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1.
Vivekananda S Sadílek M Chen X Turecek F 《Journal of the American Society for Mass Spectrometry》2004,15(7):1055-1067
Collisional neutralization of several isomeric C(4)H(7)O(2) cations is used to generate radicals that share some structural features with transient species that are thought to be produced by radiolysis of 2-deoxyribose. The title 2-hydroxyoxolan-2-yl radical (1) undergoes nearly complete dissociation when produced by femtosecond electron transfer from thermal organic electron donors dimethyl disulfide and N,N-dimethylaniline in the gas phase. Product analysis, isotope labeling ((2)H and (18)O), and potential energy surface mapping by ab initio calculations at the G2(MP2) and B3-PMP2 levels of theory and in combination with Rice-Ramsperger-Kassel-Marcus (RRKM) kinetic calculations are used to assign the major and some minor pathways for 1 dissociations. The major (approximately 90%) pathway is initiated by cleavage of the ring C-5[bond]O bond in 1 and proceeds to form ethylene and *CH(2)COOH as main products, whereas loss of a hydrogen atom forms 4-hexenoic acid as a minor product. Loss of the OH hydrogen atom forming butyrolactone (2, approximately 9%) and cleavage of the C-3[bond]C-4 bonds (<1%) in 1 are other minor pathways. The major source of excitation in 1 is by Franck-Condon effects that cause substantial differences between the adiabatic and vertical ionization of 1 (5.40 and 6.89 eV, respectively) and vertical recombination in the precursor ion 1(+) (4.46 eV). (+)NR(+) mass spectra distinguish radical 1 from isomeric radicals 2-oxo-(1H)oxolanium (3), 1,3-dioxan-2-yl (9), and 1,3-dioxan-4-yl (10) that were generated separately from their corresponding ion precursors. 相似文献
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3.
Vivekananda S Sadílek M Chen X Adams LE Turecek F 《Journal of the American Society for Mass Spectrometry》2004,15(7):1068-1079
The title radical (1) is generated in the gas-phase by collisional neutralization of carbonyl-protonated oxolan-3-one. A 1.5% fraction of 1 does not dissociate and is detected following reionization as survivor ions. The major dissociation of 1 (approximately 56%) occurs as loss of the hydroxyl H atom forming oxolan-3-one (2). The competing ring cleavages by O[bond]C-2 and C-4[bond]C-5 bond dissociations combined account for approximately 42% of dissociation and result in the formation of formaldehyde and 2-hydroxyallyl radical. Additional ring-cleavage dissociations of 1 resulting in the formation of C(2)H(3)O and C(2)H(4)O cannot be explained as occurring competitively on the doublet ground (X) electronic state of 1, but are energetically accessible from the A and higher electronic states accessed by vertical electron transfer. Exothermic protonation of 2 also produces 3-oxo-(1H)-oxolanium cation (3(+)) which upon collisional neutralization gives hypervalent 3-oxo-(1H)-oxolanium radical (3). The latter dissociates spontaneously by ring opening and expulsion of hydroxy radical. Experiment and calculations suggest that carbohydrate radicals incorporating the 3-hydroxyoxolan-3-yl motif will prefer ring-cleavage dissociations at low internal energies or upon photoexcitation by absorbing light at approximately 590 and approximately 400 nm. 相似文献
4.
Chitta R. Panda Vivekananda Chakravortty Kailash C. Dash 《Transition Metal Chemistry》1988,13(4):287-290
Summary Interaction of ZrOCl2:8H2O, [Zr4(OH)8(H2O)16]Cl8 12H2O, with the heterocyclic, aldimines (PyAlA) and heterocyclic ketimines (AcPyA) in Me2CO in the presence of HC(OEt)3 yields white amorphous compounds of the type [Zr4(OH)12(H2O)8(PyAlA)2]Cl4 and [Zr4(OH)12(H2O)10(AcPyA)]Cl4. Presumably these compounds have a tetrameric dodecahedral structure, derived from the parent. The analytical data, i.r., electrical conductance and t.g. measurements all favour the tetrameric formulation. The t.g. studies also indicate the intermediacy of complex species, which lose water and the Schiff base ligands, with increase in temperature. 相似文献
5.
Characterization by tandem mass spectrometry of stable cysteine sulfenic acid in a cysteine switch peptide of matrix metalloproteinases 总被引:1,自引:1,他引:0
Shetty V Spellman DS Neubert TA 《Journal of the American Society for Mass Spectrometry》2007,18(8):1544-1551
Cysteine sulfenic acid (Cys-SOH) is an elusive intermediate in reactive oxygen species-induced oxidation reactions of many proteins such as peroxiredoxins and tyrosine phosphatases. Cys-SOH is proposed to play a vital role in catalytic and signaling functions. The formation of cysteine sulfinic acid (Cys-SO(2)H) and cysteine sulfonic acid (Cys-SO(3)H) has been implicated in the activation of matrix metalloproteinase-7 (MMP-7) and oxidation of thiol to cysteine sulfinic acid has been associated with the autolytic cleavage of MMP-7. We have examined the formation of cysteine sulfenic acid in a synthetic peptide PRCGVPDVA, which is a cysteine switch domain of MMP-7 and other matrix metalloproteases. We have prepared the cysteine sulfenic acid containing peptide, PRC(SOH)GVPDVA, by reaction with hydroxyl radicals generated by the Fenton reaction (Fe(+2)/H(2)O(2)). We characterized this modified peptide by tandem mass spectrometry and accurate mass measurement experiments. In addition, we used 7-chloro-4-nitrobenzo-2-oxa-1,3-diazol (NBD-Cl) reagent to form an adduct with PRC(SOH)GVPDVA to provide additional evidence for the viability of PRC(SOH)GVPDVA in solution. We also characterized an intramolecular cysteine sulfinamide cross-link product PRC[S(O)N]GVPDVA based on tandem mass spectrometry and accurate mass measurement experiments. These results contribute to the understanding of a proteolytic cleavage mechanism that is traditionally associated with MMP activation. 相似文献
6.
Vivekananda Roy 《Statistics & probability letters》2012,82(1):103-108
The sandwich algorithm (SA) is an alternative to the data augmentation (DA) algorithm that uses an extra simulation step at each iteration. In this paper, we show that the sandwich algorithm always converges at least as fast as the DA algorithm, in the Markov operator norm sense. We also establish conditions under which the spectrum of SA dominates that of DA. An example illustrates the results. 相似文献
7.
4-(2-Thiazolylazo)resorcinol (TAR) is used as a chelating reagent in the reversed-phase HPLC separation and determination of transition and rare earth elements in low-alloy steels. A precolumn derivatization method is used, followed by separation on an octadecyl-bonded silica stationary phase with a sodium octane-1-sulphonate-tartaric acid mobile phase. The eluted metal chelates are detected by uv-visible spectrophotometry. The requirements for sample preparation, characterization of precolumn derivatization and the optimum conditions for the sensitive detection of metal ions after liquid chromatography (LC) separation are discussed. The influence of the pH of the chelating medium and the eluent, the concentration of TAR and the eluent were investigated. The results are compared with those of other methods such as atomic absorption spectrometry (AAS) and inductively coupled plasma atomic emission spectrometry (ICP-AES). 相似文献
8.
Prashanta K. Mishra Vivekananda Chakravortty Kailash C. Dash 《Transition Metal Chemistry》1991,16(1):73-75
Summary Zr(acac)4 undergoes ligand exchanges with various tri- and tetradentate Schiff base ligands, forming compounds of the Zr(L)2 type (H2L=tetradentate H2Sal2en, H2Sal2pn, H2Sal2
o-phen and the tridentate H2SAP) and Zr(acac)2L (H2L=H2SAN. H2SAE). Upon reaction with a combination of tri- and tetradentate ligands, Zr(acac)4 yields Zr(L)(L) complexes (H2L=H2Sal2en or H2Sal2
o-phen; H2L=H2SAN, H2SAE, or H2SAP), which have been characterised by analytical data, m.ps, electrical conductivities, i.r. and n.m.r (1H and13C) spectra, they have a coordination number of 6, 7 or 8. 相似文献
9.
A flow-injection analysis atomic absorption spectrometric (FIA-AAS) method was developed for the determination of trace amounts of arsenic, selenium and mercury in a proposed estuarine sediment standard reference material (SRM 1646a). The samples were prepared in two manners: a) A wet digestion procedure with HNO3, H2SO4, and HClO4 using a reflux column and b) A microwave-oven digestion procedure utilizing HNO3, H2SO4, and HCl for As and Se, and HNO3 for Hg. Microwave-oven digestion provides results comparable to those found by reflux column digestion and reduces the sample preparation time by a factor of 10. The proposed method employing the microwave-oven digestion procedure coupled with FIA-AAS for As and Se, and FIA-CVAAS for Hg, has detection limits of 0.15 ng As/ml, O.17 ng Se/ml and 0.15 ng Hg/ml.On leave from the Defense Metallurgical Research Laboratory, Hyderabad, India 相似文献
10.
Vivekananda Shetty Thomas A. Neubert 《Journal of the American Society for Mass Spectrometry》2009,20(8):1540-1548
We investigated the formation of hydroxyl radical (OH·) and H2O2 mediated oxidation products of a synthetic peptide, HCSAGIGRS, which is an active site sequence motif of protein tyrosine
phosphatase 1B (PTP1B). We determined that a novel cysteine sulfinamide HC[S(O)N]SAGIGRS is produced in the oxidation reaction
by Fenton reagents (Fe+2/H2O2) as well as by H2O2. These products were characterized by tandem mass spectrometry experiments on both singly and doubly charged precursor ions.
MS3 experiments using an ion trap instrument as well as LC-MS/MS experiments using a quadrupole time-of-flight (Q-TOF) instrument
demonstrated that HC[S(O)N]SAGIGRS is not a water loss product of cysteine sulfinic acid [HC(SO2H)SAGIGRS]. We also obtained data from tandem mass spectrometry experiments that provided evidence for the existence of stable
cysteine sulfenic acid [HC(SOH)SAGIGRS] in solution. A mechanism for the formation of the cysteine sulfinamide product is
proposed based on the above experimental results. The preparation and identification of cysteine sulfinamide in this study
may provide insight into the mechanism of both OH· and H2O2 induced oxidation reactions of protein tyrosine phosphatases. 相似文献