Phosphate Ester Hydrolysis by Hydroxo Complexes of Trivalent Lanthanides Stabilized by 4-Imidazolecarboxylate

Bioinspired Framework Catalysts: From Enzyme Immobilization to Biomimetic Catalysis

Enzymatic catalysis has fueled considerable interest from chemists due to its high efficiency and selectivity. However, the structural complexity and vulnerability hamper the application potentials of enzymes. Driven by the practical demand for chemical conversion, there is a long-sought quest for bioinspired catalysts reproducing and even surpassing the functions of natural enzymes. As nanoporous materials with high surface areas and crystallinity, metal-organic frameworks (MOFs) represent an exquisite case of how natural enzymes and their active sites are integrated into porous solids, affording bioinspired heterogeneous catalysts with superior stability and customizable structures. In this review, we comprehensively summarize the advances of bioinspired MOFs for catalysis, discuss the design principle of various MOF-based catalysts, such as MOF-enzyme composites and MOFs embedded with active sites, and explore the utility of these catalysts in different reactions. The advantages of MOFs as enzyme mimetics are also highlighted, including confinement, templating effects, and functionality, in comparison with homogeneous supramolecular catalysts. A perspective is provided to discuss potential solutions addressing current challenges in MOF catalysis.

Transesterification of RNA model induced by novel dinuclear copper (II) complexes with bis-tridentate imidazole derivatives

Two novel bis-tridentate imidazole derivatives were conveniently synthesized using a 'one-pot' method. Their dinuclear (Cu₂L¹Cl₄, Cu₂L²Cl₄) and mononuclear (CuL¹Cl₂, CuL²Cl₂∙H₂O) copper (II) complexes were synthesized to comparably evaluate their reactivities in the hydrolytic cleavage of 2-hydroxypropyl p-nitrophenyl phosphate (HPNP) as a classic RNA model. Single crystals of Cu₂L¹Cl₄ and Cu₂L²Cl₄ indicate that both of them are centrosymmetric, and each central copper ion is penta-coordinated. Regarding the transesterification of HPNP, both of dinuclear ones exhibited excess one order of magnitude rate enhancement in contrast with auto-hydrolysis reaction. Under comparable conditions, dinuclear complexes displayed no more than twofold increase in activity over their mononuclear analogues, which verifies the lack of binuclear cooperation effect due to long Cu-to-Cu space.

Mechanistic Insights into Promoted Hydrolysis of Phosphoester Bonds by MoO2Cl2(DMF)2

A phosphoester bond is a crucial structural block in biological systems, whose occurrence is regulated by phosphatases. Molybdenum compounds have been reported to be active in phosphate ester hydrolysis of model phosphates. Specifically, MoO₂Cl₂(DMF)₂ is active in the hydrolysis of para-nitrophenyl phosphate (pNPP), leading to heteropolyoxometalate structures. We use density functional theory (DFT) to clarify the mechanism by which these species promote the hydrolysis of the phosphoester bond. The present calculations give insight into several key aspects of this reaction: (i) the speciation of this complex prior to interaction with the phosphate (DMF release, Mo-Cl hydrolysis, and pH influence on the speciation), (ii) the competition between phosphate addition and the molybdate nucleation process, (iii) and the mechanisms by which some plausible active species promote this hydrolysis in different conditions. We described thoroughly two different pathways depending on the nucleation possibilities of the molybdenum complex: one mononuclear mechanism, which is preferred in conditions in which very low complex concentrations are used, and another dinuclear mechanism, which is preferred at higher concentrations.

Binuclear metal complexes with a novel hexadentate imidazole derivative for the cleavage of phosphate diesters and biomolecules: distinguishable mechanisms

Oxidative cleavage of phosphate diesters (HPNP, BNPP) is highly faster than the hydrolytic one by binuclear metal complexes with novel imidazole derivative, producing a non-lactone phosphate monoester due to the direct attack of free radicals.

A dual-function all-inorganic intercluster salt comprising the polycation ε-[Al13O4(OH)24(H2O)12]7+ and polyanion α-[PMo10V2O40]5- for detoxifying sulfur mustard and soman

ε-[Al13O4(OH)24(H2O)12]7+, which shares similarity with the phosphotriesterase active site ZnII-OH-ZnII, was specially chosen to interact with the cluster α-PMo10V2O405- to form a new three-dimensional intercluster, which crystallized in the monoclinic space group P21/m with Z = 2, for the decontamination of chemical warfare agents. The experimental results showed that 50 mg of the compound decontaminated 96.4% (within 120 min) and 99.5% (within 40 min) of sulfur mustard (HD) (4 μL) and soman (GD) (4 μL), respectively, in ambient conditions. The decontamination processes followed first-order reaction kinetics with a rate constant and half-life of 0.01234 min-1 and 56.15 min for HD and 0.1198 min-1 and 5.78 min for GD, respectively. It was concluded that the α-PMo10V2O405- moiety was responsible for the catalytic oxidation of HD into non-toxic sulfoxide, while the ε-[Al13O4(OH)24(H2O)12]7+ moiety was responsible for the catalytic hydrolysis of HD and GD into nontoxic hydrolysates. Besides, the compound showed notable efficacy for the decontamination of HD on guinea pig skin and of GD on Kunming mouse skin, indicating high potential for use in human skin protection and treatment.

Organophosphorus hydrolase-poly-β-cyclodextrin as a stable self-decontaminating bio-catalytic material for sorption and degradation of organophosphate pesticide

A region suffering from an attack of a nerve agent requires not only a highly sorptive material but also a fast-acting catalyst to decontaminate the lethal chemical present. The product should be capable of high sorptive capacity, selectivity and quick response time to neutralize the long lasting harmful effects of nerve agents. Herein, we have utilized organophosphorus hydrolase (OPH) as a non-toxic bio-catalytic material held in with the supporting matrix of poly-β-cyclodextrin (PCD) as a novel sorptive reinforced self-decontaminating material against organophosphate intoxication. OPH coated PCD (OPH-PCD) will not only be providing support for holding enzyme but also would be adsorbing methyl paraoxon (MPO) used as a simulant, in a host-guest inclusion complex formation. Sorption trend for PCD revealed preference towards the more hydrophobic MPO against para-nitrophenol (pNP). The results show sorption capacity of 1.26 mg/g of 100 μM MPO with PCD which was 1.7 times higher compared to pNP. The reaction rate with immobilized OPH-PCD was found to be 23% less compared to free enzyme. With the help of OPH-PCD, continuous hydrolysis (100%) of MPO into pNP was observed for a period of 24 h through packed bed reactor with good reproducibility and stability of enzyme. The long-term stability also confirmed its stable nature for the investigation period of 4 days where it maintained activity. Combined with its fast and reactive nature, the resulting self-decontaminating regenerating material provides a promising strategy for the neutralization of nerve agents and preserving the environment.

Catalytic Phosphite Hydrolysis under Neutral Reaction Conditions

Cationic phosphametallocene-based platinum(II) aqua complexes were used as efficient precatalysts for the hydrolysis of aromatic and aliphatic tertiary phosphites under neutral reaction conditions at room temperature, leading to the selective cleavage of one P-O bond of the phosphite. NMR labeling experiments combined with stoichiometric model reactions and theoretical density functional theory calculations, performed with the appropriate model compounds, shed light on the operative catalytic cycle, which comprises intramolecular water molecule transfer to the cis-coordinated phosphite molecule.

Lanthanide separation using size-selective crystallization of Ln-MOFs

We report herein an elaborate method, size-selective crystallization of Ln-MOFs, to isolate lanthanide (Ln) ions.

Phosphate Ester Bond Hydrolysis Promoted by Lanthanide-Substituted Keggin-type Polyoxometalates Studied by a Combined Experimental and Density Functional Theory Approach

Hydrolytic cleavage of 4-nitrophenyl phosphate (NPP), a commonly used DNA model substrate, was examined in the presence of series of lanthanide-substituted Keggin-type polyoxometalates (POMs) [Me₂NH₂]₁₁[Ce^III(PW₁₁O₃₉)₂], [Me₂NH₂]₁₀[Ce^IV(PW₁₁O₃₉)₂] (abbreviated as (Ce^IV(PW₁₁)₂), and K₄[EuPW₁₁O₃₉] by means of NMR and luminescence spectroscopies and density functional theory (DFT) calculations. Among the examined complexes, the Ce(IV)-substituted Keggin POM (Ce^IV(PW₁₁)₂) showed the highest reactivity, and its aqueous speciation was fully determined under different conditions of pD, temperature, concentration, and ionic strength by means of ³¹P and ³¹P diffusion-ordered NMR spectroscopy. The cleavage of the phosphoester bond of NPP in the presence of (Ce^IV(PW₁₁)₂) proceeded with an observed rate constant k_obs = (5.31 ± 0.06) × 10^-6 s^-1 at pD 6.4 and 50 °C. The pD dependence of NPP hydrolysis exhibits a bell-shaped profile, with the fastest rate observed at pD 6.4. The formation constant (K_f = 127 M^-1) and catalytic rate constant (k_c = 19.41 × 10^-5 s^-1) for the NPP-Ce(IV)-Keggin POM complex were calculated, and binding between Ce^IV(PW₁₁)₂ and the phosphate group of NPP was also evidenced by the change of the chemical shift of the ³¹P nucleus in NPP upon addition of the POM complex. DFT calculations revealed that binding of NPP to the parent catalyst Ce^IV(PW₁₁)₂ is thermodynamically unlikely. On the contrary, formation of complexes with the monomeric 1:1 species, Ce^IVPW₁₁, is considered to be more favorable, and the most stable complex, [Ce^IVPW₁₁(H₂O)₂(NPP-κO)₂]^7-, was found to involve two NPP ligands coordinated to the Ce^IVcenter of Ce^IVPW₁₁ in the monodentate fashion. The formation of such species is considered to be responsible for the hydrolytic activity of Ce^IV(PW₁₁)₂ toward phosphomonoesters. On the basis of these findings a principle mechanism for the hydrolysis of NPP by the POM is proposed.

Dual-Function Metal-Organic Framework as a Versatile Catalyst for Detoxifying Chemical Warfare Agent Simulants

The nanocrystals of a porphyrin-based zirconium(IV) metal-organic framework (MOF) are used as a dual-function catalyst for the simultaneous detoxification of two chemical warfare agent simulants at room temperature. Simulants of nerve agent (such as GD, VX) and mustard gas, dimethyl 4-nitrophenyl phosphate and 2-chloroethyl ethyl sulfide, have been hydrolyzed and oxidized, respectively, to nontoxic products via a pair of pathways catalyzed by the same MOF. Phosphotriesterase-like activity of the Zr6-containing node combined with photoactivity of the porphyrin linker gives rise to a versatile MOF catalyst. In addition, bringing the MOF crystals down to the nanoregime leads to acceleration of the catalysis.

Exploiting parameter space in MOFs: a 20-fold enhancement of phosphate-ester hydrolysis with UiO-66-NH2

The hydrolysis of nerve agents is of primary concern due to the severe toxicity of these agents. Using a MOF-based catalyst (UiO-66), we have previously demonstrated that the hydrolysis can occur with relatively fast half-lives of 50 minutes. However, these rates are still prohibitively slow to be efficiently utilized for some practical applications (e.g., decontamination wipes used to clean exposed clothing/skin/vehicles). We thus turned our attention to derivatives of UiO-66 in order to probe the importance of functional groups on the hydrolysis rate. Three UiO-66 derivatives were explored; UiO-66-NO2 and UiO-66-(OH)2 showed little to no change in hydrolysis rate. However, UiO-66-NH2 showed a 20 fold increase in hydrolysis rate over the parent UiO-66 MOF. Half-lives of 1 minute were observed with this MOF. In order to probe the role of the amino moiety, we turned our attention to UiO-67, UiO-67-NMe2 and UiO-67-NH2. In these MOFs, the amino moiety is in close proximity to the zirconium node. We observed that UiO-67-NH2 is a faster catalyst than UiO-67 and UiO-67-NMe2. We conclude that the role of the amino moiety is to act as a proton-transfer agent during the catalytic cycle and not to hydrogen bond or to form a phosphorane intermediate.

Pelopuradazole, a new imidazole derivative alkaloid from the marine bacteria Pelomonas puraquae sp. nov

One new imidazole derivative alkaloid pelopuradazole (1), together with three known alkaloids as in 3H-imidazole-4-carboxylic acid (2), 1H-pyrrole-2-carboxylic acid (3) and 2-methyl-3H-imidazole-4-carboxylic acid (4) and two known cyclo-dipeptides pelopurin A (5) and pelopurin B (6), has been isolated from the marine bacterium Pelomonas puraquae sp. nov. Pelopuradazole (1) was a new imidazole derivative alkaloid, while compounds 2, 3, 5 and 6 were firstly obtained as natural products. Compounds 1-6 were isolated from P. puraquae sp. nov. for the first time.

Simple and compelling biomimetic metal-organic framework catalyst for the degradation of nerve agent simulants

Inspired by biology, in which a bimetallic hydroxide-bridged zinc(II)-containing enzyme is utilized to catalytically hydrolyze phosphate ester bonds, the utility of a zirconium(IV)-cluster-containing metal-organic framework as a catalyst for the methanolysis and hydrolysis of phosphate-based nerve agent simulants was examined. The combination of the strong Lewis-acidic Zr(IV) and bridging hydroxide anions led to ultrafast half-lives for these solvolysis reactions. This is especially remarkable considering that the actual catalyst loading was a mere 0.045 % as a result of the surface-only catalysis observed.

Why nature really chose phosphate

Phosphoryl transfer plays key roles in signaling, energy transduction, protein synthesis, and maintaining the integrity of the genetic material. On the surface, it would appear to be a simple nucleophile displacement reaction. However, this simplicity is deceptive, as, even in aqueous solution, the low-lying d-orbitals on the phosphorus atom allow for eight distinct mechanistic possibilities, before even introducing the complexities of the enzyme catalyzed reactions. To further complicate matters, while powerful, traditional experimental techniques such as the use of linear free-energy relationships (LFER) or measuring isotope effects cannot make unique distinctions between different potential mechanisms. A quarter of a century has passed since Westheimer wrote his seminal review, 'Why Nature Chose Phosphate' (Science 235 (1987), 1173), and a lot has changed in the field since then. The present review revisits this biologically crucial issue, exploring both relevant enzymatic systems as well as the corresponding chemistry in aqueous solution, and demonstrating that the only way key questions in this field are likely to be resolved is through careful theoretical studies (which of course should be able to reproduce all relevant experimental data). Finally, we demonstrate that the reason that nature really chose phosphate is due to interplay between two counteracting effects: on the one hand, phosphates are negatively charged and the resulting charge-charge repulsion with the attacking nucleophile contributes to the very high barrier for hydrolysis, making phosphate esters among the most inert compounds known. However, biology is not only about reducing the barrier to unfavorable chemical reactions. That is, the same charge-charge repulsion that makes phosphate ester hydrolysis so unfavorable also makes it possible to regulate, by exploiting the electrostatics. This means that phosphate ester hydrolysis can not only be turned on, but also be turned off, by fine tuning the electrostatic environment and the present review demonstrates numerous examples where this is the case. Without this capacity for regulation, it would be impossible to have for instance a signaling or metabolic cascade, where the action of each participant is determined by the fine-tuned activity of the previous piece in the production line. This makes phosphate esters the ideal compounds to facilitate life as we know it.

Surface modification of gadolinium oxide thin films and nanoparticles using poly(ethylene glycol)-phosphate

The performance of nanomaterials for biomedical applications is highly dependent on the nature and the quality of surface coatings. In particular, the development of functionalized nanoparticles for magnetic resonance imaging (MRI) requires the grafting of hydrophilic, nonimmunogenic, and biocompatible polymers such as poly(ethylene glycol) (PEG). Attached at the surface of nanoparticles, this polymer enhances the steric repulsion and therefore the stability of the colloids. In this study, phosphate molecules were used as an alternative to silanes or carboxylic acids, to graft PEG at the surface of ultrasmall gadolinium oxide nanoparticles (US-Gd(2)O(3), 2-3 nm diameter). This emerging, high-sensitivity "positive" contrast agent is used for signal enhancement in T(1)-weighted molecular and cellular MRI. Comparative grafting assays were performed on Gd(2)O(3) thin films, which demonstrated the strong reaction of phosphate with Gd(2)O(3) compared to silane and carboxyl groups. Therefore, PEG-phosphate was preferentially used to coat US-Gd(2)O(3) nanoparticles. The grafting of this polymer on the particles was confirmed by XPS and FTIR. These analyses also demonstrated the strong attachment of PEG-phosphate at the surface of Gd(2)O(3), forming a protective layer on the nanoparticles. The stability in aqueous solution, the relaxometric properties, and the MRI signal of PEG-phosphate-covered Gd(2)O(3) particles were also better than those from non-PEGylated nanoparticles. As a result, reacting PEG-phosphate with Gd(2)O(3) particles is a promising, rapid, one-step procedure to PEGylate US-Gd(2)O(3) nanoparticles, an emerging "positive" contrast agent for preclinical molecular and cellular applications.

The effect of leaving group on mechanistic preference in phosphate monoester hydrolysis

We present 2-dimensional potential energy surfaces and optimised transition states (TS) for water attack on a series of substituted phosphate monoester monoanions at the DFT level of theory, comparing a standard 6-31++g(d,p) basis set with a larger triple-zeta (augmented cc-pVTZ) basis set. Small fluorinated model compounds are used to simulate increasing leaving group stability without adding further geometrical complexity to the system. We demonstrate that whilst changing the leaving group causes little qualitative change in the potential energy surfaces (with the exception of the system with the most electron withdrawing leaving group, CF(3)O(-), in which the associative pathway changes from a stepwise A(N) + D(N) pathway to a concerted A(N)D(N) pathway), there is a quantitative change in relative gas-phase and solution barriers for the two competing pathways. In line with previous studies, in the case of OCH(3), the barriers for the associative and dissociative pathways are similar in solution, and the two pathways are equally viable and indistinguishable in solution. However, significantly increasing the stability of the leaving group (decreasing proton affinity, PA) results in the progressive favouring of a stepwise dissociative, D(N) + A(N), mechanism over associative mechanisms.

Enantiomeric self-recognition in homo- and heterodinuclear macrocyclic lanthanide(III) complexes

The controlled formation of lanthanide(III) dinuclear μ-hydroxo-bridged [Ln(2)L(2)(μ-OH)(2)X(2)](n+) complexes (where X = H(2)O, NO(3)(-), or Cl(-)) of the enantiopure chiral macrocycle L is reported. The (1)H and (13)C NMR resonances of these complexes have been assigned on the basis of COSY, NOESY, TOCSY, and HMQC spectra. The observed NOE connectivities confirm that the dimeric solid-state structure is retained in solution. The enantiomeric nature of the obtained chiral complexes and binding of hydroxide anions are reflected in their CD spectra. The formation of the dimeric complexes is accompanied by a complete enantiomeric self-recognition of the chiral macrocyclic units. The reaction of NaOH with a mixture of two different mononuclear lanthanide(III) complexes, [Ln(1)L](3+) and [Ln(2)L](3+), results in formation of the heterodinuclear [Ln(1)Ln(2)L(2)(μ-OH)(2)X(2)](n+) complexes as well as the corresponding homodinuclear complexes. The formation of the heterodinuclear complex is directly confirmed by the NOESY spectra of [EuLuL(2)(μ-OH)(2)(H(2)O)(2)](4+), which reveal close contacts between the macrocyclic unit containing the Eu(III) ion and the macrocyclic unit containing the Lu(III) ion. While the relative amounts of homo- and heterodinuclear complexes are statistical for the two lanthanide(III) ions of similar radii, a clear preference for the formation of heterodinuclear species is observed when the two mononuclear complexes contain lanthanide(III) ions of markedly different sizes, e.g., La(III) and Yb(III). The formation of heterodinuclear complexes is accompanied by the self-sorting of the chiral macrocyclic units based on their chirality. The reactions of NaOH with a pair of homochiral or racemic mononuclear complexes, [Ln(1)L(RRRR)](3+)/[Ln(2)L(RRRR)](3+), [Ln(1)L(SSSS)](3+)/[Ln(2)L(SSSS)](3+), or [Ln(1)L(rac)](3+)/[Ln(2)L(rac)](3+), results in mixtures of homochiral, homodinuclear and homochiral, heterodinuclear complexes. On the contrary, no heterochiral, heterodinuclear complexes [Ln(1)L(RRRR)Ln(2)L(SSSS)(μ-OH)(2)X(2)](n+) are formed in the reactions of two different mononuclear complexes of opposite chirality.

Mononuclear and dinuclear mechanisms for catalysis of phosphodiester cleavage by alkaline earth metal ions in aqueous solution

In biological systems, enzymes often use metal ions, especially Mg(2+), to catalyze phosphodiesterolysis, and model aqueous studies represent an important avenue of examining the contributions of these ions to catalysis. We have examined Mg(2+) and Ca(2+) catalyzed hydrolysis of the model phosphodiester thymidine-5'-p-nitrophenyl phosphate (T5PNP). At 25 degrees C, we find that, despite their different Lewis acidities, these ions have similar catalytic ability with second-order rate constants for attack of T5PNP by hydroxide (k(OH)) of 4.1x10(-4)M(-1)s(-1) and 3.7x10(-4)M(-1)s(-1) in the presence of 0.30M Mg(2+) and Ca(2+), respectively, compared to 8.3x10(-7)M(-1)s(-1) in the absence of divalent metal ion. Examining the dependence of k(OH) on [M(2+)] at 50 degrees C indicates different kinetic mechanisms with Mg(2+) utilizing a single ion mechanism and Ca(2+) operating by parallel single and double ion mechanisms. Association of the metal ion(s) occurs prior to nucleophilic attack by hydroxide. Comparing the k(OH) values reveals a single Mg(2+) catalyzes the reaction by 1800-fold whereas a single Ca(2+) ion catalyzes the reaction by only 90-fold. The second Ca(2+) provides an additional 10-fold catalysis, significantly reducing the catalytic disparity between Mg(2+) and Ca(2+).

Are mixed explicit/implicit solvation models reliable for studying phosphate hydrolysis? A comparative study of continuum, explicit and mixed solvation models

Phosphate hydrolysis is ubiquitous in biology. However, despite intensive research on this class of reactions, the precise nature of the reaction mechanism remains controversial. Herein, we have examined the hydrolysis of three homologous phosphate diesters. The solvation free energy was simulated by means of either an implicit solvation model (COSMO), hybrid quantum mechanical/molecular mechanical free energy perturbation (QM/MM-FEP) or a mixed solvation model in which N water molecules were explicitly included in the ab initio description of the reacting system (where N=1-3), with the remainder of the solvent being implicitly modelled as a continuum. Here, both COSMO and QM/MM-FEP reproduce DeltaG(obs) within an error of about 1 kcal mol(-1). However, we demonstrate that in order to obtain any kind of reliable results from a mixed model, it is essential to carefully select the explicit water molecules from short QM/MM runs that act as a model for the true infinite system. Additionally, the mixed models tend to be increasingly unstable and miss larger entropic contributions as more explicit water molecules are placed into the system. Thus, our analysis indicates that this approach provides an unreliable way for modelling phosphate hydrolysis in solution.

Synthesis and luminescence properties of a trinucleotide-europium(III) complex conjugate

Two trinucleotide conjugates of the macrocyclic ligand 1,4,7-tris(carbamoylmethyl)-1,4,7,10-tetraazacyclododecane are prepared. One contains only DNA (1) and the second is a chimeric RNA/DNA conjugate (2). The synthetic methodology used to prepare the trinucleotide macrocyclic ligand conjugates is based on the introduction of a convertible nucleoside which has an electrophilic function to facilitate the attachment of any nucleophilic ligand to the 5-position of the 3-nucleoside unit. The convertible nucleoside is first treated with the macrocyclic ligand, 1,4,7,10-tetraazacyclododecane, followed by alkylation of the three remaining amine groups to give a conjugated macrocyclic ligand with three pendent amide groups. Addition of an equivalent of EuCl3 to trinucleotide (1) or (2) yields the complexes Eu(1) and Eu(2), respectively. Studies using time-resolved and steady state direct excitation luminescence spectroscopy show that Eu(III) binds to the macrocyclic moiety in 1 and in 2. The excitation peak frequency for the 7Fo5Do transition and the unexpectedly low number of water ligands in Eu(1) and Eu(2) are consistent with additional interactions of the Eu(III) macrocycle with one of the phosphate diester groups. Studies show that Eu(2) undergoes cleavage at the uridine nucleotide. The unique point of attachment of the macrocyclic complex will enable the preparation of new lanthanide nucleic acid conjugates with useful properties.