Proton-impact excitation of HeI triplet levels

We investigated the excitation of the λ(1s3d ³ D?1s2p ³ P)=588 nm line of atomic helium by proton and deuteron impact for projectile energies 10 keV≦E _p≦25 keV. In apparent contradiction to Wigner's spin conservation rule, the emission cross section does not vanish. By measuring the intensity of the impact radiation as a function of homogeneous magnetic and electric fields applied to the collision volume, it has been shown thatp- andd-impact excitation of the 1s3d ³ D level of HeI proceeds via 1snl states withl≧3, which populate the 3³ D states by cascade decays. The well-known strong singlet-triplet mixing of these 1snl states enables a population of triplet states in accord with Wigner's rule. Accordingly, we determine the excitation cross section of the 1s4f multiplet from the measured emission cross section of the 588 nm line. The field-dependent signals give evidence that predominantly substates with |m _L|≦1 are excited.     

One-Bond 13C, 13C-Spin Coupling in η4-, η3- and η2-Olefin Transition Metal Complexes

One-bond ¹³C, ¹³C-spin-coupling constants have been measured, with natural isotope abundance, in η⁴-diene, η³-allyl and η²-ene transition-metal carbonyl complexes. Typical values of ¹J(C,C) are given for Fe-, Ru- and Os-complexes with the three types of olefinic ligands. The effects of substituents and the structural significance of the C,C-coupling constants are discussed.     

Carbohydrate chain of ganglioside GM1 as a ligand: identification of the binding strategies of three 15 mer peptides and their divergence from the binding modes of growth-regulatory galectin-1 and cholera toxin

The branched pentasaccharide chain of ganglioside GM1 is a prominent cell surface ligand, for example, for cholera toxin or tumor growth-regulatory homodimeric galectins. This activity profile via protein recognition prompted us to examine the binding properties of peptides with this specificity. Our study provides insights into the mechanism of molecular interaction of this thus far unexplored size limit of the protein part. We used three pentadecapeptides in a combined approach of mass spectrometry, NMR spectroscopy and molecular modelling to analyze the ligand binding in solution. Availability of charged and hydrophobic functionalities affected the intramolecular flexibility of the peptides differently. Backfolding led to restrictions in two cases; the flexibility was not reduced significantly by association of the ligand in its energetically privileged conformations. Major contributions to the interaction energy arise from the sialic acid moiety contacting Arg/Lys residues and the N-terminal charge. Considerable involvement of stacking between the monovalent ligand and aromatic rings could not be detected. This carbohydrate binding strategy is similar to how an adenoviral fiber knob targets sialylated glycans. Rational manipulation for an affinity enhancement can now be directed to reduce the flexibility, exploit the potential for stacking and acquire the cross-linking capacity of the natural lectins by peptide attachment to a suitable scaffold.     

Transfection mediated by gemini surfactants: engineered escape from the endosomal compartment

The structure of the lipoplex formed from DNA and the sugar-based cationic gemini surfactant 1, which exhibits excellent transfection efficiency, has been investigated in the pH range 8.8-3.0 utilizing small-angle X-ray scattering (SAXS) and cryo-electron microscopy (cryo-TEM). Uniquely, three well-defined morphologies of the lipoplex were observed upon gradual acidification: a lamellar phase, a condensed lamellar phase, and an inverted hexagonal (H(II)) columnar phase. Using molecular modeling, we link the observed lipoplex morphologies and physical behavior to specific structural features in the individual surfactant, illuminating key factors in future surfactant design, viz., a spacer of six methylene groups, the presence of two nitrogens that can be protonated in the physiological pH range, two unsaturated alkyl tails, and hydrophilic sugar headgroups. Assuming that the mechanism of transfection by synthetic cationic surfactants involves endocytosis, we contend that the efficacy of gemini surfactant 1 as a gene delivery vehicle can be explained by the unprecedented observation of a pH-induced formation of the inverted hexagonal phase of the lipoplex in the endosomal pH range. This change in morphology leads to destabilization of the endosome through fusion of the lipoplex with the endosomal wall, resulting in release of DNA into the cytoplasm.     

Detection of fragment genesis in the mass spectrometer. III. Analysis of amino-acid sequence in oligopeptides and in mixtures of oligopeptides by DADI-mass spectrometry (author's transl)]

Detection of Fragment Genesis in the Mass Spectrometer. III. Analysis of Amino-Acid Sequence in Oligopeptides and in Mixtures of Oligopeptides by DADI-Mass Spectrometry. The amino acid sequence was analysed by DADI mass spectrometry in simple oligopeptides and in mixtures containing two or three peptides.     

Multicomponent coupling reactions for organic synthesis: chemoselective reactions with amide-aldehyde mixtures

The acid-catalyzed condensation chemistry of simple amides and aldehydes provides a highly prolific source of diverse reactants for irreversible follow-up reactions. Amide-aldehyde mixtures have been successfully employed in multicomponent syntheses of N-acyl alpha-amino acids (via palladium-catalyzed amidocarbonylation) and various cyclohexene, cyclohexadiene, and benzene derivatives (via the amide-aldehyde-dienophile (AAD) reaction).     

Efficient and practical Ag-catalyzed cycloadditions between arylimines and the Danishefsky diene

An efficient Ag-catalyzed method for asymmetric addition of the Danishefsky diene to various aryl imines to afford cycloadducts in > or =89% ee and > or =85% isolated yield is reported. Reactions are effected with 0.1-1 mol % catalyst (4 degrees C), and the chiral ligand is readily prepared from commercially available materials, including the inexpensive i-Leu. These catalytic asymmetric cycloadditions can be carried out without the use of solvent or with undistilled THF in air. A first generation supported chiral catalyst that effectively promotes the cycloaddition reaction and can be recycled (five cycles) is described.     

Alkaloids from marine organisms. Part 8: Isolation of bisdemethylaaptamine and bisdemethylaaptamine-9-O-sulfate from an Indonesian Aaptos sp. marine sponge

Bisdemethylaaptamine (6), a proposed biosynthetic precursor of the aaptamines has been isolated from an Aaptos sp. marine sponge harvested off the Indonesian coast, and its identity confirmed by comparison of its spectral data with that of synthetic material. Bisdemethylaaptamine-9-O-sulfate (7) was also isolated from the same source. This is the first report of a sulfated aaptamine.     

Thermal degradation studies of alkyl-imidazolium salts and their application in nanocomposites

Increasing the thermal stability of organically-modified layered silicates is one of the key points in the successful technical application of polymer-layered silicate nanocomposites on the industrial scale. To circumvent the detrimental effect of the lower thermal stability of alkyl ammonium-treated montmorillonite, a series of alkyl-imidazolium molten salts were prepared and characterized by elemental analysis, thermogravimetry (TGA) and thermal desorption mass spectroscopy (TDMS). The effect of counter ion, alkyl chain length and structural isomerism on the thermal stability of the imidazolium salts was investigated. Alkyl-imidazolium-treated montmorillonite clays were prepared by ion exchange of the imidazolium salts with Na-montmorillonite. These organically-modified clays were characterized by X-ray diffraction (XRD), TDMS and thermogravimetry coupled with Fourier transform infrared spectroscopy (TGA-FTIR), and compared to the conventional quaternary alkyl ammonium montmorillonite. Results indicate that the counter ion has an effect on the thermal stability of the imidazolium salts, and that imidazolium salts with PF₆⁻, N(SO₂CF₃)₂⁻ and BF₄⁻ anions are thermally more stable than the halide salts. A relationship was observed between the chain length of the alkyl group and the thermo-oxidative stability; as the chain length increased from propyl, butyl, decyl, hexadecyl, octadecyl to eicosyl, the stability decreased. The results also show that the imidazolium-treated montmorillonite has greater thermal stability compared to the imidazolium halide. Analysis of the decomposition products by FTIR provides an insight about the decomposition products which are water, carbon dioxide and hydrocarbons.     

The electronic structure of small lithium clusters

Ab initio molecular orbital calculations using the STO-6G and STO6-21G basis sets have been performed for the cluster series Li _n ⁺ , Li _n , and Li _n ^– (wheren=2–7). Thirty-two optimized structures are discussed and reported, many of which (especially for the anionic structures) have not yet been considered. The calculations suggest that for all three species the optimum geometries are planar. Of the two levels of theories that were investigated, STO-6G//STO-6G and STO6-21G//STO-6G, the latter hybrid theory was found to be less reliable. In particular, for the anionic structures these calculations should provide a platform from which more sophisticated, i.e., configuration interaction, geometry optimization can be performed.