄(S^tBu)₃] (2) was isolated from a self-assembly reaction of Cp¹Mo(S^tBu)₃ (1), FeCl₃, LiS^tBu, and S₈ followed by cation exchange with PPh₄Br in CH₃CN, while an analogous cluster (PPh₄)[Cp¹MoFe₃S₄(SPh)₃] (3) was obtained from the Cp¹MoCl₄/FeCl₃/LiSPh/PPh₄Br reaction system or from a ligand substitution reaction of 2 with PhSH. Treatment of 2 with benzoyl chloride gave rise to (PPh₄)[Cp¹MoFe₃S₄Cl₃] (4), which was in turn converted to (PPh₄)[Cp¹MoFe₃S₄I₃] (5) by the reaction with NaI. A neutral cubane cluster Cp¹MoFe₃S₄I₂(P^tBu₃) (6) was generated upon treating 5 with P^tBu₃. Although reduction of 4 by cobaltocene under the presence of ArNC resulted in a disproportionation of the cubane core to give Fe₄S₄(ArNC)₉Cl (7), a similar reduction reaction of 5 produced [Cp¹MoFe₃S₄(ArNC)₇]I (8), where the MoFe₃S₄ core was retained. The crystal structures of 4–6, and 8 were determined by the X-ray analysis.     

Computational chemistry of modified [MFe3S4] and [M2Fe2S4] clusters: assessment of trends in electronic structure and properties

The aim of this work is to understand the molecular evolution of iron-sulfur clusters in terms of electronic structure and function. Metal-substituted models of biological [Fe(4)S(4)] clusters in oxidation states [M(x)Fe(4-x)S(4)](3+/2+/1+) have been studied by density functional theory (M = Cr, Mn, Fe, Co, Ni, Cu, Zn, and Pd, with x = 1 or 2). Most of these clusters have not been characterized before. For those that have been characterized experimentally, very good agreement is obtained, implying that also the predicted structures and properties of new clusters are accurate. Mean absolute errors are 0.024 A for bond lengths ([Fe(4)S(4)], [NiFe(3)S(4)], [CoFe(3)S(4)]) and 0.09 V for shifts in reduction potentials relative to the [Fe(4)S(4)] cluster. All structures form cuboidal geometries similar to the all-iron clusters, except the Pd-substituted clusters, which instead form highly distorted trigonal and tetragonal local sites in compromised, pseudocuboidal geometries. In contrast to other electron-transfer sites, cytochromes, blue copper proteins, and smaller iron-sulfur clusters, we find that the [Fe(4)S(4)] clusters are very insensitive to metal substitution, displaying quite small changes in reorganization energies and reduction potentials upon substitution. Thus, the [Fe(4)S(4)] clusters have an evolutionary advantage in being robust to pollution from other metals, still retaining function. We analyze in detail the electronic structure of individual clusters and rationalize spin couplings and redox activity. Often, several configurations are very close in energy, implying possible use as spin-crossover systems, and spin states are predicted accurately in all but one case ([CuFe(3)S(4)]). The results are anticipated to be helpful in defining new molecular systems with catalytic and magnetic properties.     

Proton coupling to [4Fe-4S](2+/+) and [4Fe-4Se](2+/+) oxidation and reduction in a designed protein

The coupling of a single proton to [4Fe-4S]2+/+ oxidation/reduction in a de novo designed iron-sulfur protein maquette is presented. The reduced state pKared is 9.3, and the oxidized state pKaox is <6.5. The reduced state pKared shifts to 8.3 upon incorporation of a [4Fe-4Se]2+/+ cluster, implicating the cluster itself or its primary coordination sphere as the proton-coupling site.     

Electronic structure and intrinsic redox properties of [2Fe-2S]+ clusters with tri- and tetracoordinate iron sites

Using potentially bidentate ligands (-SC2H4NH2), we produced [2Fe-2S]+ species of different coordination geometries by fission of [4Fe-4S]2+ complexes. Even though the ligands are monodentate in the cubane complexes, both mono- and bidentate complexes were observed in the [2Fe] fission products through self-assembly because of the high reactivity of the tricoordinate iron sites. The electronic structure of the [2Fe] species was probed using photoelectron spectroscopy and density functional calculations. It was found that tetracoordination significantly decreases the electron binding energies of the [2Fe] complexes, thus increasing the reducing capability of the [2Fe-2S]+ clusters.     

Double exchange in polynuclear mixed-valence clusters. 2. Iron-sulfur proteins [Fe4S4]+ and [Fe4S4]3+

The energy states of tetrameric iron clusters with d⁶-d⁶-d⁶-d⁵ and d⁵-d⁵-d⁵-d⁶ electronic configurations, which form the metal frameworks of the [Fe₄S₄]⁺ and [Fe₄S₄]³⁺ iron-sulfur proteins, are calculated using a general approach. It is revealed that the structural distortions of these systems significantly affect the composition of the magnetic states. The effect of the isotropic Heisenberg exchange is considered. Valencia University, Spain. Bordeaux University, France. Moldova State University. Institute of Chemistry, Academy of Sciences, Moldova Republic. Translated fromZhurnal Strukturnoi Khimii, Vol. 37, No. 5, pp. 816–825, September–October, 1996. Translated by I. Izvekova     

Photosensitized [4+2]- and [2+2]-Cycloaddition Reactions of N-Sulfonylimines

The synthesis of polycyclic compounds is of high interest due to the prevalence of these motifs in drugs and natural products. Herein, we report on the stereoselective construction of 3D bicyclic scaffolds and azetidine derivatives by modulation of N-sulfonylimines to achieve either [4+2]- or [2+2]-cycloaddition reactions. The utility of the method was established by further modulation of the product. Mechanistic studies are also included, which support reaction via Dexter energy transfer.     

Escherichia coli quinolinate synthetase does indeed harbor a [4Fe-4S] cluster

Quinolinic acid is an intermediate in the biosynthesis of nicotinamide-containing redox cofactors. The ultimate step in the formation of quinolinic acid in prokaryotes is the condensation of iminosuccinate and dihydroxyacetone phosphate, which is catalyzed by the product of the nadA gene in Escherichia coli. A combination of UV-vis, M?ssbauer, and EPR spectroscopies, along with analytical methods for the determination of iron and sulfide, demonstrates for the first time that anaerobically purified quinolinate synthetase (NadA) from E. coli contains one [4Fe-4S] cluster per polypeptide. The protein is active, catalyzing the formation of quinolinic acid with a Vmax [ET]-1 of 0.01 s-1.     

Studies of inhibitor binding to the [4Fe-4S] cluster of quinolinate synthase

Stop for NadA! A [4Fe-4S] enzyme, NadA, catalyzes the formation of quinolinic acid in de?novo nicotinamide adenine dinucleotide (NAD) biosynthesis. A structural analogue of an intermediate, 4,5-dithiohydroxyphthalic acid (DTHPA), has an in?vivo NAD biosynthesis inhibiting activity in E. coli. The inhibitory effect can be explained by the coordination of DTHPA thiolate groups to a unique Fe site of the NadA [4Fe-4S] cluster.     

有机催化不对称[4+2]环加成反应*

有机催化不对称合成反应是目前国内外研究最为活跃的领域之一。不对称[4+2]环加成（Diels-Alder）反应是合成光学活性环状化合物的有效手段,目前报道的催化不对称Diels-Alder反应的有机催化剂主要有手性咪唑啉酮、手性Br?nsted酸、手性伯胺、金鸡纳碱衍生物等。本文对各类有机催化剂在有机催化不对称Diels-Alder反应中的应用研究进展,以及不对称诱导反应的机理进行了评述。