Sensing the Chirality of Dawson Lanthanide Polyoxometalates [α1-LnP2W17O61]7− by Multinuclear NMR Spectroscopy

Constructing chiral polyoxometalate assemblies via supramolecular approaches

Polyoxometalates (POMs), as a typical class of discrete metal oxide clusters that are known in inorganic and structural chemistry since long, have displayed more and more interesting applications over recent years. However, in comparison to the chemical synthesis, the photochemical, electrochemical, and magnetic properties, the structural asymmetry, and relative characteristic investigations arising therefrom are far behind even if they are very important for functional materials, especially in solution systems. One of the main reasons is that it is hard to control and maintain a stable chiral state of POMs to carry out further corresponding performances. Aiming to overcome these disadvantages, the main concerns of this review are to discuss the generation of the chirality for discrete metal oxide clusters, chirality transfer via a supramolecular approach, chirality amplification in self-assemblies, and the related functional properties such as photochromism, catalysis, and bioactivities in solutions. Considering that some previous reviews dealt with chiral structures and packing architectures in the crystalline solids of POMs, this article only concentrates on the induced chirality and material properties in solution systems, which have been more active recently but no review article has been involved in this interesting area.

Exploring polyoxometalates as non-destructive staining agents for contrast-enhanced microfocus computed tomography of biological tissues

To advance clinical translation of regenerative medicine, there is, amongst others, still need for better insights in tissue development and disease. For this purpose, more precise imaging of the 3D microstructure and spatial interrelationships of the different tissues within organs is crucial. Despite being destructive towards the sample, conventional histology still is the gold standard for structural analysis of biological tissues. It is, however, limited by 2D sections of a 3D object, prohibiting full 3D structural analysis. MicroCT has proven to provide full 3D structural information of mineralized tissues and dense biomaterials. However, the intrinsic low X-ray absorption of soft tissues requires contrast-enhancing staining agents (CESAs). In a previous study, we showed that hafnium-substituted Wells-Dawson polyoxometalate (Hf-WD POM) allows simultaneous contrast-enhanced microCT (CE-CT) visualization of bone and its marrow vascularization and adiposity. In this study, other POM species have been examined for their potential as soft tissue CESAs. Four Wells-Dawson POMs, differing in structure and overall charge, were used to stain murine long bones and kidneys. Their staining potential and diffusion rate were compared to those of Hf-WD POM and phosphotungstic acid (PTA), a frequently used but destructive CESA. Monolacunary Wells-Dawson POM (Mono-WD POM) showed similar soft tissue enhancement as Hf-WD POM and PTA. Moreover, Mono-WD POM is less destructive, shows a better diffusion than PTA, and its synthesis requires less time and cost than Hf-WD POM. Finally, the solubility of Mono-WD POM was improved by addition of lithium chloride (LiCl) to the staining solution, enhancing further the soft tissue contrast. STATEMENT OF SIGNIFICANCE: To advance clinical translation of regenerative medicine, there is, amongst others, still need for better insights in tissue development and disease. For this purpose, more precise imaging of the 3D microstructure and spatial interrelationships of the different tissues within organs is crucial. Current standard structural analysis techniques (e.g. 2D histomorphometry), however, do not allow full 3D assessment. Contrast-enhanced X-ray computed tomography has emerged as a powerful 3D structural characterization tool of soft biological tissues. In this study, from a library of Wells Dawson polyoxometalates (WD POMs), we identified monolacunary WD POM together with lithium chloride, dissolved in phosphate buffered saline, as the most suitable contrast-enhancing staining agent solution for different biological tissues without tissue shrinkage.

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.

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.

Preparation of α1- and α2-isomers of mono-Ru-substituted Dawson-type phosphotungstates with an aqua ligand and comparison of their redox potentials, catalytic activities, and thermal stabilities with Keggin-type derivatives

Both the α1- and the α2-isomers of mono-ruthenium (Ru)-substituted Dawson-type phosphotungstates with terminal aqua ligands, [α1-P2W17O61Ru(III)(H2O)](7-) (α1-RuH2O) and [α2-P2W17O61Ru(III)(H2O)](7-) (α2-RuH2O), were prepared in pure form by cleavage of the Ru-S bond of the corresponding DMSO derivatives, [α1-P2W17O61Ru(DMSO)](8-) (α1-RuDMSO) and [α2-P2W17O61Ru(DMSO)](8-) (α2-RuDMSO), respectively. Redox studies indicated that α1-RuH2O and α2-RuH2O show proton-coupled electron transfer (PCET), and the Ru(III)(H2O) species was reversibly reduced to Ru(II)(H2O) species and oxidized to Ru(IV)([double bond, length as m-dash]O) species and further to Ru(V)([double bond, length as m-dash]O) species in aqueous solution depending on the pH. Their redox potentials and thermal stabilities were compared with those of the corresponding α-Keggin-type derivatives ([α-XW11O39Ru(H2O)](n-); X = Si(4+) (n = 5), Ge(4+) (n = 5), or P(5+) (n = 4)). The basic electronic and redox features of Ru(L)-substituted Keggin- and Dawson-type heteropolytungstates (with L = H2O or O(2-)) were analyzed by means of density functional calculations. Similar to the corresponding α-Keggin-type derivatives, both α1-RuH2O and α2-RuH2O show catalytic activity for water oxidation.

Influence of the amino acid side chain on peptide bond hydrolysis catalyzed by a dimeric Zr(iv)-substituted Keggin type polyoxometalate

Structurally different dipeptides were hydrolyzed by [{α-PW₁₁O₃₉Zr-(μ-OH)(H₂O)}₂]⁸⁻. The rate constants were dependent on bulkiness and chemical nature of the dipeptide.

Selective hydrolysis of oxidized insulin chain B by a Zr(IV)-substituted Wells-Dawson polyoxometalate

We report for the first time on the selective hydrolysis of a polypeptide system by a metal-substituted polyoxometalate (POM). Oxidized insulin chain B, a 30 amino acid polypeptide, was selectively cleaved by the Zr(IV)-substituted Wells-Dawson POM, K15H[Zr(α2-P2W17O61)2]·25H2O, under physiological pH and temperature conditions in aqueous solution. HPLC-ESI-MS, LC-MS/MS, MALDI-TOF and MALDI-TOF MS/MS data indicate hydrolysis at the Phe1-Val2, Gln4-His5, Leu6-Cys(SO3H)7, and Gly8-Ser9 peptide bonds. The rate of oxidized insulin chain B hydrolysis (0.45 h(-1) at pH 7.0 and 60 °C) was calculated by fitting the integration values of its HPLC-UV signal to a first-order exponential decay function. (1)H NMR measurements show significant line broadening and shifting of the polypeptide resonances upon addition of the Zr(IV)-POM, indicating that interaction between the Zr(IV)-POM and the polypeptide takes place in solution. Circular dichroism (CD) measurements clearly prove that the flexible unfolded nature of the polypeptide was retained in the presence of the Zr(IV)-POM. The thermal stability of the Zr(IV)-POM in the presence of the polypeptide chain during the hydrolytic reaction was confirmed by (31)P NMR spectroscopy. Despite the highly negative charge of the Zr(IV)-POM, the mechanism of interaction appears to be dominated by a strong metal-directed binding between the positively charged Zr(IV) center and negatively charged amino acid side chains.

Molecular origin of the hydrolytic activity and fixed regioselectivity of a Zr(IV) -substituted polyoxotungstate as artificial protease

A multitechnique approach has been applied in order to identify the thermodynamic and kinetic parameters related to the regioselective hydrolysis of human serum albumin (HSA) promoted by the Wells-Dawson polyoxometalate (POM), K15 H[Zr(α2 -P2 W17 O61 )2 ]. Isothermal titration calorimetry (ITC) studies indicate that up to four POM molecules interact with HSA. While the first interaction site is characterized by a 1:1 binding and an affinity constant of 2×10(8)  M(-1) , the three remaining sites are characterized by a lower global affinity constant of 7×10(5)  M(-1) . The higher affinity constant at the first site is in accordance with a high quenching constant of 2.2×10(8)  M(-1) obtained for fluorescence quenching of the Trp214 residue located in the only positively charged cleft of HSA, in the presence of K15 H[Zr(α2 -P2 W17 O61 )2 ]. In addition, Eu(III) luminescence experiments with an Eu(III) -substituted POM analogue have shown the replacement of water molecules in the first coordination sphere of Eu(III) due to binding of the metal ion to amino acid side chain residues of HSA. All three interaction studies are in accordance with a stronger POM dominated binding at the positive cleft on the one hand, and interaction mainly governed by metal anchoring at the three remaining positions, on the other hand. Hydrolysis experiments in the presence of K15 H[Zr(α2 -P2 W17 O61 )2 ] have demonstrated regioselective cleavage of HSA at the Arg114Leu115, Ala257Asp258, Lys313Asp314 or Cys392Glu393 peptide bonds. This is in agreement with the interaction studies as the Arg114Leu115 peptide bond is located in the positive cleft of HSA and the three remaining peptide bonds are each located near an upstream acidic residue, which can be expected to coordinate to the metal ion. A detailed kinetic study has evidenced the formation of additional fragments upon prolonged reaction times. Edman degradation of the additional reaction products has shown that these fragments result from further hydrolysis at the initially observed cleavage positions, indicating a fixed selectivity for K15 H[Zr(α2 -P2 W17 O61 )2 ].

Regioselective hydrolysis of human serum albumin by Zr(IV)-substituted polyoxotungstates at the interface of positively charged protein surface patches and negatively charged amino acid residues

Complexes comprising the Lewis acidic Zr(IV) metal and protein binding polyoxotungstate ligands of Lindqvist-, Keggin- and Wells-Dawson-type were found to region selectively hydrolyze human serum albumin at four distinct positions. Higher reactivities were found for structures with higher polyoxometalate charges and the cleavage positions were found in protein regions of mixed charge. Both findings suggest an electrostatic nature of the observed reactivity.