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1.
A series of stable Cr(V) model complexes that mimic the binding of Cr(V) to peptide backbones at the C-terminus of proteins have been prepared for N,N-dimethylurea derivatives of the tripeptides Aib3-DMF, AibLAlaAib-DMF, and AibDAlaAib-DMF (Aib = 2-amino-2-methylpropanoic acid, DMF = N,N-dimethylformamide). The Cr(ll) precursor complexes were synthesized by the initial deprotonation of the amide and acid groups of the peptide ligands in DMF with potassium tert-butoxide in the presence of CrCl2. The Cr(II) intermediates thus formed were then immediately oxidized to Cr(V) using tert-butyl hydroperoxide. Spectroscopic and mass-spectrometric analyses of the Cr(V) complexes showed that a new metal-directed organic transformation of the ligand had occurred. This involved a DMF solvent molecule becoming covalently bound to the amine group of the peptide ligand, yielding a urea group, and a third coordinated deprotonated urea nitrogen donor. A metal-directed oxidative coupling has been proposed as a possible mechanism for the organic transformation. The Cr(V/IV) reduction potential was determined for the three Cr(V) complexes using cyclic voltammetry, and in all cases it was quasi-reversible. These are the first isolated and fully characterized Cr(V) complexes with non-sulfur-containing peptide ligands. 相似文献
2.
M. S. Bryant J. O. Lay M. P. Chiarelli 《Journal of the American Society for Mass Spectrometry》1992,3(4):360-371
An analytical strategy using fast atom bombardment (FAB) ionization and tandem mass spectrometry has been developed to determine the molecular weight and major fragment ions, and to provide limited structural characterization of low picomole levels of carcinogen-nucleoside adducts. This strategy consists of three main components: (1) the sensitivity for analysis by FAB combined with mass spectrometry is increased via chemical derivatization; (2) the nucleoside adducts are selectively detected by using constant neutral loss scans; and (3) structurally characteristic fragments are obtained by using daughter ion scans. Trimethylsilyl derivatized arylamine-nucleoside adducts have been detected at levels as low as a few picomoles by using this approach. After experimental determination of the mass of the BH 2 + fragment ion, daughter ion spectra have been used to probe the structure specificity associated with collision-activated decomposition of this fragment. With model C-8 substituted arylamine adducts [N-(deoxyguanosin-8-yl)-4-aminobiphenyl, N-(deoxyadenosin--yl)-4-aminobiphenyl, and N-(deoxyguanosin-8-yl)-2-aminofluorene], nucleoside-specific and carcinogen-specific fragmentation have been observed in daughter ion spectra. 相似文献
3.
4.
Lay Gaik Teoh 《Journal of Sol-Gel Science and Technology》2012,62(1):47-51
Mesoporous CeO2 and yttrium doped CeO2 (YDC) were prepared by a sol–gel process and characterized by a variety of techniques. XRD patterns showed that the undoped
and doped samples had a cubic fluorite structure. The grain size decreased from 24.8 to 6.1 nm at 500 °C for pure CeO2 and YDC, respectively. N2 adsorption–desorption isotherms showed that the samples possessed typical mesopore characteristics. The BET specific surface
area of the samples increased from 23.04 to 151.49 m2/g for 300 °C calcination after mesoporous CeO2 was doped with Y. It is found that the introduction of Y can inhibit the grain growth, and the presence of the pores also
can be related to this obstacle to grain growth. These results are of great significance for the control of porous microstructure,
crystallinity, and applications for the development of nanostructured mesoporous materials. 相似文献
5.
Physics and chemistry of silicene nano-ribbons 总被引:1,自引:0,他引:1
G. Le Lay B. Aufray H. Oughaddou J.-P. Biberian M.E. Dávila A. Kara 《Applied Surface Science》2009,256(2):524-529
We review our recent discovery of silicene in the form of silicon nano-ribbons epitaxially grown on silver (1 1 0) or (1 0 0) surfaces, which paves the way for the growth of graphene-like sheets. We further draw some perspectives for this unique novel material upon mild hydrogenation. 相似文献
6.
Barbara La Ferla Luigi Lay Laura Poletti Giovanni Russo Luigi Panza 《Journal of carbohydrate chemistry》2013,32(3):331-343
ABSTRACT Three glycosyllactosides, contained in the neutral fraction of human milk oligosaccharides, were synthesised in a simple and straightforward manner through a sequence based on a chemo-enzymatic approach. Lipase catalysed regioselective 6'-O-acylation of benzyl β-lactoside, followed by the introduction of an isopropylidene group and acetylation afforded, depending on the reaction conditions, compounds 4a and 4b, which allow selective access to positions 3, 3' and 6'. Glycosylation with proper donors gave trisaccharides 6, 9 and 12. 相似文献
7.
We present a low-temperature scanning tunneling microscopy study of the alpha-Sn/Si(111) surface that demonstrates the fluctuating behavior of the Sn adatoms. The dynamical fluctuation model, successfully applied in describing the alpha-Sn/Ge(111) surface, is proposed for the related alpha-Sn/Si(111) surface too, although with a much lower transition temperature. In addition, a new phenomenon appears responsible for the unexpected evidence that the average oscillation frequency remains constant at temperatures lower than 15 K, in contradiction to the Arrhenius law. We explain this phenomenon as quantum tunneling of Sn adatoms. 相似文献
8.
Liu Chongyang Gong Zhaohua Teo Kok Lay Wang Song 《Journal of Optimization Theory and Applications》2022,193(1-3):856-876
Journal of Optimization Theory and Applications - This paper considers an optimal control problem governed by nonlinear fractional-order systems with multiple time-varying delays and subject to... 相似文献
9.
Qinqin Chai Ryan Loxton Kok Lay Teo Chunhua Yang 《Nonlinear Analysis: Real World Applications》2013,14(3):1536-1550
We consider a general nonlinear time-delay system with state-delays as control variables. The problem of determining optimal values for the state-delays to minimize overall system cost is a non-standard optimal control problem–called an optimal state-delay control problem–that cannot be solved using existing optimal control techniques. We show that this optimal control problem can be formulated as a nonlinear programming problem in which the cost function is an implicit function of the decision variables. We then develop an efficient numerical method for determining the cost function’s gradient. This method, which involves integrating an auxiliary impulsive system backwards in time, can be combined with any standard gradient-based optimization method to solve the optimal state-delay control problem effectively. We conclude the paper by discussing applications of our approach to parameter identification and delayed feedback control. 相似文献
10.
The vacuum deposition of Pb onto Ag(1 1 1) gives rise to two different surface structures depending on coverage and deposition temperature. At room temperature (RT), low energy electron diffraction (LEED) reveals a sharp reconstruction completed at 1/3 Pb monolayer (ML). Beyond, a close-packed Pb(1 1 1) incommensurate overlayer develops. At low temperature (LT, ∼100 K) the incommensurate structure is directly observed whatever the coverage, corresponding to the growth of close-packed two-dimensional Pb(1 1 1) islands. Synchrotron radiation Pb 5d core-level spectra clearly demonstrate that in each surface structure all Pb atoms have essentially a unique, but different, environment. This reflects the surface alloy formation between the two immiscible metals in the reconstruction and a clear signature of the de-alloying process at RT beyond 1/3 ML coverage. 相似文献