bluuebunny/crossref_metadata_2025_split huggingface dataset:
Collision‐induced dissociation tandem mass spectrometry of desferrioxamine siderophore complexes from electrospray ionization of UO22+, Fe3+ and Ca2+ solutions
Authors: Groenewold Gary S. , Van Stipdonk Michael J. , Gresham Garold L. , Chien Winnie , Bulleigh Kellis , Howard Angela 
Published: July 2004

AbstractDesferrioxamine (DEF) is a trihydroxamate siderophore typical of those produced by bacteria and fungi for the purpose of scavenging Fe3+ from environments where the element is in short supply. Since this class of molecules has excellent chelating properties, reaction with metal contaminants such as actinide species can also occur. The complexes that are formed can be mobile in the environment. Because the natural environment is extremely diverse, strategies are needed for the identification of metal complexes in aqueous matrices having a high degree of chemical heterogeneity, and electrospray ionization mass spectrometry (ESI‐MS) has been highly effective for the characterization of siderophore–metal complexes. In this study, ESI‐MS of solutions containing DEF and either UO22+, Fe3+ or Ca2+ resulted in generation of abundant singly charged ions corresponding to [UO2(DEF − H)]+, [Fe(DEF − 2H)]+ and [Ca(DEF − H)]+. In addition, less abundant doubly charged ions were produced. Mass spectrometry/mass spectrometry (MS/MS) studies of collision‐induced dissociation (CID) reactions of protonated DEF and metal–DEF complexes were contrasted and rationalized in terms of ligand structure. In all cases, the most abundant fragmentation reactions involved cleavage of the hydroxamate moieties, consistent with the idea that they are most actively involved with metal complexation. Singly charged complexes tended to be dominated by cleavage of a single hydroxamate, while competitive fragmentation between two hydroxamate moieties increased when the doubly charged complexes were considered. Rupture of amide bonds was also observed, but these were in general less significant than the hydroxamate fragmentations. Several lower abundance fragmentations were unique to the metal examined: abundant loss of H2O occurred only for the singly charged UO22+ complex. Further, NH3 was eliminated only from the singly charged Fe3+ complex; this and fragmentation of CC and CN bonds derived from neither the hydroxamate nor the amide groups suggested that Fe3+ insertion reactions were competing with ligand complexation. In no experiments were coordinating solvent molecules observed, attached either to the intact complexes or to the fragment ions, which indicated that both intact DEF and its fragments were occupying all of the coordination sites around the metal centers. This conclusion was based on previous experiments that showed that undercoordinated UO22+ and Fe3+ readily added H2O and methanol in the ESI quadrupole ion trap mass spectrometer that was used in this study. Copyright © 2004 John Wiley & Sons, Ltd.
Quantitative analysis of 5α‐reductase inhibitors in DU145 cells using matrix‐assisted laser desorption/ ionization time‐of‐flight mass spectrometry and high‐performance liquid chromatography/tandem mass spectrometry
Authors: Kang Min‐Jung , Mathur Sonal , Hartmann Rolf W. 
Published: July 2004

AbstractN‐(Dicyclohexyl)acetylpiperidine‐4‐benzylidene‐4‐carboxylic acid (1) is an excellent in vitro inhibitor of 5α‐reductase (5αR). Compound 1 showed, however, much lower inhibition activity of 5αR in vivo than in vitro, which might be caused by poor membrane permeability. The methyl ester of 1 (1a) was therefore tested as a model prodrug to see if it has better permeability properties than the corresponding acid 1. It was also monitored that this methyl ester was cleaved into the active compound 1 within the DU145 cells. Quantitative matrix‐assisted laser desorption/ionization time‐of‐flight mass spectrometry (MALDI‐TOFMS) and high‐performance liquid chromatography/tandem mass spectrometry (HPLC/MS/MS) methods were established with reliable linearity factors (0.996 for MALDI‐TOFMS and 0.998 for HPLC/MS/MS) and reproducibility (relative standard deviation = 6.5% for MALDI‐TOFMS and 2.8% for HPLC/MS/MS). The samples for MS analysis were effectively prepared from the cell homogenates using solid‐phase extraction, with a high recovery of 90% on average. The intracellular amount of 1a (1.7 nmol) was much higher than that of 1 (0.032 nmol) in DU145 cells after 6 h of incubation. After incubation with the ester (1a), the cleaved acid (1) was detected within the cells. The concentration of acid 1 (0.045 nmol) in this experiment was higher than the acid content (0.032 nmol) after direct incubation with 1. Surprisingly, high amounts of the cleaved compound 1 were found outside the cells after 6 h of incubation with 1a. Copyright © 2004 John Wiley & Sons, Ltd.
An atmospheric pressure chemical ionization study of the positive and negative ion chemistry of the hydrofluorocarbons 1,1‐difluoroethane (HFC‐152a) and 1,1,1,2‐tetrafluoroethane (HFC‐134a) and of perfluoro‐n‐hexane (FC‐72) in air plasma at atmospheric pressure
Authors: Marotta Ester , Paradisi Cristina , Scorrano Gianfranco 
Published: July 2004

AbstractA report is given on the ionization/dissociation behavior of the title compounds within air plasmas produced by electrical corona discharges at atmospheric pressure: both positive and negative ions were investigated at different temperatures using atmospheric pressure chemical ionization mass spectrometry (APCI‐MS). CHF2CH3 (HFC‐152a) undergoes efficient ionic oxidation to C2H5O+, in which the oxygen comes from water present in the plasma. In contrast, CF3CH2F (HFC‐134a) does not produce any characteristic positive ion under APCI conditions, its presence within the plasma being revealed only as a neutral ligand in ion–molecule complexes with ions of the background (H3O+ and NO+). Analogously, the perfluorocarbon FC‐72 (n‐C6F14) does not produce significant positive ions at 30 °C: at high temperature, however, it undergoes dissociative ionization to form many product ions including C3F6+, C2F4+, CnF2n+1+ and a few families of oxygen containing cations (CnF2n+1OH2+, CnF2nOH+, CnF2n−1O+, CnF2n−1O2H2+, CnF2n−2O2H+) which are suggested to derive from CnF2n+1+ in a cascade of steps initiated by condensation with water followed by steps of HF elimination and H2O addition. Negative ions formed from the fluoroethanes CHF2CH3 and CF3CH2F (M) include complexes with ions of the background, O2−(M), O3−(M) and some higher complexes involving also water, and complexes of the fluoride ion, F−(H2O), F−(M) and higher complexes with both M and H2O also together. The interesting product O2−(HF) is also formed from 1,1‐difluoroethane. In contrast to the HFCs, perfluoro‐n‐hexane gives stable molecular anions, M−, which at low source temperature or in humidified air are also detected as hydrates, M−(H2O). In addition, in humidified air F−(H2O)n complexes are also formed. The reactions leading to all major positive and negative product ions are discussed also with reference to available thermochemical data and relevant literature reports. The effects on both positive and negative APCI spectra due to ion activation via increasing Vcone are also reported and discussed: several interesting endothermic processes are observed under these conditions. The results provide important information on the role of ionic reactions in non‐thermal plasma processes. Copyright © 2004 John Wiley & Sons, Ltd.
The propionyl cation heat of formation revisited
Authors: Harvey Zoë A. , Traeger John C. 
Published: July 2004

AbstractThe 298 K heat of formation for the propionyl cation (C2H5CO+) has been measured previously by dissociative photoionization mass spectrometry. However, recent theoretical and experimental studies involving methylketene suggest that this may be significantly underestimated, resulting in a methylketene proton affinity that is too high by ∼30 kJ mol−1. In this study, the previous m/z 57 appearance energies were carefully re‐evaluated, with various possible sources of error being investigated. These include factors such as sample purity, carbon‐13 contamination from lower energy m/z 56 processes, kinetic and/or competitive shifts, reverse activation energies, ionizing energy calibration errors and the availability of accurate supplementary thermochemical data. In addition, high‐level ab initio calculations are used to model the relevant unimolecular fragmentation processes for each of the ionized precursor molecules. As a result, it is found that only the 2‐butanone appearance energy can be used to provide a reliable value for the propionyl cation heat of formation. From the 298 K m/z 57 appearance energy of 10.199 ± 0.003 eV for 2‐butanone measured here, a value of 617.8 ± 0.9 kJ mol−1 is derived for $\Delta H^{\circ}_{{\rm f},298}(\hbox{C}_{2}\hbox{H}_{5}\hbox{CO}^{+})$, which corresponds to 845.4 ± 4.8 kJ mol−1 for the proton affinity of methylketene. This is in good agreement with previous theoretical calculations and thermokinetic proton affinity measurements, indicating that a significant upward revision to the propionyl cation heat of formation is warranted. Copyright © 2004 John Wiley & Sons, Ltd.
Matrix‐dependent cationization in MALDI mass spectrometry
Authors: Zhang Juan , Zenobi Renato 
Published: July 2004

AbstractThe matrix dependence in cationization processes, the competition between cationization and protonation and the question of whether gas‐phase cation transfer or attachment of free cations dominates in matrix‐assisted laser desorption/ionization mass spectrometry were studied. Two different sample preparation methods were employed, the dried‐droplet sample preparation and a mixture of solid matrix, analyte and salt. The latter ensures that the formation of cation adducts takes place in the gas phase. By monitoring the suppression of matrix signals for different matrices, it was found that matrices with high gas‐phase metal ion binding energies require high analyte concentrations for matrix suppression to occur. By comparing the mass spectra obtained using sinapinic acid or sinapinic methyl ester as a matrix, a correlation between cationization and deprotonation of matrix molecules was found. It is also demonstrated that attachment of free gas‐phase cations, rather than cation transfer from the cationized matrix, is the predominant process in cationization. Copyright © 2004 John Wiley & Sons, Ltd.
Peptide and protein characterization by high‐rate electron capture dissociation Fourier transform ion cyclotron resonance mass spectrometry
Authors: Tsybin Youri O. , Ramström Margareta , Witt Matthias , Baykut Gökhan , Håkansson Per 
Published: July 2004

AbstractThe analytical utility of the electron capture dissociation (ECD) technique, developed by McLafferty and co‐workers, has substantially improved peptide and protein characterization using Fourier transform ion cyclotron resonance mass spectrometry (FTICR‐MS). The limitations of the first ECD implementations on commercial instruments were eliminated by the employment of low‐energy electron‐injection systems based on indirectly heated dispenser cathodes. In particular, the ECD rate and reliability were greatly increased, enabling the combination of ECD/FTICR‐MS with on‐line liquid separation techniques. Further technique development allowed the combination of two rapid fragmentation techniques, high‐rate ECD and infrared multiphoton dissociation (IRMPD), in a single experimental configuration. Simultaneous and consecutive irradiations of trapped ions with electrons and photons extended the possibilities for ion activation/dissociation and led to improved peptide and protein characterization. The application of high‐rate ECD/FTICR‐MS has demonstrated its power and unique capabilities in top‐down sequencing of peptides and proteins, including characterization of post‐translational modifications, improved sequencing of peptides with multiple disulfide bridges and secondary fragmentation (w‐ion formation). Analysis of peptide mixtures has been accomplished using high‐rate ECD in bottom‐up mass spectrometry based on mixture separation by liquid chromatography and capillary electrophoresis. This paper summarizes the current impact of high‐rate ECD/FTICR‐MS for top‐down and bottom‐up mass spectrometry of peptides and proteins. Copyright © 2004 John Wiley & Sons, Ltd.
Simple, sensitive and rapid liquid chromatographic/electrospray ionization tandem mass spectrometric method for the quantification of lacidipine in human plasma
Authors: Ramakrishna N. V. S. , Vishwottam K. N. , Puran S. , Manoj S. , Santosh M. , Koteshwara M. 
Published: July 2004

AbstractA simple, sensitive and rapid liquid chromatographic/electrospray ionization tandem mass spectrometric method was developed and validated for the quantification of lacidipine in human plasma using its structural analogue, amlodipine, as internal standard (IS). The method involves a simple single‐step liquid–liquid extraction with tert‐butyl methyl ether. The analyte was chromatographed on an Xterra MS C18 reversed‐phase chromatographic column by isocratic elution with 20 mM ammonium acetate buffer–acetonitrile (10 : 90, v/v; pH 6) and analyzed by mass spectrometry in the multiple reaction monitoring mode. The precursor to product ion transitions of m/z 456.4 → 354.4 and m/z 409.3 → 238.3 were used to measure the analyte and the I.S., respectively. The chromatographic run time was 1.5 min and the weighted (1/x2) calibration curves were linear over the range 0.1–25 ng ml−1. Lacidipine was sensitive to temperature in addition to light. The method was validated in terms of accuracy, precision, absolute recovery, freeze–thaw stability, bench‐top stability and re‐injection reproducibility. The limit of detection and lower limit of quantification in human plasma were 50 and 100 pg ml−1, respectively. The within‐ and between‐batch accuracy and precision were found to be well within acceptable limits (<15%). The analyte was stable after three freeze–thaw cycles (deviation <15%). The average absolute recoveries of lacidipine and amlodipine (IS) from spiked plasma samples were 51.1 ± 1.3 and 50.3 ± 4.9%, respectively. The assay method described here could be applied to study the pharmacokinetics of lacidipine. Copyright © 2004 John Wiley & Sons, Ltd.
Quantitative study of degeneration and new growth of axons and synaptic endings in the chinchilla cochlear nucleus after acoustic overstimulation
Authors: Kim J.J. , Gross J. , Morest D.K. , Potashner S.J. 
Published: July 2004

AbstractTo determine if acoustic overstimulation altered synaptic connections in the cochlear nucleus, anesthetized adult chinchillas, with one ear protected by a silicone plug, were exposed for 3 hr to a 108‐dB octave‐band noise, centered at 4 kHz, and allowed to survive for periods up to 32 weeks. This exposure led to cochlear damage in the unprotected ear, mainly in the basal regions of the organ of Corti. The anterior part of the ipsilateral posteroventral cochlear nucleus consistently contained a band of degenerating axons and terminals, in which electron microscopic analysis revealed substantial losses of axons and synaptic terminals with excitatory and inhibitory cytology. The losses were significant after 1 week's survival and progressed for 16–24 weeks after exposure. By 24–32 weeks, a new growth of these structures produced a resurgence in the number of axons and terminals. The net number of excitatory endings fully recovered, but the quantity with inhibitory cytology was only partially recouped. Neuronal somata lost both excitatory and inhibitory endings at first and later recovered a full complement of excitatory but not inhibitory terminals. Dendrites suffered a net loss of both excitatory and inhibitory endings. Excitatory and inhibitory terminals with unidentified postsynaptic targets in the neuropil declined, then increased in number, with excitatory terminals exhibiting a greater recovery. These findings are consistent with a loss and regrowth of synaptic endings and with a reorganization of synaptic connections that favors excitation. © 2004 Wiley‐Liss, Inc.
Fine structure of long‐term changes in the cochlear nucleus after acoustic overstimulation: Chronic degeneration and new growth of synaptic endings
Authors: Kim J.J. , Gross J. , Potashner S.J. , Morest D.K. 
Published: July 2004

AbstractThe companion study showed that acoustic overstimulation of adult chinchillas, with a noise level sufficient to damage the cochlea, led to cytological changes and degeneration of synaptic endings in the cochlear nucleus within 1–16 weeks. In the present study, the same stimulus was used to study the long‐term effects on the fine structure of synaptic endings in the cochlear nucleus. For periods of 6 and 8 months after a single exposure to a damaging noise level, there ensued a chronic, continuing process of neurodegeneration involving excitatory and inhibitory synaptic endings. Electron microscopic observations demonstrated freshly occurring degeneration even as late as 8 months. Degeneration was widespread in the neuropil and included the synapses on the globular bushy cell, which forms part of the main ascending auditory pathway. Neurodegeneration was accompanied by newly formed synaptic endings, which repopulated some of the sites vacated previously by axosomatic endings on globular bushy cells. Many of these synaptic endings must arise from central interneurons. The findings suggest that overstimulation can induce a self‐sustaining condition of progressive neurodegeneration accompanied by a new growth of synaptic endings. Noise‐induced hearing loss thus may progress as a neurodegenerative disease with the capacity for synaptic reorganization within the cochlear nucleus. © 2004 Wiley‐Liss, Inc.
Architecture and evolution of a fjord‐head delta, western Vancouver Island, British Columbia
Authors: Gutsell Jeffrey E. , Clague John J. , Best Melvyn E. , Bobrowsky Peter T. , Hutchinson Ian 
Published: July 2004

AbstractThe architectural framework and Holocene evolution of the Zeballos fjord‐head delta on west‐central Vancouver Island was established through a multidisciplinary field‐based study. The Zeballos delta is a composite feature, consisting of an elevated, incised, late Pleistocene delta and an inset Holocene delta graded to present sea level. Both deltas have a classic Gilbert‐type tripartite architecture, with nearly flat topset and bottomset units and an inclined foreset unit. Time domain electromagnetic (TDEM) and ground‐penetrating radar (GPR) surveys, borehole data, and gravel pit exposures provided information on the internal form, lithologies and substrate of both deltas. Both sets of deltaic deposits coarsen upward from silt in the bottomset unit to gravel in the topset unit. The TDEM survey revealed a highly irregular, buried bedrock surface, ranging from 20 m to 190 m in depth, and it delineated saltwater intrusion into the deltaic sediments.Late Quaternary sea‐level change at Zeballos was inferred from delta morphology and the GPR survey. The elevated, late Pleistocene delta was constructed when the sea was about 21 m higher relative to the land than it is today. It was dissected when sea‐level fell rapidly as a result of glacio‐isostatic rebound. Relative sea‐level reached a position about 20 m below the present datum during the early Holocene. Foreset beds that overlap and progressively climb in a seaward direction and topset beds that thicken to 26 m landward imply that the delta aggraded and prograded into Zeballos Inlet during the middle and late Holocene transgression. Sea‐level may have risen above the present datum during the middle Holocene, creating a delta plain at about 4 m a.s.l. Remnants of this surface are preserved along the valley margins. Copyright © 2004 John Wiley & Sons, Ltd.
Investigation of the factors that induce analyte peak splitting in capillary electrophoresis
Authors: Chen Xiaofeng , Xie Jianping , Li Cunhong , Hu Zhide , Chen Xingguo 
Published: July 2004

AbstractPeak splitting has a detrimental effect on analyses by capillary electrophoresis. Many papers have reported it and several mechanisms have been proposed to explain the phenomenon. We investigated the electrophoretic behavior of an amphoteric analyte, levodopa, in phosphate buffer and observed a peak splitting phenomenon at moderate sample concentrations and under general analytical conditions, even without organic solvent. The dependence of effective mobility on pH was taken into account and pKa values of 2.30, 8.11, and 9.92 were obtained for levodopa. Then, we constructed pH‐dependent distribution diagrams of levodopa and phosphate species present in aqueous solution and proposed that the most relevant factors contributing to peak splitting are the presence of ionizable groups in the analyte molecule and the occurrence of ionization, yielding charged species which interacted with buffer electrolyte species in a definite pH range to form complexes. This result is different from those presented in the literature and broadens our understanding of amphoteric analyte peak splitting.
Chiral separation of omeprazole and several related benzimidazoles using supercritical fluid chromatography
Authors: del Nozal M. Jesús , Toribio Laura , Bernal José L. , Alonso Cristina , Jiménez Juan J. 
Published: July 2004

AbstractA study of the enantiomeric separation of omeprazole and several related benzimidazoles, using supercritical fluid chromatography (SFC), on the amylose based column Chiralpak AD is presented in this work. The effect of the organic modifier as well as temperature on the retention and enantioresolution was investigated. Alcohol‐type modifiers provided the best results, allowing the enantiomeric separation of all the compounds studied with resolutions that were in most cases higher than 2, and analysis times lower than 10 minutes. An investigation of the temperature effect revealed that the isoelution temperature was below the working temperature range in only two cases, and hence it was better to work at the highest temperature permitted.
Hölder continuity for quasilinear subelliptic equations in Carnot Carathéodory spaces
Authors: Di Fazio Giuseppe , Zamboni Pietro 
Published: July 2004

AbstractIn this note we prove the Harnack inequality and the Hölder continuity for weak solutions of quasilinear subelliptic equation of the form

where u belongs to Sobolev spaces with respect to a system of locally Lipschitz vector fields. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Elliptic and parabolic problems in unbounded domains
Authors: Guidotti Patrick 
Published: July 2004

AbstractWe consider elliptic and parabolic problems in unbounded domains. We give general existence and regularity results in Besov spaces and semi‐explicit representation formulas via operator‐valued fundamental solutions which turn out to be a powerful tool to derive a series of qualitative results about the solutions. We give a sample of possible applications including asymptotic behavior in the large, singular perturbations, exact boundary conditions on artificial boundaries and validity of maximum principles. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Fredholmness of singular integral operators with discrete subexponentialgroups of shifts on Lebesgue spaces
Authors: Karlovich Yuri , Silbermann Bernd 
Published: July 2004

AbstractThe paper is devoted to an application of a general local method of studying the Fredholmness of nonlocal bounded linear operators to Banach algebras of singular integral operators with piecewise continuous coefficients and discrete subexponential groups of piecewise smooth shifts acting topologically freely on composed contours. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)