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排序方式: 共有882条查询结果,搜索用时 15 毫秒
851.
852.
Dr. Katia Nchimi‐Nono Parastoo Dalvand Dr. Kuldeep Wadhwa Selbi Nuryyeva Shaikha Alneyadi Dr. Thirumurugan Prakasam Dr. Albert C. Fahrenbach Prof. John‐Carl Olsen Dr. Zouhair Asfari Carlos Platas‐Iglesias Dr. Mourad Elhabiri Prof. Ali Trabolsi 《Chemistry (Weinheim an der Bergstrasse, Germany)》2014,20(24):7334-7344
Suppression of the dimerization of the viologen radical cation by cucurbit[7]uril ( CB7 ) in water is a well‐known phenomenon. Herein, two counter‐examples are presented. Two viologen‐containing thread molecules were designed, synthesized, and thoroughly characterized by 1H DOSY NMR spectrometry, UV/Vis absorption spectrophotometry, square‐wave voltammetry, and chronocoulometry: BV 4+, which contains two viologen subunits, and HV 12+, which contains six. In both threads, the viologen subunits are covalently bonded to a hexavalent phosphazene core. The corresponding [3]‐ and [7]pseudorotaxanes that form on complexation with CB7 , that is, BV 4+?( CB 7)2 and HV 12+?( CB 7)6, were also analyzed. The properties of two monomeric control threads, namely, methyl viologen ( MV 2+) and benzyl methyl viologen ( BMV 2+), as well as their [2]pseudorotaxane complexes with CB7 ( MV 2+? CB7 and BMV 2+? CB7 ) were also investigated. As expected, the control pseudorotaxanes remained intact after one‐electron reduction of their viologen‐recognition stations. In contrast, analogous reduction of BV 4+?( CB 7)2 and HV 12+?( CB 7)6 led to host–guest decomplexation and release of the free threads BV 2( . +) and HV 6( . +), respectively. 1H DOSY NMR spectrometric and chronocoulometric measurements showed that BV 2( . +) and HV 6( . +) have larger diffusion coefficients than the corresponding [3]‐ and [7]pseudorotaxanes, and UV/Vis absorption studies provided evidence for intramolecular radical‐cation dimerization. These results demonstrate that radical‐cation dimerization, a relatively weak interaction, can be used as a driving force in novel molecular switches. 相似文献
853.
Development of Red-Shifted and Fluorogenic Nucleoside and Oligonucleotide Diarylethene Photoswitches
Theresa Kolmar Antonia Becker Ronja A. Pfretzschner Alina Lelke Prof. Dr. Andres Jäschke 《Chemistry (Weinheim an der Bergstrasse, Germany)》2021,27(69):17386-17394
The reversible modulation of fluorescence signals by light is of high interest for applications in super-resolution microscopy, especially on the DNA level. In this article we describe the systematic variation of the core structure in nucleoside-based diarylethenes (DAEs), in order to generate intrinsically fluorescent photochromes. The introduction of aromatic bridging units resulted in a bathochromic shift of the visible absorption maximum of the closed-ring form, but caused reduced thermal stability and switching efficiency. The replacement of the thiophene aryl unit by thiazol improved the thermal stability, whereas the introduction of a benzothiophene unit led to inherent and modulatable turn-off fluorescence. This feature was further optimized by introducing a fluorescent indole nucleobase into the DAE core, resulting in an effective photoswitch with a fluorescence quantum yield of 0.0166 and a fluorescence turn-off factor of 3.2. The site-specific incorporation into an oligonucleotide resulted in fluorescence-switchable DNA with high cyclization quantum yields and switching efficiency, which may facilitate future applications. 相似文献
854.
M. E. Lin A. Ramachandra R. P. Andres R. Reifenberger 《Zeitschrift für Physik D Atoms, Molecules and Clusters》1993,26(1):59-63
The quantized energy levels of electrons in supported nanometer-size Au clusters have been resolved at room temperature using field emission techniques. By studying the time dependence of the electron emission current from an individual supported cluster, information about the structural stability of the cluster can also be obtained. Studies show abrupt jumps between different emission rates that are revisited as time progresses. This phenomenon can be attributed to a rearrangement of the cluster structure and/or orientation on the substrate and provides new evidence of multiple ‘isomeric’ structures for small clusters of metallic atoms. 相似文献
855.
De Novo 3D Structure Determination from Sub‐milligram Protein Samples by Solid‐State 100 kHz MAS NMR Spectroscopy
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Dr. Vipin Agarwal Susanne Penzel Kathrin Szekely Riccardo Cadalbert Emilie Testori Andres Oss Jaan Past Prof. Ago Samoson Prof. Matthias Ernst Dr. Anja Böckmann Prof. Beat H. Meier 《Angewandte Chemie (International ed. in English)》2014,53(45):12253-12256
Solid‐state NMR spectroscopy is an emerging tool for structural studies of crystalline, membrane‐associated, sedimented, and fibrillar proteins. A major limitation for many studies is still the large amount of sample needed for the experiments, typically several isotopically labeled samples of 10–20 mg each. Here we show that a new NMR probe, pushing magic‐angle sample rotation to frequencies around 100 kHz, makes it possible to narrow the proton resonance lines sufficiently to provide the necessary sensitivity and spectral resolution for efficient and sensitive proton detection. Using restraints from such spectra, a well‐defined de novo structure of the model protein ubiquitin was obtained from two samples of roughly 500 μg protein each. This proof of principle opens new avenues for structural studies of proteins available in microgram, or tens of nanomoles, quantities that are, for example, typically achieved for eukaryotic membrane proteins by in‐cell or cell‐free expression. 相似文献
856.
Closed-loop adaptive optics in the human eye 总被引:3,自引:0,他引:3
We have developed a prototype apparatus for real-time closed-loop measurement and correction of aberrations in the human eye. The apparatus uses infrared light to measure the wave-front aberration at 25 Hz with a Hartmann-Shack sensor. Defocus is removed by a motorized optometer, and higher-order aberrations are corrected by a membrane deformable mirror. The device was first tested with an artificial eye. Correction of static aberrations takes approximately five iterations, making the system capable of following aberration changes at 5 Hz. This capability allows one to track most of the aberration dynamics in the eye. Results in living eyes showed effective closed-loop correction of aberrations, with a residual uncorrected wave front of 0.1microm for a 4.3-mm pupil diameter. Retinal images of a point source in different subjects with and without adaptive correction of aberrations were estimated in real time. The results demonstrate real-time closed-loop correction of aberration in the living eye. An application of this device is as electro-optic "spectacles" to improve vision. 相似文献
857.
Becker-Baldus J Kemp TF Past J Reinhold A Samoson A Brown SP 《Physical chemistry chemical physics : PCCP》2011,13(10):4514-4518
A new spinning-angle-encoding spin-echo solid-state NMR approach is used to accurately determine the dipolar coupling corresponding to a C-C distance over 4 ? in a fully labelled dipeptide. The dipolar coupling dependent spin-echo modulation was recorded off magic angle, switching back to the magic angle for the acquisition of the free-induction decay, so as to obtain optimum sensitivity. The retention of both ideal resolution and long-range distance sensitivity was achieved by redesigning a 600 MHz HX MAS NMR probe to provide fast angle switching during the NMR experiment: for 1.8 mm rotors, angle changes of up to ~5° in ~10 ms were achieved at 12 kHz MAS. A new experimental design that combines a reference and a dipolar-modulated experiment and a master-curve approach to data interpretation is presented. 相似文献
858.
Rafa Porba Andres de los Santos Pereira Robert Pola Siyu Jiang Ognen Pop‐Georgievski Zdeka Sedlkov Holger Schnherr 《Macromolecular bioscience》2020,20(4)
To tailor cell–surface interactions, precise and controlled attachment of cell‐adhesive motifs is required, while any background non‐specific cell and protein adhesion has to be blocked effectively. Herein, a versatile and highly reproducible antifouling surface modification based on “clickable” groups and hierarchically structured diblock copolymer brushes for the controlled attachment of cells is reported. The polymer brush architecture combines an antifouling bottom block of poly(2‐hydroxyethyl methacrylate) poly(HEMA) and an ultrathin azide‐bearing top block, which can participate in well‐established “click” reactions including the highly selective copper‐catalyzed alkyne‐azide cycloaddition (CuAAC) reaction under mild conditions. This straightforward approach allows the rapid conjugation of a cell‐adhesive, alkyne‐bearing cyclic RGD peptide motif, enabling subsequent specific attachment of NIH 3T3 fibroblasts, their extensive proliferation and confluent cell sheet formation after 48 h of incubation. The generally applicable strategy presented in this report can be employed for surface functionalization with diverse alkyne‐bearing biological moieties via CuAAC or copper‐free alkyne‐azide cycloaddition protocols, making it a versatile functionalization approach and a promising tool for tissue engineering, biomaterial implant design, and other applications that require surfaces supporting highly specific cell attachment. 相似文献
859.
860.
C. F. Moukarzel S. Gonçalves J. R. Iglesias M. Rodríguez-Achach R. Huerta-Quintanilla 《The European physical journal. Special topics》2007,143(1):75-79
Random Asset Exchange (RAE) models, despite a number of simplifying
assumptions, serve the purpose of establishing direct relationships between
microscopic exchange mechanisms and observed economical data. In this work
a conservative multiplicative RAE model is discussed in which, at each
timestep, two agents “bet” for a fraction f of the poorest agent's
wealth. When the poorest agent wins the bet with probability p, we show
that, in a well defined region of the (p,f) phase space, there is
wealth condensation. This means that all wealth ends up owned by only
one agent, in the long run. We derive the condensation conditions
analytically by two different procedures, and find results in accordance
with previous numerical estimates. In the non-condensed phase, the
equilibrium wealth distribution is a power law for small wealths. The
associated exponent is derived analytically and it is found that it tends to
-1 on the condensation interface. I turns out that wealth condensation
happens also for values of p much larger than 0.5, that is under
microscopic exchange rules that, apparently, favor the poor. We argue that
the observed “rich get richer” effect is enhanced by the
multiplicative character of the dynamics. 相似文献