Imidazolate Bridge Research Articles

Characterization of Copper(II) and Zinc(II) Complexes of Peptides Mimicking the CuZnSOD Enzyme.

Antimicrobial peptides are short cationic peptides that are present on biological surfaces susceptible to infection, and they play an important role in innate immunity. These peptides, like other compounds with antimicrobial activity, often have significant superoxide dismutase (SOD) activity. One direction of our research is the characterization of peptides modeling the CuZnSOD enzyme and the determination of their biological activity, and these results may contribute to the development of novel antimicrobial peptides. In the framework of this research, we have synthesized 10, 15, and 16-membered model peptides containing the amino acid sequence corresponding to the Cu(II) and Zn(II) binding sites of the CuZnSOD enzyme, namely the Zn(II)-binding HVGD sequence (80-83. fragments), the Cu(II)-binding sequence HVH (fragments 46-48), and the histidine (His63), which links the two metal ions as an imidazolate bridge: Ac-FHVHEGPHFN-NH2 (L1(10)), Ac-FHVHAGPHFNGGHVG-NH2 (L2(15)), and Ac-FHVHEGPHFNGGHVGD-NH2 (L3(16)). pH-potentiometric, UV-Vis-, and CD-spectroscopy studies of the Cu(II), Zn(II), and Cu(II)-Zn(II) mixed complexes of these peptides were performed, and the SOD activity of the complexes was determined. The binding sites preferred by Cu(II) and Zn(II) were identified by means of CD-spectroscopy. From the results obtained for these systems, it can be concluded that in equimolar solution, the -(NGG)HVGD- sequence of the peptides is the preferred binding site for copper(II) ion. However, in the presence of both metal ions, according to the native enzyme, the -HVGD- sequence offers the main binding site for Zn(II), while the majority of Cu(II) binds to the -FHVH- sequence. Based on the SOD activity assays, complexes of the 15- and 16-membered peptide have a significant SOD activity. Although this activity is smaller than that of the native CuZnSOD enzyme, the complexes showed better performance in the degradation of superoxide anion than other SOD mimics. Thus, the incorporation of specific amino acid sequences mimicking the CuZnSOD enzyme increases the efficiency of model systems in the catalytic decomposition of superoxide anion.

Bimetallic Carbonyl Complexes Based on Iridium and Rhodium: Useful Tools for Hydrodefluorination Reactions

AbstractA set of bimetallic complexes based on iridium and rhodium with bis(diphenylphosphino)methane, bis(di‐iso‐propylphosphino)methane, diphenyl‐2‐pyridylphosphine and 2‐(di‐iso‐propylphosphino)imidazole bridging ligands was prepared. The complexes were characterized by NMR and IR spectroscopy and studied quantum‐chemically using DFT methods. The bimetallic systems succeeded in catalytic hydrodefluorination reactions of lower fluorinated aryl fluorides using molecular hydrogen and sodiumtert‐butoxide as a base. Effects of (i) ligand variation, (ii) mono‐ vs bimetallic nuclearity, and (iii) Ir vs Rh metal identity were studied and rationalized en route to achieve an effective hydrodefluorination.

In-situ growth strategy of ZIF-8 nanocrystals on carbon fibers achieved high-efficient interface enhancement

The unique imidazole bridge structure of ZIF-8 nanocrystals makes it flexible and low elastic modulus, which can withstand more flexible crystal deformation and have superior bearing capacity and anti-wear effect. In this work, an in-situ growth strategy of ZIF-8 nanocrystals on the surface of carbon fibers (CFs) was used for the enhancement of interfacial combination and out-of-plane properties between CFs and Poly(hexahydrotriazine)s (PHT) matrix. The introduction of ZIF-8 nanocrystals functionalized CFs to disperse and transfer more load and energy, and the interface performance between two phases were characterized via mechanical and tribological tests. The results indicated that the tensile strength of ZIF-8 grafted on CFs increased due to the improved rough surface, bonding area and active chemical groups of CFs. The fracture mechanism of ZIF-8 based composites were investigated by morphology observation on the interface. In addition, the wear rate of ZIF-8 based composites reduced by 43.7% when compared to neat CF. The improvement of mechanical and tribological properties depends on the introduction of ZIF-8 which improves the interfacial adhesion between CFs and resin matrix, and further facilitates the load transfer from interface to CFs.

Self-Adaptive Dirhodium Complexes in a Metal-Organic Framework for Synthesis of N-H Aziridines.

Conformational dynamics of active sites in enzymes enable great control over the catalytic process. Herein, we constructed a metal-organic framework with conformationally dynamic active sites (Rh2-ZIF-8). The active sites in Rh2-ZIF-8 were composed of the imidazolate-bridged bimetallic center with a catalytic dirhodium moiety and structural zinc site. Even though the coordination sphere of the dirhodium species was saturated with two circularly arranged esp groups and two axial 2-MeIm ligands, it could still effectively catalyze the direct synthesis of N-H aziridines from olefins with high activity. We found that such a self-adaptive catalytic process was based on the dynamic breakage and reformation of the rhodium-zinc imidazolate bridges. Interestingly, the in situ generated dirhodium site with a unique Rh2(esp)2(2-MeIm)1 configuration was able to exhibit obviously enhanced selectivity compared to homogeneous catalyst Rh2(esp)2. Furthermore, the surrounding zinc imidazolate groups could effectively protect the dirhodium moieties from harsh environments, and this ultimately endowed it with high stability.

Bridging the Bridging Imidazolate in the Bimetallic Center of the Cu/Zn SOD1 and ALS.

Metallation status of human Cu/Zn superoxide dismutase 1 (SOD1) plays a pivotal role in the pathogenesis of amyotrophic lateral sclerosis (ALS). All of the amino acids found in the bimetallic center have been associated with ALS except for two positions. H63 which forms the bridging imidazolate ion in the bimetallic center and K136 which is not directly involved in coordination but located in the bimetallic center were not reported to be mutated in any of the identified ALS cases. In this study, we investigated the structure and flexibility of five SOD1 variants by using classical molecular dynamics simulations. These variants include three substitutions on the non-ALS-linked positions; H63A, H63R, K136A and ALS-linked positions; G37R, H46R/H48D. We have generated four systems for each variant differing in metallation and presence of the intramolecular disulfide bond. Overall, a total of 24 different dimers including the wild-type were generated and simulated at two temperatures, 298 and 400 K. We have monitored backbone mobility, fluctuations and compactness of the dimer structures to assess whether the hypothetical mutations would behave similar to the ALS-linked variants. Results showed that particularly two mutants, H63R and K136A, drastically affected the dimer dynamics by increasing the fluctuations of the metal binding loops compared with the control mutations. Further, these variants resulted in demetallation of the dimers, highlighting probable ALS toxicity that could be elicited by the SOD1 variants of H63R and K136A. Overall, this study bridges two putative SOD1 positions in the metallic center and ALS, underlining the potential use of atomistic simulations for studying disease variants.

A Ruthenium(II) Water Oxidation Catalyst Containing a pH-Responsive Ligand Framework.

The synthesis of a new RuII-based water oxidation catalyst is presented, in which a nitrophenyl group is introduced into the backbone of dpp via a pH-sensitive imidazole bridge (dpp = 2,9-di-(2′-pyridyl)-1,10-phenanthroline). This modification had a pronounced effect on the photophysical properties and led to the appearance of a significant absorption band around 441 nm in the UV–vis spectrum upon formation of the monoprotonated species under neutral conditions. Theoretical investigations could show that the main contributions to this band arise from transitions involving the imidazole and nitrophenyl motif, allowing us to determine the pKa value (6.8 ± 0.1) of the corresponding, twofold protonated conjugated acid. In contrast, the influence of the nitrophenyl group on the electrochemical properties of the catalytic center was negligible. Likewise, the catalytic performance of Ru(dppip-NO2) and its parent complex Ru(dpp) was comparable over the entire investigated pH range (dppip-NO2 = 2-(4-nitrophenyl)-6,9-di(pyridin-2-yl)-1H-imidazo[4,5-f][1,10]phenanthroline). This allowed the original catalytic properties to be retained while additionally featuring a functionalized ligand scaffold, which provides further modification opportunities as well as the ability to report the pH of the catalytic solution via UV–vis spectroscopy.

Two Co(II) coordination polymers: treatment and nursing values on acute renal failure during pregnancy by regulating the expression of aquaporin

Hydrothermal reactions of aromatic terphenyl-4,2′′,5′′,4′-tetracarboxylic acid (H4tta) and transitional metal salt Co(NO3)2·6H2O in the presence of flexible N-donor ancillary ligand afforded two novel coordination polymers, namely, {[Co(tta)0.5(m-bimb)]·H2O} n (1) and [Co2(tta)(m-bimb)2(H2O)] n (2), from solvothermal reactions in the presence of bis(imidazole) bridging linkers (m-bimb = 1,3-bis(imidazol-1-ylmethyl)benzene). The structural characterization results show that complex 1 features a 3D framework structure with a 2-fold interpenetrated (4,4)-connected bbf net, while complex 2 shows a 3D non-interpenetrated network with a novel (4,8)-connected net with the Point Symbol of (32·53·6)(34·44·59·611). Their treatment and nursing values on acute renal failure during pregnancy was evaluated and the related mechanism was explored at the same time. First of all, the content of the 24-hour urine protein was measured to determine the protective activity of the compound on kidney function. Next, the relative expression levels of the aquaporin on renal tubular endothelial cells were measured with real time RT-PCR.

Mesoporous Silica Encapsulated Metal-Organic Frameworks for Heterogeneous Catalysis

Encapsulation of metal-organic frameworks (MOFs) with a shell of mesoporous silica can boost overall performance of MOFs, offering versatility for synthetic architecture of integrated nanocatalysts with reactor-like features. Recently, Yu et al. report a green approach to make yolk-shell nanoreactors.

High efficient photocatalytic reduction of aqueous Zn2+, Pb2+ and Cu2+ ions using modified titanium dioxide nanoparticles with amino acids

Photocatalytic reduction of Zn2+, Cu2+ and Pb2+ ions were performed using modified titanium dioxide nanoparticles with histidine (His) and imidazole (IM) as the modifier and characterized by FT-IR, X-RD, FESEM, EDAX, TEM and A.A.S. techniques. The anatase phase titanium dioxide nanoparticles with the size distribution of 25–45nm were obtained by the Sol–Gel Method. In the case of Cu2+, the reduction reaction was completed in 10min. The photo-reduction yields of Pb2+ and Zn2+ ions achieved to 98% removal during 60 and 160min, respectively. The immobilized modifiers on the TiO2 surface accelerate the electron transfer from TiO2 to the metal ion via imidazole bridge upon ultraviolet light irradiation. This photocatalytic reduction could be proceeded through chemisorption, reduction and desorption processes of metal ions on the surface of modified TiO2, sequentially and repeatably. Herein, methanol was played as an artificial electron donor. These hybrid photocatalysts were reused several times without losing efficiency while the highest reduction yield was achieved for TiO2@His NPs. So, modified TiO2 with amino acids could be an excellent candidate for metal ion reduction by an inexpensive method with advantages of high performance, high activity, reusability, easily prepared, easily separate, environmentally friendly and short reaction time.

Syntheses, structural diversity, and photocatalytic-degradation properties for methylene blue of Co(ii) and Ni(ii) MOFs based on terephthalic acid and different imidazole bridging ligands

Six coordination polymers were constructed and characterized by X-ray diffraction, elemental analyses, infrared spectroscopy and thermogravimetric analysis.

Synthesis of a Nonhydrolyzable Nucleotide Phosphoroimidazolide Analogue That Catalyzes Nonenzymatic RNA Primer Extension.

We report the synthesis of guanosine 5′-(4-methylimidazolyl)phosphonate (ICG), the third member of a series of nonhydrolyzable nucleoside 5′-phosphoro-2-methylimidazolide (2-MeImpN) analogues designed for mechanistic studies of nonenzymatic RNA primer extension. The addition of a 2-MeImpN monomer to a primer is catalyzed by the presence of a downstream activated monomer, yet the three nonhydrolyzable analogues do not show catalytic effects under standard mildly basic primer extension conditions. Surprisingly, ICG, which has a pKa similar to that of 2-MeImpG, is a modest catalyst of nonenzymatic primer extension at acidic pH. Here we show that ICG reacts with 2-MeImpC to form a stable 5′–5′-imidazole-bridged guanosine-cytosine dinucleotide, with both a labile nitrogen–phosphorus and a stable carbon–phosphorus linkage flanking the central imidazole bridge. Cognate RNA primer–template complexes react with this GC-dinucleotide by attack of the primer 3′-hydroxyl on the activated N–P side of the 5′-5′-imidazole bridge. These observations support the hypothesis that 5′–5′-imidazole-bridged dinucleotides can bind to cognate RNA primer–template duplexes and adopt appropriate conformations for subsequent phosphodiester bond formation, consistent with our recent mechanistic proposal that the formation of activated 5′–5′-imidazolium-bridged dinucleotides is responsible for 2-MeImpN-driven primer extension.

Design of a Binuclear Ni(II) Complex with Large Ising-type Anisotropy and Weak Anti-Ferromagnetic Coupling.

The preparation of a binuclear Ni(II) complex with a pentacoordinate environment using a cryptand organic ligand and the imidazolate bridge is reported. The coordination sphere is close to trigonal bipyramidal (tbp) for one Ni(II) and to square pyramidal (spy) for the other. The use of the imidazolate bridge that undergoes π-π stacking with two benzene rings of the chelating ligand induces steric hindrance that stabilizes the pentacoordinate environment. Magnetic measurements together with theoretical studies of the spin states energy levels allow fitting the data and reveal a large Ising-type anisotropy and a weak anti-ferromagnetic exchange coupling between the metal ions. The magnitude and the nature of the magnetic anisotropy and the difference in anisotropy between the two metal ions are rationalized using wave-function-based calculations. We show that a slight distortion of the coordination sphere of Ni(II) from spy to tbp leads to an Ising-type anisotropy. Broken-symmetry density functional calculations rationalize the weak anti-ferromagnetic exchange coupling through the imidazolate bridge.

Synthesis, Crystal Structure, DFT Modeling and Biological Activity of a Trinuclear Copper(II) Azide Polymer Containing Imidazole and Bridging Imidazolate Ligands, [Cu3(Imz-H)4(Imz)2(N3)4

The title compound, [Cu3(Imz-H)4(Imz)2(N3)4]n (1) (Imz-H = imidazole) was prepared by the reaction of CuCl2·2H2O with imidazole and sodium azide, and its structure was determined by X-ray crystallography. The crystal structure shows that the complex exists in the form of a two dimensional polymer consisting of dimeric [Cu(Imz-H)(Imz)0.5(N3)1.5]2 and monomeric [Cu(Imz-H)2(Imz)(N3)] units. In the dimeric part each Cu(II) ion is bound to two nitrogen atoms of Imz-H or Imz− and three nitrogen atoms of azide adopting a distorted square pyramidal geometry. In the monomeric part, the copper atom is disordered over two positions and is coordinated to two imidazole molecules (Imz-H), two bridging imidazolate (Imz-) groups, and two weakly bound azide ions. In order to validate the obtained structure, the structures of two complexes; [Cu3(Imz-H)5(Imz)(HN3)4(N3)2]3+ containing one monomer and one dimeric unit as a model of 1 and a hypothetical complex, [Cu4(Imz-H)6(Imz)(HN3)4(N3)4]3+ containing only dimeric units were fully optimized at the B3LYP-D3 level of theory. The interaction energy between the dimeric and monomeric units in the former complex was found to be almost twice larger that calculated between the dimeric units in the later complex. Therefore, the theoretical results support that the experimentally determined structure (1) is energetically more preferred than that containing only the dimeric units. The natural bond orbital (NBO) analysis of the model complexes of (1) was performed and the distribution of spin density at the atoms of [Cu3(Imz-H)5(Imz)(HN3)4(N3)2]3+ and [Cu5(Imz-H)8(Imz)2(HN3)6(N3)4]4+ was discussed. The NBO charge on copper cation in the dimeric unit is 0.90 e and is smaller than that on the Cu(II) ion in the monomeric unit by 0.09 e. Antimicrobial properties of the complex were evaluated by minimum inhibitory concentration and the results showed that the complex exhibited no significant activities against the selected microorganisms.

Synthesis, characterization and activity of imidazolate-bridged and Schiff-base dinuclear complexes as models of Cu,Zn-SOD. A comparative study

Two imidazolate-bridged diCuII and CuIIZnII complexes, [CuZn(dien)2(μ-Im)](ClO4)3·MeOH (1) and [Cu2(dien)2(μ-Im)](ClO4)3 (2) (Im = imidazole, dien=diethylenetriamine), and two complexes formed with Schiff base ligands, [CuZn(salpn)Cl2] (3) and [Cu2(salbutO)ClO4] (4) (H2salpn=1,3-bis(salicylidenamino)propane, H3salbutO=1,4-bis(salicylidenamino)butan-2-ol) have been prepared and characterized. The reaction of [Cu(dien)(ImH)](ClO4)2 with [Zn(dien)(H2O)](ClO4)2 at pH≥11 yields complex 1; at lower pH, the Cu3Zn tetranuclear complex [{(dien)Cu(μ-Im)}3Zn(OH2)(ClO4)2](ClO4)3 (1a) forms as the main reaction product. X-ray diffraction of 1a reveals that the complex contains a metal centered windmill-shaped cation having three blades with a central Zn ion and three peripheral capping Cu(dien) moieties bound to the central Zn ion through three imidazolate bridges. The four complexes are able to disproportionate O2− in aqueous medium at pH7.8, with relative rates 4>1>2≫3. [Cu2(salbutO)]+ (4) is the most easily reducible of the four complexes and exhibits the highest activity among the SOD models reported so far; a fact related to the ligand flexibility to accommodate the copper ion in both CuI and CuII oxidation states and the lability of the fourth coordination position of copper facilitating stereochemical rearrangements.

Bisthienylethenes containing an imidazole bridge unit and their Ir(iii) complexes: influence of substituent groups on photochromism and luminescence

This work reports the syntheses of tBuLH and tBuLMeH, and their complexes [Ir(dfppy)2(tBuL)]·2CH3OH (1) and Ir(dfppy)2(tBuLMe)] (2), and the influence of substituent groups –C(CH3)3 and –CH3 on their photochromic and luminescence behaviors.

Syntheses, structural diversity and photocatalytic properties of various Co(ii) coordination polymers based on a “V”-shaped 1,3-di(4′-carboxyl-phenyl)benzene acid and different imidazole bridging ligands

Various polymeric structures of complexes 1–4.

Structural and magnetic diversity based on different imidazolate linkers in Cu(II)-azido coordination compounds.

This Article reports the syntheses, structural characterization, and magnetic studies of four different Cu(II)-azido compounds based on imidazole or substituted imidazole ligand. The compounds, [Cu2(μ1,1-N3)2(EtimiH)4(ClO4)2] (1) (EtimiH = 2-ethylimidazole), [Cu2(μ-Meimi(-))(MeimiH)2(μ1,1-N3)2(μ1,3-N3)]n (2) (MeimiH = 2-methylimidazole; μ-Meimi(-) is the bridging mononegative anion of 2-methylimidazole), [Cu2(μ-imi(-))(imiH)2(μ1,1-N3)2(μ1,3-N3)]n (3), and [{Cu2(μ1,1-N3)2(μ1,3-N3)(μ-imi(-))(imiH)3}·H2O]n (4) (imiH = imidazole; μ-imi(-) = bridging mononegative anion of imidazole), have been synthesized by the self-assembly of Cu(II) salts, azide ion, and the corresponding imidazole bridging ligands. By changing the substitution on the second linker (imidazole or substituted imidazole) and varying synthetic conditions, diverse structural and magnetic features have been achieved in compounds 1-4. Compound 1 has a double end-on azido bridged dinuclear core, while the other compounds (2-4) have 2D networks. Compound 2 and 3 contain 1D chains with alternate μ1,1-N3 and μ-Meimi(-) bridging, and such chains are further connected through a μ1,3-N3 bridge to result in the formation of the 2D network. Compound 4 is a novel 2D coordination polymer consisting of a zigzag 1D coordination chain having (μ1,1-N3)2, μ-imi(-), and (μ1,3-N3)2 bridging groups and the chains undergo bridging through a μ1,3-N3 group resulting in the 2D network. Temperature dependent magnetic measurements show diverse magnetic properties of 1-4. Such versatile magnetic behaviors have been correlated to the respective bridging mode of azide and the corresponding imidazole bridging ligands.

Syntheses, structures, and properties of a series of 2D and 3D coordination polymers based on trifunctional pyridinedicarboxylate and different (bis)imidazole bridging ligands

Hydrothermal reactions based on two trifunctional pyridinedicarboxylate carboxylates and transitional metal cations in the presence of three (bis)imidazole bridging ligands afford ten 2D and 3D coordination polymers.

A two-step ICT process for solvatochromic betaine pyridinium revealed by ultrafast spectroscopy, multivariate curve resolution, and TDDFT calculations

This work deals with the photophysics of a pyridinium betaine, 2-pyridin-1-yl-1H-benzimidazole (SBPa), based on a combination of steady-state, femtosecond photoionization (gas phase) and femtosecond transient absorption (solution) spectroscopic measurements, supported by (LR)-PCM-(TD)DFT calculations. Preliminary and new electrochemical results have revealed a strongly negative solvatochromic charge transfer (CT) absorption due to a S(0) → S(2) vertical transition and a weakly-solvatochromic emission due to S(1) → S(0) transition. Advanced TDDFT optimizations of the Franck-Condon states S(2)(FC) and S(1)(FC) led to two additional CT levels with planar geometry, S(2)(CT) and S(1)(CT), respectively, allowing prediction of a two-step photoinduced ICT process, i.e., S(0) → S(2)(FC) and S(2)(CT) → S(1)(CT), separated by a S(2)(FC) → S(2)(CT) back charge transfer relaxation. While the pyridinium ring is the acceptor group in both steps, two different donor groups, the benzene ring and the imidazole bridge, are involved in the excitation and internal conversion processes, respectively. Femtosecond transient absorption experiments supported by MCR-ALS decomposition confirmed indeed the contribution of two distinct CT states in the photophysics of SBPa: following excitation to the S(2)(CT) state, ultrafast production of the emissive S(1) state (the only channel observable in the gas phase) was observed to occur in competition with a further ICT process toward the S(1)(CT) state, with a time constant ranging from 300 fs to 20 ps depending on the solvent. While in aprotic media this ICT process was found to be purely solvent controlled (double polarity and viscosity dependency), in protic solvents, the influence of the hydrogen bond network has to be taken into account. Comparison with data obtained for a pre-twisted SBPa analogue led us to exclude the presence of any large-amplitude geometrical change during ICT. Analyzing the solvent dependency using the power law approach, we concluded that the S(1)(CT) state decays essentially through IC in the 3-40 ps time range whereas the emissive S(1) state decays within 130-260 ps via IC, ISC and fluorescence.

Zn induced structural aggregation patterns of β-amyloid peptides by first-principle simulations and XAS measurements

We show in this paper that in the presence of Zn ions a peculiar structural aggregation pattern of β-amyloid peptides in which metal ions are sequentially coordinated to either three or four histidines of nearby peptides is favored. To stabilize this configuration a deprotonated imidazole ring from one of the histidines forms a bridge connecting two adjacent Zn ions. Though present in zeolite imidazolate frameworks, remarkably in biological compounds this peculiar Zn-imidazolate-Zn topology is only found in enzymes belonging to the Cu,Zn-superoxide dismutase family in the form of an imidazolate bridging Cu and Zn. The results we present are obtained by combining X-ray absorption spectroscopy experimental data with detailed first-principle molecular dynamics simulations.