Experimental evidence for the existence of non-exo-anomeric conformations in branched oligosaccharides: NMR analysis of the structure and dynamics of aminoglycosides of the neomycin family

It is commonly known that the exo-anomeric effect is a major factor governing the conformational behavior of naturally occurring oligosaccharides. Conformational flexibility in these molecules mainly concerns the aglycon psi angle since phi is restricted by this stereo-electronic effect. In fact, to the best of our knowledge no case of a natural glycoside adopting a non-exo-anomeric conformation in solution has yet been reported. With respect to the flexibility among naturally occurring carbohydrates, branched type oligosaccharides including sugar residues glycosidated at contiguous positions (such as blood type carbohydrate antigens Lewis X) have been considered as the paradigm of rigid saccharides--the rigidity being enhanced by van der Waals interactions. Herein, we demonstrate unambiguously that both common beliefs are not to be generalized. For example in neomycin B, a branched oligosaccharide antibiotic, a large number of non-exo-anomeric conformations was detected in solution for the first time in naturally occurring sugars. This unusual behavior is attributed to branching. Here, polar contacts between non-vicinal sugar units lead to an enhanced flexibility of the ribose glycosidic torsion phi. The influence of sugar flexibility on RNA recognition will also be discussed.     

Investigation of the acidity of Ni/Al2O3 and Ni/SiO2?Al2O3 catalysts

Acidity of Ni-modified alumina and silica-alumina catalysts was determined using n-butylamine titration and pyridine adsorption methods. Strong influence of Ni²⁺ ions on the Brönsted acidity of silicaalumina was observed. Improved Brönsted acidity of such system was confirmed by the results of the test reaction.     

Nanoconfinement Inside Molecular Metal Oxide Clusters: Dynamics and Modified Encapsulation Behavior

Encapsulation behavior, as well as the presence of internal catalytically active sites, has been spurring the applications of a 3 nm hollow spherical metal oxide cluster {Mo₁₃₂} as an encapsulation host and a nanoreactor. Due to its well‐defined and tunable cluster structures, and nanoscaled internal void space comparable to the volumes of small molecules, this cluster provides a good model to study the dynamics of materials under nanoconfinement. Neutron scattering studies suggest that bulky internal ligands inside the cluster show slower and limited dynamics compared to their counterparts in the bulk state, revealing the rigid nature of the skeleton of the internal ligands. NMR studies indicate that the rigid internal ligands that partially cover the interfacial pore on the molybdenum oxide shells are able to block some large guest molecules from going inside the capsule cluster, which provides a convincing protocol for size‐selective encapsulation and separation.     

A weighted eigenvalue problem for the p-Laplacian plus a potential

Let Δ _p denote the p-Laplacian operator and Ω be a bounded domain in . We consider the eigenvalue problem

Diatropicity of tetraazanaphthalenes

Tetraazanaphthalenes are diatropic molecules, whose magnetic response to a magnetic field perpendicular to the molecular plane closely resembles that of naphthalene. The out-of-plane component of the magnetic susceptibility tensor and its strong anisotropy can be used as quantifiers of magnetic aromaticity. Maps showing streamlines and modulus of the current density field provide clear evidence for diatropicity of these systems. They also explain the strong anisotropy of carbon and nitrogen magnetic shielding, which is determined by the big out-of-plane component of the nuclear shielding tensor. The electronic ring currents observed in the map deshield the nuclei of ring hydrogens by enforcing the local magnetic field and diminishing the out-of-plane component of proton shielding.     

Long-time behaviour of a one-dimensional BGK model: convergence to macroscopic rarefaction waves

For one-dimensional kinetic BGK models, regarded as relaxation models for scalar conservation laws with genuinely nonlinear fluxes, we prove that the macroscopic density converges to the rarefaction wave solution of the corresponding scalar conservation law in the long time limit, and that the phase space density approaches an equilibrium distribution with the rarefaction wave as macroscopic density. The proof requires a smallness assumption on the amplitude of the rarefaction waves and uses a micro-macro decomposition of the perturbation equation.     

Convolution quadrature time discretization of fractional diffusion-wave equations

We propose and study a numerical method for time discretization of linear and semilinear integro-partial differential equations that are intermediate between diffusion and wave equations, or are subdiffusive. The method uses convolution quadrature based on the second-order backward differentiation formula. Second-order error bounds of the time discretization and regularity estimates for the solution are shown in a unified way under weak assumptions on the data in a Banach space framework. Numerical experiments illustrate the theoretical results.