Pressure-dependent ion current rectification in conical-shaped glass nanopores

Ion current rectification that occurs in conical-shaped glass nanopores in low ionic strength solutions is shown to be dependent on the rate of pressure-driven electrolyte flow through the nanopore, decreasing with increasing flow rate. The dependence of the i-V response on pressure is due to the disruption of cation and anion distributions at equilibrium within the nanopore. Because the flow rate is proportional to the third power of the nanopore orifice radius, the pressure-driven flow can eliminate rectification in nanopores with radii of ～200 nm but has a negligible influence on rectification in a smaller nanopore with a radius of ～30 nm. The experimental results are in qualitative agreement with predictions based on finite-element simulations used to solve simultaneously the Nernst-Planck, Poisson, and Navier-Stokes equations for ion fluxes in a moving electrolyte within a conical nanopore.     

Boronic acid hydrogen bonding in encapsulation complexes

Hydrogen bonding is a key determinant of much macromolecular structure in nature, but individual donor and acceptor pairs are rarely observed in solution. Their weak interactions result in nanosecond lifetimes and rapid exchange of partners. Reversible encapsulation isolates molecules in very small spaces for milliseconds to hours and allows their characterization by NMR methods. Here we report a competitive study of hydrogen-bonding functions--carboxylic acids, primary amides, and boronic acids--within a multicomponent capsular assembly. The pairwise co-encapsulation of these molecules allows the direct observation of homodimeric boronic acids and their heterodimeric complexes with carboxylic acids and primary amides. The efficiency of boronic acids as hydrogen-bonding partners derives from their adaptable structures rather than from their intrinsic acid/base properties.     

Synthesis,characterization and thermal analysis of ursolic acid solid forms

This study performed a solid‐state characterization of ursolic acid (UA) crystalline forms, a poorly water‐soluble triterpene with anticancer activity. Two new polymorphs (form I, II), two new solvates (propanol and isopropanol solvates), and a known ethanol solvate were determined and elucidated using a combination of multi‐techniques, including X‐ray single crystal and powder diffraction, Fourier transform infrared spectroscopy (FT‐IR), differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA). A colorless single crystal of UA was grown from a propanol solution, and its crystalline structure was determined through X‐ray single crystal diffraction. It was determined that the propanol solvate was crystallized in the orthorhombic space group P2₁2₁2₁ with unit‐cell parameters a = 7.17200 (8) Å, b = 12.24100 (16) Å, c = 33.8950 (4) Å and Z = 4. The ethanol solvate and propanol solvate were isomorphous crystals. The results of the thermal analysis demonstrate that form I is a meta‐stable form, while form II is a stable form that is monotropically related.     

Practical considerations for preparing polymerized phospholipid bilayer capillary coatings for protein separations

Phosphorylcholine (PC) based phospholipid bilayers have proven useful as capillary coating materials due to their inherent resistance to non-specific protein adsorption. The primary limitation of this important class of capillary coatings remains the limited long-term chemical and physical stability of the coatings. Recently, a method for increasing phospholipid coating stability in fused silica capillaries via utilization of polymerized, synthetic phospholipids was reported. Here, we expand upon these studies by investigating polymerized lipid bilayer capillary coatings with respect to separation performance including run-to-run, day-to-day and column-to-column reproducibility and long-term stability. In addition, the effects of pH and capillary inner diameter on polymerized phospholipid coated capillaries were investigated to identify optimized coating conditions. The coatings are stabilized for protein separations across a wide range of pH values (4.0–9.3), a unique property for capillary coating materials. Additionally, smaller inner diameter capillaries (≤50 μm) were found to yield marked enhancements in coating stability and reproducibility compared to wider bore capillaries, demonstrating the importance of capillary size for separations employing polymerized phospholipid coatings.     

Covalent Hypercoordination: Can Carbon Bind Five Methyl Ligands?

C(CH₃)₅⁺ is the first reported example of a five‐coordinate carbon atom bound only to separate (that is, monodentate) carbon ligands. This species illustrate the limits of carbon bonding, exhibiting Lewis‐violating “electron‐deficient bonds” between the hypercoordinate carbon and its methyl groups. Though not kinetically persistent under standard laboratory conditions, its dissociation activation barriers may permit C(CH3)₅⁺ fleeting existence near 0 K.     

Tris(Butadiene) Compounds versus Butadiene Oligomerization in Second-Row Transition Metal Chemistry: Effects of Increased Ligand Fields

The geometries, energetics, and preferred spin states of the second-row transition metal tris(butadiene) complexes (C₄H₆)₃M (M = Zr–Pd) and their isomers, including the experimentally known very stable molybdenum derivative (C₄H₆)₃Mo, have been examined by density functional theory. Such low-energy structures are found to have low-spin singlet and doublet spin states in contrast to the corresponding derivatives of the first-row transition metals. The three butadiene ligands in the lowest-energy (C₄H₆)₃M structures of the late second-row transition metals couple to form a C₁₂H₁₈ ligand that binds to the central metal atom as a hexahapto ligand for M = Pd but as an octahapto ligand for M = Rh and Ru. However, the lowest-energy (C₄H₆)₃M structures of the early transition metals have three separate tetrahapto butadiene ligands for M = Zr, Nb, and Mo or two tetrahapto butadiene ligands and one dihapto butadiene ligand for M = Tc. The low energy of the experimentally known singlet (C₄H₆)₃Mo structure contrasts with the very high energy of its experimentally unknown singlet chromium (C₄H₆)₃Cr analog relative to quintet (C₁₂H₁₈)Cr isomers with an open-chain C₁₂H₁₈ ligand.     

Einen elektrischen Widerstands-Tiegel-Ofen aus Magnesia

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