Structural Studies of Bulk to Nanosize Niobium Oxides with Correlation to Their Acidity.

Hydrated niobium oxides are used as strong solid acids with a wide variety of catalytic applications, yet the correlations between structure and acidity remain unclear. New insights into the structural features giving rise to Lewis and Brønsted acid sites are presently achieved. It appears that Lewis acid sites can arise from lower coordinate NbO5 and in some cases NbO4 sites, which are due to the formation of oxygen vacancies in thin and flexible NbO6 systems. Such structural flexibility of Nb-O systems is particularly pronounced in high surface area nanostructured materials, including few-layer to monolayer or mesoporous Nb2O5·nH2O synthesized in the presence of stabilizers. Bulk materials on the other hand only possess a few acid sites due to lower surface areas and structural rigidity: small numbers of Brønsted acid sites on HNb3O8 arise from a protonic structure due to the water content, whereas no acid sites are detected for anhydrous crystalline H-Nb2O5.

Chemosensing ensembles for monitoring biomembrane transport in real time.

The efficacy of drugs and biomolecules relies on their ability to pass through the bilayer. The development of methods to directly and sensitively monitor these membrane transport processes has remained an experimental challenge. A macrocyclic host (p-sulfonatocalix[4]arene or cucurbit[7]uril) and a fluorescent dye (lucigenin or berberine) are encapsulated as a chemosensing ensemble inside liposomes, which allows for a direct, real-time fluorescence monitoring of the passage of unlabeled bioorganic analytes. This in vitro assay is transferable to different channel proteins and analytes, has potential for fluorescence-based screening, e.g., of channel modulators, and yields the absolute kinetics of translocation. Using this new biophysical method, we observed for the first time direct rapid translocation of protamine, an antimicrobial peptide, through the bacterial transmembrane protein OmpF.