Interpretation of alkyl diselenide and selenosulfenate mass spectra

The mass spectral fragmentation of aliphatic diselenides and selenosulfenates is analyzed to gain a better understanding of the behavior of these species. The main fragmentation pathways of these species include the fragmentation along the Se-C bond, fragmentation along the Se-Se or Se-S bonds and intra-molecular rearrangements. In general, negative ionization favors the fragmentation along the Se-Se or Se-S bonds while positive ionization leads to stable molecular ions. Density functional theory calculations of bond dissociation energies and molecular orbital analysis was undertaken to explain the observed trends in molecular fragmentation. Besides the analysis of molecular fragmentation, a phenomenon of molecular association in negative electron impact and positive chemical ionization conditions was observed and investigated using a high resolution time-of-flight mass spectrometer. Molecular association that occurs during the ionization of species includes the formation of symmetrical diselenides from asymmetrical selenosulfenates and formation of alkylseleno adducts from the corresponding diselenides. For species which is hard to resolve by mass analysis, such as isobars of CHSe, CH₂Se, and CH₃Se, the isotope pattern superimposition procedure was applied to define the overlapping clusters.     

Novel Bicyclic Lactams as XaaPro Type VI beta Turn Mimics: Design, Synthesis, and Evaluation

The design, enantioselective synthesis, and structural characterization of novel bicyclic lactams as peptide mimics of the type VI beta turn is described. The mimics duplicate the conformation of the backbone and disposition of the side-chain atoms of the central two residues of the turn. The Gly L-Pro mimic, lactam 6, was prepared in good overall yield starting from (S)-2-(2'-propenyl)proline. (1)H NMR spectroscopy defined the relative stereochemistry of the substituents and conformational characteristics of the six-membered ring of the lactam; X-ray crystallographic analysis confirmed the conformational and stereochemical assignment. Examination of the crystal structure of lactam 6 revealed that the central amide bond was twisted appreciably out of planarity. The twisting of the amide bond was attributed to angle strain resulting from the presence of the sp(2)-hybridized nitrogen atom at the junction of the two rings. Alkylation of the enolate of the N,N-dimethylformamidine derivative of lactam 6 with benzyl bromide afforded stereoselectively the formamidine 11, a mimic of an L-Phe L-Pro dipeptide in the type VI turn conformation. The efficient synthetic route to highly functionalized peptidomimetics such as 11 will prove highly useful in peptide structure-function studies.     

Determination of chemical purity and isotopic composition of natural and carbon-13-labeled arsenobetaine bromide standards by quantitative1H-NMR

In this study, we report the characterization of three arsenobetaine-certified reference materials by quantitative NMR. We have synthesized an arsenobetaine bromide high-purity standard of natural isotopic composition (ABET-1) and two carbon-13-labeled isotopic standards (BBET-1 and CBET-1). Assignments of the chemical purity and isotopic composition are not trivial in the case of arsenobetaine, and in this study we utilized quantitative¹H-NMR techniques for the determination of the mass fractions (chemical purity). The isotopic purity of all three standards was also assessed by NMR from the carbon-13 satellite signals. The standards are non-hygroscopic, high-purity (ca. 0.99 g/g), and the carbon-13 enrichment for both isotopic standards is x(¹³C)≈0.99. These standards are designed for use as primary calibrators for mass spectrometric determination of arsenobetaine in environmental samples.     

Supramolecular metal-polypyridyl and Ru(II) porphyrin complexes: photophysical, electron paramagnetic resonance, and electrochemical studies

A series of mixed-metal supramolecular porphyrin arrays in which the geometry of the central metal-polypyridyl moiety defines the spatial arrangement of two or more Ru(II)-porphyrin units through axial coordination have been prepared by employing self-assembly based protocols, and their photophysical and electrochemical properties have been studied. The electrochemical properties of the constituent parts of these arrays depend only on their own chemical environment, regardless of the nuclearity and the overall charge of the compound; in this way species with predetermined redox patterns can be obtained via the synthetic control of the self-assembly process. Interestingly, several of these arrays are luminescent both at room and at low temperatures, and in many cases core-to-periphery or periphery-to-core intramolecular energy transfer processes take place according to the nature of the central metal template.     

Purity assignment for peptide certified reference materials by combining qNMR and LC-MS/MS amino acid analysis results: application to angiotensin II

Analytical and Bioanalytical Chemistry - The purity value assignment of metrologically traceable peptide reference standards requires specialized primary methods. Conventionally, amino acid...     

Solution to Informatics and Mass Spectrometry Challenge

The winner of the informatics and mass spectrometry challenge (published in issue 381/1) is: Ho Li-Ching, Teachers College, Columbia University, New York, NY, USA. The award entitles the winner to select a Springer-Verlag book from our catalogue up to a value of €75. Our Congratulations!