Phosphate Ester Cleavage Research Articles

Zn(II)-complex-based hydrolytic nanozymes prepared by nanoprecipitation for the hydrolysis of carboxylic esters and methyl parathion

Zn(II)-complex-based nanozymes have been extensively used for the cleavage of carboxylic esters and phosphate esters. Cooperativity between neighboring Zn(II) complexes is of paramount importance for the development of Zn(II)-complex-based nanozymes with excellent efficiency. Here, we have developed a controllable synthetic strategy for the construction of Zn(II)-complex-based nanozymes by using the nanoprecipitation technique, which could bring Zn(II) complexes in close proximity and regulate their microenvironments. Linear copolymers containing di(2-picolyl)amine moieties were used for the preparation of Zn(II)-complex-based nanozymes by the coordination between di(2-picolyl)amine and Zn(II) ion. These Zn(II)-complex-based nanozymes displayed excellent catalytic performance in the hydrolysis of carboxylic esters. Moreover, hydrophobic Fe3O4 nanoparticles could be incorporated into Zn(II)-complex-based nanozymes via the one-step nanoprecipitation approach, enabling the recovery of nanozymes by using a permanent magnet. Such nanozymes effectively catalyzed the hydrolysis of methyl parathion, which is highly resistant to spontaneous hydrolysis and is extremely hazardous. These nanozymes may find application in the hydrolysis of hazardous carboxylic esters and phosphate esters.

Peroxide-Selective Reduction of O2 at Redox-Inactive Rare-Earth(III) Triflates Generates an Ambiphilic Peroxide.

Metal peroxides are key species involved in a range of critical biological and synthetic processes. Rare-earth (group III and the lanthanides; Sc, Y, La-Lu) peroxides have been implicated as reactive intermediates in catalysis; however, reactivity studies of isolated, structurally characterized rare-earth peroxides have been limited. Herein, we report the peroxide-selective (93-99% O22-) reduction of dioxygen (O2) at redox-inactive rare-earth triflates in methanol using a mild metallocene reductant, decamethylferrocene (Fc*). The first molecular praseodymium peroxide ([PrIII2(O22-)(18C6)2(EG)2][OTf]4; 18C6 = 18-crown-6, EG = ethylene glycol, -OTf = -O3SCF3; 2-Pr) was isolated and characterized by single-crystal X-ray diffraction, Raman spectroscopy, and NMR spectroscopy. 2-Pr displays high thermal stability (120 °C, 50 mTorr), is protonated by mild organic acids [pKa1(MeOH) = 5.09 ± 0.23], and engages in electrophilic (e.g., oxygen atom transfer) and nucleophilic (e.g., phosphate-ester cleavage) reactivity. Our mechanistic studies reveal that the rate of oxygen reduction is dictated by metal-ion accessibility, rather than Lewis acidity, and suggest new opportunities for differentiated reactivity of redox-inactive metal ions by leveraging weak metal-ligand binding events preceding electron transfer.

Mechanistic study of carboxylic acid and phosphate ester cleavage by oximate metal complexes surpassing the limiting reactivity of highly basic free oximate anions.

Two tridentate and one tetradentate new ligands containing the terminal oxime group separated from secondary amino and pyridine groups as additional binding sites by two or three methylene groups have been prepared. Their acid-base properties, as well as the composition and stability of their complexes with Zn(ii) and Cd(ii) ions, were determined by potentiometric and spectrophotometric titrations. The X-ray structure of a Cd(ii) complex of a related tridentate oxime ligand previously studied in solution was determined. All oximate complexes show high reactivity in the cleavage of aryl acetates, paraoxon, parathion and 4-nitrophenyl diphenyl phosphate, with rate constants significantly surpassing the limiting rate constants observed for highly basic free oximate anions. The second-order rate constants for individual oximate complexes in solution are assigned to each ligand, metal cation and substrate. The results of the cleavage of 4-substituted phenyl acetates were analyzed in terms of Brønsted correlations with the leaving group pKa, which demonstrated a change in the rate determining step from the nucleophilic attack to the leaving group departure upon an increase in the leaving group basicity. The zero slope of the Brønsted correlation for the nucleophilic attack indicates transition state stabilization through electrophilic assistance by the metal ion. This interpretation is supported by metal selectivity in the relative efficiency of the cleavage of paraoxon and parathion. The existence of the alpha-effect in ester cleavage by coordinated oximates is confirmed by an analysis of the Brønsted correlations with the nucleophile basicity for metal bound oximate and alkoxo or hydroxo nucleophiles. The very high reactivity of the oximate complexes of the new ligands is attributed to transition state stabilization and to the removal of the solvational imbalance of oximate anions that impedes the expected increase in the reactivity of highly basic free anions.

IMIDAZOL E CATÁLISE: UM PAR PERFEITO

IMIDAZOLE AND CATALYSIS: A PERFFECT MATCH. The imidazole (IMZ) ring presents several chemical properties that turns it into a versatile acid-basic/nucleophilic catalyst. The discovery of the biological properties of histidine (amino acid with IMZ ring) instigates the study of this class of compounds. It is found in nature in several enzymes such as chymotrypsin and RNAses due to these properties. This review is focused on the catalytic role of IMZ towards the cleavage of acyl and phosphate esters and is structured in the following topics: chemical properties, biological catalysis (RNAses, phosphohistidines, chymotrypsin), intermolecular and intramolecular catalytic models and polymer and nanomaterials-based catalysts. Besides we discuss some properties such as tautomerism, stability, basicity and spectroscopic behavior, correlating with its reactivity in deacylation and dephosphorylation reactions. The main reaction mechanisms and kinetics promoted by IMZ as an acid, basic and nucleophilic catalysts are explored showing some examples. In addition, the importance of IMZ in enzymatic sites and their mechanisms and further combining properties of IMZ with some materials such as (bio)polymers, graphene, and cyclodextrines are covered. In this sense, with the wide IMZ data presented here, we are evidencing the perfect match between IMZ and catalysis.

Influence of pyridine oximate and quaternized pyridinium oximate ions on the hydrolysis of phosphate esters in cationic microemulsions

Kinetic studies have been performed to understand the hydrolytic potencies of oximate (2- and 4-pyridinealdoxime) and its functionalized oximate (4-(hydroxyiminomethyl)-1-alkylpyridinium bromide) ions (alkyl = C10H21 (4-C10PyOx-); alkyl = C12H25 (4-C12PyOx-)) in the cleavage of phosphate esters, diethyl p-nitrophenylphosphate (Paraoxon) and p-nitrophenyl diphenyl phosphate (PNPDPP) in a cationic (O/W) microemulsion system (ME) over a pH range 7.5 to 11.0 at 300 K. The kobs values for the reaction of paraoxon with oximate and its functionalized oximate were determined in different microemulsion composition and the kinetic rate data shows that kobs values increases with increasing water content. The specificity of different chain length of alcohols (n-butanol, n-pentanol, n-hexanol and n-octanol) was also investigated in hydrolytic reactions of paraoxon for different microemulsion composition.

Phosphate ester cleavage by a positively charged porous silica adorned with lanthanum (III)

Phosphate esters and analogs thereof are well known in chemistry and biology by their formidable stability. We used a porous silica material (PSiM) functionalized with amino groups (8.4%) and lanthanum ions (1.5%) to catalyze the cleavage of bis(2,4-dinitrophenyl)phosphate (BDNPP). The functionalized PSiM attracted the substrate from solution and accelerated its hydrolysis by 38-fold in relation to the uncatalyzed reaction at pH 8.14. After an induction period associated to hydration of the catalyst during the first use, the catalytic activity increased in the first reuse to about 600-fold. Supramolecular effects were proposed to drive the catalytic process. Kinetic studies in different pH's showed that the lanthanum ion and amino groups acted mutually in the BDNPP hydrolysis and a reaction mechanism that took this into account was proposed.

Hydrolytic Dephosphorylation of p‐Nitrophenyl Diphenyl Phosphate by Alkyl Hydroxamate Ions

AbstractThe kinetics of the hydrolysis of p‐nitrophenyl diphenyl phosphate (PNPDPP) by hydroxamate ions (R'(CO)N(RO−) such as octanohydroxamate (OHA−) and decanohydroxamate (DHA−) was investigated in dioctadecyldimethylammonium chloride (DODAC) and didodecyldimethylammonium bromide (DDAB) vesicles. The physicochemical properties of these surfactants were studied by conductivity and fluorescence measurements at 300 K. The hydrolysis of PNPDPP was studied in a vesicular system by using hydroxamate ions (OHA− and DHA−) at 300 K. The different catalytic effects of hydroxamate ions for the hydrolysis of PNPDPP in the vesicles were determined. All reactions followed pseudofirst‐order kinetics. The reactivity of DHA− was found to be higher than that of OHA− in the vesicular system toward the cleavage of phosphate ester. Further, the binding constants (K) and free energy change (∆G) for the associations of PNPDPP with DODAC and DDAB vesicles were determined spectrophotometrically as well as from the Benesi–Hildebrand (B–H) plots. The pseudophase model was applied for the quantitative treatment of the kinetic data in the vesicle systems.

Influence of octanohydroxamic acid on the association behavior of cationic surfactants: Hydrolytic cleavage of phosphate ester

The surface properties and mixed micellization behavior of cetyltrimethylammonium bromide (CTAB), tetradecyltrimethylammonium bromide (TTAB) and dodecyltrimethylammonium bromide (DTAB) with octanohydroxamic acid (OHA) have been investigated by means of conductivity and surface tension measurements in aqueous solution and borate buffer at 300K. The critical micelle concentration (cmc), surface properties such as maximum surface access (Γmax), surface pressure at the cmc (Πcmc) and minimum surface area per molecule (Amin) has been determined. The standard Gibbs free energy of micellization (ΔGm0), standard Gibbs free energy of adsorption (ΔGad0), and standard Gibbs free energy of micellization per alkyl chain (ΔGm,tail0) of cationic surfactant with OHA have been evaluated. The fluorescence quenching technique was used to estimate the aggregation number (Nagg) and packing parameter (P) for determining the structural feature of cationic surfactants in the presence of octanohydroxamic acid. The hydrolytic reaction of paraoxon with octanohydroxamic acid was studied under a cationic micellar system by using OHA− at 9.2pH and 300K. The variations of surface properties from aqueous medium to the reaction condition have also been discussed. Pseudophase model (PPM) has been fitted for the quantitative treatment of the data.

Synthesis and phosphate ester cleavage properties of copper(II) complexes of guanidinium-bridged bis(1,4,7-triazacyclononane) ligands

The synthesis and characterization of three new bis(1,4,7-triazacyclononane) ligands, L1, L2 and L3, featuring a bridging guanidinium group between two macrocyclic units, is reported. The corresponding binuclear copper(II) complexes have been studied as agents to accelerate the cleavage of P–O bonds within two model phosphodiesters, namely bis(p-nitrophenyl)phosphate (BNPP) and 2-hydroxypropyl-p-nitrophenylphosphate (HPNPP). The results of a comparative study of cleavage rates, using the mononuclear copper(II)–tacn complexes bearing single alkylguanidinium groups as a reference, revealed that the binuclear copper(II) complexes are generally less effective cleavage agents, which may be related to a tendency to form hydrolytically inactive hydroxo-bridged species at near-neutral pH and above. However, at pH 7, these complexes produced 4–18-fold increases in the rate of BNPP hydrolysis compared to the parent complex, [Cu(tacn)(OH2)2]2+. Likewise, at pH 6, the complexes cleaved HPNP 5–130-fold faster than [Cu(tacn)(OH2)2]2+, suggesting some degree of cooperative interplay between the two proximal copper(II) centers and the protonated guanidinium bridging group in promoting phosphodiester hydrolysis under these conditions.

Metallosurfactant Aggregates as Catalysts for the Hydrolytic Cleavage of Carboxylate and Phosphate Esters

Metallosurfactant aggregates have grasped a great attention of researchers worldwide owing to their unique surface activity and catalytic efficiency. They serve themselves as perfect blends of both surface science and metallo-organic chemistry. Metallosurfactant aggregates have a variety of fascinating applications like in drug delivery, membrane mimetics, in various separation methods and most importantly in catalytic cleavage of various toxic esters. Due to the resemblance in the catalytic property of metallosurfactant aggregates with natural enzymes they are broadly employed as metalloenzyme mimics for the hydrolysis of activated carboxylic esters, phosphate esters and amides in general laboratory settings. This review summarizes the design and syntheses of metallosurfactant aggregates and their role as proficient catalysts for the hydrolysis of different esters. Special emphasis is given toward the development of structurally diverse surfactant type ligands and use of variety of metal ions specifically for the hydrolytic cleavage of carboxylate and phosphate esters. Generally, the catalytic strengths of numerous synthesized metallosurfactant aggregates have been quantized by exploring kinetic investigations. Hence, a brief account on the quantitative treatment of rate data with the help of kinetic models has also been presented. Keywords: Carboxylate esters, catalysis, hydrolysis, metallosurfactant aggregates, phosphate esters, vesicles.

Macrocyclic metal complexes for metalloenzyme mimicry and sensor development.

Examples of proteins that incorporate one or more metal ions within their structure are found within a broad range of classes, including oxidases, oxidoreductases, reductases, proteases, proton transport proteins, electron transfer/transport proteins, storage proteins, lyases, rusticyanins, metallochaperones, sporulation proteins, hydrolases, endopeptidases, luminescent proteins, iron transport proteins, oxygen storage/transport proteins, calcium binding proteins, and monooxygenases. The metal coordination environment therein is often generated from residues inherent to the protein, small exogenous molecules (e.g., aqua ligands) and/or macrocyclic porphyrin units found, for example, in hemoglobin, myoglobin, cytochrome C, cytochrome C oxidase, and vitamin B12. Thus, there continues to be considerable interest in employing macrocyclic metal complexes to construct low-molecular weight models for metallobiosites that mirror essential features of the coordination environment of a bound metal ion without inclusion of the surrounding protein framework. Herein, we review and appraise our research exploring the application of the metal complexes formed by two macrocyclic ligands, 1,4,7-triazacyclononane (tacn) and 1,4,7,10-tetraazacyclododecane (cyclen), and their derivatives in biological inorganic chemistry. Taking advantage of the kinetic inertness and thermodynamic stability of their metal complexes, these macrocyclic scaffolds have been employed in the development of models that aid the understanding of metal ion-binding natural systems, and complexes with potential applications in biomolecule sensing, diagnosis, and therapy. In particular, the focus has been on "coordinatively unsaturated" metal complexes that incorporate a kinetically inert and stable metal-ligand moiety, but which also contain one or more weakly bound ligands, allowing for the reversible binding of guest molecules via the formation and dissociation of coordinate bonds. With regards to mimicking metallobiosites, examples are presented from our work on tacn-based complexes developed as simplified structural models for multimetallic enzyme sites. In particular, structural comparisons are made between multinuclear copper(II) complexes formed by such ligands and multicopper enzymes featuring type-2 and type-3 copper centers, such as ascorbate oxidase (AO) and laccase (Lc). Likewise, with the aid of relevant examples, we highlight the importance of cooperativity between either multiple metal centers or a metal center and a proximal auxiliary unit appended to the macrocyclic ligand in achieving efficient phosphate ester cleavage. Finally, the critical importance of the Zn(II)-imido and Zn(II)-phosphate interactions in Zn-cyclen-based systems for delivering highly sensitive electrochemical and fluorescent chemosensors is also showcased. The Account additionally highlights some of the factors that limit the performance of these synthetic nucleases and the practical application of the biosensors, and then identifies some avenues for the development of more effective macrocyclic constructs in the future.

O‐Nucleophilicity of Hydroxamate Ions for Cleavage of Carboxylate and Phosphate Esters in Cationic Micelles

ABSTRACTThe nucleophilic reactivities of hydroxamate (HA−) ions of the structure RCONHO− [R = CH3 (acetohydroxamate, AHA−), C6H5 (benzohydroxamate, BHA−), 2‐OHC6H4 (salicylhydroxamate, SHA−), and 4‐CH3OC6H4 (4‐methoxbenzohydroxamate, MBHA−)] for the hydrolysis of p‐nitrophenyl benzoate (PNPB), tris(3‐nitrophenyl) phosphate (TRIS), and bis(2,4‐dinitrophenyl) phosphate (BDNPP) have been examined kinetically. Over the pH range of 6.7–11.4, the α‐nucleophile (HA−) accelerates deacylation of PNPB and dephosphorylation of TRIS (in cetyltrimethylammonium bromide (CTAB) micelle, 2.0 × 10−3 M). The salicylhydroxamate ion encountered effective catalysis than AHA−, BHA−, and MBHA− ions. The monoanionic SHA− and dianionic SA2− forms of salicylhydroxamic acid are the reactive species. The hydroxamic acid concentration–dependent critical micelle concentration (cmc) and fractional ionization constant (α) and of CTAB provide qualitative information for the micellar incorporation of the hydroxamate ion. The ab initio calculations performed on the hydroxamate ions at restricted Hartree–Fock using the 6‐311G (d,p) basis set revealed the O‐nucleophilicity of hydroxamate ions toward C=O and P=O centers. On the basis of ab initio calculation, it has been concluded that hydroxamic acids can exist into E‐amide and Z‐amide forms. The large stable amide or imide anions of hydroxamate are strong nucleophilic for the esterolytic cleavage of carboxylate and phosphate esters.

Reactivity of N-benzoyl-N-phenylhydroxylamine in cationic micellar media for the cleavage of carboxylate and phosphate esters

The rate enhancement for the hydrolysis of p-nitrophenyl acetate (PNPA) and p-nitrophenyl diphenyl phosphate (PNPDPP) by N-benzoyl-N-phenylhydroxylamine (PBHA) was obtained in the presence of gemini i.e. decanediyl-1, 10-bis (cetyldimethylammonium bromide) (C16-10-C16, 2Br−) and monomeric surfactants viz. cetyldiethylethanolammonium bromide (CDEEAB), cetyldimethylethanolammonium bromide (CDMEAB), cetyl-p-toluene-sulfonate (CPTS), and cetylpyridinium bromide (CPB). The physicochemical properties of gemini and monomeric surfactants have also been investigated by conductivity and surface tension measurements. All the kinetic data has been treated by simple pseudophase model.

Physicochemical Properties and Supernucleophilicity of Oxime-Functionalized Surfactants: Hydrolytic Catalysts toward Dephosphorylation of Di- and Triphosphate Esters

Aggregation and kinetic studies have been performed to understand the hydrolytic potencies of the series of oxime-functionalized surfactants, viz., 3- hydroxyiminomethyl-1-alkylpyridinium bromide (alkyl = CnH2n+1, n = 10, 12, 14, 16, 18) in the cleavage of phosphate esters, p-nitrophenyl diphenyl phosphate (PNPDPP) and bis(2,4-dinitrophenyl) phosphate (BNDPP), in mixed micelles with cetylpyridinium bromide (CPB). Micellization and surface properties of mixed micelles functional surfactants with CPB were studied by conductivity and surface tension measurements. Acid dissociation constants (pKa) were determined, the effect of functional surfactant alkyl chain length and pH on the observed rate constant (kobs) for phosphate ester cleavage has been discussed, and the effect of substrate on the supernucleophilicities of the studied oximes was monitored. Functionalized oxime-based surfactants were proved to be supernucleophiles to attack on the P═O center of tri- and diphosphate esters. Oximes with hexadecyl alkyl chain length (3-C16) showed maximum micellar effect on the rate constants toward PNPDPP. Micellar effects were analyzed in terms of the pseudophase model.

A Series of Oxyimine-Based Macrocyclic Dinuclear Zinc(II) Complexes Enhances Phosphate Ester Hydrolysis, DNA Binding, DNA Hydrolysis, and Lactate Dehydrogenase Inhibition and Induces Apoptosis

A symmetrical macrocyclic dizinc(II) complex (1) has been synthesized by using the ligand (L(1)) [μ-11,24-dimethyl-4,7,16,19-tetraoxa-3,8,15,20-tetraazatricyclo-[20.3.1.1(10,13)] heptacosa-1(25),2,7,9,11,13(27),14,20,22(26),23-decaene-26,27-diol]. A series of unsymmetrical macrocyclic dizinc(II) complexes (2-6) has been synthesized by Schiff base condensation of bicompartmental mononuclear complex [ZnL] [μ-3,16-dimethyl-8,11-dioxa-7,12-diazadicyclo-[1.1(14,18)] heptacosa-1,3,5(20),6,12,14,16,18(19)-octacaene-19,20-diolato)zinc(II)] with various diamines like 1,2-diamino ethane (L(2)), 1,3-diamino propane (L(3)), 1,4-diamino butane (L(4)), 1,2-diamino benzene (L(5)), and 1,8-diamino naphthalene (L(6)). The ligand L(1) and all the zinc(II) complexes were structurally characterized. To corroborate the consequence of the aromatic moiety in comparison to the aliphatic moiety present in the macrocyclic ring on the phosphate ester hydrolysis, DNA binding and cleavage properties have been studied. The observed first order rate constant values for the hydrolysis of 4-nitrophenyl phosphate ester reaction are in the range from 2.73 × 10(-2) to 9.86 × 10(-2) s(-1).The interactions of complexes 1-6 with calf thymus DNA were studied by spectroscopic techniques, including absorption, fluorescence, and circular dichroism spectroscopy. The DNA binding constant values of the complexes were found in the range from 1.80 × 10(5) to 9.50 × 10(5) M(-1), and the binding affinities are in the following order: 6 > 5 > 1 > 2 > 3 > 4. All the dizinc(II) complexes 1-6 effectively promoted the hydrolytic cleavage of plasmid pBR322 DNA under anaerobic and aerobic conditions. Kinetic data for DNA hydrolysis promoted by 6 under physiological conditions give the observed rate constant (k(obs)) of 4.42 ± 0.2 h(-1), which shows a 10(8)-fold rate acceleration over the uncatalyzed reaction of ds-DNA. The comparison of the dizinc(II) complexes 1-6 with the monozinc(II) complex [ZnL] indicates that the DNA cleavage acceleration promoted by 1-6 are due to the efficient cooperative catalysis of the two close proximate zinc(II) cation centers. The ligand L(1), dizinc(II) complexes 1, 3, and 6 showed cytotoxicity in human hepatoma HepG2 cancer cells, giving IC(50) values of 117, 37.1, 16.5, and 8.32 μM, respectively. The results demonstrated that 6, a dizinc(II) complex with potent antiproliferative activity, is able to induce caspase-dependent apoptosis in human cancer cells. Cytotoxicity of the complexes was further confirmed by the lactate dehydrogenase enzyme level in HepG2 cell lysate and content media.

Phosphodiester Cleavage Properties of Copper(II) Complexes of 1,4,7-Triazacyclononane Ligands Bearing Single Alkyl Guanidine Pendants

Three new metal-coordinating ligands, L(1)·4HCl [1-(2-guanidinoethyl)-1,4,7-triazacyclononane tetrahydrochloride], L(2)·4HCl [1-(3-guanidinopropyl)-1,4,7-triazacyclononane tetrahydrochloride], and L(3)·4HCl [1-(4-guanidinobutyl)-1,4,7-triazacyclononane tetrahydrochloride], have been prepared via the selective N-functionalization of 1,4,7-triazacyclononane (tacn) with ethylguanidine, propylguanidine, and butylguanidine pendants, respectively. Reaction of L(1)·4HCl with Cu(ClO(4))(2)·6H(2)O in basic aqueous solution led to the crystallization of a monohydroxo-bridged binuclear copper(II) complex, [Cu(2)L(1)(2)(μ-OH)](ClO(4))(3)·H(2)O (C1), while for L(2) and L(3), mononuclear complexes of composition [Cu(L(2)H)Cl(2)]Cl·(MeOH)(0.5)·(H(2)O)(0.5) (C2) and [Cu(L(3)H)Cl(2)]Cl·(DMF)(0.5)·(H(2)O)(0.5) (C3) were crystallized from methanol and DMF solutions, respectively. X-ray crystallography revealed that in addition to a tacn ring from L(1) ligand, each copper(II) center in C1 is coordinated to a neutral guanidine pendant. In contrast, the guanidinium pendants in C2 and C3 are protonated and extend away from the Cu(II)-tacn units. Complex C1 features a single μ-hydroxo bridge between the two copper(II) centers, which mediates strong antiferromagnetic coupling between the metal centers. Complexes C2 and C3 cleave two model phosphodiesters, bis(p-nitrophenyl)phosphate (BNPP) and 2-hydroxypropyl-p-nitrophenylphosphate (HPNPP), more rapidly than C1, which displays similar reactivity to [Cu(tacn)(OH(2))(2)](2+). All three complexes cleave supercoiled plasmid DNA (pBR 322) at significantly faster rates than the corresponding bis(alkylguanidine) complexes and [Cu(tacn)(OH(2))(2)](2+). The high DNA cleavage rate for C1 {k(obs) = 1.30 (±0.01) × 10(-4) s(-1) vs 1.23 (±0.37) × 10(-5) s(-1) for [Cu(tacn)(OH(2))(2)](2+) and 1.58 (±0.05) × 10(-5) s(-1) for the corresponding bis(ethylguanidine) analogue} indicates that the coordinated guanidine group in C1 may be displaced to allow for substrate binding/activation. Comparison of the phosphate ester cleavage properties of complexes C1-C3 with those of related complexes suggests some degree of cooperativity between the Cu(II) centers and the guanidinium groups.

Phosphate ester cleavage promoted by a tetrameric iron(III) complex

The purple acid phosphatases (PAPs) are the only binuclear metallohydrolases where the necessity for a heterovalent active site [Fe(III)-M(II) (MisFe, Zn or Mn)] for catalysis has been established. The paradigm for the construction of PAP biomimetics, both structural and functional, is that the ligands possess characteristics which mimic those of the donor sites of the metalloenzyme and permit discrimination between trivalent and divalent metal ions. The donor atom set of the ligand 2-((2-hydroxy-5-methyl-3-((pyridin-2-ylmethylamino)methyl)benzyl)(2-hydroxybenzyl)amino)acetic acid (H(3)HPBA) mimics that of the active site of PAP although the iron(III) complex of this ligand has been characterized as the tetramer [Fe(4)(HPBA)(2)(μ-CH(3)COO)(2)(μ-O)(μ-OH)(OH(2))(2)]ClO(4)·5H(2)O. The phosphoesterase-like activity of the complex in 1:1 acetonitrile/water has now been investigated using the substrate 2,4-bis(dinitrophenyl)phosphate. The pH dependence of the catalytic rate revealed a non-symmetric bell-shaped profile, with a finite but non-zero rate at high pH. Unlike the traditional approach usually employed to analyse these bell-shaped profiles, the approach used here involved incorporating additional species which contribute to the overall activity. Employing this approach, we show that the complex has a k (cat) of 1.6 (±0.2)×10(-3)s(-1), three kinetically relevant pK (a) values of 5.3, 6.2 and 8.4, with K (M) of 7.4±0.6mM. The kinetic parameters are similar to those reported for heterovalent PAP biomimetics. Additionally, it is observed that, unlike the enzyme, the oxidation state is not the determining factor for catalytic activity.

Efficient Methanolytic Cleavage of Phosphate, Phosphonate, and Phosphonothioate Esters Promoted by Solid Supported Lanthanide Ions

The methanolyses of three neutral organophosphorus esters, namely paraoxon (O,O-diethyl O-p-nitrophenyl phosphate (12), O-ethyl O-p-nitrophenyl methylphosphonate (13), O-ethyl S-3,5-dichlorophenyl methylphosphonothioate (14, a model for VX), and an anionic phosphonate ester, O-hydrogen S-3,5-dichlorophenyl methylphosphonothioate (15, a model for a toxic hydrolytic decomposition product of VX known as EA2192)), promoted by solid supported lanthanide ion catalysts (La3+, Sm3+, Eu3+, Yb3+) were investigated at 25 °C under sspH controlled conditions in methanol. The supports were derived from commercially available chlorobenzylated silica and chloromethylated polystyrene functionalized with two chelating ligands, iminodiacetic acid (IDA) and ethylenediamine-N,N′-diacetic acid (EDDA), followed by exposure to Ln3+ ions. The catalysts showed good activity at two investigated sspH values of 8.8 and 11 (in methanol). Despite the fact that the methoxide promoted reactions for the four substrates span a range of ∼20...

A computational study towards understanding the mechanism of phosphodiester cleavage by two mononuclear Zn(ii) complexes

In this study, the hydrolytic reaction mechanism of HPNP by two effective mononuclear Zn(II) catalysts was explored by using density functional calculations. With the assumption that the active catalyst is in Zn-aqua form, the water-assisted proton transfer mode was found to be energetically the most favorable one for both the catalysts among three possible reaction pathways. The rate-limiting step on this pathway is a concerted double-proton transfer, concomitant with the nucleophilic addition of the deprotonated hydroxyl group to form a five-coordinated phosphorus intermediate. A comparison of potential energy surfaces for both catalysts shows that the three amino groups in the second coordination sphere of one catalyst play an important role in stabilizing the proton transfer (or nucleophilic addition) transition state. The two coordination spheres are found to have almost the same contributions in catalyzing the phosphate ester cleavage reaction. Also, present calculations show that the water-assisted proton transfer mechanism with the catalyst L-Zn-OH(2) is more favorable in energy than the general base pathway with the active catalyst L-Zn-OH suggested previously.

Efficient Catalytic Phosphate Ester Cleavage by Binuclear Zinc(II) Pyrazolate Complexes as Functional Models of Metallophosphatases

A series of dizinc(II) complexes based on the pyrazolate ligands 3-[(1E)-N-hydroxyethanimidoyl]-4-methyl-1H-pyrazole-5-carboxylic acid (H(3)L(1)), (1E,1'E)-1,1'-(4-methyl-1H-pyrazole-3,5-diyl)diethanone dihydrazone (HL(2)), (E,E)-(4-methyl-1H-pyrazole-3,5-diyl)bis(methylmethanone) dioxime (H(3)L(3)), (E,E)-(4-phenyl-1H-pyrazole-3,5-diyl)bis(phenylmethanone) dioxime (H(3)L(4)), and 1H-pyrazole-3,5-dicarboxylic acid (H(3)L(5)) have been synthesized and investigated as functional models of phosphoesterases, focusing on correlations between the hydrolytic activity and molecular parameters of the bimetallic core. Speciation of the various dizinc complexes in solution has been determined potentiometrically, and the structures in the solid state have been established by X-ray crystallography. The hydrolysis of two phosphoesters, an RNA model 2-hydroxypropyl-p-nitrophenyl phosphate (HPNP) and the pesticide paraoxon-ethyl (POE), promoted by the dinuclear phosphoesterase model complexes has been investigated in DMSO/buffered water (1:1) at 50 degrees C as a function of complex concentration, substrate concentration, and pH. Drastic differences in the hydrolytic activities of [Zn(2)(HL(1))(2)](0), [Zn(2)(L(2))(2)](2+), [Zn(2)(H(2)L(3))(2)](2+), and [Zn(2)(HL(5))(2)](2-) are observed and can be attributed to molecular peculiarities. Pyrazolate-bridged dinuclear zinc(II) complexes seem to provide a sufficient number of coordination sites for both activating the substrate and generating the nucleophile, where the phosphate esters are preferentially bound in a bidentate bridging fashion (in the case of HPNP) and in a monodentate fashion (in the case of POE).