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排序方式: 共有1301条查询结果,搜索用时 31 毫秒
51.
Tierui Zhang Shaoqin Liu Dirk G. Kurth Charl F. J. Faul 《Advanced functional materials》2009,19(4):642-652
A variety of functional nanostructured organic/inorganic hybrid materials from the europium‐exchanged derivative of a Preyssler‐type polyoxometalate (POM), [EuP5W30O110]12?, and functional organic surfactants were prepared by the ionic self‐assembly (ISA) route. The effect of organic surfactants on the structure, photoluminescent, electrochemical and electrochromic properties of the POM anions was investigated in detail. All obtained hybrid materials are amphotropic, i.e., exhibit both thermotropic and lyotropic liquid‐crystalline phase behaviour. Investigations of their photophysical properties have shown that the interactions of the various surfactants with the polyanions influence the coordination environments and site symmetry of Eu3+ in different ways. The functional groups in the organic surfactants significantly influence the electrochromic properties and photoluminescence of POMs. Different from normal and pyridine‐containing complexes, no photoluminescence and no electrochromism were observed from the ferrocene‐containing complexes. This may be explained in view of charge transfer between the POM anion and the ferrocenyl group. 相似文献
52.
Here, we report the construction and characterization of dual reporters, consisting of both an Escherichia coli alkaline phosphatase (AP) gene and an alpha-fragment of the beta-galactosidase (BG) gene, for studying membrane protein topology by the gene fusion approach. Each of the reporters, when fused to periplasmic domains of polytopic proteins, produces fusions with high AP activity and, when fused to cytoplasmic domains, produces fusions with high BG activity in E. coli strains capable of alpha-complementation. The dual nature of these reporters simplifies interpretation of data obtained with poorly expressed fusions and allows one to evaluate the reliability of topological data. Deleterious effects resulting from the cell's attempt to export the full-length BG are eliminated in this approach. We describe dual indicator plates that allow for discrimination between colonies bearing cytoplasmic fusions, periplasmic fusions, and no fusions. We have generated a set of fusions to the topologically well-studied lactose permease of E. coli and demonstrated that topological information generated by these new reporters is in good agreement with the existing model. We used this new methodology for the determination of membrane topology of the Rickettsia prowazekii ATP/ADP translocase (Tlc). Our results were in agreement with the proposed in silico topological model in which Tlc traverses the cytoplasmic membrane of E. coli 12 times with its N and C termini facing the cytoplasm. 相似文献
53.
Our objective was: (1) to determine the appropriate dose of new ultrasmall superparamagnetic iron oxide particles for magnetic resonance angiography (MRA). This agent comprised of a single iron oxide crystal stabilized with a carbohydrate-polyethylene glycol coat (PEG-Ferron/NC 100150 injection); (2) to determine the proper flip angle for PEG-Ferron-enhanced 3 D time-of-flight (TOF) MRA sequence; and (3) to compare the enhancement of peripheral vessels following PEG-Ferron and GdDTPA-BMA. MRA parameters were: TR/TE = 50/2.1 ms, NEX = 1, FOV = 30 x 30 x 1.8 cm, and matrix = 256 x 128 x 64. In anesthetized beagle dogs (n = 10), the effects of PEG-Ferron and GdDTPA-BMA on regional signal were monitored for 45 min and compared. The lowest dose of PEG-Ferron (0.05 mmol/kg) produced the best enhancement of primary, secondary and tertiary vessels. The flip angle 60 degrees provided better enhancement than 20 degrees on contrast enhanced images. Unlike GdDTPA-BMA, PEG-Ferron allowed prolonged delineation (> 45 min) of the pelvis and lower extremities circulation. PEG-Ferron provided greater Contrast-to-noise ratio CNR (80.2 +/- 6.2, P < 0.05) than GdDTPA-BMA (63.5 +/- 2.5). It may be possible for blood pool contrast-enhanced 3 D TOF MRA to provide valuable information for visualization of vascular tree including guiding interventions. 相似文献
54.
EJ Lee YC Hung CH Chang MC Pai HH Chen 《Canadian Metallurgical Quarterly》1998,140(6):599-604; discussion 604-5
The purpose of this study was to evaluate pre- and post-shunting haemodynamic changes and their correlation with the clinical results in normal pressure hydrocephalus (NPH). Accordingly, eleven demented patients with clinical signs suggestive of NPH received examinations of cerebral blood flow velocity (BFV) and vasomotor reactivity (VMR) by transcranial Doppler sonography with carbogen testing before and after shunt treatment. Computerized tomography (CT), clinical assessment and neuropsychological grading were performed prior to and at 3 months following surgery. A control group consisting of 10 patients was included to establish baseline data. The pre-operative CBF studies in the anterior cerebral artery (ACA) and the middle cerebral artery (MCA) revealed the NPH patients did not have significant decreases of BFVs, but had significant decreases of carbogen VMR (P < 0.05). After shunting, there were no significant changes of the BFVs as compared with the pre-shunting data. The post-shunting VMR of the ACA was significantly higher than the pre-shunting one (p < 0.05), but there was no variation in that of the MCA. Both the values of post-shunting VMR in ACA and the post-shunting increase in VMR in MCA of the 7 shunt-responsive patients who improved mentally and in other symptoms were significantly higher than those of patients without improvement (p < 0.05). In addition, the five patients with gait improvement showed significantly higher values of post-shunting VMR of ACA and the post-shunting increase of VMR for both ACA and MCA when compared with those patients without gait improvement (p < 0.05, respectively). Our study supports the view that patients with NPH had various degrees of impaired VMR in both the ACA and the MCA, but showed insignificant reduction in BFVs, indicating a compensatory mechanism of CBF over time to accommodate the subnormal state of cerebral perfusion pressure. Shunt placement would improve the VMR in responsive patients. Postoperatively, an increase of VMR tends to accompany improvement of the functional state: that in the MCA alone is associated with symptomatic improvement in mental function and that increase in VMR in both the ACA and the MCA with improvement in gait, respectively. 相似文献
55.
56.
S Agarwal CS Chandra NP Piesco HH Langkamp L Bowen C Baran 《Canadian Metallurgical Quarterly》1998,66(3):932-937
Periodontal ligament (PDL) cells maintain the attachment of the tooth to alveolar bone. These cells reside at a site in which they are challenged frequently by bacterial products and proinflammatory cytokines, such as interleukin-1beta (IL-1beta), during infections. In our initial studies we observed that IL-1beta down-regulates the osteoblast-like characteristics of PDL cells in vitro. Therefore, we examined the functional significance of the loss of the PDL cell's osteoblast-like characteristics during inflammation. In this report we show that, during inflammation, IL-1beta can modulate the phenotypic characteristics of PDL cells to a more functionally significant lipopolysaccharide (LPS)-responsive phenotype. In a healthy periodontium PDL cells exhibit an osteoblast-like phenotype and are unresponsive to gram-negative bacterial LPS. Treatment of PDL cells with IL-1beta inhibits the expression of their osteoblast-like characteristics, as assessed by the failure to express transforming growth factor beta1 (TGF-beta1) and proteins associated with mineralization, such as alkaline phosphatase and osteocalcin. As a consequence of this IL-1beta-induced phenotypic change, PDL cells become responsive to LPS and synthesize proinflammatory cytokines. The IL-1beta-induced phenotypic changes in PDL cells were transient, as removal of IL-1beta from PDL cell cultures resulted in reacquisition of their osteoblast-like characteristics and lack of LPS responsiveness. The IL-1beta-induced phenotypic changes occurred at concentrations that are frequently observed in tissue exudates during periodontal inflammation (0.05 to 5 ng/ml). The results suggest that, during inflammation in vivo, IL-1beta may modulate PDL cell functions, allowing PDL cells to participate directly in the disease process by assuming LPS responsiveness at the expense of their normal structural properties and functions. 相似文献
57.
C Vetriani DL Maeder N Tolliday KS Yip TJ Stillman KL Britton DW Rice HH Klump FT Robb 《Canadian Metallurgical Quarterly》1998,95(21):12300-12305
The discovery of hyperthermophilic microorganisms and the analysis of hyperthermostable enzymes has established the fact that multisubunit enzymes can survive for prolonged periods at temperatures above 100 degreesC. We have carried out homology-based modeling and direct structure comparison on the hexameric glutamate dehydrogenases from the hyperthermophiles Pyrococcus furiosus and Thermococcus litoralis whose optimal growth temperatures are 100 degreesC and 88 degreesC, respectively, to determine key stabilizing features. These enzymes, which are 87% homologous, differ 16-fold in thermal stability at 104 degreesC. We observed that an intersubunit ion-pair network was substantially reduced in the less stable enzyme from T. litoralis, and two residues were then altered to restore these interactions. The single mutations both had adverse effects on the thermostability of the protein. However, with both mutations in place, we observed a fourfold improvement of stability at 104 degreesC over the wild-type enzyme. The catalytic properties of the enzymes were unaffected by the mutations. These results suggest that extensive ion-pair networks may provide a general strategy for manipulating enzyme thermostability of multisubunit enzymes. However, this study emphasizes the importance of the exact local environment of a residue in determining its effects on stability. 相似文献
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Fluctuations in rates of gene expression can produce highly erratic time patterns of protein production in individual cells and wide diversity in instantaneous protein concentrations across cell populations. When two independently produced regulatory proteins acting at low cellular concentrations competitively control a switch point in a pathway, stochastic variations in their concentrations can produce probabilistic pathway selection, so that an initially homogeneous cell population partitions into distinct phenotypic subpopulations. Many pathogenic organisms, for example, use this mechanism to randomly switch surface features to evade host responses. This coupling between molecular-level fluctuations and macroscopic phenotype selection is analyzed using the phage lambda lysis-lysogeny decision circuit as a model system. The fraction of infected cells selecting the lysogenic pathway at different phage:cell ratios, predicted using a molecular-level stochastic kinetic model of the genetic regulatory circuit, is consistent with experimental observations. The kinetic model of the decision circuit uses the stochastic formulation of chemical kinetics, stochastic mechanisms of gene expression, and a statistical-thermodynamic model of promoter regulation. Conventional deterministic kinetics cannot be used to predict statistics of regulatory systems that produce probabilistic outcomes. Rather, a stochastic kinetic analysis must be used to predict statistics of regulatory outcomes for such stochastically regulated systems. 相似文献