Preyssler-type phosphotungstate is a new family of negative-staining reagents for the TEM observation of viruses

Transmission electron microscopy (TEM) is an essential method in virology because it allows for direct visualization of virus morphology at a nanometer scale. Negative staining to coat virions with heavy metal ions must be performed before TEM observations to achieve sufficient contrast. Herein, we report that potassium salts of Preyssler-type phosphotungstates (K_(15-n)[P₅W₃₀O₁₁₀Mⁿ⁺], M = Na⁺, Ca²⁺, Ce³⁺, Eu³⁺, Bi³⁺, or Y³⁺) are high-performance negative staining reagents. Additionally, we compare the staining abilities of these salts to those of uranyl acetate and Keggin-type phosphotungstate. The potassium salt of Preyssler-type phosphotungstates has the advantage of not requiring prior neutralization because it is a neutral compound. Moreover, the potassium counter-cation can be protonated by a reaction with H⁺-resin, allowing easy exchange of protons with other cations by acid–base reaction. Therefore, the counter-cations can be changed. Encapsulated cations can also be exchanged, and clear TEM images were obtained using Preyssler-type compounds with different encapsulated cations. Preyssler-type phosphotungstates may be superior negative staining reagents for observing virus. Polyoxotungstates (tungsten-oxide molecules with diverse molecular structures and properties) are thus promising tools to develop negative staining reagents for TEM observations.

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.

Polyoxometalates as sialidase mimics: selective and non-destructive removal of sialic acid from a glycoprotein promoted by phosphotungstic acid

The selective hydrolysis of the glycosidic bond between the terminal sialic acid and the penultimate sugar has been achieved in the alpha-2-HS-glycoprotein (Fetuin-A) in the presence of H₃PW₁₂O₄₀, a Keggin type polyoxometalate.

Mechanism of scrapie prion precipitation with phosphotungstate anions

The phosphotungstate anion (PTA) is widely used to facilitate the precipitation of disease-causing prion protein (PrP^Sc) from infected tissue for applications in structural studies and diagnostic approaches. However, the mechanism of this precipitation is not understood. In order to elucidate the nature of the PTA interaction with PrP^Sc under physiological conditions, solutions of PTA were characterized by NMR spectroscopy at varying pH. At neutral pH, the parent [PW₁₂O₄₀]^3– ion decomposes to give a lacunary [PW₁₁O₃₉]^7– (PW₁₁) complex and a single orthotungstate anion [WO₄]^2– (WO₄). To measure the efficacy of each component of PTA, increasing concentrations of PW₁₁, WO₄, and mixtures thereof were used to precipitate PrP^Sc from brain homogenates of scrapie prion-infected mice. The amount of PrP^Sc isolated, quantified by ELISA and immunoblotting, revealed that both PW₁₁ and WO₄ contribute to PrP^Sc precipitation. Incubation with sarkosyl, PTA, or individual components of PTA resulted in separation of higher-density PrP aggregates from the neuronal lipid monosialotetrahexosylganglioside (GM1), as observed by sucrose gradient centrifugation. These experiments revealed that yield and purity of PrP^Sc were greater with polyoxometalates (POMs), which substantially supported the separation of lipids from PrP^Sc in the samples. Interaction of POMs and sarkosyl with brain homogenates promoted the formation of fibrillar PrP^Sc aggregates prior to centrifugation, likely through the separation of lipids like GM1 from PrP^Sc. We propose that this separation of lipids from PrP is a major factor governing the facile precipitation of PrP^Sc by PTA from tissue and might be optimized further for the detection of prions.

Sectroscopic study of stability and molecular species of 12-tungstophosphoric acid in aqueous solution

The various molecular species of 12-tungstophosporic acid (WPA) in aqueous solutions of different pH values (from 1 to 11.5) were investigated by UV, IR, and NMR spectroscopy. The dependence of the attained equilibrium composition in solution on time, concentration of WPA, and type of buffer used was studied. Obtained results indicate that the buffer type and pH value greatly determine the equilibrium composition in the solution. The Keggin structure of the WPA is sustained only up to pH 1.5. With further increase in pH, the decomposition of Keggin anion does not lead directly to the monovacant lacunary anion. Between 1.5 and 2.0, the structures with 2 phosphorus atoms from the Dawson series are dominant as intermediate species. In the pH range 3.5–7.5, WPA is present in the form of the monovacant lacunary Keggin anion. These results are of special importance for the biomedical and catalytic applications of heteropoly compounds (HPCs) and for an improved understanding of the mechanism of their functioning.Key words: heteropolyacids of the Keggin structure, hydrostability, UV, IR and NMR spectroscopy.

The classic Wells-Dawson polyoxometalate, K6[alpha-P2W18O62].14H2O. Answering an 88 year-old question: what is its preferred, optimum synthesis?

The 88-year-old problem of developing a preferred, optimized synthesis of the prototype Wells-Dawson polyoxometalate, K6[alpha-P2W18O62].14H2O, is addressed herein. Specifically, six published syntheses of K6[alpha-P2W18O62].14H2O are listed and discussed, with emphasis given to the two most recent syntheses, Nadjo and co-workers' 2004 synthesis and a 1997 Inorganic Syntheses procedure by Droege, Randall, Finke et al. (hereafter D-R-F). For the starting experiment, the synthesis by Nadjo and co-workers was repeated. Next, the D-R-F synthesis and then the earlier (1984) synthesis in Droege's Ph.D. thesis were repeated and reinvestigated. The results demonstrate that the Nadjo synthesis produces over 200 g of high alpha-isomer purity (> or =97% by (31)P NMR) K6[alpha-P 2W18O62].14H2O in four steps over 8 days in 93% yield in our hands. A recrystallization step added as part of this work (for a total of five steps over 12 days) produces an increase in purity (>99%) with a concomitant loss of 8% yield (i.e., 85% overall yield) for the Nadjo-plus-recrystallization synthesis. Next, the D-R-F Inorganic Syntheses procedure was reinvestigated to determine the cause of "failed syntheses" occasionally encountered in our laboratories, the most recent and worst example to date being when one of us (C.R.G.) found 150 g of K10[alpha2-P 2W17O61] as an undesired side product when, as it turns out, the D-R-F Inorganic Syntheses procedure is followed rather than the earlier Droege synthesis. Specifically, it is shown that the problem in the Inorganic Syntheses procedure is that it ambiguously says to add 210 mL of HCl until a pH of 3-4 is reached when, in fact, it takes only 130-150 mL of HCL to reach a pH 3-4. Adding the full 210 mL of HCl ensures that a pH <2 is reached, as is required to produce isomerically pure K6[alpha-P 2W18O62].14H2O from the K 10[alpha 2-P 2W 17O 61] intermediate. The result is K6[alpha-P2W18O62].14H2O in five steps over 10 days in 82% yield and > or =97% purity. A table is provided comparing the details of the two best syntheses as reported herein: the Nadjo-plus-recrystallization synthesis and the D-R-F synthesis (with sufficient added HCl/proper pH control). That table makes apparent that the Nadjo-plus-recrystallization synthesis is improved on the basis of its better atom economy, its slightly higher product yields (85% vs 82%), slightly better purity (>99% vs >97%), and its comparable time (2 days shorter without recrystallization but 2 days longer with recrystallization) in comparison to the D-R-F synthesis with proper pH <2 control. Perhaps most importantly, some take-home messages concerning polyoxometalate synthesis illustrated by the iterative, 88 year-old quest to the best K6[alpha-P 2W18O62].14H2O synthesis are summarized and briefly discussed.

A study of 12-tungstosilicic and 12-molybdophosphoric acids in solution

The behaviour of two heteropolyacids (HPAs) with quite different stability in aqueous solutions was systematically investigated by UV, IR and NMR spectroscopy and potentiometric titration. It was shown that the Keggin structure of 12-tungstosilicic acid (H4SiW12O40, WSiA) anion was sustained over a wide range of pH from 1.0 to 7.0, while the same anion type of 12-molybdophosphoric acid (H3PMo12O40, MoPA) was present only at pH 1.0. This means that under physiological conditions WSiA is dominantly present in the form of a Kegginanion, whereas the structure of MoPA is completely decomposed to molybdate and phosphate. The obtained results are of special importance for bio-medical and catalytic applications of these compounds and for a better understanding of the mechanism of their action.