Tight upper bounds for the discrepancy of half-spaces

We show that the discrepancy of anyn-point setP in the Euclideand-space with respect to half-spaces is bounded byC _d n ^1/2−1/2d , that is, a mapping χ:P→{−1,1} exists such that, for any half-space γ, γ, |Σ _p∈P⋂γ χ(p)|≤C _d n ^1/2-1/2d . In fact, the result holds for arbitrary set systems as long as theprimal shatter function isO(m ^d ). This matches known lower bounds, improving previous upper bounds by a factor. Part of this research was done at the Third Israeli Computational Geometry Workshop in Ramot and during a visit at Tel Aviv University in December 1993. Also supported in part by Charles University Grant No. 351 and Czech Republic Grant GAČR 201/93/2167.     

Ion-chromatographic separation of metal cations in the presence of a-hydroxyisobutyric acid

Separations of metal cations on a column packed with the strongly acidic cation exchanger Separon SGX CX were investigated in the presence of -hydroxyisobutyric acid (HIBA) in the mobile phase. A retention model based on the general theory of side equilibria was elaborated and relations describing dependences of capacity factors of analytes on the compositon of the mobile phase were derived. Effects of HIBA concentration and pH of the mobile phase on the analyte retention were studied in detail. Stability constants of divalent metal cations (Cd²⁺, Co²⁺, Mn²⁺, Ni²⁺ and Zn²⁺) with HIBA were calculated from the experimental dependences of the reciprocal values of capacity factors on the ligand concentration.     

Isolierung und Struktur von Pteridinen (Lumazinen) aus Russula sp. (Täublinge; Basidiomycetes)

Isolation and Structure Elucidation of Pteridines (Lumazines) from Russula sp. (Basidiomycetes) Extensive chromatogaphic separations and spectroscopic investigations have led to the isolation and identification of several water-soluble pteridines from Russula sp., the so-called russupteridines, namely: 1-(5-amino-2-6-dioxo-1,2,3,6-tetrahydeopyrimidin-4-yl)amino-1-deoxy-D -ribitol ( 1 ; a pro-lumazine; first identification in a basidiomycete(; l-deoxy-l-(6-methyl-2-4,7-trioxo-1,2,3,4,7,8-hexahydro-pteridin-8-yl)-D -ribitol ( 3 ) and l-deoxy-1-(2,4,7-trioxo-1,2,3,4,7,8-hexahydeopteridin-8-yl)-D -ribitol ( 4 ); both compounds found for the first time in higher fungi; they belong to the components with the strongest violet-blue fluorescence in Russula sp.; riboflavine ( 6 ; now recognized as an important yellow colorant in a great many of Russula sp.); russupteridine-yellow I (= l-(6-amino-7-(N-fromylimino)-2,4-dioxo-1,2,3,4,7,8-hexahydropteridin-8-yl)-1-deoxy-D -ribitol; 5 ; a component with very strong fluorescence; the first derivative of the novel 6,7-diamino-lamazine); russupteridine-yellow IV (= l-deoxy-1-)(2,6,8-trioxo-2,4,5,6,7,8-hexahydro-1H-imidazolo[4,5-g]pteridin-4-yl)-D -ribitol (7)). Two further yellow russupteridines (yellow II and Yellow V) with very strong fluorescence have been isolated and characterized.     

Analytische Aspekte der Oxydation organischer Stickstoffverbindungen mit Chromsäure. II

Zusammenfassung Eine Halbmikromethode zur Bestimmung der an Kohlenstoff oder Sauerstoff gebundenen Nitro- und Nitrosogruppen in organischen Verbindungen wurde ausgearbeitet. Die Methode beruht auf dem oxydativen Aufschluß der Substanz mit einem Chromsäure—Schwefelsäure-Gemisch, wobei die Nitro- und Nitrosogruppen quantitativ in Salpetersäure übergehen. Diese wird mit Devarda-Legierung in alkalischem Medium zu Ammoniak reduziert, das dann im Stickstoffstrom abdestilliert und acidimetrisch titriert wird.

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Summary A semimicro method for determining nitro- and nitroso groups attached to carbon or oxygen in organic compounds has been worked out. The method is based on the oxidative decomposition of the sample with a chromic acid-sulfuric acid mixture, the nitro and the nitroso groups being quantitatively transformed into nitric acid. The latter is reduced to ammonia by means of Devarda alloy in alkaline medium, and is then distilled off in a stream of nitrogen and titrated with a standard acid solution.

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Résumé On a mis au point une méthode à l'échelle semimicro pour le dosage dans les substances organiques de groupes nitro et nitroso liés au carbone ou à l'oxygène. La méthode repose sur l'attaque oxydante de la substance par un mélange acide chromique-acide sulfurique, ce qui transforme quantitativement les groupes nitro et nitroso en acide nitrique. Au moyen de l'alliage de Devarda en milieu alcalin, on réduit celui-ci en ammoniac que l'on chasse par distillation dans un courant d'azote et que l'on titre par acidimétrie.

     

Synthesis, binding affinity, and relaxivity of target-specific MRI contrast agents

Although magnetic resonance imaging (MRI) is one of the most important imaging modalities of the central nervous system (CNS), one of the main drawbacks of MRI is its limited specificity. This can potentially be partially alleviated by target-specific contrast agents. In the present paper we describe a simple high yield synthesis of two such gadolinium-based spiperone targeted MRI contrast agents, 1a and 1b. The R₁ relaxivities of 1a and 1b were evaluated and found to be 5.94 and 8.31 mM⁻¹ s⁻¹, respectively at 9.4T, while their R₂ relaxivities at the same magnetic field were found to be 18.05 and 22.60 mM⁻¹ s⁻¹, respectively. In addition and very importantly compound 1a, which is a gadolinium-based, spiperone-targeted MRI contrast agent, was found to preserve some of the spiperone affinity toward the dopamine D2 receptor. Compounds 1a and 1b thus represent potential agents for in vitro dopamine receptor imaging using MRI in experimental models. Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Dediazoniation of Arenediazonium Ions in Homogeneous Solution. Part IX. Spectral Evidence for a Homolytic Mechanism Involving Pyridine Complexes

The mechanism of dediazoniation of arenediazonium tetrafluoroborates in 2,2,2-trifluoroethanol (TFE) is strongly dependent on the concentration of added pyridine. The added base complexes with the diazonium ion and diverts it to a homolytic pathway. Complex formation is indicated by the disappearance of the \documentclass{article}\pagestyle{empty}\begin{document}$\raise1pt\hbox{---} \mathop {\rm N}\limits^ \oplus \equiv {\rm N}\raise1pt\hbox{---}$\end{document} stretching vibration and appearance of a new band at about 1640–1690 cm^?1 ascribed to the \documentclass{article}\pagestyle{empty}\begin{document}$\raise1pt\hbox{---} {\rm N}\raise1pt\hbox{=\kern-3.45pt=} {\rm N}\raise1pt\hbox{---} \mathop {\rm N}\limits^ \oplus {\rm C}_5 {\rm H}_5$\end{document} system. UV. and NMR. results support this conclusion. Chemically induced dynamic nuclear polarization (CIDNP) experiments clearly implicate a radical-pair as an important intermediate in the decomposition of these complexes.     

Thiacalix[4]arene derivatives with proximally bridged lower rim

New types of lower rim proximally bridged thiacalix[4]arenes have been prepared by direct aminolysis of starting tetraacetate derivative in the cone conformation using aliphatic α,ω-diamines. X-ray crystallography revealed the highly preorganized array of -C(O)NH- bonds resulting in strong intramolecular hydrogen bonding between amide groups of both bridges. The length of the corresponding diamine was found to have an essential influence on the yield of these bridged molecules.     

Expanding the Scope of Hypervalent Iodine Reagents for Perfluoroalkylation: From Trifluoromethyl to Functionalized Perfluoroethyl

A series of new hypervalent iodine reagents based on the 1,3‐dihydro‐3,3‐dimethyl‐1,2‐benziodoxole and 1,2‐benziodoxol‐3‐(1H)‐one scaffolds, which contain a functionalized tetrafluoroethyl group, have been prepared, characterized, and used in synthetic applications. Their corresponding electrophilic fluoroalkylation reactions with various sulfur, oxygen, phosphorus, and carbon‐centered nucleophiles afford products that feature a tetrafluoroethylene unit, which connects two functional moieties. A related λ³‐iodane that contains a fluorophore was shown to react with a cysteine derivative under mild conditions to give a thiol‐tagged product that is stable in the presence of excess thiol. Therefore, these new reagents show a significant potential for applications in chemical biology as tools for fast, irreversible, and selective thiol bioconjugation.     

Numerical study of QCD phase diagram at high temperature and density by a histogram method

We study the QCD phase structure at high temperature and density adopting a histogram method. Because the quark determinant is complex at finite density, the Monte-Carlo method cannot be applied directly. We use a reweighting method and try to solve the problems which arise in the reweighting method, i.e. the sign problem and the overlap problem. We discuss the chemical potential dependence of the probability distribution function in the heavy quark mass region and examine the applicability of the approach in the light quark region.     

Self-assembled free-floating nanomaterials from sequence-defined polymers

Sequence-defined polymers can be programmed to self-assemble into precise nanostructures for applications in biosensing, drug delivery, optics, and molecular computation. Inspired by the natural self-assembly processes present in biological protein and DNA systems, sets of molecular design rules have emerged across materials classes as instructions to build a variety of tunable structures. This review highlights recent advances in self-assembled sequence-defined and sequence-specific polymers across peptides, peptoids, DNA, and non-biological synthetic materials, with a focus on synthesis, assembly processes and overall structure. Specifically, these self-assembled structures are free-floating, as such constructs can potentially serve as a platform for the aforementioned applications. Emphasis is placed on the molecular design of polymers that self-assemble into zero-dimensional, one-dimensional, two-dimensional, or three-dimensional nanostructures. With the development of automated syntheses and increasing control over self-assembly, future work may focus on emerging classes of compatible hybrid materials with exciting directions toward new architectures and applications.