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排序方式: 共有121条查询结果,搜索用时 234 毫秒
1.
Recent X-ray diffraction studies on α-quartz (SiO2) by Kingmaet al [1], have shown the occurrence of a reversible, crystalline-to-crystalline, phase transition just prior to amorphization
at ≈ 21 GPa. This precursor transition has also been confirmed by our recent molecular dynamics simulation study [2]. In order
to investigate the possibility of a similar behaviour in other isostructural compounds, which also undergo pressure induced
amorphization, α-GeO2 and α-AlPO4 (berlinite form) were studied using energy dispersive X-ray diffraction. In either of these materials, no such phase transition
is detected prior to amorphization. The onset of amorphization and its reversal is found to be time dependent in GeO2. 相似文献
2.
Abstract —Thymine starvation prior to 254 nm ultraviolet light (UV) exposures has been found to decrease the level of maximum photoreactivation in Escherichia coli B s-1 . The dark equilibrium level of photoreactivating enzyme-substrate complexes was determined from the levels of photoreactivation obtained with exposures to single flashes of high-intensity light. The kinetics indicate that photoreactivating enzyme concentration does not decrease as a result of thymine starvation. The UV sensitivities of normal and thymine-starved cells are found to be the same. Photoreactivation by sequential flashes shows a lesser number of total photorepairable lesions in starved cells. It is concluded that thymine starvation renders a portion of the dimers inaccessible to the photoreactivating enzyme, thus lowering the level of maximum photoreactivation. 相似文献
3.
Disc type ceramic aluminium oxide membrane has been prepared by tape casting technique. Thickness of this single layer membrane is in the range 200–300 μm. Porosity and pore size distribution have been determined by mercury porosimetry. Polymeric binder content of the green tape and the firing temperature are found to have strong influence on the average pore size, pore size distribution and overall porosity. Higher binder content promotes agglomeration of the ceramic particles, which on firing leads to wider pore size distribution and formation of closed pores. Pore coarsening is observed with increasing firing temperature. 相似文献
4.
Durgesh B. H. Alkheraif A. A. Altwijry M. K. Asiry M. A. AlShahrani I. Varrela J. Vallittu P. K. 《Mechanics of Composite Materials》2019,55(2):275-284
Mechanics of Composite Materials - Creeping of a resin-composite-based interface between the enamel and an orthodontic bracket was investigated using two testing setups — tooth-bracket and... 相似文献
5.
Brian E. Young Nandini Kundu Prof. Jonathan T. Sczepanski 《Chemistry (Weinheim an der Bergstrasse, Germany)》2019,25(34):7981-7990
As chiral molecules, naturally occurring d -oligonucleotides have enantiomers, l -DNA and l -RNA, which are comprised of l -(deoxy)ribose sugars. These mirror-image oligonucleotides have the same physical and chemical properties as that of their native d -counterparts, yet are highly orthogonal to the stereospecific environment of biology. Consequently, l -oligonucleotides are resistant to nuclease degradation and many of the off-target interactions that plague traditional d -oligonucleotide-based technologies; thus making them ideal for biomedical applications. Despite a flurry of interest during the early 1990s, the inability of d - and l -oligonucleotides to form contiguous Watson–Crick base pairs with each other has ultimately led to the perception that l -oligonucleotides have only limited utility. Recently, however, scientists have begun to uncover novel strategies to harness the bio-orthogonality of l -oligonucleotides, while overcoming (and even exploiting) their inability to Watson–Crick base pair with the natural polymer. Herein, a brief history of l -oligonucleotide research is presented and emerging l -oligonucleotide-based technologies, as well as their applications in research and therapy, are presented. 相似文献
6.
Durgesh V. Wagle Steven P. Kelley Gary A. Baker Kanishka Sikligar Jerry L. Atwood 《Angewandte Chemie (International ed. in English)》2020,59(21):8062-8065
A hexameric metal–organic nanocapsule is assembled from pyrogallol[4]arene units, which are stitched together with indium ions. This indium‐seamed capsule is the first instance of a M24L6 type hexameric coordination cage held together exclusively by trivalent metal ions. Explicitly, unlike previously reported pyrogallol[4]arene‐based metal‐seamed capsules, the current In3+ seamed capsule is entirely supported by O→In coordinate bonds. This work demonstrates the important proof of concept of the ability of pyrogallol[4]arene to react with metals in higher oxidation states to assemble into atomically‐precise hexameric coordination cages. As such, these results open up exciting avenues toward the assembly of previously unanticipated metal–organic capsules, for example offering inspiration for tackling metals exhibiting high valence states such as in the lanthanide and actinide series. 相似文献
7.
The reactions of dimeric complex [Rh(CO)2Cl]2 with hemilabile ether‐phosphine ligands Ph2P(CH2) nOR [n = 1, R = CH3 (a); n = 2, R = C2H5 (b)] yield cis‐[Rh(CO)2Cl(P ~ O)] (1) [P ~ O = η 1‐(P) coordinated]. Halide abstraction reactions of 1 with AgClO4 produce cis‐[Rh(CO)2(P ∩ O)]ClO4 (2) [P ∩ O = η 2‐(P,O)chelated]. Oxidative addition reactions of 1 with CH3I and I2 give rhodium(III) complexes [Rh(CO)(COCH3)ClI(P ∩ O)] (3) and [Rh(CO)ClI2(P ∩ O)] (4) respectively. The complexes have been characterized by elemental analyses, IR, 1H, 13C and 31P NMR spectroscopy. The catalytic activity of 1 for carbonylation of methanol is higher than that of the well‐known [Rh(CO)2I2]? species. Copyright © 2002 John Wiley & Sons, Ltd. 相似文献
8.
The present work involves the adoption of an integrated approach for the purification of lactoperoxidase from milk whey by
coupling aqueous two-phase extraction (ATPE) with ultrasound-assisted ultrafiltration. The effect of system parameters of
ATPE such as type of phase system, polyethylene glycol (PEG) molecular mass, system pH, tie line length and phase volume ratio
was evaluated so as to obtain differential partitioning of contaminant proteins and lactoperoxidase in top and bottom phases,
respectively. PEG 6000-potassium phosphate system was found to be suitable for the maximum activity recovery of lactoperoxidase
150.70% leading to 2.31-fold purity. Further, concentration and purification of enzyme was attempted using ultrafiltration.
The activity recovery and purification factor achieved after ultrafiltration were 149.85% and 3.53-fold, respectively. To
optimise productivity and cost-effectiveness of integrated process, influence of ultrasound for the enhancement of permeate
flux during ultrafiltration was also investigated. Intermittent use of ultrasound along with stirring (2 min acoustic and
2 min stirring) resulted in increased permeate flux from 0.94 to 2.18 l/m2 h in comparison to the ultrafiltration without ultrasound. The use of ultrasound during ultrafiltration resulted in increase
in flux, but there was no significant change in activity recovery and purification factor. The integrated approach involving
ATPE and ultrafiltration may prove to be a feasible method for the downstream processing of lactoperoxidase from milk whey. 相似文献
9.
When stimulated Raman pumping (SRP) is applied to a stream of isolated molecules, such as found in a supersonic molecular beam expansion, we show that SRP can neither saturate nor power broaden a molecular transition connecting two metastable levels that is resonant with the energy difference between the pump and Stokes laser pulses. Using the optical Bloch-Feynman equations, we discuss the pumping of the hydrogen molecule from H(2) (v = 0, J = 0, M = 0) to H(2) (v = 1, J = 2, M = 0) as an illustration of how coherent population return severely reduces the SRP pumping efficiency unless the pump and Stokes laser pulses are applied with an appropriate relative delay and ratio of intensities. 相似文献
10.