Low temperature space charge polarization in alkali halide crystals

Evidence is presented to indicate the presence of linear space charge polarization in single crystals of KCl and NaCl in the temperature range between liquid nitrogen temperature and 350 K. Polarization phenomena, both dipolar and space charge, were studied by means of TSPC, ITC and by the testing of Ohm's law and the Superposition principle. The results are interpreted in terms of the formation of a linear space charge, possibly caused by the trapping of ionic charge carriers within the bulk of the material.     

Optical bleaching studies of quartz

The bleaching properties of a hydrothermal, Arkansas quartz are studied in detail. The thermoluminescence (TL) and phototransferred TL (PTTL) signals are examined as a function of wavelength, illumination time and temperature at which the sample is illuminated. We have also briefly studied the effects of pre-irradiation annealing on the sample. The results of these experiments are described and their relevance to the models for TL is discussed.     

Use of combinatorial library screening to identify inhibitors of a bacterial two-component signal transduction kinase

Bacterial resistance to antibiotics is emerging as amajor concern to the medical community. Theappearance of several antibiotic-resistant strains,including multidrug-resistant Staphylococcusaureus, raises the prospect that infections by thesebacteria could soon become untreatable with currentlyavailable antibiotics. In order to address thisproblem, increased emphasis is being placed on thediscovery of novel classes of antibacterial agentsthat inhibit novel molecular targets using sources ofcompounds not yet exploited for antibiotic drugdiscovery. Novel classes of compounds can now berapidly investigated using combinatorial chemistryapproaches. This report describes the identificationof novel antibacterial compounds from a combinatoriallibrary of N-acetylated, C-amidated D-amino acidhexapeptides. This library of compounds was screenedfor inhibitors of CheA, a member of the bacterialtwo-component signal transduction kinase family. Several peptides with apparent IC₅₀ values in thelow micromolar range were identified. In addition toinhibiting CheA, these peptides inhibited mammalianprotein kinase C (from rat brain) with comparablepotency. Finally, these peptides were also found tohave significant antibacterial properties, althoughthe true mechanism by which they exhibited inhibitionof bacterial growth remains uncertain.     

The three-dimensional structure of human granzyme B compared to caspase-3, key mediators of cell death with cleavage specificity for aspartic acid in P1

BACKGROUND: Granzyme B, one of the most abundant granzymes in cytotoxic T-lymphocyte (CTL) granules, and members of the caspase (cysteine aspartyl proteinases) family have a unique cleavage specificity for aspartic acid in P1 and play critical roles in the biochemical events that culminate in cell death. RESULTS: We have determined the three-dimensional structure of the complex of the human granzyme B with a potent tetrapeptide aldehyde inhibitor. The Asp-specific S1 subsite of human granzyme B is significantly larger and less charged than the corresponding Asp-specific site in the apoptosis-promoting caspases, and also larger than the corresponding subsite in rat granzyme B. CONCLUSIONS: The above differences account for the variation in substrate specificity among granzyme B, other serine proteases and the caspases, and enable the design of specific inhibitors that can probe the physiological functions of these proteins and the disease states with which they are associated.     

Multilayer resonances sharpened by grating waveguide resonance

A multilayer planar structure comprising a highly reflective multilayer dielectric mirror and a corrugated waveguide is proposed for use as a narrow passband optical filter. The proposed filter has a much narrower linewidth than a usual Fabry–Perot cavity with two multilayer dielectric mirrors. It is shown that the narrowing of the linewidth is due to the strong spectral dependence of the phase of the wave reflected from the waveguide grating mirror. The shape of the pass band can be made symmetrical by a proper choice of the grating groove profile.     

Effect of high-dose irradiation on the optically stimulated luminescence of Al2O3:C

This paper examines the effect of high-dose irradiation on the optically stimulated luminescence (OSL) of Al2O3:C, principally on the shape of the OSL decay curve and on the OSL sensitivity. The effect of the degree of deep trap filling on the OSL was also studied by monitoring the sensitivity changes after doses of beta irradiation and after step-annealing of samples previously irradiated with high doses. The OSL response to dose shows a linear-supralinear-saturation behavior, with a decrease in the response for doses higher than those required for saturation. This behavior correlates with the sensitivity changes observed in the samples annealed only to 773 K, which show sensitization for doses up to 20-50 Gy and desensitization for higher doses. Data from the step-annealing study leads to the suggestion that the sensitization is caused by the filling of deep electron traps, which become thermally unstable at 1100-1200 K, whereas the desensitization is caused by the filling of deep hole traps, which become thermally unstable at 800-875 K, along with a concomitant decrease in the concentration of recombination centers (F+ -centers). Changes in the shape of the OSL decay curves are also observed at high doses, the decay becoming faster as the dose increases. These changes in the OSL decay curves are discussed in terms of multiple overlapping components, each characterized by different photoionization cross-sections. However, using numerical solutions of the rate equations for a simple model consisting of a main trap and a recombination center, it is shown that the kinetics of OSL process may also be partially responsible for the changes in the OSL curves at high doses in Al2O3:C. Finally, the implication of these results for the dosimetry of heavy charged particles is discussed.     

Aspochalasin H1: A New Cyclic Aspochalasin from Hawaiian Plant-Associated Endophytic Fungus Aspergillus sp. FT1307

Aspergillus is one of the most diverse genera, and it is chemically profound and known to produce many biologically active secondary metabolites. In the present study, a new aspochalasin H1 (1), together with nine known compounds (2–10), were isolated from a Hawaiian plant-associated endophytic fungus Aspergillus sp. FT1307. The structures were elucidated using nuclear magnetic resonance (NMR) (¹H, ¹H-¹H COSY, HSQC, HMBC, ROESY and 1D NOE), high-resolution electrospray ionization mass spectroscopy (HRESIMS), and comparisons with the reported literature. The absolute configuration of the new compound was established by electronic circular dichroism (ECD) in combination with NMR calculations. The new compound contains an epoxide moiety and an adjacent trans-diol, which has not been reported before in the aspochalasin family. The antibacterial screening of the isolated compounds was carried out against pathogenic bacteria (Staphylococcus aureus, Methicillin-resistant S. aureus and Bacillus subtilis). The antiproliferative activity of compounds 1–10 was evaluated against human breast cancer cell lines (MCF-7 and T46D) and ovarian cancer cell lines (A2780).     

Remarkably Slow Rotation about a Single Bond between an sp3‐Hybridised Carbon Atom and an Aromatic Ring without ortho Substituents

Look, no ortho substituents! A series of polycycles were prepared by using a three‐component Joullié–Ugi reaction. The rate of rotation about the bond between a highly hindered bridgehead and a phenyl ring with no ortho substituents was measured, and was highly dependent on the substitution. Rotamer half‐lives of up to 21 h at 298 K were observed (see figure). Rotamers resulting from this restricted rotation were isolated for the first time.

     

Concepts and approaches to in situ luminescence dating of Martian sediments

In this paper we present the concept of a robotic instrument for in situ luminescence dating of near-surface sediments on Mars. The scientific objectives and advantages to be gained from the development of such an instrument are described, and the challenges presented by the Mars surface environment to the design and operation of the instrument are outlined.