Defect dynamics observed by NMR of quadrupolar nuclei in gallium nitride

Hexagonal and cubic polytypes of bulk gallium nitride powders are characterized by 69,71Ga and 14N MAS NMR at 11.7 T. The (corrected) 71Ga chemical shifts are 333.0 and 357.5 ppm, respectively; the corresponding 14N chemical shifts are -301.8 and -297.0 ppm (all shifts referenced to 1 M gallium nitrate). The 69,71Ga nuclear quadrupole coupling constants (NQCC) in the hexagonal form are axially symmetric and agree with previous single-crystal determinations. The 71Ga MAS NMR satellite pattern envelope of the cubic form has a large Gaussian half-height width of 297 kHz, due to nonzero NQCC values induced by defects. The 14N MAS NMR spinning sideband pattern of the cubic form has a Lorentzian envelope half-height width of 17.5 kHz for the same reason. A sample containing both phases shows an unexpected marked loss of the 71Ga MAS NMR satellite transition intensity expected for the hexagonal phase. Static 71Ga-selective Hahn spin-echo measurements at the perpendicular edge of the powder pattern for the hexagonal form in this sample show a large reduction in T2, especially at higher temperatures. The partial destruction of both spin-echoes and rotational echoes is due to a chemical-exchange type process involving sites having different NQCC values.     

The palladium electrode in aqueous and nonaqueous titrimetry

Two modifications of the palladium-hydrogen electrode of Schwing and Rogers have been made and tested. A new form of “generation” electrode with a vertical palladium diaphragm and fast constant speed circulation has been devised. This form of electrode responds rapidly and has been used in a variety of acid-base titrations in aqueous and nonaqueous systems.     

Reactions of CFBr studied by laser-induced fluorescence

Laser-induced fluorescence has been used to study reactions of CFBr radicals in a discharge-flow system. Arrhenius expressions of (1.9 ± 0.6) ξ 10^?12 exp(?762± 92/T) and (1.4 ± 0.3) ξ 10^?12 exp(?533 ± 62/T) cm³ molecule ^?1s^?1 for their reactions with Cl₂ and Br₂ respectively. Upper limits were obtained for the rate of reaction of CFBr with O₂ and F₂CCFBr.     

Observations on the permanganate-arsenious oxide titration

The reduction of permanganate and the oxidation of manganous ion have been studied in a variety of acid solutions. The product appears to be an unstable complex of trivalent manganese. The manganic sulfate complex is pink in solution and the manganic arsenate complex is green. Complexes prepared in hydrochloric acid were found to be highly unstable and decomposed rapidly with the formation of free chlorine. The results of this work shed some light on the mechanism of the permanganate-arsenious oxide titration.     

The effect of carbonization heating rate on charcoal and active carbon yields

Amperometric glucose sensors have been assembled by immobilizing glucose oxidase on Nucleopore polycarbonate membranes via glutaraldehyde crosslinking. To eliminate electrochemical interferences, novel blocking membranes were used that are permeable to hydrogen peroxide, making them interference free, highly stable, reproducible, and with extended linearity for glucose up to 50 mM. Author to whom all correspondence and reprint requests should be addressed.     

The behaviour of an electrochemical detector used in liquid chromatography and continuous flow voltammetry: Part 1. Mass transport-limited current

A simple expression relating the current at a rectangular channel-type flow-through electrode to the volume flow rate of solution, cell dimensions and physical constants is derived. The expression for steady-state current is valid for laminar flow with neglect of longitudinal depolarizer diffusion. The equation is derived to extract information on the analytical utility of these cells, i.e. signal-to-noise ratio (SNR). For standard amperometric detection the optimum cell design is one in which the width of the cell is equal to the width of the electrode and the length and thickness of the cell are as small as possible, the limit of these dimensions either being physical or dictated by amplifier noise in the output. There is no optimum shape for an electrode of a given size. For standard amperometric detection with a constant cell volume, V, the optimum dimensions are given by b = ($\text{VD}\text{0.42}$U)^{$\text{1}\text{2}$}, (b = thickness, D = diffusion coefficient, -U= average volume flow rate). For flow rate-modulated operation, the optimum thickness is vanishingly small, and the electrode area (shape not critical) is given by A = 0.52 UbD^-1.     

The determination of chromium in biological materials by atomic absorption spectroscopy

Biological samples were mineralized with a mixture of nitric, sulfuric, and perchloric acids. The chromium was oxidized to chromium(VI) followed by extraction into methyl isobutyl ketone in the cold. The ketone extract was aspirated into a fuel-rich hydrogen-air flame, and the absorption recorded using the 357.9 nm line of chromium. The limit of detection for chromium was found to be 10 p.p.b. Tissue chromium levels of normal albino rats were determined. Increased chromium values were observed for animals maintained on a high chromium diet compared to those fed a deficient diet.     

The maximal sizes of faces and vertices in an arrangement

Let A be an arragement of n lines in the plane. Suppose that F₁,…,F_r are faces of A and that V,…,V_s are vertices of A. Suppose also that each F_i is a (V_j) of the lines of A intersect at V_j. Then we show that

$\text{∑}\text{i=1}\text{r}\text{t(F}{}_{\mathrm{i}}\text{+}\text{∑}\text{j=1}\text{s}\text{t(V}{}_{\mathrm{j}}\text{)?n+4}\text{r}\text{2}\text{+}\text{s}\text{2}\text{+ 2rs}$

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Synthetic antimicrobial oligomers induce a composition-dependent topological transition in membranes

Antimicrobial peptides (AMPs) are cationic amphiphiles that comprise a key component of innate immunity. Synthetic analogues of AMPs, such as the family of phenylene ethynylene antimicrobial oligomers (AMOs), recently demonstrated broad-spectrum antimicrobial activity, but the underlying molecular mechanism is unknown. Homologues in this family can be inactive, specifically active against bacteria, or nonspecifically active against bacteria and eukaryotic cells. Using synchrotron small-angle X-ray scattering (SAXS), we show that observed antibacterial activity correlates with an AMO-induced topological transition of small unilamellar vesicles into an inverted hexagonal phase, in which hexagonal arrays of 3.4-nm water channels defined by lipid tubes are formed. Polarized and fluorescence microscopy show that AMO-treated giant unilamellar vesicles remain intact, instead of reconstructing into a bulk 3D phase, but are selectively permeable to encapsulated macromolecules that are smaller than 3.4 nm. Moreover, AMOs with different activity profiles require different minimum threshold concentrations of phosphoethanolamine (PE) lipids to reconstruct the membrane. Using ternary membrane vesicles composed of DOPG:DOPE:DOPC with a charge density fixed at typical bacterial values, we find that the inactive AMO cannot generate the inverted hexagonal phase even when DOPE completely replaces DOPC. The specifically active AMO requires a threshold ratio of DOPE:DOPC = 4:1, and the nonspecifically active AMO requires a drastically lower threshold ratio of DOPE:DOPC = 1.5:1. Since most gram-negative bacterial membranes have more PE lipids than do eukaryotic membranes, our results imply that there is a relationship between negative-curvature lipids such as PE and antimicrobial hydrophobicity that contributes to selective antimicrobial activity.