Total synthesis of five thapsigargins: guaianolide natural products exhibiting sub-nanomolar SERCA inhibition

Herein we describe the total synthesis of five guaianolide natural products: thapsigargin, thapsivillosin C, thapsivillosin F, trilobolide and nortrilobolide. Prodrug derivatives of thapsigargin have shown selective in vivo cytotoxicity against prostate tumours and the need for further investigation of this phenomenon highlights the importance of these total syntheses. The first absolute stereochemical assignment of thapsivillosin C is also delineated.     

Structure and stability of Langmuir-Blodgett-Kuhn multilayers containing “hairy-rod” copolymers

The microstructure and thermal stability of Langmuir-Blodgett-Kuhn multilayers containing rod-like polyglutamate copolymers having flexible aliphatic side chains have been studied with x-ray and neutron reflectometry. Upon annealing at 84°C the multilayer structure changes in a manner which is dependent upon the degree of orientation of the rod-like backbones and therefore upon the flow dynamics of the deposition. A sample deposited with a highly convergent flow loses the bilayer structure induced by deposition, whereas samples with less highly aligned backbones may not undergo this relaxation as readily.     

Photoproduction of π+ p-pairs on the 16O nucleus and isobar configurations

The yield of the ¹⁶O(γ, π⁺ p) reaction has been measured in the region of the excitation of the Δ(1232) isobar at high momentum transfers to the residual nuclear system. The experimental data are interpreted within the model taking into account manifestations of isobar configurations in the ground state of the ¹⁶O nucleus. Direct and exchange mechanisms of the production of pions with emission of one and two nucleons, which follow from the structures of the density matrices for these reactions, have been considered. The probability of the production of the Δ-isobar in the ground state of the ¹⁶O nucleus has been empirically estimated as P _Δ = 0.019 ± 0.003 ± 0.003.     

Amperometric determination of hydrogen peroxide with a manganese dioxide film-modified screen printed carbon electrode

A carbon thick film electrode modified with an MnO₂-film is investigated as an amperometric detector for hydrogen peroxide in flow-injection analysis (FIA). At an operating potential of +0.48 V vs. Ag/AgCl catalytic oxidation of the analyte is exploited for amperometric monitoring. Experimental parameters, such as pH of the carrier, working potential, flow rate and injection volume, are optimized. The amperometric signals are linearly proportional to the concentration of H₂O₂ in the range from 0.005 to 10 mg/L, showing a detection limit (3σ) of 2.3 μg/L. The method is applied to the determination of H₂O₂ in rain water and to a simple assay to quantify glucose in human plasma. Received: 29 January 1998 / Revised: 4 May 1998 / Accepted: 13 May 1998     

Microstructural analysis of the cementation process during soiling on glass surfaces in arid and semi‐arid climates

To investigate the soiling behavior of solar energy systems like photovoltaics or concentrated solar power, glass samples were exposed to outdoor conditions in Doha, Qatar for one month. Soil formation on the glass was characterized at microstructural level using scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Further, elemental analysis of the crust was done with energy‐dispersive X‐ray spectroscopy (EDX). Small fibrous structures were found on the glass surface and dust particles, providing evidence of a cementation process leading to a strong adhesion of airborne dust particles. In contrast to the common perception, that cementation occurs via the precipitation of salt (sodium chloride) these needle structures were found to be mainly composed of oxides of Si, Mg and Al. This indicates that cementation processes in desert regions are enhanced by the growth of fibrous clay minerals.

Cross section of cemented dust particle, connected via small needles to the glass surface.     

Optical transmission through double-layer metallic subwavelength slit arrays

We present measurements of transmission of infrared radiation through double-layer metallic grating structures. Each metal layer contains an array of subwavelength slits and supports transmission resonance in the absence of the other layer. The two metal layers are fabricated in close proximity to allow coupling of the evanescent field on individual layers. The transmission of the double layer is found to be surprisingly large at particular wavelengths, even when no direct line of sight exists through the structure as a result of the lateral shifts between the two layers. We perform numerical simulations using rigorous coupled wave analysis to explain the strong dependence of the peak transmission on the lateral shift between the metal layers.     

Well-posedness and exponential equilibration of a volume-surface reaction–diffusion system with nonlinear boundary coupling

We consider a model system consisting of two reaction–diffusion equations, where one species diffuses in a volume while the other species diffuses on the surface which surrounds the volume. The two equations are coupled via a nonlinear reversible Robin-type boundary condition for the volume species and a matching reversible source term for the boundary species. As a consequence of the coupling, the total mass of the two species is conserved. The considered system is motivated for instance by models for asymmetric stem cell division.Firstly we prove the existence of a unique weak solution via an iterative method of converging upper and lower solutions to overcome the difficulties of the nonlinear boundary terms. Secondly, our main result shows explicit exponential convergence to equilibrium via an entropy method after deriving a suitable entropy entropy-dissipation estimate for the considered nonlinear volume-surface reaction–diffusion system.     

Über die Bestimmbarkeit des Verhaltens von Algorithmen

Ohne Zusammenfassung     

On the reactivity of ascomycin at the binding domain. Part 3: Reactivity of the binding domain towards diazomethane

A thorough product analysis of the reaction of ascomycin with diazomethane has been performed. Apart from the expected oxiranes (epimeric at C9), a whole range of novel derivatives bearing various modifications in the binding domain were obtained. Proposed mechanisms for their formation and stereochemical aspects are discussed.     

On the reactivity of ascomycin at the binding domain. Part 1: Liberation of the tricarbonyl portion of ascomycin

Within the binding domain, ascomycin features the unusual pattern of a masked tricarbonyl moiety, which potentially allows for high structural diversity via simple isomerisation events. Herein, methodologies, allowing the liberation of the tricarbonyl unit by blocking the 14-hydroxy group are reported.