Molecular interactions between serum albumin proteins and Keggin type polyoxometalates studied using luminescence spectroscopy

Redox-active polyoxovanadates as cofactors in the development of functional protein assemblies

The interactions of polyoxovanadates (POVs) with proteins have increasingly attracted interest in recent years due to their potential biomedical applications. This is especially the case because of their redox and catalytic properties, which make them interesting for developing artificial metalloenzymes. Organic-inorganic hybrid hexavanadates in particular offer several advantages over all-inorganic POVs. However, they have been scarcely investigated in biological systems even though, as shown in this work, hybrid hexavanadates are highly stable in aqueous solutions up to relatively high pH. Therefore, a novel bis-biotinylated hexavanadate was synthesized and shown to selectively interact with two biotin-binding proteins, avidin and streptavidin. Bridging interactions between multiple proteins led to their self-assembly into supramolecular bio-inorganic hybrid systems that have potential as artificial enzymes with the hexavanadate core as a redox-active cofactor. Moreover, the structure and charge of the hexavanadate core were determined to enhance the binding affinity and slightly alter the secondary structure of the proteins, which affected the size and speed of formation of the assemblies. Hence, tuning the polyoxometalate (POM) core of hybrid POMs (HPOMs) with protein-binding ligands has been demonstrated to be a potential strategy for controlling the self-assembly process while also enabling the formation of novel POM-based biomaterials that could be of interest in biomedicine.

Supramolecular Self-Assembly of Proteins Promoted by Hybrid Polyoxometalates

Controlling the formation of supramolecular protein assemblies and endowing them with new properties that can lead to novel functional materials is an important but challenging task. In this work, a new hybrid polyoxometalate is designed to induce controlled intermolecular bridging between biotin-binding proteins. Such bridging interactions lead to the formation of supramolecular protein assemblies incorporating metal-oxo clusters that go from several nanometers in diameter up to the micron range. Insights into the self-assembly process and the nature of the resulting biohybrid materials are obtained by a combination of Small Angle X-ray Scattering (SAXS), Transmission Electron Microscopy (TEM), and Dynamic Light Scattering (DLS), along with fluorescence, UV-vis, and Circular Dichroism (CD) spectroscopy. The formation of hybrid supramolecular assemblies is determined to be driven by biotin binding to the protein and electrostatic interactions between the anionic metal-oxo cluster and the protein, both of which also influence the stability of the resulting assemblies. As a result, the rate of formation, size, and stability of the supramolecular assemblies can be tuned by controlling the electrostatic interactions between the cluster and the protein (e.g., through varying the ionic strength of the solution), thereby paving the way toward biomaterials with tunable assembly and disassembly properties.

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.

Modular co-assembly of peptides and polyoxometalates into underwater adhesives with photoluminescence and adjustable adhesion

Supramolecular polymerization between cationic peptides and anionic polyoxometalates has emerged as a promising strategy for the creation of peptide-based biomimetic underwater adhesives. However, the extremely rigorous requirements for peptide design are an important obstacle to the fabrication of available peptide adhesives with controlled adhesion and versatile functionality. Inspired by marine sessile organisms in nature, here we reported a modular co-assembly method to easily produce peptide/polyoxometalate underwater adhesive materials through mixing two complementary cationic peptides (Pep1 and Pep2) with a single anionic polyoxometalate K6H[SiW9V3O40] in aqueous solution, which are not possible to be obtained from an individual peptide module. We demonstrated that the relatively hydrophobic Pep1 contributes to the bulk cohesion of the resulting adhesive, while the relatively hydrophilic Pep2 not only enables the interfacial adhesion but also regulates the bulk cohesion of the Pep1/Pep2/SiW9V3 adhesive. Rheological and shear adhesion tests showed that the macroscopic adhesion performance of the resulting adhesive materials could be conveniently adjusted by simply changing the molar ratio of the complementary peptide modules without any complicated peptide design. Interestingly, the luminescence properties of K11[Eu(PW11O39)2] (labelled as EuPW11) could be maintained within the Pep1/Pep2/EuPW11 adhesive even in a water environment. The lifetime of the Pep1/Pep2/EuPW11 adhesive was 2.19 ms. The fluorescence quantum yield of the Pep1/Pep2/EuPW11 adhesive was measured to be 27.46%. This study unveils that the modular co-assembly method can effectively simplify the material design of peptide/polyoxometalate underwater adhesives, which will significantly broaden the horizon of material pools and extend their availability space.

Fine-tuning non-covalent interactions between hybrid metal-oxo clusters and proteins

Interactions between the protein Hen Egg White Lysozyme (HEWL) and three different hybrid Anderson-Evans polyoxometalate clusters - AE-NH2 (δ-[MnMo6O18{(OCH2)3CNH2}2]3-), AE-CH3 (δ-[MnMo6O18{(OCH2)3CCH3}2]3-) and AE-Biot (δ-[MnMo6O18{(OCH2)3CNHCOC9H15N2OS}2]3-) - were studied via tryptophan fluorescence spectroscopy and single crystal X-ray diffraction. Quenching of tryptophan fluorescence was observed in the presence of all three hybrid polyoxometalate clusters (HPOMs), but the extent of quenching and the binding affinity were greatly dependent on the nature of the organic groups attached to the cluster. Control experiments further revealed the synergistic effect of the anionic polyoxometalate core and organic ligands towards enhanced protein interactions. Furthermore, the protein was co-crystallised with each of the three HPOMs, resulting in four different crystal structures, thus allowing for the binding modes of HPOM-protein interactions to be investigated with near-atomic precision. All crystal structures displayed a unique mode of binding of the HPOMs to the protein, with both functionalisation and the pH of the crystallisation conditions influencing the interactions. From the crystal structures, it was determined that HPOM-protein non-covalent complexes formed through a combination of electrostatic attraction between the polyoxometalate cluster and positively charged surface regions of HEWL, and direct and water-mediated hydrogen bonds with both the metal-oxo inorganic core and the functional groups of the ligand, where possible. Hence, functionalisation of metal-oxo clusters shows great potential in tuning their interactions with proteins, which is of interest for several biomedical applications.

Exploring the Reactivity of Polyoxometalates toward Proteins: From Interactions to Mechanistic Insights

The latest advances in the study of the reactivity of metal-oxo clusters toward proteins showcase how fundamental insights obtained so far open new opportunities in biotechnology and medicine. In this Perspective, these studies are discussed through the lens of the reactivity of a family of soluble anionic metal-oxo nanoclusters known as polyoxometalates (POMs). POMs act as catalysts in a wide range of reactions with several different types of biomolecules and have promising therapeutic applications due to their antiviral, antibacterial, and antitumor activities. However, the lack of a detailed understanding of the mechanisms behind biochemically relevant reactions-particularly with complex biological systems such as proteins-still hinders further developments. Hence, in this Perspective, special attention is given to reactions of POMs with peptides and proteins showcasing a molecular-level understanding of the reaction mechanism. In doing so, we aim to highlight both existing limitations and promising directions of future research on the reactivity of metal-oxo clusters toward proteins and beyond.

How to choose proper magnetic particles for bioaffinity interactions? The case for immobilised glyconanoconjugate

In this study, an assay for detection of the cancer biomarker Thomsen-nouvelle (Tn) antigen on the ELISA plates format was designed and developed. The effects of size and the interfacial density of the negative charge of magnetic beads (MBs) on the specific sensitivity of the bioaffinity interaction were studied. In particular, glyconanoconjugate, i.e. glycan Tn antigen conjugated to bovine serum albumin (BSA) was covalently immobilised on MBs for the bioaffinity detection of anti-Tn antibodies as cancer biomarkers. Six different MBs were used in the study, i.e. carboxy-modified MBs of 250 nm, 500 nm, 1000 nm and 2800 nm and epoxy-modified MBs of 2800 nm and 4500 nm. In order to evaluate which MBs are the best suited for detection of the analyte anti-Tn antibodies, sensitivities of detection (slopes from calibration curves) were calculated. Next, specific sensitivities were calculated for each type of MBs as a ratio of sensitivity of detection to the mass of MBs. From zeta potential ζ for each type of MBs, the interfacial charge density on MBs was calculated, expressed as the density of zeta potential ζ_d (ratio of zeta potential to surface area of MBs, i.e. ζ_d = ζ/A). Then, we evaluated the effect of size and ζ_d on the specific sensitivity of detection of anti-Tn antibodies in order to understand the immobilisation process on nanoscale. We also identified an optimal value of ζ_d on MBs; this was essential to achieve highly sensitive detection of the analyte, which made it possible to attain limit of detection (LOD) of (0.31 ± 0.01) ng mL^-1 or (2.10 ± 0.04) pM for analyte detection. In addition, the optimal assay configuration was highly selective and enabled reliable detection of the analyte in human serum with a recovery index in the range of 102-104%.

Expanding the Scope of Polyoxometalates as Artificial Proteases towards Hydrolysis of Insoluble Proteins

Despite the enormous importance of insoluble proteins in biological processes, their structural investigation remains a challenging task. The development of artificial enzyme-like catalysts would greatly facilitate the elucidation of their structure since currently used enzymes in proteomics largely lose activity in the presence of surfactants, which are necessary to solubilize insoluble proteins. In this study, the hydrolysis of a fully insoluble protein by polyoxometalate complexes as artificial proteases in surfactant solutions is reported for the first time. The hydrolysis of zein as a model protein was investigated in the presence of Zr(IV) and Hf(IV) substituted Keggin-type polyoxometalates (POMs), (Et₂ NH₂ )₁₀ [M(α-PW₁₁ O₃₉ )₂ ] (M = Zr or Hf), and different concentrations of the anionic surfactant sodium dodecyl sulfate (SDS). Selective hydrolysis of the protein upon incubation with the catalyst was observed, and the results indicate that the hydrolytic selectivity and activity of the POM catalysts strongly depends on the concentration of surfactant. The molecular interactions between the POM catalyst and zein in the presence of SDS were explored using a combination of spectroscopic techniques which indicated competitive binding between POM and SDS towards the protein. Furthermore, the formation of micellar superstructures in ternary POM/surfactant/protein solutions has been confirmed by conductivity and Dynamic Light Scattering measurements.

Albumin-Hyaluronan Interactions: Influence of Ionic Composition Probed by Molecular Dynamics

The lubrication mechanism in synovial fluid and joints is not yet fully understood. Nevertheless, intermolecular interactions between various neutral and ionic species including large macromolecular systems and simple inorganic ions are the key to understanding the excellent lubrication performance. An important tool for characterizing the intermolecular forces and their structural consequences is molecular dynamics. Albumin is one of the major components in synovial fluid. Its electrostatic properties, including the ability to form molecular complexes, are closely related to pH, solvation, and the presence of ions. In the context of synovial fluid, it is relevant to describe the possible interactions between albumin and hyaluronate, taking into account solution composition effects. In this study, the influence of Na⁺, Mg²⁺, and Ca²⁺ ions on human serum albumin–hyaluronan interactions were examined using molecular dynamics tools. It was established that the presence of divalent cations, and especially Ca²⁺, contributes mostly to the increase of the affinity between hyaluronan and albumin, which is associated with charge compensation in negatively charged hyaluronan and albumin. Furthermore, the most probable binding sites were structurally and energetically characterized. The indicated moieties exhibit a locally positive charge which enables hyaluronate binding (direct and water mediated).

Broadening the Scope of Polyoxometalates as Artificial Proteases in Surfactant Solutions: Hydrolysis of Ovalbumin by Zr(IV)-Substituted Keggin Complex

Development of catalysts for the selective hydrolysis of proteins is challenging, yet important for many applications in biotechnology and proteomics. The hydrolysis of hydrophobic proteins is particularly challenging, as due to their poor solubility, the use of surfactants is often required. In this study, the proteolytic potential of catalyst systems based on the Zr(IV)-substituted Keggin polyoxometalate (Et2NH2)10[Zr(PW11O39)2] (Zr-K 1:2) and three different surfactants (ionic SDS (sodium dodecyl sulfate); zwitterionic Zw3-12 (n-dodecyl-N,N-dimethyl-3-ammonio-1-propanesulfonate); and CHAPS (3-[(3-cholamidopropyl)dimethylammonio]-1-propanesulfonate)), which differ in structure and polarity, has been investigated. Hydrolysis of ovalbumin (OVA) was examined in the presence of Zr-K 1:2 and surfactants by sodium dodecyl sulfate poly(acrylamide) gel electrophoresis (SDS-PAGE), which showed the appearance of new polypeptide fragments at lower molecular weight, indicating that selective hydrolysis of OVA took place for all three catalyst systems. The same fragmentation pattern was observed, showing that the selectivity was not affected by surfactants. However, the surfactants influenced the performance of the catalyst. Hence, the interactions of OVA with surfactants and Zr-K 1:2 were investigated using different techniques such as tryptophan fluorescence, Circular Dichroism, and Dynamic Light Scattering. The speciation of the catalyst in surfactant solutions was also followed by 31P Nuclear Magnetic Resonance spectroscopy providing insight into its stability under reaction conditions.

Recent Progress in Ionic Coassembly of Cationic Peptides and Anionic Species

Peptide assembly has been extensively exploited as a promising platform for the creation of hierarchical nanostructures and tailor-made bioactive materials. Ionic coassembly of cationic peptides and anionic species is paving the way to provide particularly important contribution to this topic. In this review, the recent progress of ionic coassembly soft materials derived from the electrostatic coupling between cationic peptides and anionic species in aqueous solution is systematically summarized. The presentation of this review starts from a brief background on the general importance and advantages of peptide-based ionic coassembly. After that, diverse combinations of cationic peptides with small anions, macro- and/or oligo-anions, anionic polymers, and inorganic polyoxometalates are described. Emphasis is placed on the hierarchical structures, value-added properties, and applications. The molecular design of cationic peptides and the general principles behind the ionic coassembled structures are discussed. It is summarized that the combination of interesting and unique characteristics that arise both from the chemical diversity of peptides and the wide range of anionic species may contribute in a variety of output, including drug delivery, tissue engineering, gene transfection, and antibacterial activity. The emergent new phenomena and findings are illustrated. Finally, the outlook for the peptide-based ionic coassembly systems is also presented.

Hydrolysis of Peptide Bonds in Protein Micelles Promoted by a Zirconium(IV)-Substituted Polyoxometalate as an Artificial Protease

The development of artificial proteases is challenging, but important for many applications in modern proteomics and biotechnology. The hydrolysis of hydrophobic or unstructured proteins is particularly difficult due to their poor solubility, which often requires the presence of surfactants. Herein, it is shown that a zirconium(IV)-substituted Keggin polyoxometalate (POM), (Et₂ NH₂ )₁₀ [Zr(α-PW₁₁ O₃₉ )₂ ] (1), is able to selectively hydrolyze β-casein, which is an intrinsically unstructured protein at pH 7.4 and 60 °C. Four surfactants (sodium dodecyl sulfate (SDS), N-dodecyl-N,N-dimethyl-3-ammonio-1-propanesulfonate (ZW3-12), 3-[(3-cholamidopropyl)dimethylammonio]-1-propanesulfonate (CHAPS), and polyethylene glycol tert-octylphenyl ether (TX-100)), which differ in the nature of their polar groups, were investigated for their role in influencing the selectivity and efficiency of protein hydrolysis. Under experimental conditions, β-casein forms micellar structures in which the hydrophilic part of the protein is water accessible and able to interact with 1. Identical fragmentation patterns of β-casein in the presence of 1 were observed through SDS poly(acrylamide) gel electrophoresis both in the presence and absence of surfactants, but the rate of hydrolysis varied, depending on the nature of surfactant. Whereas TX-100 surfactant, which has a neutral polar head, caused only a slight decrease in the hydrolysis rate, stronger inhibition was observed in the presence surfactants with charges in their polar heads (CHAPS, ZW3-12, SDS). These results were consistent with those of tryptophan fluorescencequenching studies, which showed that the binding between β-casein and 1 decreased with increasing repulsion between the POM and the polar heads of the surfactants. In all cases, the micellar structure of β-casein was not significantly affected by the presence of POM or surfactants, as indicated by circular dichroism spectroscopy.

Multivalent ions and biomolecules: Attempting a comprehensive perspective

Ions are ubiquitous in nature. They play a key role for many biological processes on the molecular scale, from molecular interactions, to mechanical properties, to folding, to self-organisation and assembly, to reaction equilibria, to signalling, to energy and material transport, to recognition etc. Going beyond monovalent ions to multivalent ions, the effects of the ions are frequently not only stronger (due to the obviously higher charge), but qualitatively different. A typical example is the process of binding of multivalent ions, such as Ca2+ , to a macromolecule and the consequences of this ion binding such as compaction, collapse, potential charge inversion and precipitation of the macromolecule. Here we review these effects and phenomena induced by multivalent ions for biological (macro)molecules, from the "atomistic/molecular" local picture of (potentially specific) interactions to the more global picture of phase behaviour including, e. g., crystallisation, phase separation, oligomerisation etc. Rather than attempting an encyclopedic list of systems, we rather aim for an embracing discussion using typical case studies. We try to cover predominantly three main classes: proteins, nucleic acids, and amphiphilic molecules including interface effects. We do not cover in detail, but make some comparisons to, ion channels, colloidal systems, and synthetic polymers. While there are obvious differences in the behaviour of, and the relevance of multivalent ions for, the three main classes of systems, we also point out analogies. Our attempt of a comprehensive discussion is guided by the idea that there are not only important differences and specific phenomena with regard to the effects of multivalent ions on the main systems, but also important similarities. We hope to bridge physico-chemical mechanisms, concepts of soft matter, and biological observations and connect the different communities further.

Selective Hydrolysis of Transferrin Promoted by Zr-Substituted Polyoxometalates

The hydrolysis of the iron-binding blood plasma glycoprotein transferrin (Tf) has been examined at pH = 7.4 in the presence of a series of Zr-substituted polyoxometalates (Zr-POMs) including Keggin (Et2NH2)10[Zr(PW11O39)2]∙7H2O (Zr-K 1:2), (Et2NH2)8[{α-PW11O39Zr-(μ-OH) (H2O)}2]∙7H2O (Zr-K 2:2), Wells-Dawson K15H[Zr(α2-P2W17O61)2]·25H2O (Zr-WD 1:2), Na14[Zr4(α-P2W16O59)2(μ3-O)2(μ-OH)2(H2O)4]·57H2O (Zr-WD 4:2) and Lindqvist (Me4N)2[ZrW5O18(H2O)3] (Zr-L 1:1), (nBu4N)6[(ZrW5O18(μ-OH))2]∙2H2O (Zr-L 2:2)) type POMs. Incubation of transferrin with Zr-POMs resulted in formation of 13 polypeptide fragments that were observed on sodium dodecyl sulfate poly(acrylamide) gel electrophoresis (SDS-PAGE), but the hydrolysis efficiency varied depending on the nature of Zr-POMs. Molecular interactions between Zr-POMs and transferrin were investigated by using a range of complementary techniques such as tryptophan fluorescence, circular dichroism (CD), 31P-NMR spectroscopy, in order to gain better understanding of different efficiency of investigated Zr-POMs. A tryptophan fluorescence quenching study revealed that the most reactive Zr-WD species show the strongest interaction toward transferrin. The CD results demonstrated that interaction of Zr-POMs and transferrin in buffer solution result in significant secondary structure changes. The speciation of Zr-POMs has been followed by 31P-NMR spectroscopy in the presence and absence of transferrin, providing insight into stability of the catalysts under reaction condition.

Nanocomposites of ionic copolymer integrating Gd-containing polyoxometalate as a multiple platform for enhanced MRI and pH-response chemotherapy

Nanocomposites from the co-assemblies of block copolymers and a gadolinium-grafting inorganic cluster were constructed as a multifunctional platform for MRI enhancement, drug loading, and environment-response release at local positions.

Exploring Wells-Dawson Clusters Associated With the Small Ribosomal Subunit

The polyoxometalate P₂W₁₈O626-, the Wells-Dawson cluster, stabilized the ribosome sufficiently for the crystallographers to solve the phase problem and improve the structural resolution. In the following we characterize the interaction of the Wells-Dawson cluster with the ribosome small subunit. There are 14 different P₂W₁₈O626- clusters interacting with the ribosome, and the types of interactions range from one simple residue interaction to complex association of multiple sites including backbone interactions with a Wells-Dawson cluster. Although well-documented that bridging oxygen atoms are the main basic sites on other polyoxometalate interaction with most proteins reported, the W=O groups are the main sites of the Wells-Dawson cluster interacting with the ribosome. Furthermore, the peptide chain backbone on the ribosome host constitutes the main sites that associate with the Wells-Dawson cluster. In this work we investigate the potential of one representative pair of closely-located Wells-Dawson clusters being a genuine Double Wells-Dawson cluster. We found that the Double Wells-Dawson structure on the ribosome is geometrically sound and in line with other Double Wells-Dawson clusters previously observed in the solid state and solution. This information suggests that the Double Wells-Dawson structure on the ribosome is real and contribute to characterization of this particular structure of the ribosome.

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).

Interactions between polyoxometalates and biological systems: from drug design to artificial enzymes

Polyoxometalates have long been studied in a variety of biological applications. Interactions between the highly charged POM molecules and biological molecules frequently occur through hydrogen-bonding and electrostatic interactions. Tellurium-centred Anderson-Evans POMs show exceptional promise as crystallization agents, while acidic and metal-substituted POMs may provide interesting alternatives to enzymes in proteomics applications. While POMs also show interesting results in a number of medicinal applications, for example as anti-amyloid agents for the treatment of Alzheimer's disease and as anti-tumoral agents, their use is often impeded by their toxicity. Many recent studies have therefore focussed on POM-functionalization to reduce toxicity and increase activity by addition of biological targeting molecules.

Synthesis of the first Zn6-hexagon sandwich-tungstoantimonate via rearrangement of a non-lacunary Krebs-type polyoxotungstate

For the first time, the Krebs-POM archetype was used as a non-lacunary precursor expanding the rarely reported hexagon sandwich-POM family.

A novel synthetic pathway to obtain the first Zn₆ hexagon tungstoantimonate [(Zn(H₂O))₆(B-α-SbW₉O₃₃)₂]^6– ([Zn₆-α-SbW₉] (1)) via rearrangement of a non-lacunary Krebs-POM precursor (C₁₂N₄H₁₁)₄K₄[(Mn(H₂O)₃)₂((Mn_0.5W_0.5)O₂)₂(B-β-SbW₉O₃₃)₂] ([Mn-β-SbW₉]) has been developed. Addition of ortho-phenylenediamine (opda) in order to optimize the synthesis of [Mn-β-SbW₉] led to the crystallization of a novel Krebs-type Zn-POM (C₁₂N₄H₁₁)₄Na₅[((Zn₀.₈W_0.2)(H₂O)₃)₂((Zn_0.2W_0.8)O₂)₂(B-β-SbW₉O₃₃)₂] ([(Zn/W)₂-β-SbW₉] (2)) comprising four disordered Zn-centers after replacement of MnCl₂ with ZnCl₂. The compounds were characterized 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 UV-vis spectroscopy and ESI-mass spectrometry. The Krebs-POM archetype and the Zn₆ hexagon tungstoantimonate have been investigated towards their interactions with Human Serum Albumin (HSA) as a model protein using SDS-PAGE and trypthophan fluorescence quenching.

Direct observation of the ZrIV interaction with the carboxamide bond in a noncovalent complex between Hen Egg White Lysozyme and a Zr-substituted Keggin polyoxometalate

The successful cocrystallization of the noncovalent complex formed between (Et2NH2)8[{α-PW11O39Zr-(μ-OH)(H2O)}2]·7H2O Keggin polyoxometalate (2) and Hen Egg White Lysozyme (HEWL) protein is reported. The resulting structural model revealed interaction between monomeric [Zr(PW11O39)]4−(1), which is a postulated catalytically active species, and the protein in two positions in the asymmetric unit. The first position (occupancy 36%) confirms the previously observed binding sites on the protein surface, whereas the second position (occupancy 14%) provides novel insights into the hydrolytic mechanisms of ZrIV-substituted polyoxometalates. The new interaction site occurs at the Asn65 residue, which is directly next to the Asp66–Gly67 peptide bond that was identified recently as a cleavage site in the polyoxometalate-catalysed hydrolysis of HEWL. Furthermore, in this newly discovered binding site, the monomeric polyoxometalate 1 is observed to bind directly to the side chain of the Asn65 residue. This binding of ZrIV as a Lewis-acid metal to the carbonyl O atom of the Asn65 side chain is very similar to the intermediate state proposed in density functional theory (DFT) studies in which ZrIV activates the peptide bond via interaction with its carbonyl O atom, and can be thus regarded as a model for interaction between ZrIV and a peptide bond.

Polyoxometalates: more than a phasing tool in protein crystallography

Protein crystallography is the most widely used method for determining the molecular structure of proteins and obtaining structural information on protein–ligand complexes at the atomic level. As the structure determines the functions and properties of a protein, crystallography is of immense importance for nearly all research fields related to biochemistry. However, protein crystallography suffers from some major drawbacks, whereby the unpredictability of the crystallization process represents the main bottleneck. Crystallization is still more or less a ‘trial and error’ based procedure, and therefore, very time and resource consuming. Many strategies  have been developed in the past decades to improve or enable the crystallization of proteins, whereby the use of so-called additives, which are mostly small molecules that make proteins more amenable to crystallization, is one of the most convenient and successful methods. Most of the commonly used additives are, however, restricted to particular crystallization conditions or groups of proteins. Therefore, a more universal additive addressing a wider range of proteins and being applicable to a broad spectrum of crystallization conditions would represent a significant advance in the field of protein crystallography. In recent years, polyoxometalates (POMs) emerged as a promising group of crystallization additives due to their unique structures and properties. In this regard, the tellurium-centered Anderson–Evans polyoxotungstate [TeW₆O₂₄]⁶⁻ (TEW) showed its high potential as crystallization additive. In this lecture text, the development of POMs as tools in protein crystallography are discussed with a special focus on the so far most successful cluster TEW.

Selectivity and Reactivity of ZrIV and CeIV Substituted Keggin Type Polyoxometalates Toward Cytochrome c in Surfactant Solutions

In this paper we investigate the effect of three different types of surfactants, on the hydrolysis of Cytochrome c (Cyt c), a predominantly α helical protein containing a heme group, promoted by [Ce(α PW₁₁O₃₉)2]10- (CeK) and [Zr(α PW₁₁O₃₉)2]10- (ZrK) polyoxometalates. In the presence of SDS, Zw3 12, or CHAPS surfactants, which are commonly used for solubilizing hydrophobic proteins, the specificity of CeK or ZrK toward hydrolysis of Cyt c does not change. However, the hydrolysis rate of Cyt c by CeK was increased in the presence of SDS, but decreased in the presence of CHAPS, and was nearly inhibited in the presence of Zw3 12. The Circular dichroism and Tryptophan fluorescence spectroscopy have shown that the structural changes in Cyt c caused by surfactants are similar to those caused by POMs, hence the same specificity in the absence or presence of surfactants was observed. The results also indicate that for Cyt c hydrolysis to occur, large unfolding of the protein is needed in order to accommodate the POMs. While SDS readily unfolds Cyt c, the protein remains largely folded in the presence of CHAPS and Zw3 12. Addition of POMs to Cyt c solutions in CHAPS results in unfolding of the structure allowing the interaction with POMs to occur and results in protein hydrolysis. Zw3 12, however, locks Cyt c in a conformation that resists unfolding upon addition of POM, and therefore results in nearly complete inhibition of protein hydrolysis.

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.

Selective Targeting of Proteins by Hybrid Polyoxometalates: Interaction Between a Bis-Biotinylated Hybrid Conjugate and Avidin

The Keggin-type polyoxometalate [γ-SiW₁₀O₃₆]^8- was covalently modified to obtain a bis-biotinylated conjugate able to bind avidin. Spectroscopic studies such as UV-vis, fluorimetry, circular dichroism, coupled to surface plasmon resonance technique were used to highlight the unique interplay of supramolecular interactions between the homotetrameric protein and the bis-functionalized polyanion. In particular, the dual recognition mechanism of the avidin encompasses (i) a complementary electrostatic association between the anionic surface of the polyoxotungstate and each positively charged avidin subunit and (ii) specific host-guest interactions between each biotinylated arm and a corresponding pocket on the tetramer subunits. The assembly exhibits peroxidase-like reactivity and it was used in aqueous solution for L-methionine methyl ester oxidation by H₂O₂. The recognition phenomenon was then exploited for the preparation of layer-by-layer films, whose structural evolution was monitored in situ by ATR-FTIR spectroscopy. Finally, cell tracking studies were performed by exploiting the specific interactions with a labeled streptavidin.

Polyoxometalate-Coated Magnetic Nanospheres for Highly Selective Isolation of Immunoglobulin G

Polyoxometalate [{a-PW₁₁O₃₉Zr(μ-OH)(H₂O)}₂]^8- (POM1) is first prepared by sandwiching Zr^IV among 2 mono-lacunary α-Keggin polyoxometalates, and then novel magnetic nanoparticles (NPs), Fe₃O₄@polyethyleneimine (PEI)@POM1, are fabricated by coating POM1 onto the surface of magnetic Fe₃O₄@PEI NPs under electrostatic interaction. The obtained Fe₃O₄@PEI@POM1 NPs are characterized by Fourier transform infrared, zeta potential, vibrating sample magnetometer, transmission electron microscopy, energy-dispersive X-ray spectroscopy, and X-ray diffraction. Ascribed to the hydrogen-bonding and electrostatic interactions, the NPs exhibit high adsorption selectivity toward IgG, and the adsorption capacity is high up to 304 mg g^-1 under optimal adsorption conditions. By using 0.01% cetyl trimethylammonium bromide to strip the adsorbed protein species, an elution efficiency of 95% is achieved. The feasibility of Fe₃O₄@PEI@POM1 NPs in real-world sample assay has been demonstrated by the selective isolation of IgG heavy chain and light chain from human serum, as confirmed by the sodium dodecyl sulfate polyacrylamide gel electrophoresis assay.

Evidence That Speciation of Oxovanadium Complexes Does Not Solely Account for Inhibition of Leishmania Acid Phosphatases

Leishmaniasis is an endemic disease affecting a diverse spectra of populations, with 1.6 million new cases reported each year. Current treatment options are costly and have harsh side effects. New therapeutic options that have been previously identified, but still underappreciated as potential pharmaceutical targets, are Leishmania secreted acid phosphatases (SAP). These acid phosphatases, which are reported to play a role in the survival of the parasite in the sand fly vector, and in homing to the host macrophage, are inhibited by orthovanadate and decavanadate. Here, we use L. tarentolae to further evaluate these inhibitors. Using enzyme assays, and UV-visible spectroscopy, we investigate which oxovanadium starting material (orthovanadate or decavanadate) is a better inhibitor of L. tarentolae secreted acid phosphatase activity in vitro at the same total moles of vanadium. Considering speciation and total vanadium concentration, decavanadate is a consistently better inhibitor of SAP in our conditions, especially at low substrate:inhibitor ratios.

Three new Strandberg-type phenylphosphomolybdate supports for immobilizing horseradish peroxidase and their catalytic oxidation performances

Three new Strandberg-type polyoxometalate TM-(PhP)2Mo5 supports for immobilizing HRP showed high adsorption capacity and good catalytic oxidation activity.

Toxicity evaluation of two polyoxotungstates with anti-acetylcholinesterase activity

A toxicity evaluation of two Keggin-type heteropolytungstates, K₇[Ti₂PW₁₀O₄₀]·6H₂O and K₆H[SiV₃W₉O₄₀]·3H₂O, with different inhibitory potencies toward acetylcholinesterase activity (IC₅₀ values of 1.04×10^-6 and 4.80×10^-4mol/L, respectively) was performed. Wistar albino rats were orally treated with single doses (5 and 50mg/kg) of both investigated compounds. The biochemical parameters of renal (serum urea and creatinine) and liver function (direct and total bilirubin, alanine transaminase, and aspartate aminotransferase) were determined after 24h and 14days. A histopathological analysis of liver tissue was carried out 14days after the polyoxotungstate administration. Both applied doses of the investigated compounds did not induce statistically significant alterations of the renal function markers. However, the polyoxotungstate treatment caused an increase in the activities of serum alanine transaminase and aspartate aminotransferase in a time- and concentration-dependent manner, although statistically significant changes in bilirubin concentrations were not observed. Furthermore, the detected hepatotoxic effect was confirmed by histhopathological analysis that suggested some reversible liver tissue damage two weeks after the treatment, especially in the case of K₆H[SiV₃W₉O₄₀]·3H₂O. Accordingly, the toxicity of these two polyoxotungstates with anti-acetylcholinesterase effect cannot be considered as a severe one, but their potential clinical application would require a more complex toxicological study.

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, cytotoxicity and antitumour mechanism investigations of polyoxometalate doped silica nanospheres on breast cancer MCF-7 cells

Polyoxometalates (POMs) have shown the potential anti-bacterial, anti-viral and anti-tumor activities. In order to improve their physiological stability and antitumour activity for medical application, K2Na[AsIIIMo6O21(O2CCH2NH3)3]·6H2O doped silica nanospheres (POM@SiO2) with diameters of ~40 nm have been synthesized by the water-in-oil microemulsion method in this study. The obtained spheres were morphologically uniform nanosized and nearly monodispersed in solution. The nanoparticles had high entrapment efficiency, which was upto 46.2% by the inductively coupled plasma mass spectrometry (ICP-MS) analysis and POMs slowly released from the nanospheres both in the PH 7.4 and 5.5 phosphate buffer saline (PBS) solutions in 60 h. The in vitro MTT assays of particles on MCF-7 cell line (a human breast adenocarcinoma cell line) exhibited enhanced antitumor activity compared to that of plain polyoxometalate. The IC50 value of the POM@SiO2 nanoparticles was 40.0 μg/mL at 24 h calculated by the encapsulated POM concentration, which was much lower comparing to that of 2.0 × 104 μg/mL according to the pure POM. And the SiO2 shells showed low inhibitory effect at the corresponding concentration. Confocal images further indicated the cell morphology changes and necrosis. Flow cytometric analysis showed nanoparticles induced the apoptosis by arresting the cells in S phase and western blot analysis indicated they promoted apoptosis by inhibiting the Bcl-2 protein. Moreover, the study of interactions between human serum albumin (HSA) and the nanoparticles indicated the fluorescence quenching was static, and the nanoparticles were likely to bind to HSA and changed its conformation.

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.

Co-assembly of polyoxometalates and peptides towards biological applications

The synergistic self-assembly of biomolecules with polyoxometalates (POMs) has recently been considered as an effective approach to construct nano-biomaterials with diverse structures and morphologies towards applications in drug delivery, controlled release, tissue engineering scaffolds, and biomineralization, due to the unique features of the clusters in addition to many well-known inorganic nanoparticles. This review presents an overview of recent work focusing on the noncovalent co-assembly of peptides and POMs as well as their biological applications. In the co-assemblies triggered by the interaction between the components significant advantages are observed that POMs or peptides alone do not possess; examples include chiral recognition of hybrid metal oxides, the quick hydrolysis of peptides, and enhanced inhibition of Aβ aggregation. Finally, we outline a brief perspective on possible unresolved issues and future opportunities in this field.

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.

Measuring the bioactivity and molecular conformation of typically globular proteins with phenothiazine-derived methylene blue in solid and in solution: A comparative study using photochemistry and computational chemistry

Methylene blue is a phenothiazine agent, that possesses a diversity of biomedical and biological therapeutic purpose, and it has also become the lead compound for the exploitation of other pharmaceuticals such as chlorpromazine and the tricyclic antidepressants. However, the U.S. Food and Drug Administration has acquired cases of detrimental effects of methylene blue toxicities such as hemolytic anemia, methemoglobinemia and phototoxicity. In this work, the molecular recognition of methylene blue by two globular proteins, hemoglobin and lysozyme was characterized by employing fluorescence, circular dichroism (CD) along with molecular modeling at the molecular scale. The recognition of methylene blue with proteins appears fluorescence quenching via static type, this phenomenon does cohere with time-resolved fluorescence lifetime decay that nonfluorescent protein-drug conjugate formation has a strength of 10(4)M(-1), and the primary noncovalent bonds, that is hydrogen bonds, π-conjugated effects and hydrophobic interactions were operated and remained adduct stable. Meantime, the results of far-UV CD and synchronous fluorescence suggest that the α-helix of hemoglobin/lysozyme decreases from 78.2%/34.7% (free) to 58.7%/23.8% (complex), this elucidation agrees well with the elaborate description of three-dimensional fluorescence showing the polypeptide chain of proteins partially destabilized upon conjugation with methylene blue. Furthermore, both extrinsic fluorescent indicator and molecular modeling clearly exhibit methylene blue is situated within the cavity constituted by α1, β2 and α2 subunits of hemoglobin, while it was located at the deep fissure on the lysozyme surface and Trp-62 and Trp-63 residues are nearby. With the aid of computational analyses and combining the wet experiments, it can evidently be found that the recognition ability of proteins for methylene blue is patterned upon the following sequence: lysozyme<hemoglobin<albumin. Basically, the distinction originates from different spatial structures of proteins and noncovalent interactions between proteins and methylene blue. In addition, biological relevance of the biorecognition of methylene blue with proteins was briefly discussed. We hope that this study could provide further standpoint so that one explore the biological activity of methylene blue and also phenothiazines.

Reactivity of Dimeric Tetrazirconium(IV) Wells-Dawson Polyoxometalate toward Dipeptide Hydrolysis Studied by a Combined Experimental and Density Functional Theory Approach

Detailed kinetic studies on the hydrolysis of glycylglycine (Gly-Gly) in the presence of the dimeric tetrazirconium(IV)-substituted Wells-Dawson-type polyoxometalate Na14[Zr4(P2W16O59)2(μ3-O)2(OH)2(H2O)4] · 57H2O (1) were performed by a combination of (1)H, (13)C, and (31)P NMR spectroscopies. The catalyst was shown to be stable under a broad range of reaction conditions. The effect of pD on the hydrolysis of Gly-Gly showed a bell-shaped profile with the fastest hydrolysis observed at pD 7.4. The observed rate constant for the hydrolysis of Gly-Gly at pD 7.4 and 60 °C was 4.67 × 10(-7) s(-1), representing a significant acceleration as compared to the uncatalyzed reaction. (13)C NMR data were indicative for coordination of Gly-Gly to 1 via its amide oxygen and amine nitrogen atoms, resulting in a hydrolytically active complex. Importantly, the effective hydrolysis of a series of Gly-X dipeptides with different X side chain amino acids in the presence of 1 was achieved, and the observed rate constant was shown to be dependent on the volume, chemical nature, and charge of the X amino acid side chain. To give a mechanistic explanation of the observed catalytic hydrolysis of Gly-Gly, a detailed quantum-chemical study was performed. The theoretical results confirmed the nature of the experimentally suggested binding mode in the hydrolytically active complex formed between Gly-Gly and 1. To elucidate the role of 1 in the hydrolytic process, both the uncatalyzed and the polyoxometalate-catalyzed reactions were examined. In the rate-determining step of the uncatalyzed Gly-Gly hydrolysis, a carboxylic oxygen atom abstracts a proton from a solvent water molecule and the nascent OH nucleophile attacks the peptide carbon atom. Analogous general-base activity of the free carboxylic group was found to take place also in the case of polyoxometalate-catalyzed hydrolysis as the main catalytic effect originates from the -C═O···Zr(IV) binding.