1,720,962 research outputs found
FLUO 25-Chirality recognition in dimers of alpha-hydroxy carboxylic acids: The influence of fluorination, esterification, and aromatic substitution
No full textFLUO 25-Chirality recognition in dimers of alpha-hydroxy carboxylic acids: The influence of fluorination, esterification, and aromatic substitution
No full textElementary peptide motifs in the gas phase: FTIR aggregation study of formamide, acetamide, N-methylformamide, and N-methylacetamide
No full textCold, isolated peptide model compounds and their aggregates are generated in pulsed supersonic jet expansions and detected by FTIR spectroscopy in the amide-A region, complemented by amide-I spectra. The most stable, symmetric dimer of formamide is unambiguously assigned in the gas phase for the first time, also by comparison to the analogous acetamide dimer. Efficient quenching of a hot-state Fermi resonance by cooling of the dimers is invoked. As the preferred relative orientation of the C=O and N-H groups in N-methylated formamide and acetamide is trans, these compounds show a fundamentally different dimerization pattern. Their most stable dimers, which would be analogous to those of formamide and acetamide, remain undetected as a consequence of kinetic control in the jet. Accurate benchmark quantities for multidimensional vibrational treatments of these peptide models are derived, and the influence of methyl groups on the N-H stretching dynamics is discussed.Fonds der Chemischen Industrie; DFG research training group [782
Variations of bite angle and coupling patterns in double hydrogen bonds: The case of oxime dimers
No full textstretching excitation. It is faster than in alpha-hydroxyester or alcohol dimers but slower than in carboxylic acid dimers. For this purpose, supersonic jet FTIR and Raman spectra of free and Ar-nanocoated acetoxime monomers, dimers and trimers are presented. By comparison to heated nozzle cyclohexanone oxime spectra, to condensed phase data and to quantum chemical calculations, a Fermi resonance description of the O-H stretching dynamics in the symmetric, doubly hydrogen-bonded dimers is obtained. C=N stretching and librational O-H bending modes are predicted to participate in the coupling. Hydrogen bond topologies and cooperativity issues in larger oxime clusters are discussed. (c) 2007 Elsevier B.V. All rights reserved
Chirality Recognition in Menthol and Neomenthol: Preference for Homoconfigurational Aggregation
No full textChirality-dependent sublimation of alpha-(trifluoromethyl)-lactic acid: Relative vapor pressures of racemic, eutectic, and enantiomerically pure forms, and vibrational spectroscopy of isolated (S,S) and (S,R) dimers
No full textA mass-spectrometric determination of the sublimation pressure diagram of alpha-(trifluoromethyl)-lactic acid as a function of its enantiomeric composition at 293 K shows that the racemic crystals have a 38 +/- 15% higher volatility than the enantiomerically pure crystals and the sublimation eutectic has a 55 +/- 10% higher vapor pressure. These data indicate a possibility for thermodynamically controlled enantiomeric enrichment of the residual material via sublimation of samples of higher than 35 +/- 10% ee. The vibrational spectroscopy of isolated (S,S) and (S,R) dimers in supersonic jets shows that chirality recognition effects are restricted to larger clusters. This is a consequence of the rigid planar carboxylic acid binding motif. (C) 2009 Elsevier B.V. All rights reserved.Fonds der Chemischen Industrie; DFG [782
Adaptive Aggregation of Peptide Model Systems
No full textJet-cooled infrared spectra of acetylated glycine, alanine, and dialanine esters and their dimers are reported in the amide A and amide I-III regions. They serve as particularly simple peptide aggregation models and are found to prefer a single backbone conformation in the dimer that is different from the most stable monomer backbone conformation. In the case of alanine, evidence for topology-changing chirality discrimination upon dimer formation is found. The jet spectroscopic results are compared to gas phase spectra and quantum chemical calculations. They provide reliable benchmarks for the evaluation of the latter in the field of peptide interactions.Fonds der Chemischen Industrie; DFG research training group 78
Low Temperature Infrared Spectroscopy Study of Pyrazinamide: From the Isolated Monomer to the Stable Low Temperature Crystalline Phase
Full text linkA structural and spectroscopic analysis of the anti-tuberculosis drug pyrazinamide (PZA) was carried out. The PZA molecule was predicted theoretically to possess two conformers differing by internal rotation around the C−C(═O) bond, with the E conformer (Cs symmetry point group; N−C−C═O dihedral: 180°) being ca. 30 kJ mol−1 more stable than the Z form (C1 point group; N−C−C═O dihedral: ca. ± 42°). In consonance with both the large energy difference and low energy barrier between the Z and E conformers, upon isolation in low temperature argon and xenon matrices, only the E form could be observed and characterized spectroscopically. In the argon matrix, this conformer was found to exist in at least three matrix sites, of different stability. In a supersonic jet, besides the monomer (E), the most stable dimer of PZA with two equivalent NH···O═ hydrogen bonds could also be identified. Its spectrum reveals rapid energy flow out of the excited NH stretching mode mediated by one of the heteroatoms in the ring. Finally, the IR spectra of the amorphous solid resulting from fast cooling of the vapor of the compound (initially in the α crystalline phase) onto the cold substrate of the cryostat (10 K) and of the crystalline phase resulting from warming the amorphous solid were also recorded and interpreted. The obtained crystalline phase was found to be the thermodynamically most stable δ polymorph of PZA
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
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