Monoesterase activity of a purple acid phosphatase mimic with a cyclam platform

Hydrolysis of Nerve Agent Simulants Accelerated by Stimuli-Responsive Dinuclear Catalysts

The ability to control the catalytic activity of enzymes in chemical transformations is essential for the design and development of artificial catalysts. Herein, we report the synthesis and characterization of functional ligands featuring two 1,4,7,10-tetraazacyclododecane units linked by an azobenzene group and their corresponding dinuclear Zn(II) complexes. We show that the configuration switching (E/Z) of the azobenzene spacer in the ligands and their dinuclear Zn(II) complexes is reversibly controlled by irradiation with UV and visible light. The Zn(II)-metal complexes are light-responsive catalysts for the hydrolytic cleavage of nerve agent simulants, i.e., p-nitrophenyl diphenyl phosphate and methyl paraoxon. The catalytic activity of the Z-isomers of the dinuclear Zn(II) complexes outperformed that of the E-counterparts. Moreover, combining the less active E-isomers with gold nanoparticles induced an enhancement in the hydrolysis rate of p-nitrophenyl diphenyl phosphate. Kinetic analysis has shown that the catalytic site appears to involve a single metal ion. We explain our results by considering the different desolvation effects occurring in the catalyst's configurations in the solution and the catalytic systems involving gold nanoparticles.

Synthesis, characterization and computational investigation of the phosphatase activity of a dinuclear Zinc(II) complex containing a new heptadentate asymmetric ligand

We report the synthesis of a new asymmetric heptadentate ligand based on the 1,3-diaminopropan-2-ol backbone. The ligand 3-[[3-(bis-pyridin-2-ylmethyl-amino)-2-hydroxy-propyl]-(2-carbamoyl-ethyl)-amino]-propionamide (HL1) contains two amide and two pyridine groups attached to the 1,3-diaminopropan-2-ol core. Reaction between HL1 and Zn(ClO₄)₂.6H₂O resulted in the formation of the dinuclear [Zn₂(L1)(μ-OAc)](ClO₄)₂ complex, characterized by single crystal X-ray diffraction, ¹H, ¹³C and ¹⁵N NMR, ESI-(+)-MS, CHN elemental analysis as well as infrared spectroscopy. The phosphatase activity of the complex was studied in the pH range 6-11 employing pyridinium bis(2,4-dinitrophenyl)phosphate (py(BDNPP)) as substrate. The complex exhibited activity dependent on the pH, presenting an asymmetric bell shape profile with the highest activity at pH 9; at high pH ligand exchange is rate-limiting. The hydrolysis of BDNPP^- at pH 9 displayed behavior characteristic of Michaelis-Menten kinetics, with k_cat = 5.06 × 10^-3 min^-1 and K_m = 5.7 ± 1.0 mM. DFT calculations map out plausible reaction pathways and identify a terminal, Zn(II)-bound hydroxide as likely nucleophile.

Biomimetics for purple acid phosphatases: A historical perspective

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.

Hydrolytic activity of new bioinspired MnIIIMnII and FeIIIMnII complexes as mimetics of PAPs: Biological and environmental interest

Coordination compounds that mimic Purple Acid Phosphatases (PAPs) have drawn attention in the bioinorganic field due to their capacity to cleave phosphodiester bonds. However, their catalytic activity upon phosphate triesters is still unexplored. Thus, we report the synthesis and characterization of two binuclear complexes, [Mn^IIMn^III(L₁)(OAc)₂]BF₄ (1) and [Mn^IIFe^III(L₁)(OAc)₂]BF₄ (2) (H₂L₁ = 2-[N,N-bis-(2- pyridilmethyl)aminomethyl]-4-methyl-6-[N-(2-hydroxy-3-formyl-5-methylbenzyl)-N-(2-pyridylmethyl)aminomethyl]phenol), their hydrolytic activity and antioxidant potential. The complexes were fully characterized, including the X-Ray diffraction (XRD) of 1. Density functional theory (DFT) calculations were performed to better understand their electronic and structural properties and phosphate conjugates. The catalytic activity was analyzed for two model substrates, a diester (BDNPP) and a triester phosphate (DEDNPP). The results suggest enhancement of the hydrolysis reaction by 170 to 1500 times, depending on the substrate and complex. It was possible to accompany the catalytic reaction of DEDNPP hydrolysis by phosphorus nuclear magnetic resonance (³¹P NMR), showing that both 1 and 2 are efficient catalysts. Moreover, we also addressed that 1 and 2 present a relevant antioxidant potential, protecting the yeast Saccharomyces cerevisiae, used as eukaryotic model of study, against the exposure of cells to acute oxidative stress.

Antitumor activity via apoptotic cell death pathway of water soluble copper(II) complexes: effect of the diamino unit on selectivity against lung cancer NCI-H460 cell line

The cytotoxicity against five human tumor cell lines (THP-1, U937, Molt-4, Colo-205 and NCI-H460) of three water soluble copper(II) coordination compounds containing the ligands 3,3'-(ethane-1,2-diylbis(azanediyl))dipropanamide (BCEN), 3,3'-(piperazine-1,4-diyl)dipropanamide (BPAP) or 3,3'-and (1,4-diazepane-1,4-diyl)dipropanamide (BPAH) are reported in this work. The ligands contain different diamine units (ethylenediamine, piperazine or homopiperazine) and two propanamide units attached to the diamine centers, resulting in N₂O₂ donor sets. The complex containing homopiperazine unit presented the best antiproliferative effect and selectivity against lung cancer cell line NCI-H460, showing inhibitory concentration (IC₅₀) of 58 μmol dm^-3 and Selectivity Index (SI) > 3.4. The mechanism of cell death promoted by the complex was investigated by Sub-G1 cell population analysis and annexin V and propidium iodide (PI) labeling techniques, suggesting that the complex promotes death by apoptosis. Transmission electron microscopy investigations are in agreement with the results presented by mitochondrial membrane potential analysis and also show the impairment of other organelles, including endoplasmic reticulum.

Copper(ii) complexes of N-propargyl cyclam ligands reveal a range of coordination modes and colours, and unexpected reactivity

The coordination chemistry of N-functionalised cyclam ligands has a rich history, yet cyclam derivatives with pendant alkynes are largely unexplored. This is despite the significant potential and burgeoning application of N-propargyl cyclams and related compounds in the creation of diversely functionalised cyclam derivatives via copper-catalysed azide-alkyne 'click' reactions. Herein we describe single crystal X-ray diffraction and spectroscopic investigations of the coordination chemistry of copper(ii) complexes of cyclam derivatives with between 1 and 4 pendant alkynes. The crystal structures of these copper complexes unexpectedly reveal a range of coordination modes, and the surprising occurrence of five unique complexes within a single recrystallisation of the tetra-N-propargyl cyclam ligand. One of these species exhibits weak intramolecular copper-alkyne coordination, and another is formed by a surprising intramolecular copper-mediated hydroalkoxylation reaction with the solvent methanol, transforming one of the pendant alkynes to an enol ether. Multiple functionalisation of the tetra-N-propargyl ligand is demonstrated via a 'tetra-click' reaction with benzyl azide, and the copper-binding behaviour of the resulting tetra-triazole ligand is characterised spectroscopically.

Recent Advances in Heterogeneous Catalytic Systems Containing Metal Ions for Phosphate Ester Hydrolysis

Organophosphates are a class of organic compounds that are important for living organisms, forming the building blocks for DNA, RNA, and some essential cofactors. Furthermore, non-natural organophosphates are widely used in industrial applications, including as pesticides; in laundry detergents; and, unfortunately, as chemical weapons agents. In some cases, the natural degradation of organophosphates can take thousands of years; this longevity creates problems associated with handling and the storage of waste generated by such phosphate esters, in particular. Efforts to develop new catalysts for the cleavage of phosphate esters have progressed in recent decades, mainly in the area of homogeneous catalysis. In contrast, the development of heterogeneous catalysts for the hydrolysis of organophosphates has not been as prominent. Herein, examples of heterogeneous systems are described and the importance of the development of heterogeneous catalysts applicable to organophosphate hydrolysis is highlighted, shedding light on recent advances related to different solid matrices that have been employed.

Synthesis, Magnetic Properties, and Catalytic Properties of a Nickel(II)-Dependent Biomimetic of Metallohydrolases

A dinickel(II) complex of the ligand 1,3-bis(bis(pyridin-2-ylmethyl)amino)propan-2-ol (HL1) has been prepared and characterized to generate a functional model for nickel(II) phosphoesterase enzymes. The complex, [Ni2(L1)(μ-OAc)(H2O)2](ClO4)2·H2O, was characterized by microanalysis, X-ray crystallography, UV-visible, and IR absorption spectroscopy and solid state magnetic susceptibility measurements. Susceptibility studies show that the complex is antiferromagnetically coupled with the best fit parameters J = -27.4 cm-1, g = 2.29, D = 28.4 cm-1, comparable to corresponding values measured for the analogous dicobalt(II) complex [Co2(L1)(μ-OAc)](ClO4)2·0.5 H2O (J = -14.9 cm-1 and g = 2.16). Catalytic measurements with the diNi(II) complex using the substrate bis(2,4-dinitrophenyl)phosphate (BDNPP) demonstrated activity toward hydrolysis of the phosphoester substrate with K m ~10 mM, and k cat ~0.025 s-1. The combination of structural and catalytic studies suggests that the likely mechanism involves a nucleophilic attack on the substrate by a terminal nucleophilic hydroxido moiety.

A direct method for the N-tetraalkylation of azamacrocycles

An efficient protocol for the direct synthesis of N-tetraalkylated derivatives of the azamacrocycles cyclam and cyclen has been developed, using a partially miscible aqueous–organic solvent system with propargyl bromide, benzyl bromide, and related halides. The method works most effectively when the reaction mixture is shaken, not stirred. A crystal structure of the N-tetrapropargyl cyclam derivative 1,4,8,11-tetra(prop-2-yn-1-yl)-1,4,8,11-tetraazacyclotetradecane diperchlorate is reported.

Synthesis and characterization of FeIII(μ-OH)ZnII complexes: effects of a second coordination sphere and increase in the chelate ring size on the hydrolysis of a phosphate diester and DNA

Effects of a second coordination sphere and of the chelate ring size in Fe^III(μ-OH)Zn^II complexes properties and catalysis.

A Bioinspired Dinickel(II) Hydrolase: Solvent Vapor-Induced Hydrolysis of Carboxyesters under Ambient Conditions

From the perspective of synthetic metallohydrolases, a phenoxo-bridged dinickel(II) complex [Ni^II₂(L)(H₂O)₂(CH₃OH)][ClO₄]·CH₃OH (1) (H₃L = 2,6-bis[{{(5-bromo-2-hydroxybenzyl)(N',N″-(dimethylamino)ethyl)}amino}methyl]-4-methylphenol) has been synthesized and structurally characterized. The presence of a vacant coordination site and a weakly bound water molecule provides the scope for substrate binding to act as a metallohydrolase model. Ethyl acetate vapor diffusion at 298 K to a CH₃CN/CH₃OH solution of 1 results in the formation of a pentanuclear acetato-bridged complex [Ni^II₅(H₂L)₂(μ₃-OH)₂(μ-O₂CCH₃)₄][ClO₄]₂·CH₃CO₂C₂H₅ (2), demonstrating for the first time the metal-coordinated water-promoted hydrolysis of a carboxyester at room temperature. When the crystals of 1, moistened with a few drops of ethyl acetate, were kept for ethyl acetate vapor diffusion, it transforms into a monoacetato-bridged complex [Ni^II₂(HL)(μ-O₂CCH₃)(H₂O)₂][ClO₄]·4H₂O (3). This kind of solvent (vapor)-induced single-crystal-to-single-crystal structural transformation concomitant with the hydrolysis of external substrate (ethyl acetate) is unprecedented. Reaction of H₃L with 2 equiv of Ni^II(O₂CCH₃)₂·4H₂O, followed by the usual workup, and recrystallization from CH₂Cl₂ led to the isolation of [Ni^II₂(H₂L)(μ-O₂CCH₃)₂][ClO₄]·CH₂Cl₂·2H₂O (4). Complex 4 is structurally different from 3, confirming that the reaction of Ni^II(O₂CCH₃)₂·4H₂O with H₃L is a different phenomenon from the hydrolysis of ethyl acetate, promoted by Ni^II-coordinated water in 1. Complex 1 is also capable of hydrolyzing ethyl propionate to a propionato-bridged complex [Ni^II₂(HL)(μ-O₂CCH₂CH₃)(H₂O)₂][ClO₄] (5). For the hydrolytic phenomena mentioned above, the coordinated ligand donor sites (phenolate and tertiary amine) provide a microenvironment around the dinickel(II) center to facilitate efficient stoichiometric hydrolysis of ethyl acetate and ethyl propionate under ambient conditions. Temperature-dependent magnetic studies of dimeric complexes 1, 4, and 5 reveal the presence of moderate antiferromagnetic coupling: J = -25.0(1) cm^-1 for 1, J = -20.0(1) cm^-1 for 4, and J = -18.80(8) cm^-1 for 5. For pentanuclear complex 2, three types of magnetic-exchange interactions, two ferromagnetic (J_a = +16.02 cm^-1, and J_b = +9.02 cm^-1) and an antiferromagnetic (J_c = -49.7 cm^-1), have been identified.

Metallohydrolase biomimetics with catalytic and structural flexibility

The phosphatase activity of zinc complexes containing six- and seven-dentate ligands was evaluated through kinetic and³¹P NMR studies.

Synthesis, self-assembly, and catalytic activity of histidine-based structured lipopeptides for hydrolysis reactions in water

When interfacial catalysis and organocatalysis meet: self-assembling histidine-based lipopeptides catalyse ester hydrolysis in water, depending on aggregation.

Spectroscopic and mechanistic studies of dinuclear metallohydrolases and their biomimetic complexes

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.

Highly efficient synthetic iron-dependent nucleases activate both intrinsic and extrinsic apoptotic death pathways in leukemia cancer cells

The nuclease activity and the cytotoxicity toward human leukemia cancer cells of iron complexes, [Fe(HPClNOL)Cl2]NO3 (1), [Cl(HPClNOL)Fe(μ-O)Fe(HPClNOL)Cl]Cl2·2H2O (2), and [(SO4)(HPClNOL)Fe(μ-O)Fe(HPClNOL)(SO4)]·6H2O (3) (HPClNOL=1-(bis-pyridin-2-ylmethyl-amino)-3-chloropropan-2-ol), were investigated. Each complex was able to promote plasmid DNA cleavage and change the supercoiled form of the plasmid to circular and linear ones. Kinetic data revealed that (1), (2) and (3) increase the rate of DNA hydrolysis about 278, 192 and 339 million-fold, respectively. The activity of the complexes was inhibited by distamycin, indicating that they interact with the minor groove of the DNA. The cytotoxic activity of the complexes toward U937, HL-60, Jukart and THP-1 leukemia cancer cells was studied employing 3-(4,5-dimethythiazol-2-yl)-2,5-diphenyl tetrazolium bromide (MTT), fluorescence and electronic transmission microscopies, flow cytometry and a cytochrome C release assay. Compound (2) has the highest activity toward cancer cells and is the least toxic for normal ones (i.e. peripheral blood mononuclear cells (PBMCs)). In contrast, compound (1) is the least active toward cancer cells but displays the highest toxicity toward normal cells. Transmission electronic microscopy indicates that cell death shows features typical of apoptotic cells, which was confirmed using the annexin V-FITC/PI (fluorescein isothiocyanate/propidium iodide) assay. Furthermore, our data demonstrate that at an early stage during the treatment with complex (2) mitochondria lose their transmembrane potential, resulting in cytochrome C release. A quantification of caspases 3, 9 (intrinsic apoptosis pathway) and caspase 8 (extrinsic apoptosis pathway) indicated that both the intrinsic (via mitochondria) and extrinsic (via death receptors) pathways are involved in the apoptotic stimuli.