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.

K6[P2Mo18O62] as DNase-Mimetic Artificial Nucleases to Promote Extracellular Deoxyribonucleic Acid Degradation in Bacterial Biofilms

In the current study, the DNase-like activity of the Dawson-type polyoxometalate K6[P2Mo18O62] was explored. The obtained findings demonstrated that K6[P2Mo18O62] could effectively cleave phosphoester bonds in the DNA model substrate (4-nitrophenyl phosphate) and result in the degradation of plasmid DNA. Moreover, the application potential of this Dawson-type polyoxometalate as a DNase-mimetic artificial enzyme to degrade extracellular DNA (eDNA) in Escherichia coli (E. coli) bacterial biofilm was explored. The results demonstrated that K6[P2Mo18O62] exhibited high cleavage ability toward eDNA secreted by E. coli and thus eradicated the bacterial biofilm. In conclusion, Dawson-type polyoxometalate K6[P2Mo18O62] possessed desirable DNase-like activity, which could serve as a bacterial biofilm eradication agent by cleaving and degrading eDNA molecules.

Polyoxometalate K6[P2Mo18O62] Inactivates Escherichia coli O157:H7 by Inducing recA Expression and Apoptosis-like Bacterial Death

Polyoxometalates have emerged as promising bactericidal agents. In the current study, the bactericidal activity of polyoxometalate K₆[P₂Mo₁₈O₆₂] against Escherichia coli (E. coli) O157:H7 and its possible underlying mechanisms were explored. The obtained results demonstrated that K₆[P₂Mo₁₈O₆₂] could effectively kill E. coli O157:H7 at millimolar levels. Moreover, K₆[P₂Mo₁₈O₆₂] treatment also induced significant increases in recA protein expression and further triggered characteristic apoptosis-like bacterial death events such as DNA fragmentation and phosphatidylserine exposure. In conclusion, polyoxometalate K₆[P₂Mo₁₈O₆₂] possesses a desirable antibacterial activity, and induction of bacterial apoptosis-like death might be involved in its underlying bactericidal mechanisms.

Trisubstituted 4f- and 4d tungstoantimonates as artificial phosphoesterases for nerve agent degradation

Three new trisubstituted 4f- and 4d tungstoantimonates (TA) K3Na21[(M(CH3COO))3(HPO3)(WO4)(SbW9O33)3]·nH2O {M3(HPO3)Sb3W28} (M = GdIII, YIII, YbIII, n = 35-36) were synthesized using a double-template synthetic approach. Following their characterization in the solid state employing single- and powder X-ray diffraction (XRD), IR-spectroscopy, and elemental - and thermogravimetric analyses (TGA), {M3(HPO3)Sb3W28} were subjected to a comprehensive set of solution characterization methods including UV/vis- and multinuclear 31P and 13C NMR spectroscopy. All representatives were shown to be highly active, recyclable, and stable Lewis-acid catalysts towards the nerve agent simulant O,O-dimethyl O-(4-nitrophenyl) phosphate (DMNP) at neutral pH (in Tris-HCl [125 mM] at pD 7.0 25 °C). Control experiments showing catalytic activity of the unsubstituted trilacunary TA [SbW9O33]9- suggest the non-innocence of Tris in the DMNP hydrolysis for the first time.

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.

Rapid Hydrolysis of Organophosphates Induced by U(IV) Nanoparticles: A Kinetic and Mechanistic Study using Spectroscopic Analysis

The heterogeneous interactions of colloidal U particles with organophosphates, leading to the formation of U-phosphate minerals, can retard the migration of U in contaminated sites. Here, we studied the hydrolytic mechanism of p-nitrophenyl phosphate (NPP) on the surfaces of tetravalent uranium nanoparticles (U(IV)NPs), resulting in the formation of U-phosphate precipitates. Our study shows that the reaction rate of NPP hydrolysis is significantly enhanced by U(IV)NPs through a multi-step heterogeneous reaction on the particle surfaces. The end products of the reaction were identified as U(IV)NPs-aggregates with surface-bound phosphates. Colloidal properties, such as high positive values of the zeta-potential (>+30 mV) and large surface areas of U(IV)NPs due to their unique cluster structures consisting of relatively small primary UO2(cr)-particles, are correlated with their reactivity towards hydrolysis reaction. Reaction kinetic modeling studies using spectrophotometric data indicated the presence of two distinct reaction intermediates as the surface complexes of NPP on U(IV)NPs. We suggest the involvement of the NPP inner-sphere complexes in the rate-determining step based on the results obtained by analyzing the ATR-FTIR spectra and the surface-enhanced infrared absorption of NPP bound to substrate surfaces.

Cleaving DNA-model phosphodiester with Lewis acid-base catalytic sites in bifunctional Zr-MOFs

Organophosphates exist in many biomolecules. The design of artificial nucleases for efficient P-O bond cleavage is essential for the fields of genetic engineering and molecular biology. Herein, metal-organic frameworks (MOFs) with cooperatively isolated multi-catalytic active sites were utilized as heterogeneous catalysts for the hydrolytic cleavage of bis(p-nitrophenyl) phosphate (BNPP).

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).

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.

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 survey of the different roles of polyoxometalates in their interaction with amino acids, peptides and proteins

This perspective provides a comprehensive description of the different roles of POMs in their interaction with relevant biological molecules.