bluuebunny/crossref_metadata_2025_split huggingface dataset:
Is there a bridging ligand in metal‐substituted zinc β‐lactamases? A spectroscopic and theoretical answer
Authors: Estiú Guillermina L. , Rasia Rodolfo M. , Cricco Julia A. , Vila Alejandro J. , Zerner Michael C. 
Published: August 2002

AbstractThe spectral features of Co(II)‐substituted metallo‐β‐lactamases were analyzed. Nuclear magnetic resonance (NMR) evidence is provided that shows no magnetic coupling between the two metal ions in Co(II),Co(II)‐βLII (β‐lactamase II from Bacillus cereus), suggesting that no bridging ligand connects them. Ultraviolet–visible (UV–vis) data are rationalized on the basis of semiempirical quantum chemical calculations of the intermediate neglect of differential overlap type (ZINDO/S). The binuclear active sites of the enzymes from B. cereus (βLII) and B. fragilis (CcrA) were modeled from the protein data bank (PDB) coordinates, and calculations were performed at the multireference configuration interaction level. The active site geometries of these enzymes were examined using the experimental electronic spectra as a guide. The model which best fits to the spectroscopic data lacks a bridging solvent molecule, in agreement with the NMR data. The model could also reproduce spectral changes seen in the mixed Zn(II),Co(II) adduct. The bands in the visible range are associated with ligand field transitions at the high‐affinity site, whereas the UV‐calculated features originate in ligand‐to‐metal charge transfer to the open shell Co(II) in the low‐affinity site. These results indirectly support the hypothesis that the Co(II),Co(II) derivative of CcrA is also unbridged, in contrast with the structure of the native zinc enzyme. These results indicate that the existence of a bridging ligand is not necessary for metallo‐β‐lactamase activity, and that the second zinc ion is not essential for lowering the pKa of the bound water. © 2002 Wiley Periodicals, Inc. Int J Quantum Chem 88: 118–132, 2002
Theoretical study of p‐methacryloyl‐aminophenylarsonic acid
Authors: Hernández J. M. , Herrera Ana M. , García‐Serrano J. , Rivas‐Silva J. F. 
Published: August 2002

AbstractConformations of p‐methacryloylaminophenylarsonic acid (p‐MAPHA) are determined through molecular mechanics and DFT/B3LYP calculations. Solvation effects are studied within the self‐consistent isodensity continuum model (SCI‐PCM). The stationary points were found to correspond to minima as verified by the analysis of vibrational frequencies in the molecule. The molecular optical absorption was obtained by using different solvent environments. The present results are in good agreement with the available experimental data. © 2002 Wiley Periodicals, Inc. Int J Quantum Chem, 2002
Pseudorotational dynamics of H3+ and Li3+
Authors: Hagelberg F. 
Published: August 2002

AbstractThe origin of the pseudoprecession phenomenon is investigated through a computational study of the time evolution of H3+ and Li3+ by electron nuclear dynamics theory. In particular, the pseudorotation of both molecules is shown to induce a spatial rotation, which in turn leads to Coriolis coupling of the two orthogonal nuclear shape deformation modes. This effect is rooted in an anisotropy of the molecular ground state potential energy surface that is caused by the interaction between the D3h ground state and a twofold degenerate first excited state. Computations are performed for a variety of vibrational energies. In addition, the impact of the anharmonicity of the ground state potential surface on the shape deformation modes and the coupling between them is discussed. © 2001 John Wiley & Sons, Inc. Int J Quantum Chem, 2001
Investigation of the structural units of germanium sulfide and selenide by quantum chemical methods
Authors: Fejes István , Billes Ferenc 
Published: August 2002

AbstractThis article deals with the modeling of the structural units (clusters) of germanium sulfide and germanium selenide glasses by quantum chemical (ab initio Hartree–Fock) methods. Clusters of different sizes were investigated. Geometric parameters and vibrational frequencies of these structural units were calculated. The quantum chemical calculations were followed by normal coordinate analysis. Based on the yielded results, the vibrational spectra of the clusters were simulated. The results for germanium sulfide and germanium selenide were compared. It was concluded that in the spectral regions where germanium sulfide is not applicable for fiber optics, germanium selenide or different germanium sulfide–selenides are suitable to replace it. © 2001 John Wiley & Sons, Inc. Int J Quantum Chem, 2001
AIM study of pyrimidyl carbocyclic analogues of nucleosides based on cyclopentene rings
Authors: González‐Moa María J. , Terán Carmen , Mosquera Ricardo A. 
Published: August 2002

AbstractA topological charge electron density analysis using the atoms in molecules (AIM) theory with HF/6‐31++G**// HF/6‐31G* wave functions was performed oncis‐pyrimidyl andcis‐5/6‐halopyrimidyl derivatives of 1,2‐disubstituted analogues of carbocyclic nucleosides. In these analogues, which belong to an important class of antiviral or antitumoral compounds, the usual ribose or deoxyribose is replaced by a carbocycle with a hydroxymethyl group and a heterocyclic base on adjacent carbons and a double bond between positions 2 and 3 of the carbocycle. The results obtained demonstrate that 5‐halogenation does not alter the relative energy sequence of the conformers and gives rise to no meaningful distortion of the electron distribution in the cyclopentene ring. In fact, the difference in the total electron population of this cycle between the parent and any of the halogenated molecules is never larger than 7×10−3a.u. It has been found that the electron population of the αCatom in the chloro derivatives is enlarged with respect to the unhalogenated species. The 5‐halogenation results in a destabilization of the C4, C5, and C6 atoms of the heterocyclic base. According to the AIM computed energy properties this destabilization originates from larger interelectronic repulsions in the C5 basin that are consistent with its reduced volume and its lower first moment of the electron density. On the other hand, 6‐halogenation produces a significant modification of the conformational equilibrium, modifying the relative energy sequence of conformers. The electronic properties are also significantly modified by 6‐halogenation. The charge transfer from the cyclopentene ring to the base reaches 0.71 a.u. in the 6‐fluoro derivative and 0.69 a.u. in the 6‐chloro derivative. © 2002 John Wiley & Sons, Inc. Int J Quantum Chem 86: 67–78, 2002
Perturbation of the intramolecular hydrogen bonds of glucose by Cu+association
Authors: Boutreau Laurence , Tortajada Jeanine , Luna Alberto , Alcamí Manuel , Mó Otilia , Yáñez Manuel 
Published: August 2002

AbstractA density functional theory study of glucose and glucose–Cu+complexes has been performed to investigate the changes undergone by the set of intramolecular hydrogen bonds of the neutral system upon Cu+association. The geometries of the different species investigated were optimized at the B3LYP/6‐31G(d,p) level. The same level of theory was used to obtain the harmonic vibrational frequencies and to analyze the electron charge density by means of the atoms in molecules theory. We have shown that the interaction with Cu+strongly perturbs the set of intramolecular hydrogen bonds of the neutral. Some of these changes are a direct consequence of the conformational changes induced by the metal, which result in the breaking of some of the existing bonds or in the formation of new ones. The most important point, however, is that the intramolecular hydrogen bonds that remain are perturbed to a different extent. In general, all hydrogen bonds in which the OH donor interacts directly with the metal cation are significantly stabilized while the remaining ones become weaker. These changes influence the relative stability of the complexes as well as its capacity to interact with other systems. © 2001 John Wiley & Sons, Inc. Int J Quantum Chem, 2001
Method/basis set dependence of the traceless quadrupole moment calculation for N2, CO2, SO2, HCl, CO, NH3, PH3, HF, and H2O
Authors: Höfinger Siegfried , Wendland Martin 
Published: August 2002

AbstractDipole moments and traceless quadrupole moments for a training set of nine molecules—N2, CO2, SO2, HCl, CO, NH3, PH3, HF, and H2O—were deduced from the final optimized density matrices of a series of ab initio calculations, thereby employing different levels of theory with varying basis set quality. All the results were obtained in a true ab initio sense, meaning that at a certain combination of method/basis set, the configurations of the molecules were first subjected to a geometry optimization and only afterwards the dipole and traceless quadrupole moments were derived for the final relaxed structures. All results could be compared to experimental data, thus the quality of the different models with respect to applicability and reliability of calculating molecular moments without any knowledge of structural details of the equilibrium geometries could be evaluated. Generally, a combination of methods that take into account correlation effects, together with a high‐level basis set, was found to yield molecular moments close to the experimental values. © 2001 John Wiley & Sons, Inc. Int J Quantum Chem, 2001
Optimisation of the quantitative determination of chlormequat by matrix‐assisted laser desorption/ionisation mass spectrometry
Authors: Horak Jeannie , Werther Wolfgang , Schmid Erich R. 
Published: August 2002

AbstractThe plant growth regulator chlormequat, an involatile quaternary ammonium salt, has been quantified by matrix‐assisted laser desorption/ionisation time‐of‐flight mass spectrometry (MALDI‐TOFMS). Restrictions for quantitative MALDI‐TOFMS analysis, such as irreproducible crystallisation and unsatisfactory laser stability, have been overcome by the application of two synthesised isotopically labelled standards and the optimisation of the measurement protocol. Data acquisition at constant laser power was compared to data acquisition at approximately constant ion abundance of the relevant ions (analyte and internal standards). Data acquisition at constant ion abundance performed better and enabled a high number of consecutive firings to the same sample deposition area. Furthermore an increased sample‐to‐sample repeatability and a high reproducibility over several weeks without re‐calibration have been attained by this method. Linearity over three orders of magnitude (0.05 to 30 ng/µL chlormequat), with a correlation coefficient of 0.9997, was achieved using [13C3]‐chlormequat as internal standard. Limit of detection and limit of determination were determined to be in the low pg/µL range for pure standard solutions. Thin‐layer chromatography was applied for the removal of high amounts of choline, which is often present in plant tissue extracts and can adversely affect the ionisation and detection of chlormequat by MALDI‐TOFMS. The use of two internal standards ([13C3]‐ and [2H9]‐chlormequat) enabled direct quantification and simultaneous control of the recovery. Copyright © 2001 John Wiley & Sons, Ltd.
A study of the electrospray ionisation and ion‐trap fragmentation of [M − H]− ions of new 3,5‐disubstituted tetrahydro‐2H‐1,3,5‐thiadiazin‐2‐thiones
Authors: Martínez‐Alvarez Roberto , Martín Nazario , Seoane Carlos , Suárez Margarita , Pérez Rolando , Rodríguez Hortensia , Kayali Nour 
Published: August 2002

AbstractThe electrospray ionisation (ESI) in negative mode of the pharmacologically significant 3,5‐disubstituted tetrahydro‐2H‐1,3,5‐thiadiazin‐2‐thiones, and their subsequent fragmentations using an ion‐trap mass spectrometer, have been investigated. Experiments on sequential product ion fragmentations (MSn) were performed in order to elucidate the degradation pathways for these compounds. The data presented show that the fragmentation of the even‐electron [M − H]− ions could proceed through an internal nucleophilic substitution displacement. Decarboxylation and extrusion of carbon disulfide are other fragmentations observed. Copyright © 2001 John Wiley & Sons, Ltd.
Using septum‐capped vials with continuous‐flow isotope ratio mass spectrometric analysis of atmospheric CO2 for Keeling plot applications
Authors: Tu Kevin P. , Brooks Paul D. , Dawson Todd E. 
Published: August 2002

AbstractRecent advances in continuous‐flow isotope ratio mass spectrometry have made possible automated analysis of δ13C and δ18O in CO2 in air from 10‐mL septum‐capped vials. With the potential to collect, transport, and analyze a large number of air samples using these small inexpensive vials, the opportunity exists for routine and statistically robust sampling of CO2 for Keeling plot applications.With the above‐mentioned objectives in mind, we measured the precision of carbon and oxygen stable isotope ratios in the CO2 of air within septum‐capped vials using a Finnigan MAT Deltaplus XL continuous‐flow isotope ratio mass spectrometer (CF‐IRMS) interfaced to a GasBench II. The standard deviation of 45 δ13CPDB and δ18OVSMOW measurements from five vials filled with identical tank air was 0.08‰ and 0.14‰, respectively. This translated into a measurement precision (standard error; SE) of 0.01‰ δ13CPDB and 0.02‰ δ18OVSMOW. This precision was similar to that obtained with the tank air introduced directly into the GasBench II by bypassing the vials/autosampler (also with a SE = 0.01‰ for both δ13CPDB and δ18OVSMOW), suggesting minimal vial‐related effects with no storage time at ambient pressure and CO2 concentration. Further, puncturing the butyl rubber septa during filling just prior to analysis did not affect the precision (SE = 0.01‰ for both δ13CPDB and δ18OVSMOW, n = 45).After nine days of storage in vials with unpunctured septa, neither δ13CPDB nor δ18OVSMOW changed significantly, although the precision decreased to ∼0.04‰ for both δ13CPDB and δ18OVSMOW. A greater decrease in precision was observed for vials with punctured septa (from 0.01‰ to 0.10‰ SE for both δ13CPDB and δ18OVSMOW). Further, whereas vials with punctured septa showed no change in δ18OVSMOW, δ13CPDB decreased by 0.07‰/day (r2 = 0.96). In practice, such storage‐related effects could be accounted for based on concurrent changes in reference standards.These results suggest that air may be collected and stored in 10‐mL septum‐capped vials and analyzed in an automated fashion with the Gas Bench II/Deltaplus with sufficient precision for Keeling plot applications. Further, a relatively high sample throughput of about 4 vials per hour or about 100 vials or 2000 injections per day offers the potential for high precision with large sample sizes. Higher throughput should permit experimental advances that were not possible before. Copyright © 2001 John Wiley & Sons, Ltd.
Isoelectric focusing nonporous silica reversed‐phase high‐performance liquid chromatography/electrospray ionization time‐of‐flight mass spectrometry: a three‐dimensional liquid‐phase protein separation method as applied to the human erythroleukemia cell‐line
Authors: Wall Daniel B. , Kachman Maureen T. , Gong Siyuan S. , Parus Stephen J. , Long Michael W. , Lubman David M. 
Published: August 2002

AbstractA liquid‐phase three‐dimensional protein separation method has been developed that is used to separate the cytosolic fraction of a HEL cell lysate via isoelectric focusing (IEF), nonporous silica (NPS) reversed‐phase high‐performance liquid chromatography (RP‐HPLC) and electrospray ionization time‐of‐flight mass spectrometry (ESI‐TOFMS), respectively. Several hundred unique protein molecular weights were observed in a pI range from 4.8 to 8.5 and a mass range from 5 to 85 kDa. Proteins were positively identified by analysis of the pI (±0.5 pI units), an intact protein molecular weight (±150 ppm), and peptide mass mapping results. Using the molecular weight (MW) and peptide mapping results of identified proteins it was possible to characterize their posttranslational (PTMs) and/or sequence modifications. PTMs were detected on both forms of cytosolic actin, heat shock 90 β, HINT and α‐enolase. Sequence modifications or conflicts were observed for β‐and γ‐actin, ATP β‐synthase and heat shock 90 β. IEF‐NPS‐RP‐HPLC/ESI‐TOFMS was used to determine experimental pI, MW and relative hydrophobicity values for each protein detected. This data was used to generate a 2‐D pI‐MS protein map, where proteins are displayed according to their pI and molecular weight. Protein molecular weight peaks are represented as bands in the 2‐D pI‐MS image where the gray scale of each band is proportional to the intensity of the protein molecular weight peak. In addition, a third hydrophobicity dimension (%B) was added as the % acetonitrile elution to generate a 3‐D pI‐MS‐%B plot where each protein can be tagged according to three parameters. Copyright © 2001 John Wiley & Sons, Ltd.
Investigation of reduction of Cu(II) complexes in positive‐ion mode electrospray mass spectrometry
Authors: Gianelli Luca , Amendola Valeria , Fabbrizzi Luigi , Pallavicini Piersandro , Mellerio Giorgio G. 
Published: August 2002

AbstractThe electrospray ionization mass spectrometry (ESI‐MS) behavior of seven Cu(II) complexes with tetradentate ligands has been studied. An unexpected reduction process, in positive ion mode, of the Cu oxidation state was observed, and shown to be due to charge transfer between the metal complex and the solvent molecules in the gas phase. Ion trap collision‐induced dissociation experiments and deuterated solvents were used to support the proposed mechanism that is not a common electrochemical redox reaction at the ESI tip, but a gas‐phase process. A series of solvents (acetonitrile, methanol, ethanol, propanol and iso‐butanol) were tested, and a correlation between ionization energy (IE) and the amount of Cu(I) produced in ESI has been demonstrated for the alcohols, although some other solvent properties should also be taken into account. The electrochemical reduction potential of the complexes in solution is also an important parameter, since complexes more easily reduced in solution are also easier to reduce in the gas phase. Copyright © 2001 John Wiley & Sons, Ltd.
Determination of azaspiracids in shellfish using liquid chromatography/tandem electrospray mass spectrometry
Authors: Furey Ambrose , Braña‐Magdalena Ana , Lehane Mary , Moroney Cian , James Kevin J. , Satake Masayuki , Yasumoto Takeshi 
Published: August 2002

AbstractAzaspiracid (AZA1), a recently discovered marine toxin, is responsible for the new human toxic syndrome, azaspiracid poisoning (AZP), which is caused by the consumption of contaminated shellfish. A new, sensitive liquid chromatography/mass spectrometry (LC/MS) method has been developed for the determination of AZA1 and its analogues, 8‐methylazaspiracid (AZA2) and 22‐demethylazaspiracid (AZA3). Separation of these toxins was achieved using reversed‐phase LC and coupled, via an electrospray ionisation (ESI) source, to an ion‐trap mass spectrometer. Spectra showed the protonated molecules, [M + H]+, and their major product ions, due to the sequential loss of two water molecules, [M + H − H2O]+, [M + H − 2H2O]+, in addition to fragment ions that are characteristic of these cyclic polyethers. A highly specific and sensitive LC/MS3 analytical method was developed and, using shellfish extracts containing AZA1, the detection limit (S/N = 3) was 4 pg on‐column, corresponding to 0.8 ng/mL. Using the protocol presented here, this is equivalent to 0.37 ng/g shellfish tissue and good linear calibrations were obtained for AZA1 in shellfish extracts (average r2 = 0.9988). Good reproducibility was achieved with % RSD values (N = 5) ranging from 1.5% (0.75 µg/mL) to 4.2% (0.05 µg/mL). An efficient procedure for the extraction of toxins from shellfish aided the development of a rapid protocol for the determination of the three predominant azaspiracids. Copyright © 2002 John Wiley & Sons, Ltd.
2H‐ and 13C‐Labelled tracers compared for kinetic studies of ascorbic acid metabolism in man: a factor analytical approach
Authors: Bluck Leslie J. C. , Jones Kerry S. , Bates Christopher J. 
Published: August 2002

AbstractA recent report that a 13C stable isotope method can be used to measure the kinetics of ascorbic acid uptake and distribution in man has raised some interesting questions with regard to the physiological interpretation of the data obtained, in particular the sizes of the ascorbate distribution spaces. In order to prove that this result is not a function of the label used we have compared the behaviour of two different tracers to see if they are likely to give comparable results for the kinetic parameters. Volunteers received an oral dose of ascorbic acid, half of which was labelled with 2H, and half of which was labelled with 13C. Blood samples were taken over the course of the next 48 h, and ascorbic acid mass spectra measured by gas chromatography/mass spectrometry (GC/MS). Principal component analysis was used to investigate the number of factors required to explain the total variations observed in the ratios of the molecular isotopomer cluster over the time course of the experiment. Theoretical cracking patterns were then used as test vectors in target transformation and as the basis for subsequent combination to determine tracer/tracee ratios.Two factors were found sufficient to account for the observed cracking pattern variations within experimental error. These were identified as (1) the spectrum of unlabelled (endogenous) ascorbic acid, and (2) a linear combination of the spectra of the two labelled species used. The absence of a third factor in the decomposition indicates that there is no difference in the behaviour of the 13C‐ and 2H‐labelled tracers. Target testing allowed the tracer/tracee ratios to be determined using calculated cracking patterns, and produced equivalent results to conventional methods. Our experience in this work indicates that factor analysis has a useful place in many kinetic studies of this kind, either with one or many labelled species. Copyright © 2002 John Wiley & Sons, Ltd.
A stochastic analysis of robust estimation algorithms in H∞ with rational basis functions
Authors: Akçay Hüseyin 
Published: August 2002

AbstractIn this paper, a two‐stage nonlinear identification algorithm parameterized in terms of rational basis functions with fixed basis poles is studied when disturbances are subject to mild stochastic assumptions. The two‐stage algorithm is the archetype for robust estimation algorithms in H∞ and its first stage is linear‐in‐data. Conditions for the consistency of both the stages are derived. It is shown that the two‐stage algorithm enjoys a better stochastic as well as deterministic performance than those of the linear algorithms. Copyright © 2001 John Wiley & Sons, Ltd.