The Influence of Intermolecular Interactions on Second-Order Susceptibilities of Molecular Crystals: Application to the m-Nitroaniline Crystal

The effects of intermolecular interactions on the second-order susceptibilities (⁽²⁾) of molecular crystals are studied by using a semiempirical quantum–chemical method. These effects are discussed on the basis of the super-molecular approach. The influence of the electrostatic interactions between adjacent unit cells on the tensor components of ⁽²⁾ was modeled by using the technique of cumulative atomic multipole moments (CAMMs).     

Palladium-catalyzed synthesis of novel optically active tryptophan analogues

[reaction: see text] Both unsaturated proline derivatives and optically active tryptophan analogues have been obtained via Pd-catalyzed cyclization of aniline-containing acetylenic amino acids. The side chain length of the cyclization precursor determines which one of the two possible products will be formed.     

Evaluation of a dual electrospray ionization/atmospheric pressure chemical ionization source at low flow rates (∼50 µL/min) for the analysis of both highly and weakly polar compounds

The atmospheric pressure ionization (API) source for a commercial mass spectrometer was modified to operate as a dual source in both the electrospray ionization (ESI) and atmospheric pressure chemical ionization (APCI) techniques by simultaneously utilizing the electrospray probe and the corona discharge needle. A switching box was designed to operate in either manual or programmable modes to permit rapid switching between ionization techniques without changing sources, probes, or breaking vacuum. The source can be operated using the following ionization techniques: ESI only, APCI only, ESI/APCI simultaneously, and ESI/APCI alternatingly. The optimum operating conditions for these ionization techniques were similar to the manufacturer’s original specifications except that the APCI flow rate was lower (～50 µL/min versus 1000 µL/min) and externally heated nebulizing gas was found to be desirable. A four-component mixture, introduced by flow injection, was used to demonstrate the versatility of the dual ESI/APCI source.     

Combining gel and capillary electrophoresis, nano-LC and mass spectrometry for the elucidation of post-translational modifications of Trichoderma reesei cellobiohydrolase I

N-Glycosylation of cellobiohydrolase I from the fungus Trichoderma reesei (strain Rut-C30) is studied using a combination of electrophoretic, chromatographic and mass spectrometric techniques. As four potential N-glycosylation sites and several uncharged and phosphorylated high-mannose glycans are present, a large number of glycoforms and phospho-isoforms can be expected. Isoelectric focusing both in gel and in capillary format was successfully applied for the separation of the phospho-isoforms. They were extracted in their intact form from the gel and subsequently analysed by nanospray-Q-TOF-MS, thereby making use of a powerful two-dimensional technique. Nano-LC/MS/MS on a Q-Trap MS further allowed the determination of the glycosylation sites. As a novel approach, an oxonium ion was used in precursor ion scanning for selective detection of glycopeptides containing phosphorylated high-mannose glycans.     

Possible role of relativistic effects in the plasticity of the coordination geometry of cadmium(II). A voltammetric study of the stability of the complexes of cadmium(II) with 12-crown-4,15-crown-5 and 18-crown-6 in aqueous solution and the structures of [Cd(benzo-18-crown-6)(NCS)(2)] and [K(18-crown-6)][Cd(SCN)(3)

A differential pulse voltammetric study of complexes of Cd(II) and Pb(II) with crown ethers is reported. Measured log K(1) values for Cd(II) with 18-crown-6 (1,4,7,10,13,16-hexaoxacyclooctadecane), 15-crown-5 (1,4,7,10,13-pentaoxacyclopentadecane), and 12-crown-4 (1,4,7,10-tetraoxacyclododecane) are respectively 2.53 (+/-0.06), 1.97 (+/-0.07), and 1.72 (+/-0.08) and for Pb(II) with 18-crown-6 is 4.17 (+/-0.03), all at 25 degrees C in 0.1 M LiNO(3). Cd(II) is smaller than is usually associated with strong bonding with crown ethers. The high log K(1) values for Cd(2+) with crown ethers found here are discussed in terms of distortion of Cd(II) by relativistic effects. The resulting plasticity of the coordination geometry of the Cd(II) ion allows it to meet the metal ion size requirements of all the crown ethers, allowing high log K(1) values to occur. Crystal structures for [Cd(bz-18-crown-6)(SCN)(2)] (1) (bz-18-crown-6 = benzo-1,4,7,10,13,16-hexaoxacyclooctadecane) and [K(18-crown-6)][Cd(SCN)(3)] (2) are reported. 1 was triclinic, space group P1, a = 8.5413(2), b = 10.0389(2), and c = 13.4644(2) A, alpha = 94.424(1), beta = 102.286(1), and gamma = 93.236(1) degrees, Z = 2, and final R = 0.023. 2 was orthorhombic, space group Cmc2(1), a = 14.7309(3), b = 15.1647(3), and c = 10.6154(2) A, Z = 4, and final R = 0.020. In 1, the Cd occupies the cavity of the bz-18-crown-6 with long average Cd-O bond lengths of 2.65 A and is N-bonded to the thiocyanates with short average Cd-N bonds of 2.12 A. In [Cd(bz-18-crown-6)(SCN)(2)], the linear coordination involving the Cd and the two N-bonded thiocyanate groups in 1 is discussed in terms of the role of relativistic effects in the tendency to linear coordination geometry in group 12 metal ions. In 2 Cd forms a polymeric structure involving thiocyanate bridges between Cd atoms and K(+) occupies the cavity of the crown ether. 2 highlights the fact that cadmium is almost never S-bonded to thiocyanate except in bridging thiocyanates.     

Emulsion polymerization of styrene and methyl methacrylate using a hydrophobically modified inulin and comparison with other surfactants

The use of a new class of graft polymer surfactants, based on inulin, in emulsion polymerization of poly(methyl methacrylate) (PMMA) and polystyrene (PS) particles is described. PS and PMMA were synthesized by emulsion polymerization, and stable particles with a high monomer content (50 wt %) were obtained with a very small amount of polymeric surfactant ([surfactant]/[monomer] = 0.0033). The latex dispersions were characterized by dynamic light scattering and by transmission electron microscopy to obtain the average particle size and the polydispersity index, and the stability was determined by turbidimetry measurements and expressed in terms of critical coagulation concentration. The last section gives a comparison of PMMA particles prepared by emulsion polymerization using classical surfactants from different types as emulsifiers with that obtained using the copolymer surfactant. It shows the superiority of INUTEC SP1 as it is the only one that allows stable particles at 20 wt % monomer content, with a smaller ratio [surfactant]/[monomer] = 0.002.     

Exact mass measurement of product ions for the structural confirmation and identification of unknown compounds using a quadrupole time-of-flight spectrometer: a simplified approach using combined tandem mass spectrometric functions

This article describes a simple method to perform lock mass corrected accurate mass measurements in tandem mass spectrometry (MS/MS) with a quadrupole time-of-flight (Q-TOF) mass spectrometer. The experimental approach consists of using the protonated molecule of a known compound, which is measured in a MS/MS function using low collision energy (no fragmentation), as mass calibrator. The unknown compound is acquired in MS/MS mode albeit using high collision energy. After the acquisition, the two MS/MS spectra of unknown and mass calibrator are combined, and the fragments of the unknown are lock mass corrected by using the protonated molecule of the mass calibrator. To prove this concept, 10 compounds were analyzed using this approach, the fragments interpreted and, where possible, related to structural data available in the literature. All the unequivocally assigned fragments were accurately mass measured with mass errors within appropriate limits, i.e. for m/z values <200 with a mass tolerance of 3 mDa while for m/z > 200 the mass tolerance is expressed as 10 ppm.     

Monitoring of optical isomers of chiral alcohols and derivatives by chiral gas chromatography. Effect of derivatization on the enantio-differentiation

Summary Gas chromatographic chiral separation of several chiral 2-and 3-alkanols and diols was studied both in their free hydroxyl and in their trimethylsilyl ether forms. First, the derivatization procedure was verified through the identification of the trimethylsilyl ethers formed on the basis of their mass spectra and optimized to obtain quantitative reaction. The optimized procedure was applied to the trimethylsilylation of racemic mixtures of various hydroxyl compounds. The silylation was found to be highly effective in the improvement of the separation of the individual enantiomers. The major advantages of the derivatization process can be summarized as: (i) excellent baseline separation of the enantiomers of the silyl ethers was achieved in contrast to the parent OH-containing compounds, (ii) the sensitivity of detection highly increased, (iii) the separations do not show any significant concentration dependence and finally (iv) the analysis time needed decreased significantly. Presented at: Balaton Symposium on High-Performance Separation Methods, Siófok, Hungary, September 3–5, 1997     

Capillary electrophoretic studies of acid-base properties of sanguinarine and chelerythrine alkaloids

Capillary zone electrophoresis with UV detection was used for determination of dissociation constants of alkaloids sanguinarine and chelerythrine. Despite the limited solubility of the uncharged forms of the alkaloids resulting in insufficient analytical signal at higher pH the reliable dissociation constants were obtained when acidified samples containing low amount of the alkaloid were injected into the capillary. The precipitation of the alkaloid in the capillary induced by injecting sample of low pH into the background electrolyte of higher pH does not affect the mobility of the alkaloid if its concentration injected exceeds the solubility only to a small extent. Dissociation constants (pK(R+)) of sanguinarine and chelerythrine calculated to 8.3 +/- 0.1 and 9.2 +/- 0.1, respectively, are relevant to Good buffers of ionic strength of 30 mM.