Kinetic-coulometric determination of mercury in biological samples

A simple approximate calculation method is given which permits determina-tion of the maximal scan rates of potential allowable for distortion-free recording of current-voltage curves with X-Y recorders. For the calculations only the response time of the recorder, the wave shape and the scan rate of potential need be known. Stationary mercury electrodes and rapid polarography with a dropping mercury electrode at controlled drop times were examined. Electroanalytical implications are discussed, with particular emphasis on the rapid a.c. polarographic method with short controlled drop times and on stationary-electrode fundamental and second harmonic a.c. voltammetry. Theoretically and experimentally it has been shown that an X-Y recorder with 0.5–1.0-s response time can be used for scan rates up to about (50/nt') mV s^-1 with a.c. techniques and about (100/nt') mV s^-1 with d.c. polarography (t'= response time of recorder, n = number of electrons).     

Radiation degradation of poly(arylene ether ketone)s

The resistance of five poly(arylene ether ketone)s with related chemical structures to degradation by ionizing radiation has been studied by ESR spectroscopy and yields of volatile products. All of the polymers showed high resistance to radiation with low yields of radicals after irradiation in vacuum at 77 K (when up to 84% of the radicals were identified as radical anions) and much lower yields at 300 K. The yields of volatile products were much less than reported for poly(arylene sulfone)s [1, 2]. Methyl substitution on a main-chain aromatic ring decreased the radiation resistance, but methane only comprised 10% of the volatile products from the methyl-substituted polymers. A polymer containing an isopropylidene group in the main chain and a substituent aromatic carbonyl showed significantly decreased radiation resistance. Extremely low radical yields were obtained after irradiation in air at 300 K, contrary to many polymers. XPS analysis showed an increase in C–O bonds on the surface after irradiation in air.     

The minimum genus of a two-point universal graph

A graph G is two-point universal if, given any two vertices A and B, there is a vertex jointed to both, a vertex joined to neither, a vertex joined to A but not B, and a vertex joined to B but not A. Erdös asked whether there is an infinite family of such graphs of some genus γ. In this note, we show that the number of vertices of a two-point universal graph of genus γ satisfies n ≤ 216(2γ + 1)² so that there are most finitely many of each genus.     

A dedicated glutathione S-transferase mediates carbon-sulfur bond formation in gliotoxin biosynthesis

Gliotoxin is a virulence factor of the human pathogen Aspergillus fumigatus , the leading cause of invasive aspergillosis. Its toxicity is mediated by the unusual transannular disulfide bridge of the epidithiodiketopiperazine (ETP) scaffold. Here we disclose the critical role of a specialized glutathione S-transferase (GST), GliG, in enzymatic sulfurization. Furthermore, we show that bishydroxylation of the diketopiperazine by the oxygenase GliC is a prerequisite for glutathione adduct formation. This is the first report of the involvement of a GST in enzymatic C-S bond formation in microbial secondary metabolism.     

The coulometric determination of salicylate in serum

Constant-current coulometry can be used for the determination of salicylate in 100 μg of blood serum. The titration is preceded by extraction of acidified serum with ethylene dichloride followed by removal of the salicylate to an aqueous solution of pH 7.5 tris(hydroxymethyl)aminomethane buffer which also contains copper(II) sulfate to enhance recovery. A residual titration with bromine as titrant followed by addition of potassium thiocyanate is used because of the slow bromination of salicylate. Serum samples containing salicylate in the range 4–60 mg/100 ml were analyzed. Recoveries of about 90% were obtained.     

Electrical and ionic conductivity effects on magic-angle spinning nuclear magnetic resonance parameters of CuI

We investigate experimentally and theoretically the effects of two different types of conductivity, electrical and ionic, upon magic-angle spinning NMR spectra. The experimental demonstration of these effects involves (63)Cu, (65)Cu, and (127)I variable temperature MAS-NMR experiments on samples of γ-CuI, a Cu(+)-ion conductor at elevated temperatures as well as a wide bandgap semiconductor. We extend previous observations that the chemical shifts depend very strongly upon the square of the spinning-speed as well as the particular sample studied and the magnetic field strength. By using the (207)Pb resonance of lead nitrate mixed with the γ-CuI as an internal chemical shift thermometer we show that frictional heating effects of the rotor do not account for the observations. Instead, we find that spinning bulk CuI, a p-type semiconductor due to Cu(+) vacancies in nonstoichiometric samples, in a magnetic field generates induced AC electric currents from the Lorentz force that can resistively heat the sample by over 200 °C. These induced currents oscillate along the rotor spinning axis at the spinning speed. Their associated heating effects are disrupted in samples containing inert filler material, indicating the existence of macroscopic current pathways between micron-sized crystallites. Accurate measurements of the temperature-dependence of the (63)Cu and (127)I chemical shifts in such diluted samples reveal that they are of similar magnitude (ca. 0.27 ppm/K) but opposite sign (being negative for (63)Cu), and appear to depend slightly upon the particular sample. This relationship is identical to the corresponding slopes of the chemical shifts versus square of the spinning speed, again consistent with sample heating as the source of the observed large shift changes. Higher drive-gas pressures are required to spin samples that have higher effective electrical conductivities, indicating the presence of a braking effect arising from the induced currents produced by rotating a conductor in a homogeneous magnetic field. We present a theoretical analysis and finite-element simulations that account for the magnitude and rapid time-scale of the resistive heating effects and the quadratic spinning speed dependence of the chemical shift observed experimentally. Known thermophysical properties are used as inputs to the model, the sole adjustable parameter being a scaling of the bulk thermal conductivity of CuI in order to account for the effective thermal conductivity of the rotating powdered sample. In addition to the dramatic consequences of electrical conductivity in the sample, ionic conductivity also influences the spectra. All three nuclei exhibit quadrupolar satellite transitions extending over several hundred kilohertz that reflect defects perturbing the cubic symmetry of the zincblende lattice. Broadening of these satellite transitions with increasing temperature arises from the onset of Cu(+) ion jumps to sites with different electric field gradients, a process that interferes with the formation of rotational echoes. This broadening has been quantitatively analyzed for the (63)Cu and (65)Cu nuclei using a simple model in the literature to yield an activation barrier of 0.64 eV (61.7 kJ/mole) for the Cu(+) ion jumping motion responsible for the ionic conductivity that agrees with earlier results based on (63)Cu NMR relaxation times of static samples.