Fast atom bombardment and tandem mass spectrometry of quaternary pyridinium salt-type tryptophan derivatives

Fast atom bombardment and collision-induced dissociation tandem mass spectrometry were used to study the fragmentation of quaternary pyridinium salt-type amides of tryptophan (α-amino-3-indolepropionic acid) esters and their analogs which incorporate the α-nitrogen into the quaternary pyridinium structure. By cleavage directly at the pyridine nitrogen, the 1-alkyl-substituted nicotinamides decompose exclusively to a carbocation, which then becomes the intermediate to further fragments. Rearrangement of the 3-indolepropionate-2-yl carbocations may involve a five- to seven-membered ring expansion, which generates alternative fragmentation pathways; the formation of an even-electron and a radical cation, respectively. In trigonellyl amide-type tryptophan derivatives, fragmentation of the pyridinium ion proceeds on multiple pathways induced by the positive charge which may not be localized on the quaternary nitrogen, and isomerization to a dihydropyridinyl structure is probably involved. Besides the formation of protonated nicotinamide and alkene from tryptophan amides that contain methylene or ethylene units between the amino and the quaternary pyridinium nitrogens, a fragmentation route leading to the carbocation identical with that of the 1-alkyl-substituted nicotinamides has also been revealed.     

Struktur und circular-dichroismus einiger steroidoxime

The correlation between the syn-anti geometry and circular dichroism of several O-benzyl or O-isopropyl substituted steroidoximes 1–22 was investigated. A simple sector rule was found for the longest wavelength CD band of the unsaturated oximes 1–13. The correlations are not so clear for the unsaturated derivatives 14–22.     

Theoretical Insights into the Formation, Structure, and Energetics of Some Cyclodextrin Complexes

To investigate the physicochemical aspects relevant for the formation of various cyclodextrin inclusion complexes and to search for corresponding general structure–complex-stability relationships, stability data of 1 : 1 complexes for 179, 310, and 51 guest molecules with unsubstituted -, -, and -cyclodextrin were collected. Statistical analysis using structure-based parameters such as molecular size, hydrophobicity, rotatable bonds, electronic properties, and the presence or absence of more than 150 various functional or structural moieties were performed. The complexation thermodynamics could be well described within the framework of our recently introduced molecular size-based model for nonassociative liquids. With increasing guest size, 1 : 1 complex stability, as measured by ln K or G⁰, increases linearly up to a size limit characteristic for each CD, and the corresponding slopes and intercepts are in agreement with those predicted by the model. For larger structures, values level off and are scattered around an average value depending on shape, goodness of fit, and possibly lipophilicity and some specific effects (e.g. such as those caused by presence of phenol functionality). The complexation between -cyclodextrin and certain large steroidal guest molecules, especially a brain-targeted estradiol chemical delivery systems (E₂-CDS) that is under clinical development, was investigated in details based on fully relaxed semiempirical AM1 quantum chemical calculations. A deformation index (DI) of the CD ring computed using these fully optimized host-guest geometries could be used to characterize the conformational change of the guest.     

Determination of chloride, sulfate and nitrate in drinking water by microchip electrophoresis

We report on a method for simultaneous determination of chloride, sulfate and nitrate in drinking water by microchip electrophoresis. By adjusting a pH value of 4.2 in the aspartate background electrolyte, and by applying separation mechanisms via the ionic strength effect (bis-tris propane) and association equilibria (N-dodecyl-N,N-dimethyl-3-ammonio-1-propanesulfonate), a complete resolution of analytes from other co-migrating constituents is accomplished. In addition, isotachophoretic preconcentration of nitrate was implemented directly on the microchip. The ions were detected via conductivity measurements after injecting 900-nL samples. The limits of detection range from 40 to 120?μg?L^-1. The method displays a high reproducibility of the migration velocities under suppressed hydrodynamic and electroosmotic flow and therefore allows for a precise quantitation of analytes. The total analysis time is <450?s, and the working range is up to 60?mg?L^-1 for chloride and sulfate, and of up to 20?mg?L^-1 for nitrate. Filtration and degassing are the only sample treatment steps prior to analysis. The use of an internal standard enabled an easy chip-to-chip transfer of the method.

AM1-based model system for estimation of brain / blood concentration ratios

Estimation of brain uptake of solutes and drugs would be useful in designing novel therapeutic agents. Given the problems associated with fragment-based models, including the need for strict additivity, we used AM1-derived molecular parameters in attempting to generate an equation for predicting the log of the brain-to-blood concentration ratios (log B:B) for a collection of compounds. Sequential fitting of the data using 10 AM1-derived variables indicated that dipole generated the most predictive one-term equation, dipole and log P, the best two-parameter system, dipole, log P, and molecular weight produced the optimal three-term model, and dipole, log P, molecular weight, and HOMO energy resulted in the best four-parameter paradigm. These systems were statistically similar to those generated using general solvation equations, but with the use of fewer parameters. An expanded parameter set may generated models of even better predictability. © 1996 John Wiley & Sons, Inc.     

Determination of crystallite particle size distribution of polymers from WAXS

A method for the calculation of crystallite size distributions from the profile of wide-angle x-ray reflections is developed. The influence of lattice distortions on the profile is taken into account. The information about the lattice distortions is obtained from the measurement of the integral widths of a number of reflections. The method is applied to samples of (ethylen-1 hexen) copolymers. The change of crystallite size distributions in lateral directions with increasing temperature (20–121°C) is measured. Recrystallization processes at temperatures near the melting point are observed. © 1996 John Wiley & Sons, Inc.     

H- and C-NMR as tools to study aluminium coordination chemistry—aqueous Al(III)-citrate complexes

Based on the use thermodynamic and solid state structure data, ¹H- and ¹³C-NMR is a very useful tool to understand the conformational and dynamic behavior of complexes containing organic ligands in solution. In this paper we describe shortly the possibilities of the assignation of the spectra by means of modern NMR techniques. From the assigned spectra the scalar and dipolar couplings make it possible to determine the orientation of the ligand around the metal ion and the distances between hydrogen atoms in space. Aluminium-citrate complexes are reviewed as examples. It is shown that with the armory of correlation NMR spectroscopy unique insight can be obtained in the behavior of Al-citrate species even if oligomers are present in the solution.     

Unitarity bounds in the vector condensate model of electroweak interactions

We replace the standard model scalar doublet by a doublet of vector fields and generate masses by dynamical symmetry breaking. Oblique radiative corrections are small if the new vector bosons (B ⁺,B ⁰) are heavy. In this note it is shown that the model has a low momentum scale and above Λ?2 TeV it does not respect the perturbative unitarity. From tree-graph unitarity the allowed region ofB ⁺ (B ⁰) mass is estimated asm _B ⁺≥333 GeV (m _B ⁰≥373 GeV) at Λ=1 TeV.