bluuebunny/crossref_metadata_2025_split huggingface dataset:
A density functional study of the interconversion of carbonyls and alcohols in solution: Comparison of reaction mechanisms involving NADPH, histidine, and tyrosine
Authors: Várnai Péter , Richards W. Graham 
Published: August 2002

AbstractSeveral reaction mechanisms for the interconversion of carbonyls and alcohols in solution are examined. The model system includes acetaldehyde, 1‐methyl‐1,4‐dihydropiridine as a hydride donor, and either 4‐methyl‐phenol or 4‐methyl‐imidazolium as a possible proton donor. This system is of relevance to understanding the catalytic mechanism of many oxidoreductase enzymes which use NADPH cofactor and tyrosine or histidine as general acids. The energetic changes are evaluated by constrained geometry optimization using density functional theory (B3LYP/6‐31+G*) coupled to a Poisson–Boltzmann solvation scheme. The results show that a stepwise mechanism with an initial hydride transfer to the aldehyde is favored for both proton donors over the alternative stepwise mechanism, in which there is an initial proton transfer to the aldehyde. The possibility of a concerted mechanism is also considered, but the corresponding activation energy exceeds those of the stepwise processes. © 2001 John Wiley & Sons, Inc. Int J Quantum Chem 84: 276–281, 2001
Three conformers observed and characterized in 1‐hexyne
Authors: Atticks Karissa , Bohn Robert K. , Michels H. Harvey 
Published: August 2002

AbstractThe microwave spectrum of 1‐hexyne (n‐butyl acetylene) reveals the presence of three conformers that have been carefully characterized. 1‐Hexyne can be viewed as propane with terminal ethynyl and methyl substituents. These substituents can be arranged in anti–anti (AA), anti–gauche (AG), gauche–anti (GA), gauche–gauche (GG cis), and gauche–gauche′ (GG trans) conformations. The first three conformers are those observed experimentally and their rotational constants have been determined. The GG conformers, in both cis and trans arrangements have not been positively identified to date. We have determined the structural parameters and energies of all five possible conformations through ab initio quantum mechanical calculations at the HF, MP2, and B3LYP levels of theory. The calculated energies of these five conformers are very close, and a change in the relative stability of these conformers is found at different levels of theory. We find that inclusion of electron correlation effects is critical to our understanding of the energy ordering of these conformers. © 2001 John Wiley & Sons, Inc. Int J Quantum Chem, 2001
High‐resolution analysis of a 144‐membered pyrazole library from combinatorial solid phase synthesis by using electrospray ionisation Fourier transform ion cyclotron resonance mass spectrometry
Authors: Schmid Dietmar G. , Grosche Philipp , Jung Günther 
Published: August 2002

AbstractA compound library consisting of 144 pyrazole carboxylic acids and six sublibraries consisting of 24 components was analysed using electrospray ionisation Fourier transform ion cyclotron resonance mass spectrometry (ESI‐FTICR‐MS). The library was synthesised by the split‐mix method and investigated by direct infusion analysis by which 134 compounds were detected. FTICR‐MS is predestined for the direct characterisation of complex compound libraries because of its outstanding mass resolution and mass accuracy. However, discrimination within the electrospray ionisation process sometimes leads to signal suppression and thus to misinterpretation of the synthetic results. Using micro‐HPLC/MS we were able to assign all 144 compounds including all pairs of isobaric pyrazoles. We also show that, due to partial separation, FTICR‐MS is indispensable for proper detection of co‐eluting compounds. Copyright © 2001 John Wiley & Sons, Ltd.
Electrospray ionization in the study of sol‐gel processes: the polycondensation of Ti(O‐n‐Bu)4 in the presence of Si(OEt)4
Authors: Cristoni Simone , Armelao Lidia , Gross Silvia , Tondello Eugenio , Traldi Pietro 
Published: August 2002

AbstractThe polycondensation of Ti(O‐n‐Bu)4 in presence of Si(OEt)4 (Et = C2H5, Bu = C4H9) has been studied by electrospray ionization of their mixtures in methanol. Series of oligomers due to Ti(OMe)4 (Me = CH3) polycondensation have been detected and represent the most abundant species. This result can be explained by the ready alcoholysis of Ti(O‐n‐Bu)4 and by its fast kinetics of oligomerization. However a series of Ti,Si‐containing oligomers, with molecular weights in the range 400–1300 Da, has also been detected. Their structures have been characterized by isotopic cluster analysis and MS/MS experiments, proving that they originate by reaction of Ti‐OH and Si‐OEt moieties. Copyright © 2001 John Wiley & Sons, Ltd.
The solvent shells of cluster ions produced by direct electric field extraction from glycerol/water mixtures
Authors: Balakin A. A. , Dodonov A. F. , Novikova L. I. , Talrose V. L. 
Published: August 2002

AbstractElectric field extraction of gaseous negative ions directly from water/glycerol solutions by use of a track membrane technique was investigated. The distributions of numbers of solvent molecules in the extracted cluster ions for different compounds were obtained. It is shown that the extraction mechanism is a direct field‐stimulated evaporation of cluster ions from liquid, with a subsequent loss of several solvent molecules in the vacuum. For relatively simple ions a good correspondence of results was obtained with a continuous medium model. It was found that the number of solvent molecules in a cluster shell, for more complicated ions such as amino acids, is significantly greater than that for halide ions or ions of simple organic acids. An increase in the number of solvent molecules in the case of amino acid negative ions is rationalized in terms of the existence of several charged groups, each of which gives an additional contribution to the cluster shell. Copyright © 2001 John Wiley & Sons, Ltd.
Simultaneous determination of betamethasone and dexamethasone residues in bovine liver by liquid chromatography/tandem mass spectrometry
Authors: Van den hauwe O. , Perez J. Castro , Claereboudt J. , Peteghem C. Van 
Published: August 2002

AbstractIn this study a new method for the simultaneous confirmation of betamethasone and dexamethasone residues in bovine liver is presented. A Quattro LCZ triple quadrupole mass spectrometer, equipped with an atmospheric pressure ionization (API) source, was coupled to a high performance liquid chromatograph (HPLC) system. Spiked liver samples were first extracted with acetonitrile, and the extracts were purified on C‐18 columns. LC separations were performed on a Hypercarb column, with acetonitrile/water (90:10, v/v, +0.3% formic acid) as the mobile phase. Retention times for dexa‐ and betamethasone were 6.60 and 8.50 min, respectively. Fluorometholone had a retention time of 6.70 min and was used as the internal standard. The detection of the analytes was performed in the multiple reaction monitoring (MRM) mode. The assay was linear over the range of 0.5 to 8 µg/kg for both analytes. The estimated determination limits were 0.2 µg/kg for both beta‐ and dexamethasone and the quantification limits were 0.4 µg/kg for dexamethasone and 0.3 µg/kg for betamethasone. Analysis precision at 1, 2 and 4 µg/kg was lower than 6.1% (relative standard deviation, RSD) and accuracy was at least 97.5%. Recoveries at 1, 2 and 4 µg/kg ranged between 56 and 69%. Copyright © 2001 John Wiley & Sons, Ltd.
Detection of chlordanes by positive ion chemical ionization in an ion trap: a comparitive study of the non‐conventional reagents acetonitrile, acrylonitrile and dichloromethane
Authors: Zencak Zdenek , Oehme Michael , Skopp Sonja 
Published: August 2002

AbstractThe detection of some chlordane compounds (heptachlor, cis‐/trans‐chlordane and cis‐/trans‐nonachlor) by positive ion chemical ionization (PICI) in an ion trap was studied using acetonitrile, acrylonitrile and dichloromethane as non‐conventional reagent gases. These reagent gases initiated specific fragmentation reactions and resulted in different response factors. All reagent gases enabled detection limits in the low‐pg range for heptachlor, whereas the detection limits of cis‐/trans‐chlordane and cis‐/trans‐nonachlor were in the mid‐pg range. Additionally, the acetonitrile and dichloromethane PICI mass spectra of the cis‐ and trans‐stereoisomers of chlordane and nonachlor were different. Copyright © 2001 John Wiley & Sons, Ltd.
Violation of frozen shell approximation in dissociative electron capture by halogenated anthraquinones
Authors: Asfandiarov N. L. , Fal'ko V. S. , Lukin V. G. , Nafikova E. P. , Pshenichnyuk S. A. , Fokin A. I. , Lomakin G. S. , Chizhov Yu. V. 
Published: August 2002

AbstractA series of halogenated anthraquinone (AQ) derivatives has been studied by means of electron capture negative ion (NI) mass spectrometry (ECNI‐MS). 1Cl‐AQ and 2Br‐AQ display dramatically steep positive temperature dependencies of Hal− ion abundance in the low electron energy region. Molecular NI intensity decreases rapidly with increasing temperature in the case of 1I‐AQ. In the case of 2Br‐AQ, a metastable NI peak (m/z 22.9) corresponding to the process BrAQ− → Br− + AQ0 was recorded. This means that the characteristic dissociation lifetime of the molecular NI Br‐AQ− is at least ∼25 µs at the energy ∼0.67 eV in the low‐temperature spectrum (T ∼80 °C), and at the energy ∼0.13 eV in the hot spectrum (T ∼290 °C). Together with the observed temperature dependence of the 2Br‐AQ curves of effective yield (CEY), this proves that this anion dissociates according to Coulson's model. The same halogen anion behavior is observed in the case of 1Cl‐AQ. There are three consecutive stages in the process of molecular NI dissociation of Cl‐ and Br‐substituted AQ, namely, electron capture into the empty π‐orbital by means of the shape resonance mechanism, followed by a radiationless transition into the ground electronic π‐state of the anion, as predicted by Compton in the case of the parabenzoquinone molecule, and, finally, a fluctuative dissociation of the molecular NI accompanied by the transition from the π‐term into the σ‐term, so‐called predissociation. Calculations show reasonable agreement with the experimental data. In the case of 1I‐AQ, an effect of inversion of empty levels in the process of electron capture by the molecule takes place, a violation of the so‐called frozen shell approximation. The phenomenon found may be of significance not only in the case of ECNI‐MS, but also in other experimental investigations using low‐energy electron‐molecule and ion‐molecule collisions. Copyright © 2001 John Wiley & Sons, Ltd.
Conformational changes in β‐endorphin as studied by electrospray ionization mass spectrometry
Authors: Lin Hui , Dass Chhabil 
Published: August 2002

AbstractBecause of a wide range of physiological functions, the structure of β‐endorphin (BE) is of great interest. In this study, conformational changes in BE induced by methanol are explored with electrospray ionization‐mass spectrometry (ESI‐MS). Differences in the charge‐state distribution (CSD) and the extent of hydrogen/deuterium (H/D) exchange were used to monitor the conformational changes. The latter experiments were conducted via time‐resolved ESI‐MS in a continuous‐flow apparatus. Both these techniques demonstrate that BE exists in a random coil open structure in aqueous media, but it acquires a more compact conformation with increased concentration of methanol. The H/D exchange experiments reveal that BE forms 61% α‐helix in mixed solvents. Copyright © 2001 John Wiley & Sons, Ltd.
A method for the identification of the double‐bond position of isomeric linear tetradecenols and related compounds based on mass spectra of dimethyl disulfide derivatives
Authors: Yuan Gu , Yan Jichao 
Published: August 2002

AbstractA simple method is presented for calculation of the double‐bond position in linear tetradecenols based on mass spectral data of dimethyl disulfide derivatives. In this approach, the m/z ratios of the molecular ion and of one (or both) of the two most abundant fragment ions were utilized to calculate the double‐bond position, without the requirement to identify both fragment ions resulting from carbon‐carbon cleavage across what was originally the double bond. The approach was tested with mass spectra of dimethyl disulfide derivatives of 12 isomeric tetradecenols, and the double‐bond position in each isomer was successfully identified by this method. The method was shown to work also for the corresponding acetates. Copyright © 2001 John Wiley & Sons, Ltd.
Application of sulfur as a matrix for laser desorption/ionization in the characterization of halogenated fullerenes
Authors: Streletskiy Alexey V. , Kouvitchko Igor V. , Esipov Stanislav E. , Boltalina Olga V. 
Published: August 2002

AbstractThe application of sulfur as a matrix for matrix‐assisted laser desorption/ionization time‐of‐flight mass spectrometry (MALDI‐TOFMS) analysis of highly chlorinated and fluorinated fullerenes is reported. Control over fluorofullerene fragmentation which resulted in the domination of the molecular peak C60F36− was achieved, with the optimal matrix‐to‐analyte ratio found to be 1000:1. We suggest the possible mechanism of the molecular ion formation according to the charge transfer between the sulfur anions and C60F36. For the first time the LDI and MALDI mass spectra of the highly chlorinated fullerene C60Clx(xmax ≈ 32) are presented. The formation of odd chlorine ions (positive and negative) is observed. We conclude that use of sulfur as a matrix leads to a significant decrease in fragmentation of the halogenated fullerenes. Copyright © 2001 John Wiley & Sons, Ltd.
Fine structure of the stability diagram and the amplitude of ion oscillation within hyperboloidal mass spectrometers
Authors: Sheretov Ernst P. , Philippov Igor V. , Karnav Tatiana B. , Fedosov Edgar V. , Ivanov Vladimir W. 
Published: August 2002

AbstractThis article shows that the dependence of the amplitude of ion oscillation within a hyperboloidal mass spectrometers (HMS) as a function of working point location on the stability diagram (in this case all working points lie at one scan line) exhibits a structure with spikes: the amplitude decreases drastically in some points and these dips look like ‘reversed spikes’. It is shown that the dips appear at particular values of the stability parameter, β0, that correspond to intersections of the scan line with lines of parametric resonances (quasi‐stability lines) described by us previously. This effect governs the trapping efficiency of ions injected into a radio‐frequency ion trap from an external source. Copyright © 2002 John Wiley & Sons, Ltd.
Differential isotopic mass splitting as a mass spectrometric tool for identifying protease substrates
Authors: McKeown Stephen C. , Heudi Olivier , Kay Corinne , Marshall Peter 
Published: August 2002

AbstractA method is described whereby stable isotopic signatures were partially incorporated into both termini of a peptide sequence giving rise to a characteristic cluster of four peaks in the mass spectral analysis. Cleavage of this peptide by a protease between the labeled positions generates two fragments both displaying their own individual signature peaks. The event of protease cleavage of the peptide was monitored by the changes in clusters within the spectrum. We believe that this technique could be used to aid the discovery of new cleavage substrates for proteases. Additionally, the analysis can be automated with dedicated software designed to select and interpret the data since all peaks of interest contain predefined signatures and can be easily distinguished from background noise. Copyright © 2002 John Wiley & Sons, Ltd.
Tri‐α‐naphthylbenzene as a crystalline or glassy matrix for matrix‐assisted laser desorption/ionization: a model system for the study of effects of dispersion of polymer samples at a molecular level
Authors: Bauer Barry J. , Guttman Charles M. , Liu Da‐wei , Blair William R. 
Published: August 2002

AbstractTri‐α‐naphthylbenzene (TαNB) can exist as either a crystalline or glassy solid at ambient temperatures, making it a unique matrix in matrix‐assisted laser desorption/ionization (MALDI) spectroscopy. Electrosprayed TαNB is crystalline and has a melting point of 180 ± 2 °C, as measured by differential scanning calorimetry (DSC). A glass of TαNB is obtained upon heating above the crystalline melting point with a glass transition temperature of 68 ± 2 °C having no remaining crystallinity. MALDI samples containing mass fraction 1% polystyrene (PS) are run in both the crystalline and amorphous states. In the crystalline state, there is a strong spectrum typical of PS, but upon melting and quenching to the glassy state, the MALDI signal disappears. If the transparent, amorphous sample is treated with 1‐butanol, it becomes white, and the MALDI signal returns. DSC shows that the 1‐butanol treatment leads to the return of some of the crystallinity. Small angle neutron scattering (SANS) shows that the crystalline state has large aggregations of PS while the amorphous state has molecularly dispersed PS molecules. MALDI gives strong signals only when there are large aggregations of polymer molecules, with individually dispersed molecules producing no signal. Published in 2002 by John Wiley & Sons, Ltd.
On the sample‐complexity of ℋ︁∞ identification
Authors: Venkatesh S. R. , Dahleh M. A. 
Published: August 2002

AbstractIn this paper we derive the sample complexity for discrete time linear time‐invariant stable systems described in the ℋ︁∞ topology. The problem set‐up is as follows: the ℋ︁∞ norm distance between the unknown real system and a known finitely parameterized family of systems is bounded by a known real number. We can associate, for every feasible real system, a model in the finitely parameterized family that minimizes the ℋ︁∞ distance. The question now arises as to how long a data record is required to identify such a model from noisy input–output data. This question has been addressed in the context of l1, ℋ︁2 and several other topologies, and it has been shown that the sample‐complexity is polynomial. Nevertheless, it turns out that for the ℋ︁∞ topology the sample‐complexity in the worst case can be infinite. Copyright © 2001 John Wiley & Sons, Ltd.
On the structure of clique‐free graphs
Authors: Prömel Hans Jürgen , Schickinger Thomas , Steger Angelika 
Published: August 2002

AbstractUsing a clever inductive counting argument Erdős, Kleitman and Rothschild showed in 1976 that almost all triangle‐free graphs are bipartite, i.e., that the cardinality of the two graph classes is asymptotically equal. In this paper we investigate the structure of the “few” triangle‐free graphs which are not bipartite. As it turns out, with high probability, these graphs are bipartite up to a few vertices. More precisely, almost all of them can be made bipartite by removing just one vertex. Almost all others can be made bipartite by removing two vertices, and then three vertices and so on. We also show that similar results hold if we replace “triangle‐free” by K𝓁+1‐free and “bipartite” by 𝓁‐partite. © 2001 John Wiley & Sons, Inc. Random Struct. Alg., 19, 37–53, 2001
Analysis of the x‐ray photoelectron energy‐loss background in silicides
Authors: Castle J. E. , Salvi A. M. , Guascito M. R. 
Published: August 2002

AbstractIntermetallic compounds provide a means to investigate the mutual influence of two elements on the photoelectron fine structure and background that have developed as a result of bond formation. In the case of the aluminides we have shown that aluminium acquires a background structure that is typical of elements having a 3d valence band and concluded that there is a strong ‘d’ component in the bonding structure arising from pd hybridization. In this paper we report a similar analysis of the background structure in two silicides, MoSi2 and FeSi2.Both silicon and molybdenum in MoSi2 are very similar in background structure to that in their respective pure elemental states. Moreover, the plasmon structure that appears is similar to that of silicon itself. Thus we conclude that in this material ‘p’ character is an important feature of the bond. In the case of FeSi2 the situation is different. Here, silicon gains a pronounced background step, similar to that of iron, and the plasmon structure becomes weaker. This behaviour is analogous to that of FeAl and we conclude that this is evidence of the participation of the ‘d’ states of iron in bond formation. Copyright © 2001 John Wiley & Sons, Ltd.
Scanning force microscopy of DNA metallization
Authors: Seidel Ralf , Mertig Michael , Pompe Wolfgang 
Published: August 2002

AbstractThe nucleation and growth of platinum nanoparticles at single DNA molecules, leading to a template‐directed formation of micrometre‐long cluster chains, is investigated by scanning force microscopy (SFM). The cluster chains are synthesized by incubation of DNA with K2PtCl4 and subsequent chemical reduction of the platinum complexes. Scanning force microscopy is applied for observation of the different stages of cluster formation, because it provides the unique possibility to image DNA and metallic nanoparticles simultaneously. Finder grids were used to investigate exactly the same cluster chains with SFM and transmission electron microscopy, thus combining the advantages of both microscopic methods to study the process of DNA metallization. Copyright © 2002 John Wiley & Sons, Ltd.
Enrichment of Al in the topmost surface of AlN crystalline film prepared by post‐irradiation
Authors: Mizuhara Y. , Mitsuhashi R. , Nagatomi T. , Takai Y. , Inoue M. , Shimizu R. 
Published: August 2002

AbstractCrystalline AlN film that was prepared by post‐irradiation under specific conditions has been examined by reflection high‐energy electron diffraction (RHEED) and ion scattering spectroscopy (ISS). The RHEED pattern clearly indicated that an AlN crystalline structure is grown on an Al substrate under irradiation of 12 keV N2+ ions impinged on the Al surface at normal incidence. However, ISS revealed that the topmost surface is not the stoichiometric composition corresponding to AlN, but is significantly Al‐rich. Copyright © 2001 John Wiley & Sons, Ltd.
A holistic approach to competence development
Authors: Sundberg Lennart 
Published: August 2002

AbstractPragmatically seeing competence as a combination of a person's knowledge, experiences and abilities, it is concluded that it is not possible to directly develop another person's competence. It is just possible to set the scene, to provide the tools and act like a catalyst. A holistic and systemic approach in six steps is designed with the ambition to handle competence development in that way. Practical experiences from applying the developed methodology in a number of SMEs show that those basic assumptions at large have been correct. A method of cooperation between a university department and a practice‐oriented consultant is also developed within the project. Copyright © 2001 John Wiley & Sons, Ltd.
Competence development and learning organizations: a critical analysis of practical guidelines and methods
Authors: Löfstedt Ulrica 
Published: August 2002

AbstractIn critically analysing eight research papers about competence development in organizations, it is found that systemic models, methods and approaches have a great deal to offer in the field of competence development in SMEs. However, the proposed methods and approaches are still rather tentative and theoretically based. Hence, they need to be further empirically tested and adapted to practical use. It is also desirable to further develop and elaborate all the ideas raised in the papers in order to bring them to full maturity. Finally, in order to meet all the requirements in the European Commission's ADAPT programme, further research and development is needed concerning how to combine and extend the original methods discussed in this analysis. Copyright © 2001 John Wiley & Sons, Ltd.