Ring size effects. III. Carbonium ion stability as a factor in the course of the pinacol rearrangement

The formation of the initial carbonium ion in the pinacol rearrangement will be the major factor in determining product composition when the reaction is carried out at 0°C with concentrated sulfuric acid.     

Investigation of bicyclic thioketones as triggers for liquid crystal optical switches

The axially chiral bicyclic thioketones 11 and 15 were prepared and investigated for suitability as chiroptical triggers in a liquid crystal optical switch. Irradiation of partially resolved 15 with unpolarized light leads to its conversion to the racemic form (photoracemization). However, irradiation of racemic thioketones 11 and 15 with circularly polarized light does not lead to detectable photoresolution. The lack of photoresolution was traced to inefficiency in intramolecular, through-bond triplet energy transfer. These thioketones are not suitable for use as phototriggers.     

The amperometric detection of thyroid hormones following reverse-phase high-performance liquid chromatography

The principle of an assay of the major thyroid hormones by an electrochemical technique is demonstrated. The separation of 3,3',5-triiodothyronine, 3,3',5'-triiodothyronine, and thyroxine, by reverse-phase high-performance liquid chromatography is followed by their electrochemical oxidation in a thin-layer electrochemical detection cell with a low-temperature isotropic carbon working electrode. The limits of detection found were in the subnanogram range with linear response in the ranges 0–125 ng for T₃ and 0–500 ng for T₄. The approach makes the simultaneous assay of total serum thyroid hormones feasible.     

Separation of phospholipids by high-performance liquid chromatography: all major classes, including ethanolamine and choline plasmalogens, and most minor classes, including lysophosphatidylethanolamine

High-performance liquid chromatographic methods for the separation and quantitation of phospholipids were developed and shown to give sensitive, reliable measurements of tissue phospholipids, including difficult-to-resolve pairs such as choline plasmalogen (plasmenylcholine) and phosphatidylcholine, choline glycerophospholipids and sphingomyelin, phosphatidylinositol and phosphatidylserine, and phosphatidylserine and lysophosphatidylethanolamine. Separations of most phospholipids including those mentioned above are more complete than in existing procedures, and require only 40 min per injection. Utilization of the hexane-2-propanol-water system has an advantage over separation techniques that employ acidic solvents in that the plasmalogens are not hydrolyzed and a less degradative environment for labile lipids is provided. Further, a rapid high-performance liquid chromatographic procedure for the separation of intact ethanolamine plasmalogen (plasmenylethanolamine) from phosphatidylethanolamine was developed. Previous procedures have required derivatized samples or acid hydrolysis of the plasmalogen vinyl ether linkage. A slight modification of the primary method (method I) increases the resolution of lysophosphatidylethanolamine from other classes (method II). A third modification (method III) can replace the standard silicic acid column separation of lipids into neutral, glycolipid, and phospholipid fractions.     

Determination of Monoterpenes in Oleoresin: A Chemosystematic Study of the Interaction between Fraser Fir (Abies fraseri) and Balsam Woolly Adelgid (Adelges piceae)

The Fraser fir (Abies fraseri), a conifer native to high elevation sites in the southern Appalachians, has suffered severe mortality attributed to an exotic insect, the balsam woolly adelgid (BWA) (Adelges piceae). Fraser fir, like many other plants, produce volatile organic compounds called monoterpenes in a variety of tissues, including the cortical oleoresin found in blisters on the bark. The purpose of this study was to perform a chemosystematic study to evaluate whether differences in monoterpene composition of Fraser fir oleoresin were observed as a function of the season sampled, the presence or absence of BWA infestation, the health of a tree, and the susceptibility of trees to BWA, based on geographic location. Oleoresin was collected in September and December, 1994, at two mountains, Mount Rogers, where the fir were relatively healthy, and Roan Mountain, where heavy to moderate mortality has been observed. The oleoresin samples were dissolved in methylene chloride, and the monoterpenes were determined by gas chromatography. This method was shown to have acceptable precision for chemosystematic studies for five monoterpenes and the total of all monoterpenes. The levels of monoterpenes in oleoresin were statistically the same for most compounds in September and December, although the total monoterpenes were higher in September at Mount Rogers and α-pinene was higher in December at Roan Mountain. Infested and uninfested fir had the same levels of oleoresin monoterpenes at both mountains. No differences in monoterpene levels were observed at Mount Rogers with healthy and unhealthy fir, but at Roan Mountain, the total monoterpene concentrations were different in these categories. A comparison of monoterpene concentrations in oleoresin from Mount Rogers and Roan Mountain showed statistically significant differences for 3-carene, β-phellandrene, and total monoterpenes.     

Liquid chromatographic separation of the thyroid hormones

The liquid Chromatographic separation of 3,3',5-triiodothyronine, 3,3',5'-triiodothyronine, and thyroxine with a nonpolar stationary phase was studied as a function of pH, temperature, organic content of the mobile phase, and ionic strength using aqueous phosphate—acetonitrile, aqueous phosphate—methanol, and aqueous phosphate— n-propanol mobile phase systems. It was demonstrated that the quality of the thyroid hormone separations, as determined by normalized peak capacity values, was unchanged with temperature, remained relatively constant with increasing ionic strength, and was affected to the greatest extent by changes in pH and organic modifier content of the mobile phase. Chromatographic behavior of the compounds studied as a function of these variables was found to be consistent with existing Chromatographic theory and/or empirical observations. Recommended conditions are aqueous phosphate—methanol mobile phase, pH 2–5 (aqueous portion), and high temperature (60–70°C).     

Octylgermane on gold: synthesis, oxidation, and pattern formation

Chemisorbed monolayers of octylgermane (C8H17GeH3) on gold have been formed by vapor deposition in ultrahigh vacuum. The monolayer has been characterized by X-ray photoelectron and reflection absorption infrared spectroscopies (XPS and RAIRS) and scanning tunneling microscopy (STM). XPS data indicate the monolayer can be oxidized by exposure to ozone. STM images exhibit a complex pattern which can be modeled as strain-mediated spinodal decomposition.     

Microfabricated ISEs: critical comparison of inherently conducting polymer and hydrogel based inner contacts

A rigorous side by side comparison of miniature planar potassium-selective electrodes with hydrogel and potassium hexacyanoferrate(II)/(III) doped polypyrrole (PPy/FeCN) based inner contacts is presented. The planar electrodes were manufactured by screen printing as four- and five-site arrays on ceramic substrates. These electrode arrays were incorporated into a flow-through cell, which could accommodate nine electrode sites. Two identical flow cells were connected in series and the effect of the inner contacts on the analytical performance of the respective electrodes has been critically evaluated. The time necessary to reach steady state conditions has been determined and the effect of experimental parameters (temperature, ambient light intensity, CO₂, and O₂ concentration of the sample) on the potential stability of the electrodes was analyzed. At controlled temperature, the drift of the planar potassium electrodes with hydrogel and PPy/FeCN solid contact were 0.11±0.02 mV h⁻¹ and 0.03±0.007 mV h⁻¹, respectively. The experimental data proved that there is no aqueous film formation between the PPy/FeCN film and the potassium-selective solvent polymeric membrane.     

Lipid bilayer disruption by polycationic polymers: the roles of size and chemical functional group

Polycationic polymers are used extensively in biology to disrupt cell membranes and thus enhance the transport of materials into the cell. The highly polydisperse nature of many of these materials makes obtaining a mechanistic understanding of the disruption processes difficult. To design an effective mechanistic study, a monodisperse class of polycationic polymers, poly(amidoamine) (PAMAM) dendrimers, has been studied in the context of supported dimyristoylphosphatidylcholine (DMPC) lipid bilayers using atomic force microscopy (AFM). Aqueous solutions of amine-terminated generation 7 (G7) PAMAM dendrimers caused the formation of 15-40-nm-diameter holes in lipid bilayers. This effect was significantly reduced for smaller G5 dendrimers. For G3, no hole formation was observed. In addition to dendrimer size, surface chemistry had a strong influence on dendrimer-lipid bilayer interactions. In particular, acetamide-terminated G5 did not cause hole formation in bilayers. In all instances, the edges of bilayer defects proved to be points of highest dendrimer activity. A proposed mechanism for the removal of lipids by dendrimers involves the formation of dendrimer-filled lipid vesicles. By considering the thermodynamics, interaction free energy, and geometry of these self-assembled vesicles, a model that explains the influence of polymer particle size and surface chemistry on the interactions with lipid membranes was developed. These results are of general significance for understanding the physical and chemical properties of polycationic polymer interactions with membranes that lead to the transport of materials across cell membranes.