排序方式: 共有93条查询结果,搜索用时 15 毫秒
41.
In this study the rapid growth of sII H(2) hydrate within 20 min of post formation quenching towards liquid nitrogen (LN(2)) temperature is presented. Initially at 72 MPa and 258 K, hydrate samples would cool to the conditions of ~60 MPa and ~90 K after quenching. Although within the stability region for H(2) hydrate, new hydrate growth only occurred under LN(2) quenching of the samples when preformed hydrate "seeds" of THF + H(2) were in the presence of unconverted ice. The characterization of hydrate seeds and the post-quenched samples was performed with confocal Raman spectroscopy. These results suggest that quenching to LN(2) temperature, a common preservation technique for ex situ hydrate analysis, can lead to rapid unintended hydrate growth. Specifically, guest such as H(2) that may otherwise need sufficiently long induction periods to nucleate, may still experience rapid growth through an increased kinetic effect from a preformed hydrate template. 相似文献
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Amadeu H. Iglesias Luiz F. A. Santos Fabio C. Gozzo 《Journal of the American Society for Mass Spectrometry》2009,20(4):557-566
The use of chemical crosslinking is an attractive tool that presents many advantages in the application of mass spectrometry
to structural biology. The correct assignment of crosslinked peptides, however, is still a challenge because of the lack of
detailed fragmentation studies on resultant species. In this work, the fragmentation patterns of intramolecular crosslinked
peptides with disuccinimidyl suberate (DSS) has been devised by using a set of versatile, model peptides that resemble species
found in crosslinking experiments with proteins. These peptides contain an acetylated N-terminus followed by a random sequence
of residues containing two lysine residues separated by an arginine. After the crosslinking reaction, controlled trypsin digestion
yields both intra- and intermolecular crosslinked peptides. In the present study we analyzed the fragmentation of matrix-assisted
laser desorption/ionization-generated peptides crosslinked with DSS in which both lysines are found in the same peptide. Fragmentation
starts in the linear moiety of the peptide, yielding regular b and y ions. Once it reaches the cyclic portion of the molecule, fragmentation was observed to occur either at the following peptide
bond or at the peptide crosslinker amide bond. If the peptide crosslinker bond is cleaved, it fragments as a regular modified
peptide, in which the DSS backbone remains attached to the first lysine. This fragmentation pattern resembles the fragmentation
of modified peptides and may be identified by common automated search engines using DSS as a modification. If, on the other
hand, fragmentation happens at the peptide bond itself, rearrangement of the last crosslinked lysine is observed and a product
ion containing the crosslinker backbone and lysine (m/z 222) is formed. The detailed identification of fragment ions can help the development of softwares devoted to the MS/MS data
analysis of crosslinked peptides. 相似文献
43.
We show that certain mechanical systems, including a geodesic flow in any dimension plus a quasi-periodic perturbation by a potential, have orbits of unbounded energy.The assumptions we make in the case of geodesic flows are:
- (a)
- The metric and the external perturbation are smooth enough.
- (b)
- The geodesic flow has a hyperbolic periodic orbit such that its stable and unstable manifolds have a tranverse homoclinic intersection.
- (c)
- The frequency of the external perturbation is Diophantine.
- (d)
- The external potential satisfies a generic condition depending on the periodic orbit considered in (b).
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Daniel Alfonso Spudeit Marcel Piovezan Maressa D. Dolzan Jacqueline Pereira Vistuba Mônia Stremel Azevedo Luciano Vitali Marcone Augusto Leal Oliveira Ana Carolina Oliveira Costa Gustavo Amadeu Micke 《Electrophoresis》2013,34(24):3333-3340
A rapid method for the simultaneous determination of free glycerol (FG) and total glycerol (TG) in biodiesel by CE using a short‐end multiple injection (SE/MI) configuration system is described. The sample preparation for FG involves the extraction of glycerol with water and for TG a saponification reaction is carried out followed by extraction as in the case of FG. The glycerol extracted in both cases is submitted to periodate oxidation and the iodate ions formed are measured on a CE‐SE/MI system. The relevance of this study lies in the fact that no analytical procedure has been previously reported for the determination of TG (or of FG and TG simultaneously) by CE. The optimum conditions for the saponification/extraction process were 1.25% KOH and 25°C, with a time of only 5 min, and biodiesel mass in the range of 50.0–200.0 mg can be used. Multiple injections were performed hydrodynamically with negative pressure as follows: 50 mbar/3s (FG sample); 50 mbar/6s (electrolyte spacer); 50 mbar/3s (TG sample). The linear range obtained was 1.55–46.5 mg/L with R2> 0.99. The LOD and LOQ were 0.16 mg/L and 0.47 mg/L, respectively for TG. The method provides acceptable throughput for application in quality control and monitoring biodiesel synthesis process. In addition, it offers simple sample preparation (saponification process), it can be applied to a variety biodiesel samples (soybean, castor, and waste cooking oils) and it can be used for the determination of two key parameters related to the biodiesel quality with a fast separation (less than 30 s) using an optimized CE‐SE/MI system. 相似文献
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Walsh MR Rainey JD Lafond PG Park DH Beckham GT Jones MD Lee KH Koh CA Sloan ED Wu DT Sum AK 《Physical chemistry chemical physics : PCCP》2011,13(44):19951-19959
Interest in describing clathrate hydrate formation mechanisms spans multiple fields of science and technical applications. Here, we report findings from multiple molecular dynamics simulations of spontaneous methane clathrate hydrate nucleation and growth from fully demixed and disordered two-phase fluid systems of methane and water. Across a range of thermodynamic conditions and simulation geometries and sizes, a set of seven cage types comprises approximately 95% of all cages formed in the nucleated solids. This set includes the ubiquitous 5(12) cage, the 5(12)6(n) subset (where n ranges from 2-4), and the 4(1)5(10)6(n) subset (where n also ranges from 2-4). Transformations among these cages occur via water pair insertions/removals and rotations, and may elucidate the mechanisms of solid-solid structural rearrangements observed experimentally. Some consistency is observed in the relative abundance of cages among all nucleation trajectories. 5(12) cages are always among the two most abundant cage types in the nucleated solids and are usually the most abundant cage type. In all simulations, the 5(12)6(n) cages outnumber their 4(1)5(10)6(n) counterparts with the same number of water molecules. Within these consistent features, some stochasticity is observed in certain cage ratios and in the long-range ordering of the nucleated solids. Even when comparing simulations performed at the same conditions, some trajectories yield swaths of multiple adjacent sI unit cells and long-range order over 5 nm, while others yield only isolated sI unit cells and little long-range order. The nucleated solids containing long-range order have higher 5(12)6(2)/5(12) and 5(12)6(3)/4(1)5(10)6(2) cage ratios when compared to systems that nucleate with little long-range order. The formation of multiple adjacent unit cells of sI hydrate at high driving forces suggests an alternative or addition to the prevailing hydrate nucleation hypotheses which involve formation through amorphous intermediates. 相似文献
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Trapero A Alfonso I Butters TD Llebaria A 《Journal of the American Chemical Society》2011,133(14):5474-5484
Four diastereomeric series of N-alkylated [6+5] bicyclic isoureas having hydroxyl substituents mimicking glucose hydroxyl groups have been synthesized as potential β-glucocerebrosidase (GCase) inhibitors with the aim of developing pharmacological chaperones for enzyme deficiency in Gaucher disease (GD). The bicyclic compounds differ either by the configuration of the ring fusion carbon atoms or by the nature of the N-alkyl substituents. When assayed for effects on GCase activity, the isoureas displayed selective inhibition of GCase with low micromolar to nanomolar IC(50)'s in isolated enzyme experiments. One of the series of isoureas, a family having a specific cis ring fusion, exhibited strong inhibition of recombinant GCase activity with K(i) values in the 2-42 nM range. In addition, the [6+5] bicyclic guanidine derivatives with a substitution pattern analogous to the most active isoureas were also found to be potent inhibitors of GCase with K(i) values between 3 and 10 nM. Interestingly, the active bicyclic isoureas and guanidines also behaved as GCase inhibitors in wild-type human fibroblasts at nanomolar concentrations. The potential of these compounds as pharmaceutical chaperones was determined by analyzing their capacity for increasing GCase activity in GD lymphoblasts derived from N370S and L444P variants, two of the most prevalent Gaucher mutations. Six compounds were selected from the different bicyclic isoureas and guanidines obtained that increased GCase activity by 40-110% in N370S and 10-50% in L444P cells at low micromolar to nanomolar concentrations following a 3 day incubation. These results describe a promising series of potent GCase ligands having the cellular properties required for pharmacological chaperones. 相似文献
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