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71.
72.
Nitriles pose an interesting problem to the explanatory powers of organic chemistry because, despite the favorable overall thermodynamics of hydrolysis to the corresponding amides, the reactions are inherently slow. The rate determining step is hydration of the nitrile to give the imidic acid, which quickly tautomerizes to the amide. In terms of Marcus Theory, the intrinsic barriers for acid and base‐catalyzed hydration are higher for nitriles than for amides, which are themselves slow reactions. It is remarkable that hydration of a nitrile, for which the free energy change is close to zero except for polyhaloacetonitriles, is much slower than hydration of an amide, which is energetically unfavorable. This can be explained by No Barrier Theory in terms of the high energetic cost of the geometrical distortions in “one thing at a time” corner species. There are no experimental equilibrium constants for this initial hydration step, so we have determined them computationally. The free energy change for the initial hydration is small; it is the fast and energetically downhill second step, tautomerization to the amide, which makes the overall hydrolysis of nitrile to amide thermodynamically favorable. Very few of the pKa values needed in the acid and base‐catalyzed mechanisms are known, so we used linear free energy relations and treat the parent pKa values as adjustable parameters. This procedure leads to pKa values in accord with expectation based on such data as are available and permits calculation of rate constants in satisfactory agreement with experiment. Copyright © 2013 John Wiley & Sons, Ltd. 相似文献
73.
Elemental surface enrichment is important for functionalities of flame-synthesized particle materials, but its mechanism is poorly understood. In this paper, a mechanism associated with particle melting-solidification is proposed based on an experimental study. Y2O3:Eu particles were generated by flame assisted spray pyrolysis (FASP), using H2/O2 flames or H2/air flames. The particles were analyzed by transmission electron microscopy (TEM), X-ray diffraction (XRD), inductively coupled plasma mass spectrometry (ICP-MS), and X-ray photoelectron spectroscopy (XPS). H2/O2 flames resulted in particles with Eu surface enrichment, i.e. the surface Eu concentration was several times higher than the overall Eu concentration; there was no elemental surface enrichment in particles from H2/air flames. The Eu surface enrichment in H2/O2 flames was attributed to elemental partitioning during solidification of molten Y2O3:Eu particles; in H2/air flames the particles did not melt and hence there was no elemental surface enrichment. The findings of this study suggest that elemental surface enrichment may be a common phenomenon for binary metal oxide particles that experience melting-solidification. Such particles should be examined for elemental surface enrichment, both for understanding their functionality and for their potential biological effects. 相似文献
74.
Haidong Xu Chungyoul Choe Seung-Hun Shin Sung-Won Park Ho-Shik Kim Seung-Hyun Jung Seon-Hee Yim Tae-Min Kim Yeun-Jun Chung 《Experimental & molecular medicine》2014,46(5):e97
Although it has been suggested that kinesin family member 14 (KIF14) has oncogenic potential in various cancers, including hepatocellular carcinoma (HCC), the molecular mechanism of this potential remains unknown. We aimed to elucidate the role of KIF14 in hepatocarcinogenesis by knocking down KIF14 in HCC cells that overexpressed KIF14. After KIF14 knockdown, changes in tumor cell growth, cell cycle and cytokinesis were examined. We also examined cell cycle regulatory molecules and upstream Skp1/Cul1/F-box (SCF) complex molecules. Knockdown of KIF14 resulted in suppression of cell proliferation and failure of cytokinesis, whereas KIF14 overexpression increased cell proliferation. In KIF14-silenced cells, the levels of cyclins E1, D1 and B1 were profoundly decreased compared with control cells. Of the cyclin-dependent kinase inhibitors, the p27Kip1 protein level specifically increased after KIF14 knockdown. The increase in p27Kip1 was not due to elevation of its mRNA level, but was due to inhibition of the proteasome-dependent degradation pathway. To explore the pathway upstream of this event, we measured the levels of SCF complex molecules, including Skp1, Skp2, Cul1, Roc1 and Cks1. The levels of Skp2 and its cofactor Cks1 decreased in the KIF14 knockdown cells where p27Kip1 accumulated. Overexpression of Skp2 in the KIF14 knockdown cells attenuated the failure of cytokinesis. On the basis of these results, we postulate that KIF14 knockdown downregulates the expression of Skp2 and Cks1, which target p27Kip1 for degradation by the 26S proteasome, leading to accumulation of p27Kip1. The downregulation of Skp2 and Cks1 also resulted in cytokinesis failure, which may inhibit tumor growth. To the best of our knowledge, this is the first report that has identified the molecular target and oncogenic effect of KIF14 in HCC. 相似文献
75.
Synthesis,Characterization, Self‐Assembly,Gelation, Morphology and Computational Studies of Alkynylgold(III) Complexes of 2,6‐Bis(benzimidazol‐2′‐yl)pyridine Derivatives 下载免费PDF全文
King‐Chin Yim Elizabeth Suk‐Hang Lam Dr. Keith Man‐Chung Wong Dr. Vonika Ka‐Man Au Dr. Chi‐Chiu Ko Dr. Wai Han Lam Prof. Dr. Vivian Wing‐Wah Yam 《Chemistry (Weinheim an der Bergstrasse, Germany)》2014,20(32):9930-9939
A novel class of alkynylgold(III) complexes of the dianionic ligands derived from 2,6‐bis(benzimidazol‐2′‐yl)pyridine (H2bzimpy) derivatives has been synthesized and characterized. The structure of one of the complexes has also been determined by X‐ray crystallography. Electronic absorption studies showed low‐energy absorption bands at 378–466 nm, which are tentatively assigned as metal‐perturbed π–π* intraligand transitions of the bzimpy2? ligands. A computational study has been performed to provide further insights into the nature of the electronic transitions for this class of complexes. One of the complexes has been found to show gelation properties, driven by π–π and hydrophobic–hydrophobic interactions. This complex exhibited concentration‐ and temperature‐dependent 1H NMR spectra. The morphology of the gel has been characterized by transmission electron microscopy (TEM) and scanning electron microscopy (SEM). 相似文献
76.
H. Bhang S. Ajimura K. Aoki A. Banu T. Fukuda O. Hashimoto J. I. Hwang S. Kameoka B. H. Kang E. H. Kim J. H. Kim M. Kim T. Maruta Y. Miura Y. Miyake T. Nagae M. Nakamura S. N. Nakamura H. Noumi S. Okada Y. Okayasu H. Outa H. Park P. K. Saha Y. Sato M. Sekimoto S. Shin T. Takahashi H. Tamura K. Tanida A. Toyota K. Tsukada T. Watanabe H. J. Yim 《The European Physical Journal A - Hadrons and Nuclei》2007,33(3):259-263
Recent exclusive coincidence measurements of non-mesonic weak decays (NMWD) reported for the ratio of the partial decay width
of neutron-induced-to-proton-induced NMWD, Γn/Γp , values of 0.45±0.11±0.03 and 0.51±0.13±0.04 for 5lam and 12
Λ
C , respectively. These observations agree well with the improved theoretical Γn/Γp ratios which are in the range of 0.3-0.7. It appears that the long-standing discrepancy between the experimental and theoretical
values of Γn/Γp has finally been solved. However, when compared to the results of intra-nuclear cascade (INC) calculations, the observed
numbers of both single nucleons and coincident nucleon pairs are strongly quenched. The quenching of the proton yield observed
previously has been interpreted as an increase of the Γn/Γp ratio. On the other hand, significant contributions from the two-nucleon-induced three-body process ΛNN → nNN are predicted. Indeed, the angular correlation of the emitted nucleon pairs in the NMWD of 12
Λ
C showed not only decay events in back-to-back kinematics, but also events with non-back-to-back kinematics. In this paper
we show that the difficulties to extract the correct Γn/Γp ratio from the proton spectra is related to the three-body weak-interaction process which strongly quenches the nucleon yields. 相似文献
77.
H. S. Jeon P. S. Dixit H. Yim M. S. Kent K. Shin S. Satija 《Journal of Polymer Science.Polymer Physics》2002,40(23):2653-2660
The interfacial structure and properties of immiscible deuterated polystyrene (dPS)/epoxy bilayer films were investigated with neutron reflectivity as functions of the composition of the epoxy layer, the thickness of the dPS layer, and the annealing time. We have found that the interfacial width and its growth rate depend strongly on the compositions of the epoxy layer but only weakly on the thickness of the dPS layer. The effect of the resin/crosslinker composition on the interfacial width and its growth rate is likely due to the different near‐surface structures that result for different epoxy stoichiometries. For an ultra‐thin dPS film (thickness = 2Rg), the data suggest a slight suppression of the growth of the interfacial width that could be due to confinement effects for the long‐chain molecules such as have been previously reported for a thickness of less than approximately 4Rg, where Rg is the radius of gyration of polymer molecules. © 2002 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 40: 2653–2660, 2002 相似文献
78.
Heung‐Seop Yim Mi‐Ra Kim Gi‐Hyeon Sung Hyun‐A Chung Jin‐Kook Lee Sang‐Gyeong Lee Yong‐Jin Yoon 《Journal of heterocyclic chemistry》2008,45(1):215-220
trans‐2‐(2‐Aryl‐ or heteroarylvinyl)‐4,5‐dichloropyridazin‐3(2H)‐ones 3 were synthesized from 4,5‐dichloropyridazin‐3(2H)‐one via 2 step. The photochemical behavior of 3 in THF, methylene chloride, acetonitrile and methanol is dependent on the kind of aryl or heterocyclic ring and the solvent polarity 相似文献
79.
Dejun?Guo Joseph?R.?Bourne Hesheng?Wang Woosoon?Yim Kam?K.?LeangEmail author 《Journal of Nonlinear Science》2017,27(4):1235-1256
This paper presents the design and implementation of an adaptive-repetitive visual-servo control system for a moving high-flying vehicle (HFV) with an uncalibrated camera to monitor, track, and precisely control the movements of a low-flying vehicle (LFV) or mobile ground robot. Applications of this control strategy include the use of high-flying unmanned aerial vehicles (UAVs) with computer vision for monitoring, controlling, and coordinating the movements of lower altitude agents in areas, for example, where GPS signals may be unreliable or nonexistent. When deployed, a remote operator of the HFV defines the desired trajectory for the LFV in the HFV’s camera frame. Due to the circular motion of the HFV, the resulting motion trajectory of the LFV in the image frame can be periodic in time, thus an adaptive-repetitive control system is exploited for regulation and/or trajectory tracking. The adaptive control law is able to handle uncertainties in the camera’s intrinsic and extrinsic parameters. The design and stability analysis of the closed-loop control system is presented, where Lyapunov stability is shown. Simulation and experimental results are presented to demonstrate the effectiveness of the method for controlling the movement of a low-flying quadcopter, demonstrating the capabilities of the visual-servo control system for localization (i.e.,, motion capturing) and trajectory tracking control. In fact, results show that the LFV can be commanded to hover in place as well as track a user-defined flower-shaped closed trajectory, while the HFV and camera system circulates above with constant angular velocity. On average, the proposed adaptive-repetitive visual-servo control system reduces the average RMS tracking error by over 77% in the image plane and over 71% in the world frame compared to using just the adaptive visual-servo control law. 相似文献
80.
Martina Tholen Joshua J. Yim Katarzyna Groborz Euna Yoo Brock A. Martin Nynke S. van den Berg Marcin Drag Matthew Bogyo 《Angewandte Chemie (Weinheim an der Bergstrasse, Germany)》2020,132(43):19305-19314
Fluorescently quenched probes that are specifically activated in the cancer microenvironment have great potential application for diagnosis, early detection, and surgical guidance. These probes are often designed to target specific enzymes associated with diseases by direct optimization using single purified enzymes. However, this can result in painstaking chemistry efforts to produce a probe with suboptimal performance when applied in vivo. We describe here an alternate, unbiased activity-profiling approach in which whole tissue extracts are used to directly identify optimal peptide sequences for probe design. Screening of tumor extracts with a hybrid combinatorial substrate library (HyCoSuL) identified a combination of natural and non-natural amino-acid residues that was used to generate highly efficient tumor-specific probes. This new strategy simplifies and enhances the process of probe optimization without any a priori knowledge of enzyme targets and has the potential to be applied to diverse disease states using clinical or animal-model tissue samples. 相似文献