Showing papers in "ChemInform in 1990"

Electrospray Ionization for Mass Spectrometry of Large Biomolecules

Abstract: Electrospray ionization has recently emerged as a powerful technique for producing intact ions in vacuo from large and complex species in solution. To an extent greater than has previously been possible with the more familiar "soft" ionization methods, this technique makes the power and elegance of mass spectrometric analysis applicable to the large and fragile polar molecules that play such vital roles in biological systems. The distinguishing features of electrospray spectra for large molecules are coherent sequences of peaks whose component ions are multiply charged, the ions of each peak differing by one charge from those of adjacent neighbors in the sequence. Spectra have been obtained for biopolymers including oligonucleotides and proteins, the latter having molecular weights up to 130,000, with as yet no evidence of an upper limit.

Crown thioether chemistry

Abstract: The synthetic, structural, and coordination chemistry of crown thioethers with both transition and p-block metal ions is reviewed comprehensively through December 1988. Emphasis falls upon the electronic structures and redox properties induced in metal ions by coordination to crown thioethers. Examples include stabilization of mononuclear Rh(II), Pt(III), and low-spin octahedral Co(II). A subsidiary theme concerns the influence of ligand conformation in determining both the binding efficacy and the qualitative coordination chemistry associated with a given crown thioether. The review concludes with a view toward potential future applications of crown thioethers in catalysis, in sequestration or biological delivery of heavy metal ions, and in fundamental studies directed toward rational design of ligands.

Application of Zinc Oxide Varistors

Abstract: This paper deals with the application of ZnO varistors—an area which has not been treated in a systematic way in the literature. The paper starts with a brief description of the fundamental properties comprising the electrical behavior as well as the physics, chemistry, and microstructure of the varistor. These properties then form the basis for defining the application parameters that are directly related to the nonlinear current-voltage characteristics of the varistor. This paper provides a detailed description of these parameters and their relation to microstructure and the processing of the varistor. Finally, a discussion is presented on the reliability of the varistor by considering a grain-boundary defect model which explains both the instability and the stability under use conditions.

A Cleavage Method which Minimizes Side Reactions Following Fmoc Solid Phase Peptide Synthesis.

Abstract: The success of solid phase peptide synthesis utilizing 9-fluorenylmethoxycarbonyl (Fmoc) amino acids is often limited by deleterious side reactions which occur during TFA peptide-resin cleavage and side-chain deprotection. The majority of these side reactions modify susceptible residues, such as Trp, Tyr, Met, and Cys, with TFA-liberated side-chain protecting groups and linkers. The purpose of this study was to assess the relative effectiveness of various scavengers in suppressing these side reactions. We found that the cleavage mixture 82.5% TFA : 5% phenol : 5% H2O : 5% thioanisole : 2.5% EDT (Reagent K) was maximally efficient in inhibiting a great variety of side reactions. Synthesis and cleavage of 10 peptides, each containing 20-50 residues, demonstrated the complementarity of Fmoc chemistry with Reagent K for efficient synthesis of complex peptides.

Spectroscopic characterization of minerals and their surfaces

Abstract: Spectroscopic Characterization of Minerals and Their Surfaces: An Overview Thin-Film Elemental Analyses for Precise Characterization of Minerals High-Resolution Transmission Electron Microscopy Applied to Clay Minerals Skeletal Versus Nonbiogenic Carbonates: VN-IR (0.3-2.7- m) Reflectance Properties Photoluminescence, Candoluminescence, and Radical Recombination Luminescence of Minerals Three-Dimensional Thermoluminescence in Minerals Induced Thermoluminescence and Cathodoluminescence Studies of Meteorites: Relevance to Structure and Active Sites in Feldspar Emission of Particles and Photons from the Fracture of Minerals and Inorganic Materials What Excites Triboluminescence? ]5]7Fe-Bearing Oxide, Silicate, and Aluminosilicate Minerals: Crystal Structure Trends in Mossbauer Spectra Crystal Chemistry, Electronic Structures, and Spectra of Fe Sites in Clay Minerals: Applications to Photochemistry and Electron Transport Dissociation and Recombination of Positive Holes in Minerals Variable Oxidation States of Iron in the Crystal Structure of Smectite Clay Minerals Surface Activities of Clays Probing the Surface of Clays in Aqueous Suspension by Fluorescence Spectroscopy of Proflavine Multinuclear Magnetic Resonance Studies of Structure and Dynamics at the Interface of Clay Materials Near-Infrared Correlation Spectroscopy: Quantifying Iron and Surface Water in a Series of Variable Cation-Exchanged Montmorillonite Clays Raman and FT-IR Spectra of the Kaolinite-Hydrazine Intercalate Reactive Cr-O Sites: Catalytic Properties of Chromia-Pillared Montmorillonite and Preliminary Study Results

Sulfur-Centered Three-Electron Bonded Radical Species

Abstract: We all know that it is not possible to stabilize a He2 molecule. Any interaction of the filled He is orbitals would lead to the establishment of bonding a and antibonding σ* energy levels, both doubly occupied. As σ*, for quantum mechanical reasons, is raised a little bit more than σ is lowered relative to the original atomic energy levels the net result of this electronic σ/σ* configuration would be a slight repulsion of the two atoms. The situation becomes significantly different though if one of the antibonding σ* electrons is removed. Open image in new window

Metal homoenolates from siloxycyclopropanes

Abstract: The potential of metal homoenolates as multifunctional reagents in organic synthesis has become recognized in the last decade. Ring opening of siloxycyclopropanes by Lewis acidic metals currently provides the best route to the metal homoenolates, which are either isolable as a stable complex or exist only as a transient organometallic species. This review summarizes, for the first time, the research activities of metal homoenolate chemistry. Other synthetically useful routes to metal homoenolates are also described.

Molecular “Light Switch” for DNA: Ru(bpy)2(dppz)2+.

Abstract: Considerable research has focused on the development of nonradioactive probes for nucleic acids. Extensive photophysical studies indicate that Ru(phen){sub 3}{sup 2+} bound to double-helical DNA displays an increase in luminescence owing to intercalation; emission from the metal-to-ligand charge transfer (MLCT) excited state decays as a biexponential with one lifetime of 2 {mu}s attributed to the intercalative form and a second lifetime of 0.6 {mu}s (indistinguishable from the free species) assigned to the surface bound form. Here we report the application of a novel transition-metal complex as a true molecular light switch for DNA. This probe is Ru(bpy){sub 2}(dppz){sup 2+} (bpy = 2,2{prime}-bipyridine, dppz = dipyrido(3,2-a:2{prime},3{prime}-c)phenazine), which shows no photoluminescence in aqueous solution at ambient temperatures, but displays intense photoluminescence in the presence of double-helical DNA, to which the complex binds avidly.

Formation of Hydrocarbons in the Electrochemical Reduction of Carbon Dioxide at a Copper Electrode in Aqueous Solution

Abstract: Electroreduction of CO2 at Cu in aqueous inorganic electrolytes was studied by means of voltammetric, coulometric and chronopotentiometric measurements. CO, CH4, C2H4, EtOH and PrnOH are produced at ambient temperatures. Formation of CO predominates at less negative potentials (more positive than –1.2 V vs. NHE); hydrocarbons and alcohols are favourably produced below –1.3 V vs. NHE, where the Faradaic efficiency of CO drops. CO, formed as an intermediate from CO2, is adsorbed on the Cu electrode, interfering with cathodic hydrogen formation. The adsorption strength of CO on Cu is very weak as compared with that on Pt. Adsorbed CO is reduced to Hydrocarbons and alcohols at more negative potentials. The product distribution from CO2 depends strongly upon the electrolytes employed. Formation of C2H4 and alcohols is favoured in KCl, K2SO4, KClO4 and dilute HCO–3 solutions, whereas CH4 is preferentially produced in relatively concentrated HCO–3 and phosphate solutions. The product selectivity depends upon availability of hydrogen or protons on the surface, which is controlled by pH at the electrode. The pH at the electrode is greatly affected by the electrolyte, since OH– is released in the electrode reactions. The production of hydrocarbons and alcohols is discussed in comparison with the mechanism of the Fishcher–Tropsch reaction.

Hydrogen Bonding. Part 10. A Scale of Solute Hydrogen-Bond Basicity Using log K Values for Complexation in Tetrachloromethane.

Abstract: A scale of solute hydrogen-bond acidity has been constructed using equilibrium constants (as log K values) for complexation of series of acids (i) against a given base in dilute solution in tetrachloromethane, equation (A). Forty-five such equations have been solved to yield LB and DB, log Ki=LB log KAHI+DB(A) values characterising the base, and log KAH values that characterise the acid. In this analysis, use has been made of the novel observation that all the lines in equation (A) intersect at a given point where log K= log KAH=–1.1 with K on the molar scale. Some 190 log KAH values that constitute a reasonably general scale of solute hydrogen-bond acidity have been obtained. It is shown that there is no general connection between log KAH; and any proton-transfer quantities, although certain family dependences are obtained. A number of acid-base combinations are excluded from equation (A), and alternative log KAHE values have been determined for such cases. The general log KAH values may be transformed into α2H values suitable for use in multiple linear-regression analysis through the equation α2H=(log KAH+ 1.1)/4.636.

Crystallization as a separations process

Abstract: Crystallization was studied as a separation process in various applications. Two applications of interest are flue-gas desulfurization and phosphate removal from waste water.