Surfactant adsorption at the salt/water interface: comparing the conformation and interfacial water structure for selected surfactants

We report in situ spectroscopic measurements monitoring the adsorption of a series of carboxylate surfactants onto the surface of the semisoluble, ionic solid fluorite (CaF2). We employ the surface-specific technique, vibrational sum-frequency spectroscopy (VSFS), to examine the effect that surfactant adsorption has on the bonding interactions and orientation of interfacial water molecules through the alteration of the electric properties in the interfacial region. In addition, we report on the chain length and headgroup dependence of the formation of hydrophobic self-assembled monolayers on the surface of the solid phase. Differences in chain length and headgroup functionality lead to large changes in the adsorption behavior and structuring of the monolayers formed and the interactions of interfacial water molecules with these monolayers. Fundamental studies such as these are essential for understanding the mechanisms involved in the surfactant adsorption process, information that is important for industrially relevant processes such as mineral ore flotation, waste processing, and petroleum recovery.     

Investigation of pyrrolizidine alkaloids and their N-oxides in commercial comfrey-containing products and botanical materials by liquid chromatography electrospray ionization mass spectrometry

Pyrrolizidine alkaloids (PAs) and their N-oxides are found in several plant families throughout the world. PAs are potentially toxic to the liver and/or lungs in humans and may cause acute liver failure, cirrhosis, pneumonitis, or pulmonary hypertension. PAs are also carcinogenic to animals, and they have been linked to the development of hepatocellular and skin squamous cell carcinomas as well as liver angiosarcomas. According to experimental studies, the quantity of PAs in some herbal teas and dietary supplements is sufficient to be carcinogenic in exposed individuals. A method for the extraction and identification of PAs and their N-oxides in botanical materials and commercial comfrey-containing products has been developed using liquid chromatography electrospray ionization mass spectrometry. Following optimization of the extraction procedure and the chromatographic conditions, the method was applied to the analysis of 10 herbal remedies. All of the products that were labeled to contain comfrey were found to contain measurable quantities of PAs.     

Ag+ labeling: a convenient new tool for the characterization of hydrogen-bonded supramolecular assemblies by MALDI-TOF mass spectrometry

Herein we describe our results on the characterization of a wide variety of different hydrogen-bonded assemblies by means of a novel matrix-assisted laser desorption ionization time-of-flight mass spectrometry (MALDI-TOF MS) technique with Ag+ labeling. The labeling technique with Ag+ ions is extremely mild and provides a nondestructive way to generate charged assemblies that can be detected by mass spectrometry. Up to now more than 25 different single (1(3).2(3)), double (3(3).2(6)), and tetrarosettes (4(3).2(12)) have been successfully characterized by the use of this method. The success of the method entirely depends on the presence of a suitable binding site for the Ag+ ion. A variety of functionalities has been identified that provide strong binding sites for Ag+, either acting in a cooperative way (pi-arene and pi-alkene donor functionalities) or individually (cyano and crown ether functionalities). The method works well for assemblies with molecular weights between 2,000 and 8,000 Da, and most likely far beyond this limit.     

Photochemical thallimetric oxidations-estimation of formic acid

A simple, rapid and convenient redox method has been developed for the estimation of formic acid. Formic acid is photochemically oxidized with thallium(III) in the presence of bromide as catalyst, and the thallium(I) formed is determined by titration with potassium bromate. The procedure can also be used for the estimation of thallium(III) with formic acid as reductant.     

Synthesis of novel retinoid X receptor-selective retinoids

Retinoids 1-5 have been identified as potent RXR agonists for evaluation in the treatment of non-insulin-dependent (type II) diabetes mellitus (NIDDM). Highly convergent syntheses of 1-5 have been developed. The core tetrahydronaphthalene 7, employed in the synthesis of 1 and 2, was prepared in 98% yield using an AlCl(3)-catalyzed (0.03 equiv) Friedel-Crafts alkylation of toluene with 2,5-dichloro-2,5-dimethylhexane 6. A nitromethane-mediated Fridel-Crafts acylation of 7 with chloromethylnicotinate 9 was developed to prepare ketone 10 in 68% yield. Chelate-controlled addition of MeMgCl to 10 followed by dehydration afforded olefin 11 in 65% yield. Cyclopropanation of 11 with trimethylsulfoxonium ylide, followed by saponification, completed a five-step synthesis of 1 in 33% yield. FeCl(3)-catalyzed (0.05 equiv) Friedel-Crafts acylation of 7 with chloromethylterephthalate 14 afforded ketone 15 in 81% yield. Saponification of 15 and reaction with 50% aqueous NH(2)OH in AcOH afforded a 9:1 mixture of cis and trans oximes, from which the desired cis-oxime 2 was isolated in 43% yield. The core bromo-dihydronaphthalene 29 required for the synthesis of 3-5 was prepared by a Shapiro reaction. Transmetalation of 29 and reaction with Weinreb amides 30b or 36 afforded ketones 32 and 37, which were converted into 3-5 using chemistry comparable to the tetrahydronaphthylene series. Suzuki coupling of boronic acids 41 and 42 with vinyl triflate 43 provided an alternative approach to the synthesis of this class of compounds.     

Mechanism of silver-mediated di-tert-butylsilylene transfer from a silacyclopropane to an alkene

Kinetic studies of the reactions of cyclohexene silacyclopropane 1 and monosubstituted alkenes in the presence of 5 mol % of (Ph3P)2AgOTf suggested a possible mechanism for silver-mediated di-tert-butylsilylene transfer. The kinetic order in cyclohexene silacyclopropane 1 was determined to be one. Inverse kinetic saturation behavior (rate inhibition) was observed in monosubstituted alkene and cyclohexene concentrations. Saturation kinetic behavior in catalyst concentration was observed. A reactive intermediate, a silylsilver complex, was observed using low temperature 29Si NMR spectroscopy. Competition experiments between substituted styrenes and a deficient amount of 1 correlated well with the Hammett equation and provided a rho value of -0.62 +/- 0.02 using sigmap constants. These data support a mechanism involving reversible silver-promoted di-tert-butylsilylene extrusion from 1 followed by irreversible concerted electrophilic attack of the silylsilver intermediate on the alkene.     

Structural analysis of phosphatidylcholines by post-source decay matrix-assisted laser desorption/ionization time-of-flight mass spectrometry

The utility of post-source decay (PSD) matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF-MS) was investigated for the structural analysis of phosphatidylcholine (PC). PC did not produce detectable negative molecular ion from MALDI, but positive ions were observed as both [PC+H](+) and [PC+Na](+). The PSD spectra of the protonated PC species contained only one fragment corresponding to the head group (m/z 184), while the sodiated precursors produced many fragment ions, including those derived from the loss of fatty acids. The loss of fatty acid from the C-1 position (sn-1) of the glycerol backbone was favored over the loss of fatty acid from the C-2 position (sn-2). Ions emanating from the fragmentation of the head group (phosphocholine) included [PC+Na-59](+), [PC+Na-183](+) and [PC+Na-205](+), which corresponded to the loss of trimethylamine (TMA), non-sodiated choline phosphate and sodiated choline phosphate, respectively. Other fragments reflecting the structure of the head group were observed at m/z 183, 146 and 86. The difference in the fragmentation patterns for the PSD of [PC+Na](+) compared to [PC+H](+) is attributed to difference in the binding of Na(+) and H(+). While the proton binds to a negatively charged oxygen of the phosphate group, the sodium ion can be associated with several regions of the PC molecule. Hence, in the sodiated PC, intermolecular interaction of the negatively charged oxygen of the phosphate group, along with sodium association at multiple sites, can lead to a complex and characteristic ion fragmentation pattern. The preferential loss of sn-1 fatty acid group could be explained by the formation of an energetically favorable six-member ring intermediate, as apposed to the five-member ring intermediate formed prior to the loss of sn-2 fatty acid group.