Total synthesis of myxothiazols, novel bis-thiazole beta-methoxyacrylate-based anti-fungal compounds from myxobacteria

Convergent total syntheses of myxothiazols A and Z are described. The syntheses are based on elaboration of the (S)-E,E-diene thioamide 22, conversion of 22 into the bis-thiazole 27 and Wittig reactions between 27c and the aldehyde 30. The substituted beta-methoxyacrylate aldehyde 30 was produced via an Evans asymmetric aldol protocol or via the 2H-pyran-2-one 31. An E-selective Wittig reaction between the ylide derived from the phosphonium salt 27c and the (+)-aldehyde 30 led to (+)-myxothiazol Z (1b), and a corresponding reaction with the (+/-)-acrylamide aldehyde 44 gave (+/-)-myxothiazol A (1a). Complementary studies led to synthesis of the ester 47b, corresponding to myxothiazol R and myxothiazol S.     

Approaches for coupling solid-phase microextraction to nanospray

Biocompatible solid-phase microextraction (SPME) devices were prepared using two restricted access materials (RAM) as the SPME coating. The restricted access materials were immobilized on steel and platinum wires. The selective coating eliminated most of the matrix interference, which allowed the coupling to mass spectrometry without further purification. The SPME devices were interfaced to mass spectrometry by electronanospray. Several experimental set-ups are described and discussed herein. For the in situ extraction of peptides from the tryptic digests, trypsin was immobilized both on steel wires and on the inside wall of a vial. The devices were incubated together with the RAM-SPME devices and a protein (casein) solution. After the protein digestion, the resulting peptides were analyzed by SPME/nanospray. The vial approach provided the best results; up to eight peptides could be identified which corresponds to a sequence coverage of 58%. The limit of detection of SPME/nanospray for the extraction of peptides from an aqueous solution was about 50 fmol/mL. The results demonstrate that the direct coupling of SPME to nanospray can reduce analysis time and is an attractive alternative to conventional approaches like Zip-Tip purification.     

New approximate formulae for the retardation factors of nematic liquid crystals with simple uniaxial anisotropy

Abstract

Simple formulae which yield a close approximation to the exact analytic solution for the longitudinal and transverse relaxation times T _|| and T _⊥ of the components of the dipole moment of a nematic liquid crystal with simple uniaxial anisotropy are presented. The new formula for T _|| yields a substantial correction to the Meier-Saupe formula [1966, Molec. Crystals, 1, 515]. It appears that both the longitudinal and transverse relaxation processes may be accurately described by a single Debye type relaxation mechanism with relaxation times T _|| and T _⊥.     

Spiral-path high-sensitivity silicon photonic wire molecular sensor with temperature-independent response

We demonstrate a new silicon photonic wire waveguide evanescent field (PWEF) sensor that exploits the strong evanescent field of the transverse magnetic mode of this high-index-contrast, submicrometer-dimension waveguide. High sensitivity is achieved by using a 2 mm long double-spiral waveguide structure that fits within a compact circular area of 150 microm diameter, facilitating compatibility with commercial spotting apparatus and the fabrication of densely spaced sensor arrays. By incorporating the PWEF sensor element into a balanced waveguide Mach-Zehnder interferometer circuit, a minimum detectable mass of approximately 10 fg of streptavidin protein is demonstrated with near temperature-independent response.     

Headspace stir bar sorptive extraction-gas chromatography/mass spectrometry characterization of the diluted vapor phase of cigarette smoke delivered to an in vitro cell exposure chamber

Advanced smoke generation systems, such as the Borgwaldt RM20S^® smoking machine used in combination with the BAT exposure chamber, allow for the generation, dilution and delivery of fresh cigarette smoke to cell or tissue cultures for in vitro cell culture analyses. Recently, our group confirmed that the Borgwaldt RM20S^® is a reliable tool to generate and deliver repeatable and reproducible exposure concentrations of whole smoke to in vitro cultures [1]. However, the relationship between dose and diluted smoke components found within the exposure chamber has not been characterized. The current study focused on the development of a headspace stir bar sorptive extraction (HSSE) method to chemically characterize some of the vapor phase components of cigarette smoke generated by the Borgwaldt RM20S^® and collected within a cell culture exposure chamber. The method was based on passive sampling within the chamber by HSSE using a Twister™ stir bar. Following exposure, sorbed analytes were recovered using a thermal desorption unit and a cooled injection system coupled to gas chromatograph/mass spectrometry for identification and quantification. Using the HSSE method, sixteen compounds were identified. The desorption parameters were assessed using ten reference compounds and the following conditions led to the maximal response: desorption temperature of 200 °C for 2 min with cryofocussing temperature of −75 °C. During transfer of the stir bars to the thermal desorption system, significant losses of analytes were observed as a function of time; therefore, the exposure-to-desorption time interval was kept at the minimum of 10 ± 0.5 min. Repeatability of the HSSE method was assessed by monitoring five reference compounds present in the vapor phase (10.1–12.9% RSD) and n-butyl acetate, the internal standard (18.5% RSD). The smoke dilution precision was found to be 17.2, 6.2 and 11.7% RSD for exposure concentrations of 1, 2 and 5% (v/v) cigarette vapor phase in air, respectively. A linear response of analyte abundance was observed as a function of dilution. Extrapolation to 100% (v/v) cigarette vapor phase, i.e., undiluted smoke, gave yields for the five compounds ranging from 6 to 450 ng for 10 min exposure.     

Parametric Study of a Pressure Swing Adsorption Process

The performance of a pressure swing adsorption (PSA) process for production of high purity hydrogen from a binary methane-hydrogen mixture is simulated using a detailed, adiabatic PSA model. An activated carbon is used for selective adsorption of methane over hydrogen. The effects of various independent process variables (feed gas pressure and composition, purge gas pressure and quantity, configuration of process steps) on the key dependent process variables (hydrogen recovery at high purity, hydrogen production capacity) are evaluated. It is demonstrated that many different combinations of PSA process steps, their operating conditions, and the feed gas conditions can be chosen to produce an identical product gas with different hydrogen recovery and productivity.     

Weyl invariance and the origins of mass

By a uniform and simple Weyl invariant coupling of scale and matter fields, we construct theories that unify massless, massive, and partially massless excitations. Masses are related to tractor Weyl weights, and Breitenlohner–Freedman stability bounds in anti de Sitter amount to reality of these weights. The method relies on tractor calculus – mathematical machinery allowing Weyl invariance to be kept manifest at all stages. The equivalence between tractor and higher spin systems with arbitrary spins and masses is also considered.     

Tight frames generated by finite nonabelian groups

Let $\cal H$ be a Hilbert space of finite dimension d, such as the finite signals ? ₂(d) or a space of multivariate orthogonal polynomials, and n?≥?d. There is a finite number of tight frames of n vectors for $\cal H$ which can be obtained as the orbit of a single vector under the unitary action of an abelian group G (of symmetries of the frame). Each of these so called harmonic frames or geometrically uniform frames can be obtained from the character table of G in a simple way. These frames are used in signal processing and information theory. For a nonabelian group G there are in general uncountably many inequivalent tight frames of n vectors for $\cal H$ which can be obtained as such a G-orbit. However, by adding an additional natural symmetry condition (which automatically holds if G is abelian), we obtain a finite class of such frames which can be constructed from the character table of G in a similar fashion to the harmonic frames. This is done by identifying each G-orbit with an element of the group algebra ?G (via its Gramian), imposing the condition in the group algebra, and then describing the corresponding class of tight frames.     

Interference effect in scattering loss of high-index-contrast planar waveguides caused by boundary reflections

We present theoretical and experimental results on an interference effect caused by boundary reflections on the optical scattering loss in high-index-contrast planar waveguides. Analytical expressions for the polarization-dependent scattering loss are derived using a surface Green's function. For high-index-contrast waveguides of submicrometer dimensions a significant deviation from accepted theory arises, including scattering loss suppression owing to a thin-film interference effect. Our theoretical predictions are confirmed by loss measurements on silicon-on-insulator channel waveguides.