Thermal and mass spectrometric fragmentation of esters of deuterated long chain carboxylic acids

A method combining thermal fragmentation and mass spectrometry for the determination of the position of double bonds in an unsaturated ester is presented. The thermal fragmentation of methyl esters of deuterated long chain carboxylic acids yields a homologous series of olefins plus a homologous series of unsaturated esters. The positions of the deuterium atoms in the original ester are revealed by the deuterium content of its fragments as determined by mass spectrometry. Therefore, the positions of double bonds of a polyunsaturated acid can be determined by pyrolysis after saturation by deuterium. The structures of the unsaturated fragments are ascertained by mass spectrometric method, and the formation of the ion [M – 32] in the mass spectrometric fragmentation of unsaturated methyl esters is studied by means of deuterium labeling.     

Autocorrelated 13c-13c double quantum coherence two-dimensional nmr spectroscopy: Utilization of a modified version of the technique as an adjunct in the total assignment of the 1h- and 13c-nmr spectra of the mutagen phenanthro[3,4-b]thiophene

Development of successively higher field nmr spectrometers has facilitated the study of increasingly more complex molecules, although smaller molecules such as phenanthro[3,4-b]thiophene still offer very substantial assignment problems because of the highly congested nature of their ¹H- and ¹³C-nmr spectra. Assignments of such spectra, if they are to be unequivocal, frequently require the utilization of two-dimensional nmr spectroscopic techniques. Total assignments of the ¹H- and ¹³C-nmr spectra of phenanthro[3,4-b]thiophene are reported. Assignments were based on a conventional high resolution 500 MHz ¹H-nmr spectrum, autocorrelated two-dimensional ¹H-nmr spectra (COSY), two-dimensional ¹H-¹³C chemical shift correlation spectra and a modified version of autocorrelated ¹³C-¹³C double quantum coherence two-dimensional nmr spectroscopy. From NOE measurements, a separation of 1.99 Å between H1 and H11 was computed, suggesting that phenanthro[3,4-b]thiophene has a pronounced helical conformation in solution.     

Characterization of nonderivatized plant cell walls using high-resolution solution-state NMR spectroscopy

A recently described plant cell wall dissolution system has been modified to use perdeuterated solvents to allow direct in-NMR-tube dissolution and high-resolution solution-state NMR of the whole cell wall without derivatization. Finely ground cell wall material dissolves in a solvent system containing dimethylsulfoxide-d(6) and 1-methylimidazole-d(6) in a ratio of 4:1 (v/v), keeping wood component structures mainly intact in their near-native state. Two-dimensional NMR experiments, using gradient-HSQC (heteronuclear single quantum coherence) 1-bond (13)C--(1)H correlation spectroscopy, on nonderivatized cell wall material from a representative gymnosperm pinus taeda (loblolly pine), an angiosperm Populus tremuloides (quaking aspen), and a herbaceous plant Hibiscus cannabinus (kenaf) demonstrate the efficacy of the system. We describe a method to synthesize 1-methylimidazole-d(6) with a high degree of perdeuteration, thus allowing cell wall dissolution and NMR characterization of nonderivatized plant cell wall structures.     

Application of double-spike isotope dilution for the accurate determination of Cr(III), Cr(VI) and total Cr in yeast

A method is presented for the simultaneous determination of Cr(III) and Cr(VI) in yeast using species-specific double-spike isotope dilution (SSDSID) with anion-exchange liquid chromatography (LC) separation and sector field inductively coupled plasma mass spectrometric (SF-ICP-MS) detection. Total Cr is quantitated using ID SF-ICP-MS. Samples were digested on a hot plate at 95±2 °C for 6 h in an alkaline solution of 0.5 M NaOH and 0.28 M Na₂CO₃ for the determination of Cr(III) and Cr(VI), whereas microwave-assisted decomposition with HNO₃ and H₂O₂ was used for the determination of total Cr. Concentrations of 2,014±16, 1,952±103 and 76±48 mg kg⁻¹ (one standard deviation, n=4, 3, 3), respectively were obtained for total Cr, Cr(III) and Cr(VI) in the yeast sample. Significant oxidation of Cr(III) to Cr(VI) (24.2±7.6% Cr(III) oxidized, n=3) and reduction of Cr(VI) to Cr(III) (37.6±6.5% Cr(VI) reduced, n=3 ) occurred during alkaline extraction and subsequent chromatographic separation at pH 7. Despite this significant bidirectional redox transformation, quantitative recoveries for both Cr(III) and Cr(VI) were achieved using the SSDSID method. In addition, mass balance between total Cr and the sum of Cr(III) and Cr(VI) concentrations was achieved. Method detection limits of 0.3, 2 and 30 mg kg⁻¹ were obtained for total Cr, Cr(VI) and Cr(III), respectively, based on a 0.2-g sub-sample.     

A comparison of the binding of metal complexes to duplex and quadruplex DNA

Electrospray ionisation mass spectrometry (ESI-MS) and circular dichroism (CD) spectroscopy were used to compare the binding of mononuclear nickel, ruthenium and platinum complexes to double stranded DNA (dsDNA) and quadruplex DNA (qDNA). CD studies provided evidence for the binding of intact complexes of all three metal ions to qDNA. ESI mass spectra of solutions containing platinum or ruthenium complexes and qDNA showed evidence for the formation of non-covalent complexes consisting of intact metal molecules bound to DNA. However, the corresponding spectra of solutions containing nickel complexes principally contained ions consisting of fragments of the initial nickel molecule bound to qDNA. In contrast ESI mass spectra of solutions containing nickel, ruthenium or platinum complexes and dsDNA only showed the presence of ions attributable to intact metal molecules bound to DNA. The fragmentation observed in mass spectral studies of solutions containing nickel complexes and qDNA is attributable to the lower thermodynamic stability of the former metal complexes relative to those containing platinum or ruthenium, as well as the slightly harsher instrumental conditions required to obtain spectra of qDNA. This conclusion is supported by the results of tandem mass spectral studies, which showed that ions consisting of intact nickel complexes bound to qDNA readily undergo fragmentation by loss of one of the ligands initially bound to the metal. The ESI-MS results also demonstrate that the binding affinity of each of the platinum and ruthenium complexes towards qDNA is significantly less than that towards dsDNA.     

A comparative study of myo-inositol quantification using LCmodel at 1.5 T and 3.0 T with 3 D 1H proton spectroscopic imaging of the human brain

Myo-inositol is a strongly coupled system and resonates at four chemical shift positions. At 1.5 T, only the singlet component at 3.57 ppm is detected. However, at 3 T this resonance is resolved into its components at 3.55 ppm and 3.61 ppm. Due to the increased spectral resolution and signal-to-noise ratio, it is anticipated that the quantification of myo-inositol should improve at 3 T. Using data from normal controls and the LCmodel quantification procedure, we found that the quantification precision, reproducibility and detection sensitivity of myo-inositol is significantly better at 3 T relative to 1.5 T.     

Enhanced mass resolution method development, validation and assay application to support preclinical studies of a new drug candidate

A very highly sensitive and highly selective liquid chromatographic/tandem mass spectrometric (LC/MS/MS) method was developed to evaluate and quantify a new drug candidate in different biological matrices. Following a simple plasma protein precipitation using acetonitrile, the post-treatment samples were analyzed on a C18 column interfaced with a new generation of triple-quadrupole mass spectrometer. The recently introduced triple-quadrupole mass spectrometer, the TSQ Quantum Ultra, with enhanced mass-resolution capability, demonstrated improved sensitivity (0.05 ng/mL), coupled with suitable accuracy and precision, over a broad linear dynamic range (0.05-1000 ng/mL). A comparison of the assay performance data (dynamic range, calibration curve equation, precision and accuracy) of the enhanced resolution method against a unit resolution method under optimized conditions showed the performance improvement of the enhanced mass resolution method for bioanalytical high-throughput applications. The enhanced mass resolution method herein described was successfully applied to the evaluation of the pharmacokinetic profile of a new drug candidate in rat, rabbit and dog plasma samples.     

Boron 1s photoelectron spectrum of 11BF3: vibrational structure and linewidth

The boron 1s photoelectron spectrum of (11)BF(3) has been measured at a photon energy of 400 eV and a resolution of about 55 meV. The pronounced vibrational structure seen in the spectrum has been analyzed to give the harmonic and anharmonic vibrational frequencies of the symmetric stretching mode, 128.1 and 0.15 meV, as well as the change in equilibrium BF bond length upon ionization, -5.83 pm. A similar change in bond length has been observed for PF(3) and SiF(4), but a much smaller change for CF(4). Theoretical calculations for BF(3) that include the effects of electron correlation give results that are in reasonable accord with the experimental values. The Lorentzian (lifetime) width of the boron 1s core hole in BF(3) is found to be 72 meV, comparable to the value of 77 meV that has been reported for CF(4).     

One-pot synthesis of new symmetric and asymmetric xanthene dyes

Symmetric and asymmetric xanthene dyes have been prepared by a convenient one-step procedure from aldehyde and diol or m-aminophenol precursors using concentrated phosphoric acid as a solvent. This protocol provides access to water-soluble dyes with large Stoke’s shifts and far-red fluorescence emission. These compounds are envisioned as components of fluorescence-based sensors for a variety of imaging applications.     

Microfluidic devices for energy conversion: planar integration and performance of a passive, fully immersed H2-O2 fuel cell

We describe the fabrication and performance of a passive, microfluidics-based H2-O2 microfluidic fuel cell using thin film Pt electrodes embedded in a poly(dimethylsiloxane) (PDMS) device. The electrode array is fully immersed in a liquid electrolyte confined inside the microchannel network, which serves also as a thin gas-permeable membrane through which the reactants are fed to the electrodes. The cell operates at room temperature with a maximum power density of around 700 microW/cm(2), while its performance, as recorded by monitoring the corresponding polarization curves and the power density plots, is affected by the pH of the electrolyte, its concentration, the surface area of the Pt electrodes, and the thickness of the PDMS membrane. The best results were obtained in basic solutions using electrochemically roughened Pt electrodes, the roughness factor, R(f), of which was around 90 relative to a smooth Pt film. In addition, the operating lifetime of the fuel cell was found to be longer for the one using higher surface area electrodes.