Rhodium catalysis has been extensively used for
-C-H functionalization reactions, and successfully extended to
-C-H functionalization. Its application to
-C-H activation remains an unmet challenge. ...Herein we disclose the first example of such a reaction, with the Rh-catalyzed
-C-H olefination of arenes. The use of a Si-linked cyanobiphenyl unit as a traceless directing group leads to highly
-selective arene-olefin couplings.
It is becoming increasingly apparent that the secondary coordination sphere can have a crucial role in determining the functional properties of biomimetic metal complexes. We have therefore designed ...and prepared a variety of ligands as metallo-hydrolase mimics, where hydrogen bonding in the second coordination sphere is able to influence the structure of the primary coordination sphere and the substrate binding. The assessment of a structure–function relationship is based on derivates of 2,6-bis{bis(pyridin-2-ylmethyl)aminomethyl}-4-methylphenol (HBPMP = HL1) and 2-{bis(pyridin-2-ylmethyl)aminomethyl}-6-{(2-hydroxybenzyl)(pyridin-2-ylmethyl)aminomethyl}-4-methylphenol (H2BPBPMP = H2L5), well-known phenolate-based ligands for metallo-hydrolase mimics. The model systems provide similar primary coordination spheres but site-specific modifications in the secondary coordination sphere. Pivaloylamide and amine moieties were chosen to mimic the secondary coordination sphere of the phosphatase models, and the four new ligands H3L2, H3L3, HL4, and H4L6 vary in the type and geometric position of the H-bond donors and acceptors, responsible for the positioning of the substrate and release of the product molecules. Five dinuclear ZnII complexes were prepared and structurally characterized in the solid, and four also in solution. The investigation of the phosphatase activity of four model complexes illustrates the impact of the H-bonding network: the Michaelis–Menten constants (catalyst–substrate binding) for all complexes that support hydrogen bonding are smaller than for the reference complex, and this generally leads to higher catalytic efficiency and higher turnover numbers.
The species originally reported as Co(bipy)2O2BOH·B5O6(OH)4·H3BO3·H3O·H2O, a Co(II) complex containing a chelated O2BOH2– ligand, is shown to be Co(bipy)2O2CO·B5O6(OH)4·H3BO3·2H2O, a Co(III) ...complex containing a chelated O2CO2– ligand. This was confirmed by 1H and 13C NMR, MS, IR, and an X-ray crystal structure.
Binuclear metallohydrolases are a structurally diverse group of enzymes that use binuclear metal ion centers to catalyze the hydrolysis of amides and esters of carboxylic and phosphoric acids. A ...current understanding of metallohydrolase-catalyzed reactions is presented.
The OP (organophosphate)-degrading enzyme from Agrobacterium radiobacter (OpdA) is a binuclear metallohydrolase able to degrade highly toxic OP pesticides and nerve agents into less or non-toxic ...compounds. In the present study, the effect of metal ion substitutions and site-directed mutations on the catalytic properties of OpdA are investigated. The study shows the importance of both the metal ion composition and a hydrogen-bond network that connects the metal ion centre with the substrate-binding pocket using residues Arg254 and Tyr257 in the mechanism and substrate specificity of this enzyme. For the Co(II) derivative of OpdA two protonation equilibria (pKa1 ~5; pKa2 ~10) have been identified as relevant for catalysis, and a terminal hydroxide acts as the likely hydrolysis-initiating nucleophile. In contrast, the Zn(II) and Cd(II) derivatives only have one relevant protonation equilibrium (pKa ~4-5), and the μOH is the proposed nucleophile. The observed mechanistic flexibility may reconcile contrasting reaction models that have been published previously and may be beneficial for the rapid adaptation of OP-degrading enzymes to changing environmental pressures.
An enhanced understanding of the metal ion binding and active site structural features of phosphoesterases such as the glycerophosphodiesterase from Enterobacter aerogenes (GpdQ), and the ...organophosphate degrading agent from Agrobacterium radiobacter (OpdA) have important consequences for potential applications. Coupled with investigations of the metalloenzymes, programs of study to synthesise and characterise model complexes based on these metalloenzymes can add to our understanding of structure and function of the enzymes themselves. This review summarises some of our work and illustrates the significance and contributions of model studies to knowledge in the area.
Biomimetics hold potential for varied applications in biotechnology and medicine but have also attracted particular interest as benchmarks for the functional study of their more complex biological ...counterparts, e.g. metalloenzymes. While many of the synthetic systems adequately mimic some structural and functional aspects of their biological counterparts the catalytic efficiencies displayed are mostly far inferior due to the smaller size and the associated lower complexity. Nonetheless they play an important role in bioinorganic chemistry. Numerous examples of biologically inspired and informed artificial catalysts have been reported, designed to mimic a plethora of chemical transformations, and relevant examples are highlighted in reviews and scientific reports. Herein, we discuss biomimetics of the metallohydrolase purple acid phosphatase (PAP), examples of which have been used to showcase synergistic research advances for both the biological and synthetic systems. In particular, we focus on the seminal contribution of our colleague Prof. Ademir Neves, and his group, pioneers in the design and optimization of suitable ligands that mimic the active site of PAP.
Synopsis: Biomimetics hold potential for varied applications in biotechnology and medicine but have also attracted interest as simple models for their complex biological counterparts. The development of biomimetic complexes of purple acid phosphatase (PAP), in particular the contributions towards these designs by prof. Ademir Neves, is showcased in this minireview. Display omitted
•Prof. Ademir Neves pioneered the synthesis of mimics for purple acid phosphatases.•Biomimetics reproduce characteristic purple color of the enzyme.•Their ability to cleave DNA illustrates their potential as chemotherapeutic.•Second sphere modifications led to improved substrate binding.
Cadmium(II) complexes of ethyl 4-hydroxy-3,5-bis(((2-hydroxyethyl)(pyridin-2-ylmethyl)amino)methyl)benzoate (CO2EtH3 L1) and ethyl ...4-hydroxy-3,5-bis(((2-methoxyethyl)(pyridin-2-ylmethyl)amino)methyl)benzoate (CO2EtHL2) are described. The two ligands possess an ethyl ester (CO2Et-) at the position para to the phenolic −OH; CO2EtHL2, with methyl ether donors in contrast to potentially nucleophilic alkoxide donors in CO2EtH3 L1, offers a direct comparison of potential ligand-centered nucleophiles. The complex with CO2EtH3 L1 was characterized using 1H and 13C NMR spectroscopy, mass spectrometry and microanalysis; X-ray crystallography defined a tetranuclear structure Cd4(CO2EtH2 L1)2(CH3COO)3.75Cl0.25(H2O)2(PF6)2. Functional studies of the cadmium(II) complexes were undertaken with the substrates bis(2,4-dinitrophenyl)phosphate (BDNPP), and nitrocefin to assess their phosphatase and β-lactamase activities, respectively. The complexes with CO2EtH3 L1 and CO2EtHL2 are competent phosphoesterase mimics with K M = 9.4 ± 2.1 mM and 10.1 ± 3.4 mM, k cat = 9.4 ± 0.2 × 10–3 s–1 and 9.7 ± 2.7 × 10–3 s–1, respectively. Use of a solvent mixture containing H2 18O/H2 16O in the reaction with BDNPP showed that for the complex with CO2EtH3 L1 the 18O label was incorporated in the reaction product suggesting that the nucleophile involved is a Cd–OH moiety and not a metal bound alkoxide; for CO2EtHL2 the presence of the methyl-ether dictates that the active nucleophile must also be a hydroxide. The cadmium(II) complex with CO2EtH3 L1 was furthermore found to be a competent β-lactamase mimic with k cat = 1.39 × 10–2 ± 3 × 10–3 s–1, K M = 0.11 ± 0.03 mM, and pK a = 7.9 ± 0.1. Mass spectral evidence suggested that the active nucleophile in this reaction is the alkoxide; lack of β-lactamase activity of the complex with CO2EtHL2 supports this assignment. Similar to enzyme-catalyzed reactions, a blue reaction intermediate in the β-lactamase reaction of the CO2EtH3 L1 complex was also identified. It is proposed that the Cd(II) complexes of CO2EtH3 L1 and CO2EtHL2 react identically as phosphatases, with a terminal hydroxide as the nucleophile; the former exhibits β-lactamase activity with the alkoxide as a nucleophile, while the latter, without a potentially nucleophilic alkoxide, is inactive.
Annually thousands of people die or suffer from organophosphate (pesticide) poisoning. In order to remove these toxic compounds from the environment, the use of enzymes as bioremediators has been ...proposed. We report here a Ser127Ala mutant based on the enzyme glycerophosphodiesterase (GpdQ) from Enterobacter aerogenes. The mutant, with improved metal binding abilities, has been immobilized using glutaraldehyde on PAMAM dendrimer-modified magnetite nanoparticles. The immobilized system was characterized using elemental analysis as well as infrared, transmission electron and X-ray photoelectron spectroscopies. The amount of GpdQ that was immobilized with the optimized procedure was 1.488nmol per g MNP. A kinetic assay has been designed to evaluate the activity of the system towards organophosphoester substrates. The specific activity towards BPNPP directly after immobilization was 3.55μmolmg−1min−1, after one week 3.39μmolmg−1min−1 and after 120days 3.36μmolmg−1min−1, demonstrating that the immobilized enzyme was active for multiple cycles and could be stored on the nanoparticles for a prolonged period.
We report that a mutant with improved metal binding abilities and based on the enzyme glycerophosphodiesterase (GpdQ) from Enterobacter aerogenes has been immobilized on modified magnetite nanoparticles. The enzyme was successfully immobilized on the nanoparticles, was active for multiple cycles and could be stored on the nanoparticles for a prolonged period. Display omitted
•Mutant of glycerophosphodiesterase (GpdQ) from Enterobacter aerogenes is reported.•The mutant has been immobilized on PAMAM dendrimer-modified magnetite nanoparticles.•A kinetic assay has been designed using organophosphoester substrates.•GpdQ was successfully immobilized on the nanoparticles and active for multiple cycles.
Binuclear metallohydrolases are a large family of enzymes that require two closely spaced transition metal ions to carry out a plethora of hydrolytic reactions. Representatives include purple acid ...phosphatases (PAPs), enzymes that play a role in bone metabolism and are the only member of this family with a heterovalent binuclear center in the active form (Fe3+–M2+, M = Fe, Zn, Mn). Other members of this family are urease, which contains a di-Ni2+ center and catalyzes the breakdown of urea, arginase, which contains a di-Mn2+ center and catalyzes the final step in the urea cycle, and the metallo-β-lactamases, which contain a di-Zn2+ center and are virulence factors contributing to the spread of antibiotic-resistant pathogens. Binuclear metallohydrolases catalyze numerous vital reactions and are potential targets of drugs against a wide variety of human disorders including osteoporosis, various cancers, antibiotic resistance, and erectile dysfunctions. These enzymes also tend to catalyze more than one reaction. An example is an organophosphate (OP)-degrading enzyme from Enterobacter aerogenes (GpdQ). Although GpdQ is part of a pathway that is used by bacteria to degrade glycerolphosphoesters, it hydrolyzes a variety of other phosphodiesters and displays low levels of activity against phosphomono- and triesters. Such a promiscuous nature may have assisted the apparent recent evolution of some binuclear metallohydrolases to deal with situations created by human intervention such as OP pesticides in the environment. OP pesticides were first used approximately 70 years ago, and therefore the enzymes that bacteria use to degrade them must have evolved very quickly on the evolutionary time scale. The promiscuous nature of enzymes such as GpdQ makes them ideal candidates for the application of directed evolution to produce new enzymes that can be used in bioremediation and against chemical warfare. In this Account, we review the mechanisms employed by binuclear metallohydrolases and use PAP, the OP-degrading enzyme from Agrobacterium radiobacter (OPDA), and GpdQ as representative systems because they illustrate both the diversity and similarity of the reactions catalyzed by this family of enzymes. The majority of binuclear metallohydrolases utilize metal ion-activated water molecules as nucleophiles to initiate hydrolysis, while some, such as alkaline phosphatase, employ an intrinsic polar amino acid. Here we only focus on catalytic strategies applied by the former group.
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