“Trimethyl lock” facilitated spirocyclizations: A structural analysis

A trimethyl lock was shown earlier to significantly facilitate certain cyclization reactions. Our renewed interests in such facile cyclization systems stemmed from their potential applications in the preparation of redox-, esterase-, and phosphatase-sensitive prodrugs. Furthermore, such systems have also been used for the development of redox-sensitive protecting groups for amines and alcohols. However, there is an undesirable spirocyclization reaction associated with certain trimethyl lock facilitated cyclization systems. In an effort to probe factors important for the controlling of the undesirable spirocyclization reaction, we undertook the X-ray crystallographic studies of the such trimethyl lock facilitated spirocyclization systems. The spirocyclization product was crystallized from ethyl acetate/hexane mixture (space group: P2₁/c,a=13.467(1),b=13.297(2),c=13.626(8) Å, =92.120(6)°. It was found that spirocyclization results in release of steric congestion caused by the trimethyl lock to a larger degree than the desired lactonization. However, this release of steric congestion itself is not enough to bring about the cyclization. A kinetically reactive nucleophile is also essential for the initiation of the spirocyclization reaction. This is consistent with the fact that such spirocyclization reactions require specific base catalysis.     

Probing the helical content of growth hormone-releasing factor analogs using electrospray ionization mass spectrometry

A series of growth hormone-releasing factor analogs have been studied by both circular dichroism and electrospray ionization mass spectrometry (ESI/MS). The peptides are 32 residues long and are known to adopt a random-coil structure in aqueous solution but become increasingly helical as the proportion of organic solvent is increased. Deuterium exchange was observed as an increase in mass of the peptide, as measured by ESI/MS. Rates of exchange were measured and half-lives calculated for analogs containing amino acid substitutions designed to promote or discourage helix formation. Exchange was slower in peptides that are helical (as shown by circular dichroism) than in randomly coiled peptides. Solution conditions that favor helix formation also produced slower exchange rates. These studies suggest that ESI/MS can provide date about the extent and stability of helix formation.     

The influence of gas-phase etch removal,deposition temperature and substrate properties on the crystallographic perfection of silicon epitaxial layers

An investigation of the defect structure of silicon homoepitaxial layers, as influenced by gas phase etching, deposition temperature, and substrate properties, is presented. The minimum values of etch removal and deposition temperature necessary for sufficient layer perfection have been determined. The defect and impurity distribution in the substrate, as formed during a preceeding arsenic buried layer processing, have been evaluated and are discussed here with regard to their influence on the epitaxial layer perfection.     

Zuckerarten

Ohne Zusammenfassung     

Nanostructures on La-doped SrTiO3 surfaces

SrTiO(3)(100) single crystals with high donor dopant concentrations (5 at% La) were annealed at 1000 degrees C for up to 150 h in ultrahigh vacuum (UHV). By applying scanning tunneling microscopy (STM) nanostructures are observed on top of the surface with typical diameters of 20 nm and typical heights of 8 nm. To characterize their electronic structure and chemical composition, the surface was analyzed by metastable impact electron spectroscopy (MIES), ultraviolet photoelectron spectroscopy (UPS), scanning tunneling spectroscopy (STS), and depth profiling Auger electron spectroscopy (AES). Investigations of the stoichiometry suggest that the secondary phases consist of LaTiO(3). We present a defect chemistry model which attempts to explain the observed effects.     

Determination of the standard Gibbs free energies of formation of the silicates of cobalt,magnesium, and strontium by e.m.f. measurements

Standard Gibbs free energies of formation of the silicates in the quasi-binaries: CoO and SiO₂, MgO and SiO₂, and SrO and SiO₂ were determined with the help of solid-state galvanic cells in the temperature ranges 1273 to 1673 K, 1373 to 1573 K, and 1323 to 1473 K, respectively. Stabilized ZrO₂ was used as an electrolyte for the CoO and SiO₂ system. For the MgO and SiO₂ system MgF₂, and for the SrO and SiO₂ system SrF₂, was used as an electrolyte embedded in a silicate-matrix phase.     

S-homoadenosyl-L-cysteine and S-homoadenosyl-L-homocysteine. Synthesis and binding studies of hon-hydrolyzed substrate analogues with S-adenosyl-L-homocysteine hydrolase

Treatment of homoadenosine [9-(5-deoxy-beta-D-ribo-hexofuranosyl)adenine] with thionyl chloride and pyridine in acetonitrile gave 6'-chloro-6'-deoxyhomoadenosine, which underwent nucleophilic displacement with L-cysteine or L-homocysteine to give homologated analogues of S-adenosyl-L-homocysteine. Each amino acid in aqueous sodium hydroxide at 60 degrees C gave excellent conversion from the chloronucleoside, and adsorption on Amberlite XAD-4 resin provided more convenient isolation than prior methods. Weak binding of these non-hydrolyzed analogues to S-adenosyl-L-homocysteine hydrolase was observed.     

SIMS analysis of poorly conducting surfaces

Summary Charge effects perturb the analysis of poorly conducting materials (glass, ceramics, polymers etc.) via modern surface physics methods which use charged primary and/or secondary particles. This is also true for secondary ion mass spectrometry (SIMS). The use of neutral primary particles (Neutral Primary Beam-SIMS = NPB-SIMS) reduces such perturbations appreciably: positive as well as negative secondary ions may be analyzed. The original distribution of very mobile species in the matrix to be analyzed will not be changed if certain conditions are observed. With this in mind the analysis of chemical and isotopic concentration profiles is possible as they are encountered in corrosion processes, tracer diffusion experiments, and surface engineering or semiconductor technology.

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SIMS-Analyse von Isolatoroberflächen