Separation of leucine and isoleucine by electrospray ionization-high field asymmetric waveform ion mobility spectrometry-mass spectrometry

An electrospray ionization (ESI) source was used to generate gas-phase molecular anions of the amino acids leucine and isoleucine ((M–H)⁻; m/z −130), which were separated by high- field asymmetric waveform ion mobility spectrometry (FAIMS) and detected by quadrupole mass spectrometry (MS). This combination of ESI-FAIMS-MS enabled selective determination of either amino acid in mixtures that contained at least a 625-fold excess of the other. Comparisons with conventional ESI-MS showed a 50-fold improvement in the signal to background ratio for a 1 μM solution of leucine.     

Simultaneous Measurement of Hydrocarbons and Sulfur Compounds using Flame Ionization and Sulfur Chemiluminescence Detection for Sulfur Simulated Distillation

A gas chromatographic system for the simultaneous acquisition of hydrocarbon and sulfur chromatograms was developed. Detection of sulfur compounds is achieved using a sulfur chemiluminescence detector (SCD) mounted in series with a flame ionization detector (FID). A constant fraction of the effluent of the FID is transferred to the SCD by means of a fixed restrictor. Unlike previous versions of this approach, the FID is not used to generate the chemiluminescent sulfur species. Rather, the FID is operated under optimum conditions for hydrocarbon analysis and a furnace is used to generate the chemiluminescent sulfur species. The system permits dual acquisition of the hydrocarbon and sulfur signals in a single analysis with a single column, since the detectors are operated in a serial fashion. The application of sulfur simulated distillation using this approach was examined, since this requires simultaneous universal and sulfur selective detection. Precision of absolute response of both the FID and SCD was typically less than 2% RSD for a standard reference material.     

Synthesis of divinylbenzene–maleic anhydride microspheres using precipitation polymerization

Narrow disperse micron-range divinylbenzene-maleic anhydride microspheres have been prepared in near quantitative yields using precipitation polymerization. A variety of solvents were investigated for use as the reaction medium with a 40:60 mixture of methyl ethyl ketone and heptane providing the best results. The effects of solvent composition on particle size and morphology and monomer loading effects were also investigated. Particle size decreased with increasing solvency (increasing MEK fraction) while increases in monomer loading caused larger particle sizes. © 1998 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 36: 2223–2227, 1998     

Electrophilic aromatic substitution. 14. [1] A kinetic,NMR, and Raman study of the aluminum chloride-catalyzed reaction of p-toluene-sulfonyl chloride with benzene and toluene in dichloromethane

Vacuum line kinetic studies of the reaction of p-toluenesulfonyl chloride and benzene or toluene, using aluminum chloride as the catalyst and dichloromethane as the solvent were determined at 25°C by means of gas chromatography. The reaction is first-order in arene, tosyl chloride, and in AlCl₃ as catalyst. Noncompetitive results are k_T/k_B=22±7 with a product sulfone isomer distribution: ortho, 14±1%; meta, 4.3±0.2%; and para 82±1%. With hexadeuteriobenzene k_H/k_D was determined to be 1.8±0.1. Rate constant ratios and product isomer distributions were also determined competitively: with AlCl₃, k_T/k_B=30±2; % ortho, 13±1; % meta, 4.0±0.5; % para, 84±3; with SbCl₅, k_T/k_B=40±4; % ortho 10.3±0.4; % meta, 4.7±0.2; and % para, 85.0±0.5. The k_T/k_B ratio for AlCl₃ and the meta sulfone product percentages for both AlCl₃ and SbCl₅ are considerably higher than those reported in the literature. NMR and Raman studies suggest a molecular complex between p-tosyl chloride and AlCl₃, with coordination through oxygen as the dominant species and the probable electrophile in CH₂Cl₂. A reaction mechanism consistent with the kinetic and spectroscopic results is proposed. © 1998 John Wiley & Sons, Inc. Int J Chem Kinet 30: 367–372,1998     

A fast and Space-efficient boundary element method for computing electrostatic and hydration effects in large molecules

At present, there are two widely used approaches for computing molecular hydration and electrostatic effects within the continuum approximation: the finite difference method, in which the electric potential is directly computed on a cubic grid, and the induced polarization charge or boundary element method, in which an induced charge distribution is first computed on the molecular surface and in which solvation effects are then calculated by reference to the reaction field arising from this induced surface charge. While the induced surface charge approach has a number of advantages over finite differences, especially in the computation of hydration forces and solvent stabilization, the applications of this technique have been largely restricted to small molecules. This is primarily due to the very large system of equations that results when the surface of a macromolecule is discretized into elements small enough to ensure an acceptable level of numerical accuracy within the continuum model. This article describes a new algorithm for implementing boundary element calculations within the continuum model. The essence of our approach is only to compute explicitly those interactions between surface elements that are relatively close together and to approximate long-range interactions by grid-based multipole expansion. The resulting system of equations has a relatively sparse coefficient matrix and requires disk storage that increases linearly with molecular surface area. The technique has numerous applications in the analysis of solvation effects in large molecules, especially in the area of conformational analysis, where it is critical to accurately estimate the global hydration energy for the entire structure. © 1996 by John Wiley & Sons, Inc.     

Hexagonal terpyridine--ruthenium and -iron macrocyclic complexes by stepwise and self-assembly procedures

Methods for the self-assembly, as well as directed construction, of hexaruthenium metallomacrocycles employing bisterpyridine building blocks are described. Self-assembly is effected by a combination of equimolar mixtures of bismetalated and nonmetalated bis(terpyridinyl) monomers each possessing the requisite planar, 60 degrees, terpyridine-metal-terpyridine connectivity. Stepwise synthesis of the identical hexamer is also discussed and used to aid in verification of the self-assembled product. Preparation and analysis of the related FeII metallomacrocycle are detailed and its TEM image confirms the hexameric structure. Characterization of the metalated products includes cyclic voltammetry along with the routine analytical techniques.     

Selective ortho methylation of nitroheteroaryls by vicarious nucleophilic substitution

An efficient and scalable three-step one-pot approach to 6-methyl-5-nitroisoquinoline (1) from inexpensive 5-nitroisoquinoline, utilizing the vicarious nucleophilic substitution (VNS) as a key step, is described. The optimized reaction conditions can be applied to a limited number of other aromatic and heteroaromatic nitro compounds. Attempts to understand the observed selectivity in the VNS step led to the discovery of two new reaction pathways under VNS conditions, one leading to an isoxazole and the other resulting in the formal cyclopropanation of an aromatic nitro compound.