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191.
The biaryl unit is extensively presented in many classes of natural products, such as polyketides, terpenes, lignanes,coumarins, flavonoids, tannins, and many alkaloids.[1] It has long been recognized that an intramolecular oxidative phenolic or nonphenolic coupling reaction serves as the key step in the biosynthesis of these natural products, and the non-enzymic analogue of this transformation can lead the elegantly simple syntheses of these compounds.[2] During the last decade, a large number of oxidative coupling reagents, such as ferric chloride (FeCl3), phenyliodine(Ⅲ)bis(trifluoroacetate) (PIFA), lead(Ⅳ) tetraacetate [Pb(OAc)4], thallium(Ⅲ) triflouroacetate (TTFA), as well as vanadium oxytrifluoride (VOF3),[3,4] have been developed for this target. However, extensive application of this synthetic potential has been limited by low yields and unexpected side reactions usually encountered. 相似文献
192.
Hong‐Ze Gao Zhong‐Min Su Chun‐Sheng Qin Ri‐Gen Mo Yu‐He Kan 《International journal of quantum chemistry》2004,97(6):992-1001
Bis(2‐methyl‐8‐quinolinolato)aluminum(III) hydroxide complex (AlMq2OH) is used in organic light‐emitting diodes (OLEDs) as an electron transport material and emitting layer. By means of ab initio Hartree–Fock (HF) and density functional theory (DFT) B3LYP methods, the structure of AlMq2OH was optimized. The frontier molecular orbital characteristics and energy levels of AlMq2OH have been analyzed systematically to study the electronic transition mechanism in AlMq2OH. For comparison and calibration, bis(8‐quinolinolato)aluminum(III) hydroxide complex (Alq2OH) has also been examined with these methods using the same basis sets. The lowest singlet excited state (S1) of AlMq2OH has been studied by the singles configuration interaction (CIS) method and time‐dependent DFT (TD‐DFT) using a hybrid functional, B3‐LYP, and the 6‐31G* basis set. The lowest singlet electronic transition (S0 → S1) of AlMq2OH is π → π* electronic transitions and primarily localized on the different quinolate ligands. The emission of AlMq2OH is due to the electron transitions from a phenoxide donor to a pyridyl acceptor from another quinolate ligand including C → C and O → N transference. Two possible electron transfer pathways are presented, one by carbon, oxygen, and nitrogen atoms and the other via metal cation Al3+. The comparison between the CIS‐optimized excited‐state structure with the HF ground‐state structure indicates that the geometric shift is mainly confined to the one quinolate and these changes can be easily understood in terms of the nodal patterns of the highest occupied and lowest unoccupied molecular orbitals. On the basis of the CIS‐optimized structure of the excited state, TD‐B3‐LYP calculations predict an emission wavelength of 499.78 nm. An absorption wavelength at 380.79 nm on the optimized structure of B3LYP/6‐31G* was predicted. They are comparable to AlMq2OH 485 and 390 nm observed experimentally for photoluminescence and UV‐vis absorption spectra of AlMq2OH solid thin film on quartz, respectively. Lending theoretical corroboration to recent experimental observations and supposition, the reasons for the blue‐shift of AlMq2OH were revealed. © 2003 Wiley Periodicals, Inc. Int J Quantum Chem, 2004 相似文献
193.
15-甲基-双环[10,3,0]十五碳-1(12)-烯-13-酮的单晶属空间群P2_1/n,晶胞参数:a=0.4838(1)nm,b=1.3949(2)nm,c=2.1645(4)nm,β=92.07(1)°,Z=4.其中平面五员环呈刚性。另一个含烯十二员环的实际构象与分子力学计算的最稳定构象一致。 相似文献
194.
Determination of proteins with Alizarin Red S by Rayleigh light scattering technique 总被引:1,自引:0,他引:1
A new protein determination method by enhanced Rayleigh light scattering (RLS) technique has been developed. In acid condition (pH=3.60), RLS of 1,2-dihydroxyanthraquinone-3-sulfonate (Alizarin Red S) can be greatly enhanced by addition of proteins, resulting in two characteristic peaks, 360 and 505 nm, respectively. The new protein assay is based on the RLS enhancement and spectrum change. The optimum condition for the reaction was investigated. The linear range is 0.20-24.9 μg ml−1 for BSA and 0.20-15.5 μg ml−1 for HSA. The detection limits (S/N=3) are 9.59 ng ml−1 for BSA and 9.51 ng ml−1 for HSA. The results of determination for human serum samples were comparable to those obtained by Bradford method. The binding stoichiometry was determined. 相似文献
195.
Zhong Q Han X He L Beesley TE Trahanovsky WS Armstrong DW 《Journal of chromatography. A》2005,1066(1-2):55-70
Chiral stationary phases (CSPs) based on polymeric (R,R)- or (S,S)-1,2-diaminocyclohexane (DACH) derivatives are synthesized. When bonded to 5 microm porous spherical silica gel, the poly (trans-1,2-cyclohexanediyl-bis acrylamide) based poly-cyclic amine polymer (P-CAP) stationary phases is proved to be effective chiral stationary phases that could be used in the normal-phase mode, polar organic mode and with halogenated solvents mobile phases, if desired. Since these are entirely synthetic CSPs, the elution order of all enantiomers can be reversed between the (R,R) P-CAP and (S,S) P-CAP columns. Because of the high loading of chiral selectors, the columns exhibit very high sample capacities. Thus, P-CAP columns are useful for preparative and semi-preparative enantiomeric separations. The application of these CSPs and optimization of their separations are discussed. 相似文献
196.
Zhang B Li JF Zhong QL Ren B Tian ZQ Zou SZ 《Langmuir : the ACS journal of surfaces and colloids》2005,21(16):7449-7455
Core-shell Au-Pt nanoparticles were synthesized by using a seed growth method and characterized by transmission electron microscopy, X-ray diffraction, and UV-vis spectroscopy. Au(core)-Pt(shell)/GC electrodes were prepared by drop-coating the nanoparticles on clean glassy carbon (GC) surfaces, and their electrochemical behavior in 0.5 M H2SO4 revealed that coating of the Au core by the Pt shell is complete. The electrooxidation of carbon monoxide and methanol on the Au(core)-Pt(shell)/GC was also examined, and the results are similar to those obtained on a bulk Pt electrode. High quality surface-enhanced Raman scattering (SERS) spectra of both adsorbed CO and thiocyanate were observed on the Au(core)-Pt(shell)/GC electrodes. The potential-dependent SERS features resemble those obtained on electrochemically roughened bulk Pt or Pt thin films deposited on roughened Au electrodes. For thiocyanate, the C-N stretching frequency increases with the applied potential, yielding two distinctly different dnu(CN)/dE. From -0.8 to -0.2 V, the dnu(CN)/dE is ca. 50 cm(-1)/V, whereas it is 90 cm(-1)/V above 0 V. The bandwidth along with the band intensity increases sharply above 0 V. At the low-frequency region, Pt-NCS stretching mode at 350 cm(-1) was observed at the potentials from -0.8 to 0 V, whereas the Pt-SCN mode at 280 cm(-1) was largely absent until around 0 V and became dominant at more positive potentials. These potential-dependent spectral transitions were attributed to the adsorption orientation switch from N-bound dominant at the negative potential region to S-bound at more positive potentials. The origin of the SERS activity of the particles is briefly discussed. The study demonstrates a new method of obtaining high quality SERS on Pt-group transition metals, with the possibility of tuning SERS activity by varying the core size and the shell thickness. 相似文献
197.
The interactions between proteins and Ponceau 4R (PR) in aqueous solution have been studied by the techniques of resonance light scattering (RLS) spectroscopy, the absorption spectroscopy, zeta potential assay and circular dichroism (CD) spectrum. The dry PR can assemble on the surface of protein via electrostatic and hydrophobic forces to produce an associated compound of protein-PR, this compound can enhance the RLS of protein. Based on this fact, a simple, rapid, and sensitive method has been developed for the determination of proteins at nanogram level by RLS technique with a common spectrofluorimeter. Under optimum conditions, the linear range is 0.10-39.2 μg mL−1 for the determination of both bovine serum albumin (BSA) and human serum albumin (HSA). The detection limits (S/N = 3) are 6.96 ng mL−1 for BSA and 5.71 ng mL−1 for HSA, respectively. There is almost no interference from amino acids, most of the metal ions, and other coexistent substances. The method has been satisfactorily applied to the direct determination of the total protein in human serum. 相似文献
198.
A series of new five-coordinated ionic organotin(IV) complexes with general formula [Q][Me2Sn(μ2-SCH2COO)Cl](Q = diethylammonium, triethylammonium, di-i-propylammonium, tripropylammonium, tri-n-butylammonium, pyrimidium, 3-picolinium, methylphenylammonium, dimethylphenylammonium) were synthesized by the reaction of mercaptoacetic acid with dimethyltin dichloride in the presence of an organic base. These complexes have been characterized by elemental analyses, IR and 1H NMR spectroscopies. The crystal structure of [(n-Pr)3NH][Me2Sn(μ2-SCH2COO)Cl] was determined by X-ray crystallography. The structure consists of an anion part, and a tri-n-propylammonium cation part as a counterion. The tin atom has a distorted cis-tbp geometry with two carbon and one sulfur atoms occupying the equatorial positions and the O atom and Cl atom occupying the axial positions. The organotin anion and its counterion are connected through a hydrogen bond between the N atom in the ammonium and the O atom of the carbonyl group with a N-O length of 2.766 Å. 相似文献
199.
An electrochemical synthesis of oxazol‐2‐ones and imidazol‐2‐ones has been developed via 5‐exo‐dig cyclization of propargylic carbamates‐ and ureas‐derived amidyl radicals. The electrosynthesis relies on the dual function of 2,2,6,6‐tetramethylpiperidin‐ 1‐yl)oxyl (TEMPO) as a redox mediator for amidyl radical formation and an oxygen atom donor. The reactions are conducted under mild conditions using a simple setup and provide convenient access to functionalized oxazol‐2‐ones and imidazol‐2‐ones from readily available materials. 相似文献
200.
A new program to characterize polyethylene glycol-modified (PEGylated) proteins is outlined using capillary zone electrophoresis (CZE). PEGylated ribonuclease A and lysozyme were selected as examples. Five separation procedures were compared to select out the mixed buffer of acetonitrile-water (1:1, v/v) at pH 2.5 as the best to characterize the PEGylated proteins without sample pretreatment. Polyethylene oxide (PEO) with a high molecular mass of 8 x 10(6) was applied to rinse the capillary to form a dynamic coating which would decrease the undesirable proteins adsorbed to the inner wall of the silica. The electroosmotic flow (EOF) mobility of the five procedures was determined, respectively. It is found that acetonitrile is mainly responsible for the good resolution of PEGylated proteins with the help of PEO coating in the semi-aqueous system. The low EOF mobility and current in the semi-aqueous system might also have some responsibility for the high resolution. The semi-aqueous procedure described in this paper also demonstrates higher resolution of natural proteins than aqueous ones. 相似文献