排序方式: 共有59条查询结果,搜索用时 15 毫秒
31.
Ruth A. Perez Nils Rehmann Sheila Crain Patricia LeBlanc Cheryl Craft Shawna MacKinnon Kelley Reeves Ian W. Burton John A. Walter Philipp Hess Michael A. Quilliam Jeremy E. Melanson 《Analytical and bioanalytical chemistry》2010,398(5):2243-2252
The production and certification of a series of azaspiracid (AZA) calibration solution reference materials is described. Azaspiracids were isolated from contaminated mussels, purified by preparative liquid chromatography and dried under vacuum to the anhydrous form. The purity was assessed by liquid chromatography–mass spectrometry and nuclear magnetic resonance spectroscopy. The final concentration of each AZA in a CD3OH stock solution was determined by quantitative NMR spectroscopy. This solution was then diluted very accurately in degassed, high purity methanol to a concentration of 1.47?±?0.08 μmol/L for CRM-AZA1, 1.52?±?0.05 μmol/L for CRM-AZA2, and 1.37?±?0.13 μmol/L for CRM-AZA3. Aliquots were dispensed into argon-filled glass ampoules, which were immediately flame-sealed. The calibration solutions are suitable for method development, method validation, calibration of liquid chromatography or mass spectrometry instrumentation and quality control of shellfish monitoring programs. 相似文献
32.
Srinu Bodige Parameshwar Ravula Kali Charan Gulipalli Srinivas Endoori Purna Koteswara Rao Cherukumalli Narendra Sharath Chandra JN 《合成通讯》2013,43(17):2219-2234
AbstractA novel series pyrrolo[3,2-b]pyridine-3-carboxamide linked 2-methoxypyridine derivatives have been designed, synthesized and confirmed by FT-IR, 1H NMR, 13C NMR, 19F NMR, MS, and elemental analysis. The synthesized compounds were screened for their antitubercular activity using microplate alamar blue assay method and antibacterial activity. Among the tested compounds, 4- fluorophenyl (8m), 4- chlorophenyl (8n) and 4-methoxyphenyl (8i) showed potent anti-TB activity (3.12?µg/mL) in comparison with reference drug, Pyrazinamide ((3.12?µg/mL). In addition, all compounds were docked into DprE1 (PDB code: 4KW5) to explore their binding interactions at the active site. The compounds exhibited essential key interactions as that of reported DprE1 inhibitors and hence, the synthesized compounds may be considered as molecular scaffolds for antitubercular activity. Compounds, 4-chlorophenyl (8n) and 4-flurophenyl (8m) showed significant antibacterial activity against Escherichia coli and Staphylococcus aureus strains. In silico prediction of toxicities, druglikeness and drug score profiles of the tested compounds are promising. 相似文献
33.
Abstract The development of area detectors such as the imaging plate (IP) allows the use of angle-dispersive techniques for powder diffraction studies at high pressure. Integration of the 2-d pattern greatly improves the powder averaging and signal-to-noise ratio, making it possible to carry out full Rietveld refinements of crystal structures in high-pressure phases. An IP system is being developed at the Synchrotron Radiation Source (SRS), Daresbury, and the current status of this system -hardware and software -is described. Recent results on La2CuO4 and lnSb demonstrate the effectiveness and advantages of the techniques. 相似文献
34.
The structure of self-assembled monolayers presenting aromatic rings at a surface is studied by near edge x-ray absorption fine structure spectroscopy (NEXAFS). Fluorine substitution at asymmetric positions in the aromatic rings is used to generate a layer of dipoles at the surface of the monolayer. We find that fluorine substituted aromatic rings are more ordered than unsubstituted aromatic rings by a factor of two based on the polarization dependence of the lowest C 1s to pi* transition, which is associated with transitions from phenyl carbons attached to hydrogens. This result is consistent with the influence of dipole-dipole interactions and quadrupolar interactions between the aromatic groups due to the substitution of fluorine atoms. The work also serves to illustrate how subtle variations in the orientation of an end group of a self-assembled monolayer can be determined by using NEXAFS. 相似文献
35.
制备了五种对水稳定性好、带-SO3H官能团的磺酸类BrФnsted酸离子液体,用核磁共振(NMR)、红外光谱(IR)、电喷雾质谱(ESI-MS)、热重分析(TG)等表征手段对制备的离子液体进行了表征。结果表明,制备的离子液体与预期设计的结构一致,离子液体纯度大于95%;热重分析发现离子液体具有高的热稳定性和较宽的液态范围,其分解温度均高于300℃;五种离子液体均存在四种离子存在形式,H 可以单独以离子形式存在,并不是通常所认为的仅有两种离子存在形式。另外,研究了离子液体在常用溶剂中的溶解性,发现制备的离子液体易溶于水、甲醇,不溶于乙醚、甲苯和乙酸乙酯。 相似文献
36.
Single molecule fluorescence imaging incorporated with optical tweezers and a laminar flow cell has been used to monitor the kinetic process of DNA condensation induced by spermidine. It was found that at least two steps were involved in the condensation process of the hydrodynamically-stretched linear DNA; a lag period followed by a rapid collapse of DNA. The lag time increased with the flow speed and the collapse time remained short within the range of the flow speed studied. The effect of salt concentration on the condensation process was examined, and the results suggest that the longer lag time observed in the higher salt buffer probably results from the displacement of bound cations and rearrangement of spermidine on the DNA. The flow-speed dependence of the lag time suggests that a nucleation event at the free end of the DNA, i.e. formation of a loop, may play a vital role in the kinetic process of condensation. 相似文献
37.
Troitzsch RZ Martyna GJ McLain SE Soper AK Crain J 《The journal of physical chemistry. B》2007,111(28):8210-8222
The structure of aqueous L-proline amino acid has been the subject of much debate centering on the validity of various proposed models, differing widely in the extent to which local and long-range correlations are present. Here, aqueous proline is investigated by atomistic, replica exchange molecular dynamics simulations, and the results are compared to neutron diffraction and small angle neutron scattering (SANS) data, which have been reported recently (McLain, S.; Soper, A.; Terry, A.; Watts, A. J. Phys. Chem. B 2007, 111, 4568). Comparisons between neutron experiments and simulation are made via the static structure factor S(Q) which is measured and computed from several systems with different H/D isotopic compositions at a concentration of 1:20 molar ratio. Several different empirical water models (TIP3P, TIP4P, and SPC/E) in conjunction with the CHARMM22 force field are investigated. Agreement between experiment and simulation is reasonably good across the entire Q range although there are significant model-dependent variations in some cases. In general, agreement is improved slightly upon application of approximate quantum corrections obtained from gas-phase path integral simulations. Dimers and short oligomeric chains formed by hydrogen bonds (frequently bifurcated) coexist with apolar (hydrophobic) contacts. These emerge as the dominant local motifs in the mixture. Evidence for long-range association is more equivocal: No long-range structures form spontaneously in the MD simulations, and no obvious low-Q signature is seen in the SANS data. Moreover, associations introduced artificially to replicate a long-standing proposed mesoscale structure for proline correlations as an initial condition are annealed out by parallel tempering MD simulations. However, some small residual aggregates do remain, implying a greater degree of long-range order than is apparent in the SANS data. 相似文献
38.
Daniel Crain Jianping Zheng Christopher Sulyma Corina Goia Dan Goia Dipankar Roy 《Journal of Solid State Electrochemistry》2012,16(8):2605-2615
Lithium manganese oxide (LMO), mechano-chemically modified by ball-milling, is a potentially useful active material for high-power-density cathodes of lithium ion batteries. The present work investigates the electrochemical characteristic of a cathode prepared from a controlled mixture of nano- and micrometric LMO particles processed in this approach. The nanoparticles in the mixture support surface-localized insertion/extraction of Li and thus increase the cathode charge/discharge rates. The LMO micro-particles promote cathode cyclability by stabilizing the coexisting nanoparticles against segregation and strong electrolyte reactions. The underlying mechanisms of these effects are studied here using voltammetry, galvanostatic cycling, Ragone plot construction, and electrochemical impedance spectroscopy. The relative timescales of charge transfer and diffusion of Li+ within the LMO lattice are determined, and the criteria for material utilization during rapid charge–discharge are examined. 相似文献
39.
We show that the anomalous negative excess entropy of mixing characteristic of aqueous lower alcohols containing hydrophobic groups is quantitatively consistent with a model exploiting only the experimentally observed molecular-scale segregation of the components across the entire concentration range. The simple model presented here, which uses plausible interatomic distances as its only free parameters, obviates the need to invoke "iceberg" or other water restructuring concepts which, though frequently postulated in explaining the hydrophobic interaction, are unsupported by recent experiments. 相似文献
40.
Allison SK Bates SP Crain J Martyna GJ 《The journal of physical chemistry. B》2006,110(42):21319-21326
Classical molecular dynamics simulations of aqueous N-methylacetamide (NMA) have been performed across a concentration range at 308 K. This peptidic fragment molecule is a useful model for investigating water/peptide hydrogen bond competition. The simulations predict considerable NMA self-association even at low concentrations with a concentration-dependent increase in the ratio of branched to linear clusters. Water-mediated NMA contacts are a feature of this regime, manifested by an unexpected increase in the number of short NMA oxygen contacts arising from water bridge motifs. In contrast, bulk water structure is significantly disrupted by the addition of even small quantities of NMA. With increases in NMA concentration water molecules become progressively more isolated, forming dimers and trimers hydrogen-bonded to NMA. The mixture in this concentration regime may therefore offer a minimal model system for certain structural properties of interior water buried in protein cavities and hydrogen-bonded to mainchain peptide groups. 相似文献