All-Inorganic Polyoxometalates Act as Superchaotropic Membrane Carriers

Polyoxometalates (POMs) are known antitumoral, antibacterial, antiviral, and anticancer agents and considered as next-generation metallodrugs. Herein, a new biological functionality in neutral physiological media, where selected mixed-metal POMs are sufficiently stable and able to affect membrane transport of impermeable, hydrophilic, and cationic peptides (heptaarginine, heptalysine, protamine, and polyarginine) is reported. The uptake is observed in both, model membranes as well as cells, and attributed to the superchaotropic properties of the polyoxoanions. In view of the structural diversity of POMs these findings pave the way toward their biomedical application in drug delivery or for cell-biological uptake studies with biological effector molecules or staining agents.

The Chaotropic Effect as an Assembly Motif in Chemistry

Following up on scattered reports on interactions of conventional chaotropic ions (for example, I- , SCN- , ClO4- ) with macrocyclic host molecules, biomolecules, and hydrophobic neutral surfaces in aqueous solution, the chaotropic effect has recently emerged as a generic driving force for supramolecular assembly, orthogonal to the hydrophobic effect. The chaotropic effect becomes most effective for very large ions that extend beyond the classical Hofmeister scale and that can be referred to as superchaotropic ions (for example, borate clusters and polyoxometalates). In this Minireview, we present a continuous scale of water-solute interactions that includes the solvation of kosmotropic, chaotropic, and hydrophobic solutes, as well as the creation of void space (cavitation). Recent examples for the association of chaotropic anions to hydrophobic synthetic and biological binding sites, lipid bilayers, and surfaces are discussed.

Preparation of Preyssler-type Phosphotungstate with One Central Potassium Cation and Potassium Cation Migration into the Preyssler Molecule to form Di-Potassium-Encapsulated Derivative

A mono-potassium cation-encapsulated Preyssler-type phosphotungstate, [P5W30O110K]14- (1), was prepared as a potassium salt, K14[P5W30O110K] (1a), by heating mono-bismuth- or mono-calcium-encapsulated Preyssler-type phosphotungstates (K12[P5W30O110Bi(H2O)] or K13[P5W30O110Ca(H2O)]) in acetate buffer. Characterization of the potassium salt 1a by single-crystal X-ray structure analysis, 31P and 183W nuclear magnetic resonance (NMR) spectroscopy, Fourier transform infrared spectroscopy, high-resolution electrospray ionization mass spectroscopy, and elemental analysis revealed that one potassium cation is encapsulated in the central cavity of the Preyssler-type phosphotungstate molecule with a formal D 5h symmetry. Density functional theory calculations have confirmed that the potassium cation prefers the central position of the cavity over a side position, in which no water molecules are coordinated to the encapsulated potassium cation. 31P NMR and cyclic voltammetry analyses revealed the rapid protonation-deprotonation of the oxygens in the cavity compared to that of other Preyssler-type compounds. Heating of 1a in the solid state afforded a di-K+-encapsulated compound, K13[P5W30O110K2] (2a), indicating that a potassium counter-cation is introduced in one of the side cavities, concomitantly displacing the internal potassium ion from the center to a second side cavity, thus providing a new method to encapsulate an additional cation in Preyssler compounds.