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

Antimicrobial and Antiviral Nanofibers Halt Co-Infection Spread via Nuclease-Mimicry and Photocatalysis

The escalating spread of drug-resistant bacteria and viruses is a grave concern for global health. Nucleic acids dominate the drug-resistance and transmission of pathogenic microbes. Here, imidazolium-type poly(ionic liquid)/porphyrin (PIL-P) based electrospun nanofibrous membrane and its cerium (IV) ion complex (PIL-P-Ce) are developed. The obtained PIL-P-Ce membrane exhibits high and stable efficiency in eradicating various microorganisms (bacteria, fungi, and viruses) and decomposing microbial antibiotic resistance genes and viral nucleic acids under light. The nuclease-mimetic and photocatalytic mechanisms of the PIL-P-Ce are elucidated. Co-infection wound models in mice with methicillin-resistant S. aureus and hepatitis B virus demonstrate that PIL-P-Ce integrate the triple effects of cationic polymer, photocatalysis, and nuclease-mimetic activities. As revealed by proteomic analysis, PIL-P-Ce shows minimal phototoxicity to normal tissues. Hence, PIL-P-Ce has potential as a "green" wound dressing to curb the spread of drug-resistant bacteria and viruses in clinical settings.

Reactivity of metal-oxo clusters towards biomolecules: from discrete polyoxometalates to metal-organic frameworks

Metal-oxo clusters hold great potential in several fields such as catalysis, materials science, energy storage, medicine, and biotechnology. These nanoclusters of transition metals with oxygen-based ligands have also shown promising reactivity towards several classes of biomolecules, including proteins, nucleic acids, nucleotides, sugars, and lipids. This reactivity can be leveraged to address some of the most pressing challenges we face today, from fighting various diseases, such as cancer and viral infections, to the development of sustainable and environmentally friendly energy sources. For instance, metal-oxo clusters and related materials have been shown to be effective catalysts for biomass conversion into renewable fuels and platform chemicals. Furthermore, their reactivity towards biomolecules has also attracted interest in the development of inorganic drugs and bioanalytical tools. Additionally, the structural versatility of metal-oxo clusters allows for the efficiency and selectivity of the biomolecular reactions they promote to be readily tuned, thereby providing a pathway towards reaction optimization. The properties of the catalyst can also be improved through incorporation into solid supports or by linking metal-oxo clusters together to form Metal-Organic Frameworks (MOFs), which have been demonstrated to be powerful heterogeneous catalysts. Therefore, this review aims to provide a comprehensive and critical analysis of the state of the art on biomolecular transformations promoted by metal-oxo clusters and their applications, with a particular focus on structure-activity relationships.

Polyoxometalates as Potential Artificial Enzymes toward Biological Applications

Artificial enzymes, as alternatives to natural enzymes, have attracted enormous attention in the fields of catalysis, biosensing, diagnostics, and therapeutics because of their high stability and low cost. Polyoxometalates (POMs), a class of inorganic metal oxides, have recently shown great potential in mimicking enzyme activity due to their well-defined structure, tunable composition, high catalytic efficiency, and easy storage properties. This review focuses on the recent advances in POM-based artificial enzymes. Different types of POMs and their derivatives-based mimetic enzyme functions are covered, as well as the corresponding catalytic mechanisms (where available). An overview of the broad applications of representative POM-based artificial enzymes from biosensing to theragnostic is provided. Insight into the current challenges and the future directions for POMs-based artificial enzymes is discussed.

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, Characterization, and Phosphoesterase Activity of a Series of 4f- and 4d-Sandwich-Type Germanotungstates [(n-C4H9)4N]l/mH2[(M(H2O)3)(γ-GeW10O35)2] (M = CeIII, NdIII, GdIII, ErIII, l = 7; ZrIV, m = 6)

We report on a family of five new 4f- and 4d-doped sandwich-type germanotungstates with the general formula [(n-C₄H₉)₄N]_l/mH₂[(M(H₂O)₃)(γ-GeW₁₀O₃₅)₂]·3(CH₃)₂CO [M(H₂O)₃(GeW₁₀)₂] (M = Ce^III, Nd^III, Gd^III, Er^III, l = 7; Zr^IV, m = 6), which have been synthesized at room temperature in an acetone–water mixture. Among the compound series, [Zr(H₂O)₃(GeW₁₀)₂]^8–, which has been obtained in the presence of 30% H₂O₂, represents the first example of a 4d-substituted germanotungstate incorporating the intact dilacunary [γ-Ge^IVW₁₀O₃₆]^8– building block. All compounds were characterized thoroughly in the solid state by single-crystal and powder X-ray diffraction (XRD), IR spectroscopy, thermogravimetric analysis (TGA), and elemental analysis and in solution by NMR and UV–vis spectroscopy. The phosphoesterase activity of [Ce(H₂O)₃(GeW₁₀)₂]^9– and [Zr(H₂O)₃(GeW₁₀)₂]^8– toward the model substrates 4-nitrophenyl phosphate (NPP) and O,O-dimethyl O-(4-nitrophenyl) phosphate (DMNP) was monitored with ¹H- and ³¹P-NMR spectroscopy revealing an acceleration of the hydrolytic reaction by an order of magnitude (k_corr = 3.44 (±0.30) × 10^–4 min^–1 for [Ce(H₂O)₃(GeW₁₀)₂]^9– and k_corr = 5.36 (±0.05) × 10^–4 min^–1 for [Zr(H₂O)₃(GeW₁₀)₂]^8–) as compared to the uncatalyzed reaction (k_uncat = 2.60 (±0.10) × 10^–5 min^–1). [Ce(H₂O)₃(GeW₁₀)₂]^9– demonstrated improved antibacterial activity toward Moraxella catarrhalis (MIC 32 μg/mL), compared to the unsubstituted [GeW₁₀O₃₆]^8– POM (MIC 64 μg/mL).

We report on the synthesis and characterization of five new monosubstituted 4f- and 4d-germanotungstates [(n-C₄H₉)₄N]_l/mH₂[(M(H₂O)₃)(γ-GeW₁₀O₃₅)₂]·3(CH₃)₂CO [M(H₂O)₃(GeW₁₀)₂] (M = Ce^III, Nd^III, Gd^III, Er^III, l = 7; Zr^IV; m = 6). The phosphoesterase properties of [Ce(H₂O)₃(GeW₁₀)₂]⁹⁻ and [Zr(H₂O)₃(GeW₁₀)₂]⁸⁻ were investigated by probing the hydrolytic activity toward 4-nitrophenyl phosphate (NPP) and O,O-dimethyl O-(4-nitrophenyl) phosphate (DMNP). Antibacterial tests revealed inhibiting activity of [Ce(H₂O)₃(GeW₁₀)₂]⁹⁻ against Moraxella catarrhalis.

Effects of the Metal Ion on the Mechanism of Phosphodiester Hydrolysis Catalyzed by Metal-Cyclen Complexes

In this study, mechanisms of phosphodiester hydrolysis catalyzed by six di- and tetravalent metal-cyclen (M-C) complexes (Zn-C, Cu-C, Co-C, Ce-C, Zr-C and Ti-C) have been investigated using DFT calculations. The activities of these complexes were studied using three distinct mechanisms: (1) direct attack ( DA ), (2) catalyst-assisted ( CA ), and (3) water-assisted ( WA ). All divalent metal complexes (Zn-C, Cu-C and Co-C) coordinated to the BNPP substrate in a monodentate fashion and activated its scissile phosphoester bond. However, all tetravalent metal complexes (Ce-C, Zr-C, and Ti-C) interacted with BNPP in a bidentate manner and strengthened this bond. The DA mechanism was energetically the most feasible for all divalent M-C complexes, while the WA mechanism was favored by the tetravalent complexes, except Ce-C. The divalent complexes were found to be more reactive than their tetravalent counterparts. Zn-C catalyzed the hydrolysis with the lowest barrier among all M-C complexes, while Ti-C was the most reactive tetravalent complex. The activities of Ce-C and Zr-C, except Ti-C, were improved with an increase in the coordination number of the metal ion. The structural and mechanistic information provided in this study will be very helpful in the development of more efficient metal complexes for this critical reaction.

Selective Hydrolysis of Ovalbumin Promoted by Hf(IV)-Substituted Wells-Dawson-Type Polyoxometalate

The reactivity and selectivity of Wells-Dawson type polyoxometalate (POM), K₁₆[Hf(α₂-P₂W₁₇O₆₁)₂]·19H₂O (Hf1-WD2), have been examined with respect to the hydrolysis of ovalbumin (OVA), a storage protein consisting of 385 amino acids. The exact cleavage sites have been determined by Edman degradation experiments, which indicated that Hf1-WD2 POM selectively cleaved OVA at eight peptide bonds: Phe13-Asp14, Arg85-Asp86, Asn95-Asp96, Ala139-Asp140, Ser148-Trp149, Ala361-Asp362, Asp362-His363, and Pro364-Phe365. A combination of spectroscopic methods including ³¹P NMR, Circular Dichroism (CD), and Tryptophan (Trp) fluorescence spectroscopy were employed to gain better understanding of the observed selective cleavage and the underlying hydrolytic mechanism. ³¹P NMR spectra have shown that signals corresponding to Hf1-WD2 gradually broaden upon addition of OVA and completely disappear when the POM-protein molar ratio becomes 1:1, indicating formation of a large POM/protein complex. CD demonstrated that interactions of Hf1-WD2 with OVA in the solution do not result in protein unfolding or denaturation even upon adding an excess of POM. Trp fluorescence spectroscopy measurements revealed that the interaction of Hf1-WD2 with OVA (K_q = 1.1 × 10⁵ M⁻¹) is both quantitatively and qualitatively slightly weaker than the interaction of isostructural Zr-containing Wells-Dawson POM (Zr1-WD2) with human serum albumin (HAS) (K_q = 5.1 × 10⁵ M⁻¹).

Protein-Assisted Formation and Stabilization of Catalytically Active Polyoxometalate Species

The effect of the protein environment on the formation and stabilization of an elusive catalytically active polyoxometalate (POM) species, K₆ [Hf(α₂ -P₂ W₁₇ O₆₁ )] (1), is reported. In the co-crystal of hen egg-white lysozyme (HEWL) with 1, the catalytically active monomeric species is observed, originating from the dimeric 1:2 POM form, while it is intrinsically unstable under physiological pH conditions. The protein-assisted dissociation of the dimeric POM was rationalized by means of DFT calculations. The dissociation process is unfavorable in bulk water, but becomes favorable in the protein-POM complex due to the low dielectric response at the protein surface. The crystal structure shows that the monomeric form is stabilized by electrostatic and water-mediated hydrogen bonding interactions with the protein. It interacts at three distinct sites, close to the aspartate-containing hydrolysis sites, demonstrating high selectivity towards peptide bonds containing this residue.

Polyoxometalates as artificial nucleases: hydrolytic cleavage of DNA promoted by a highly negatively charged ZrIV-substituted Keggin polyanion

A highly negatively charged binuclear Zr^IV-substituted Keggin polyoxometalate [{α-PW₁₁O₃₉Zr(μ-OH)(H₂O)}₂]^8- (ZrK 2 : 2) has been shown to promote the hydrolytic cleavage of phosphoester bonds in the supercoiled plasmid pUC19 DNA under physiological pH and temperature, giving relaxed and linear forms of pUC19 as hydrolysis products. The interaction between ZrK 2 : 2 and DNA was experimentally proven by circular dichroism (CD) spectroscopy and ³¹P diffusion ordered NMR spectroscopy.

Spectroscopic Study of the Interaction between Horse Heart Myoglobin and Zirconium(IV)-Substituted Polyoxometalates as Artificial Proteases

A recent study [Angew. Chem. Int. Ed. 2015, 54, 7391-7394] has shown that horse heart myoglobin (HHM) is selectively hydrolyzed by a range of zirconium(IV)-substituted polyoxometalates (POMs) under mild conditions. In this study, the molecular interactions between the Zr-POM catalysts and HHM are investigated by using a range of complementary techniques, including circular dichroism (CD), UV/Vis spectroscopy, tryptophan fluorescence spectroscopy, and ¹ H and ³¹ P NMR spectroscopy. A tryptophan fluorescence quenching study reveals that, among all examined Zr-POMs, the most reactive POM, 2:2 Zr^IV -Keggin, exhibits the strongest interaction with HHM. ³¹ P NMR spectroscopy studies show that this POM dissociates in solution, resulting in the formation of a monomeric 1:1 Zr^IV -Keggin structure, which is likely to be a catalytically active species. In the presence of Zr^IV -POMs, HHM does not undergo complete denaturation, as evidenced by CD, UV/Vis, tryptophan fluorescence, and ¹ H NMR spectroscopy. CD spectroscopy shows a gradual decrease in the α-helical content of HHM upon addition of Zr^IV -POMs. The largest effect is observed in the presence of a large Zr^IV -Wells-Dawson structure, whereas small Zr^IV -Lindqvist POM has the least influence on the decrease in the α-helical content of HHM. In all cases, the Soret band at λ=409 nm is maintained in the presence of all examined Zr-POMs, which indicates that no conformational changes in the protein occur near the heme group.

Discovery and isolation of the trans-isomers of two 1 : 2-type lanthanide-containing monolacunary Dawson-type tungstophosphates: [LnIII(α2-P2W17O61)2]17− (Ln = La, Ce)

Two novel trans-isomers of 1 : 2-type lanthanide-containing monolacunary Dawson-type tungstophosphates [Ln(α₂-P₂W₁₇O₆₁)₂]¹⁷⁻ (Ln = La^III (1), Ce^III (2)) were successfully isolated and characterized.