Versatile inlet system for on-line compound-specific deltaD and delta13C gas chromatography-oxidation/reduction-isotope ratio mass spectrometry analysis of gaseous mixtures

Compound-specific deltaD and delta13C analyses of gas mixtures are useful indicators of geochemical and environmental factors. However, the relative concentrations of individual components in gas mixtures (e.g., H2, CO2, methane, ethane, propane, i-butane, n-butane) may vary over several orders of magnitude. The determination of hydrogen and carbon compound-specific stable isotope ratios requires that the hydrogen and carbon dioxide produced from each separated component has a concentration adjusted to match the dynamic range of the stable isotope mass spectrometer. We present a custom-built gas sampling and injection system (GASIS) linked with a Delta Plus XP mass spectrometer that provides flexibility, ease of operation, and economical use of small gas samples with wide ranges of analyte concentrations. The overall on-line GC-ox/red-IRMS (Gas Chromatography - oxidation/reduction - Isotope Ratio Mass Spectrometry) system consists of (i) a customized GASIS inlet system and (ii) two alternative reactors, namely an oxidative Cu-Ni-Pt reactor at 950 degrees C for production of CO2 and a reductive graphitized Al2O3 reactor at 1420 degrees C for production of H2. In addition, the system is equipped with (iii) a liquid nitrogen spray-cooling unit for cryo-GC-focusing at -20 degrees C, and (iv) a Nafion dryer for removal of water vapor from product CO2. The three injection loops of the GASIS inlet allow flexibility in the volume of injected analyte gas (e.g., from 0.06 to 500 microL) in order to measure reproducible deltaD and delta13C values for gases at concentrations ranging from 100% down to 10 ppm. We calibrate our GC-ox/red-IRMS system with two isotopically distinct methane references gases that are combusted off-line and characterized using dual-inlet IRMS.     

Chiral separation of highly negatively charged enantiomers by capillary electrophoresis

The separation of two highly negatively charged enantiomeric organic disulfates containing two chiral centers was investigated by capillary electrophoresis using cyclodextrin based chiral selectors added to the run buffer. The optimum separation for the enantiomers was achieved in less than 3 min at 25 degrees C with a run buffer of 10 mM glycine pH 2.4 and 5 mM QA-beta-CD, which is a positively charged quaternary ammonium beta-cyclodextrin derivative. The method resulted in baseline resolution, excellent linearity, and highly reproducible migration times allowing facile evaluation of the enantiomeric purity of the individual isomers. Detection limits for the enantiomeric pair were determined to be 0.3 ng/microl (S/N = 3). The nature of the selector-enantiomer interaction and a quantitative measurement of the apparent stability constants that governed chiral discrimination of the enantiomers with QA-beta-CD were also investigated by UV-Vis spectroscopy and electrospray ionization mass spectrometry.     

In Appreciation¶A Lifetime of Connections: Otto Herbert Schmitt, 1913 - 1998

Otto H. Schmitt was born in St. Louis, Missouri, in 1913. As a youth, he displayed an affinity for electrical engineering but also pursued a wide range of other interests. He applied his multi-disciplinary talents as an undergraduate and graduate student at Washington University, where he worked in three departments: physics, zoology, and mathematics. For his doctoral research, Schmitt designed and built an electronic device to mimic the propagation of action potentials along nerve fibers. His most famous invention, now called the Schmitt trigger, arose from this early research. Schmitt spent most of his career at the University of Minnesota, where he did pioneering work in biophysics and bioengineering. He also worked at national and international levels to place biophysics and bioengineering on sound institutional footings. His years at Minnesota were interrupted by World War II. During that conflict - and the initial months of the Cold War to follow - Schmitt carried out defense-related research at the Airborne Instruments Laboratory in New York. Toward the end of his career at Minnesota, Schmitt coined the term biomimetics. He died in 1998. RID="*" ID="*"Jon M. Harkness received his Ph.D. degree in the history of science from the University of Wisconsin in 1996. During the spring of 2002, he is an adjunct assistant professor of the history of medicine at the University of Minnesota.     

A study of the thermal denaturation of ribonuclease S by electrospray ionization mass spectrometry

The thermal stability of ribonuclease S (RNase S), an enzymatically active noncovalent complex composed of a 2166-u peptide (S-peptide) and a 11,534-u protein (S-protein), was investigated by electrospray ionization mass spectrometry (ESI-MS) and capillary electrophoresis ESI-MS (CE-ESI-MS). The intensities of peaks corresponding to the RNase S complex were inversely related to both the applied nozzle-skimmer (or capillary-skimmer) voltage bias in the atmosphere-vacuum interface and the temperature of the RNase S solution. By using a heated metal capillary-skimmer interface and a room temperature solution of RNase S, the intensities of RNase S molecular ion peaks were observed to decrease with increasing metal capillary temperature. Mass spectrometric studies with both the nozzle-skimmer and capillary-skimmer interface designs allowed determination of phenomenological enthalpies for dissociation of the RNase S complex in both solution and for the electrosprayed microdroplet-gas phase species. Intact RNase S complex could also be detected with CE-ESI-MS separations by using a 10-mM ammonium bicarbonate (pH 7.9) solution as the electrophoretic buffer. These studies provide new insights into the stability of multiply charged noncovalent complexes in the gas phase and the mass spectrometric conditions required for such studies, and suggest that information regarding solution properties can be obtained by ESI-MS.     

N-activated β-lactams as versatile reagents for acyl carrier protein labeling

Acyl carrier proteins are critical components of fatty acid and polyketide biosynthesis. Their primary function is to shuttle intermediates between active sites via a covalently bound phosphopantetheine arm. Small molecules capable of acylating this prosthetic group will provide a simple and reversible means of introducing novel functionality onto carrier protein domains. A series of N-activated β-lactams are prepared to examine site-specific acylation of the phosphopantetheine-thiol. In general, β-lactams are found to be significantly more reactive than our previously studied β-lactones. Selectivity for the holo over apo-form of acyl carrier proteins is demonstrated indicating that only the phosphopantetheine-thiol is modified. Incorporation of an N-propargyloxycarbonyl group provides an alkyne handle for conjugation to fluorophores and affinity labels. The utility of these groups for mechanistic interrogation of a critical step in polyketide biosynthesis is examined through comparison to traditional probes. In all, we expect the probes described in this study to serve as valuable and versatile tools for mechanistic interrogation.     

Classical chemical concepts from ab initio SCF calculations

Alternative definitions of bond order, valency, gross orbital populations and total atomic charge for SCF wavefunctions are compared. It is found that there are sound theoretical and numerical reasons for preferring definitions based on the Löwdin density matrix.     

Sigmatropic Rearrangement of Vinyl Aziridines: Expedient Synthesis of Cyclic Sulfoximines from Chiral Sulfinimines

A novel rearrangement of 2‐vinyl aziridine 2‐carboxylates to unusual chiral cyclic sulfoximines is described herein. The method allows the synthesis of substituted cyclic sulfoximines in high yields with complete stereocontrol, and tolerates a wide substrate scope. A one‐pot process starting directly from sulfinimines provides access to complex chiral sulfoximines in only two steps from commercially available aldehydes. A mechanistic hypothesis and synthetic application in the formal synthesis of trachelanthamidine, by transformation of a cyclic sulfoximine into a pyrroline, is also disclosed.     

Enhanced nucleation, smoothness and conformality of ultrananocrystalline diamond (UNCD) ultrathin films via tungsten interlayers

Extremely smooth (6 nm RMS roughness over 4 μm²), thin (100 nm), and continuous ultrananocrystalline diamond (UNCD) films were synthesized by microwave plasma chemical vapor deposition using a 10 nm tungsten (W) interlayer between the silicon substrate and the diamond film. These UNCD films possess a high content of sp³-bonded carbon. The W interlayer significantly increased the initial diamond nucleation density, thereby lowering the surface roughness, eliminating interfacial voids, and allowing thinner UNCD films to be grown. This structural optimization enhances the films’ properties and enables its integration with a wide variety of substrate materials.     

Strategies for advanced applications of permutation-inversion groups to the microwave spectra of molecules with large amplitude motions

This article presents permutation-inversion group-theoretical strategies and recipes aimed at helping a high-resolution molecular spectroscopist use the existing pedagogical literature to carry out their own treatment of the basic symmetry questions in rotating molecules with large-amplitude vibrational motions. Topics addressed include: determination of the feasible permutation-inversion group and its symmetry species and character table; a general equation defining coordinates that can describe translation, overall rotation, large-amplitude vibrations and small-amplitude vibrations for a large class of floppy molecules; and the determination of symmetry species for basis functions and selection rules for operators written in these coordinates. The article is intended to be more advanced than existing pedagogical works, but it still leaves many important topics untreated.