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.

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.

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.

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 ß-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, ß-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 ß-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 ß-casein and 1 decreased with increasing repulsion between the POM and the polar heads of the surfactants. In all cases, the micellar structure of ß-casein was not significantly affected by the presence of POM or surfactants, as indicated by circular dichroism spectroscopy.

Mononuclear gold(III) complexes with phenanthroline ligands as efficient inhibitors of angiogenesis: A comparative study with auranofin and sunitinib.

Gold(III) complexes with 1,7- and 4,7-phenanthroline ligands, [AuCl3(1,7-phen-κN7)] (1) and [AuCl3(4,7-phen-κN4)] (2) were synthesized and structurally characterized by spectroscopic (NMR, IR and UV-vis) and single-crystal X-ray diffraction techniques. In these complexes, 1,7- and 4,7-phenanthrolines are monodentatedly coordinated to the Au(III) ion through the N7 and N4 nitrogen atoms, respectively. In comparison to the clinically relevant anti-angiogenic compounds auranofin and sunitinib, gold(III)-phenanthroline complexes showed from 1.5- to 20-fold higher anti-angiogenic potential, and 13- and 118-fold lower toxicity. Among the tested compounds, complex 1 was the most potent and may be an excellent anti-angiogenic drug candidate, since it showed strong anti-angiogenic activity in zebrafish embryos achieving IC50 value (concentration resulting in an anti-angiogenic phenotype at 50% of embryos) of 2.89µM, while had low toxicity with LC50 value (the concentration inducing the lethal effect of 50% embryos) of 128µM. Molecular docking study revealed that both complexes and ligands could suppress angiogenesis targeting the multiple major regulators of angiogenesis, such as the vascular endothelial growth factor receptor (VEGFR-2), the matrix metalloproteases (MMP-2 and MMP-9), and thioredoxin reductase (TrxR1), where the complexes showed higher binding affinity in comparison to ligands, and particularly to auranofin, but comparable to sunitinib, an anti-angiogenic drug of clinical relevance.

Mononuclear gold(iii) complexes with l-histidine-containing dipeptides: tuning the structural and biological properties by variation of the N-terminal amino acid and counter anion.

Gold(iii) complexes with different l-histidine-containing dipeptides, [Au(Gly-l-His-NA,NP,N3)Cl]Cl·3H2O (1a), [Au(Gly-l-His-NA,NP,N3)Cl]NO3·1.25H2O (1b), [Au(l-Ala-l-His-NA,NP,N3)Cl][AuCl4]·H2O (2a), [Au(l-Ala-l-His-NA,NP,N3)Cl]NO3·2.5H2O (2b), [Au(l-Val-l-His-NA,NP,N3)Cl]Cl·2H2O (3), [Au(l-Leu-l-His-NA,NP,N3)Cl]Cl (4a) and [Au(l-Leu-l-His-NA,NP,N3)Cl][AuCl4]·H2O (4b), have been synthesized and structurally characterized by spectroscopic (1H NMR, IR and UV-vis) and single-crystal X-ray diffraction techniques. The antimicrobial efficiency of these gold(iii) complexes, along with K[AuCl4] and the corresponding dipeptides, was evaluated against the broad panel of Gram-positive and Gram-negative bacteria and fungi, displaying their moderate inhibiting activity. Moreover, the cytotoxic properties of the investigated complexes were assessed against the normal human lung fibroblast cell line (MRC5) and two human cancer, cervix (HeLa) and lung (A549) cell lines. None of the complexes exerted significant cytotoxic activity; nevertheless complexes that did show selectivity in terms of cancer vs. normal cell lines (2a/b and 4a/b) have been evaluated using zebrafish (Danio rerio) embryos for toxicity and antiangiogenic potential. Although the gold(iii) complexes achieved an antiangiogenic effect comparable to the known angiogenic inhibitors auranofin and sunitinib malate at 30-fold higher concentrations, they had no cardiovascular side effects, which commonly accompany auranofin and sunitinib malate treatment. Finally, binding of the gold(iii) complexes to the active sites of both human and bacterial (Escherichia coli) thioredoxin reductases (TrxRs) was demonstrated by conducting a molecular docking study, suggesting that the mechanism of biological action of these complexes can be associated with their interaction with the TrxR active site.