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111.
Jin Li Dou‐sheng Zhang Xiao‐meng Chong Chang‐qin Hu 《Rapid communications in mass spectrometry : RCM》2010,24(14):2143-2150
The structural fragment ions of nine cephalosporins were studied by electrospray ionization quadrapole trap mass spectrometry (Q‐Trap MSn) in positive mode. The influence of substituent groups in the 3‐position on fragmentation pathway B, an α‐cleavage between the C7? C8 single bond, coupled with a [2,4]‐trans‐Diels‐Alder cleavage simultaneously within the six‐membered heterocyclic ring, was also investigated. It was found that when the substituent groups were methyl, chloride, vinyl, or propenyl, fragmentations belonging to pathway B were detected; however, when the substituents were heteroatoms such as O, N, or S, pathway B fragmentation was not detected. This suggested that the [M–R3]+ ion, which was produced by the bond cleavage within the substituent group at the 3‐position, had a key influence on fragmentation pathway B. This could be attributed to the strong electronegativity of the heteroatoms (O, N, S) that favors the production of the [M–R3]+ ion. Moreover, having the positive charge of the [M–R3]+ ion localized on the nitrogen atom in the 1‐position changed the electron density distribution of the heterocyclic structure, which prohibits a [2,4]‐reverse‐Diels‐Alder fragmentation and as a result fragmentation pathway B could not occur. The influence of the substituent group in the 3‐position was determined by the intensity ratio (e/d) of ions produced by fragmentation pathway A, a [2,2]‐trans‐Diels‐Alder cleavage within the quaternary lactam ring, including the breaking of the amide bond and the C6? C7 single bond (ion d), and fragmentation pathway B (ion e). The results indicate that the electronegativity of the substituent group was a key influencing factor of pathway B fragmentation intensity, because the intensity ratio (e/d) is higher for a chlorine atom, a vinyl, or a propenyl group than that of a methyl group. This study provided some theoretical basis for the identification of cephalosporin antibiotics and structural analysis of related substances in drugs. Copyright © 2010 John Wiley & Sons, Ltd. 相似文献
112.
J. Yang H. Zhang S.M. Wang C.G. Lin D.Q. Shi S.X. Dou 《Physica C: Superconductivity and its Applications》2011,471(7-8):233-236
YBa2Cu3O7?δ (YBCO) superconductors were coated on the CeO2/YSZ/Y2O3 buffered Ni-5at%W tapes by a reel-to-reel pulsed laser deposition (PLD). The process of a multi-layer deposition of YBCO film was explored. X-ray diffraction texture measurements showed good both in-plane and out of plane crystalline orientations in YBCO films. The average values calculated at a full width at half maximum (FWHM) of the peaks from phi-scans (φ) and omega (ω) scans for one meter-long YBCO tape were 7.49° and 4.71°, respectively. The critical current (Ic) was over 200 A/cm-width at 77 K and under self-field for meter-long YBCO tape. The critical transition temperature of the YBCO tape was typically as 90.1 K with 0.5 K transition widths. 相似文献
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W. B. Dou T. Shen Z. L. Sun 《International Journal of Infrared and Millimeter Waves》1994,15(7):1247-1253
The scattering of a Gaussian beam by a ferrite cylinder is analyzed using two methods. The spatial pictures in the amplitude of the total Gaussian beam near the cylinder resulting from the two methods are both clarified and compared. The scattering by a ferrite cylinder coating a dielectric sleeve is also presented. 相似文献
115.
Myosin V and myosin VI are two classes of two-headed molecular motors of the myosin superfamily that move processively along helical actin filaments in opposite directions. Here we present a hand-over-hand model for their processive movements. In the model, the moving direction of a dimeric molecular motor is automatically determined by the relative orientation between its two heads at free state and its head‘s binding orientation on track filament.This determines that myosin V moves toward the barbed end and myosin VI moves toward the pointed end of actin.During the moving period in one step, one head remains bound to actin for myosin V whereas two heads are detached for myosin VI: the moving manner is determined by the length of neck domain. This naturally explains the similar dynamic behaviours but opposite moving directions of myosin VI and mutant myosin V (the neck of which is truncated to only one-sixth of the native length). Because of different moving manners, myosin VI and mutant myosin V exhibit significantly broader step-size distribution than native myosin V. However, all the three motors give the same mean step size of -36nm (the pseudo-repeat of actin helix). All these theoretical results are in agreement with previous experimental ones. 相似文献
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Wang WC Zhong DY Zhu J Kalischewski F Dou RF Wedeking K Wang Y Heuer A Fuchs H Erker G Chi LF 《Physical review letters》2007,98(22):225504
We report a generally applicable method to pattern organic molecules on mesoscopic scales. In our method, organic molecular beam deposition was conducted on substrate surfaces prepatterned with materials to which the organic molecules have larger binding energies in comparison to the substrate. Fully uniform nucleation control at these predefined locations can be achieved by an appropriate selection of the growth parameters including temperature and deposition rate. The physical mechanisms involved are studied by Monte Carlo simulations and stand in good agreement with the experimental findings. 相似文献
118.
Qingping Dou Haitao Ma Gang Jia Zhanguo Chen Kun Cao Tiechen Zhang 《Optics & Laser Technology》2007,39(3):647-651
The cubic boron nitride (cBN) is a kind of artificial electro-optic (EO) crystal, and we have not found any relative reports so far. Because the artificial synthetic cBN wafers are very small and hard, the wafers cannot be cut into rectangular slabs. The polarizer-sample-λ/4 retardation plate (compensator)-analyzer (PSCA) transverse EO modulator has to be adjusted to the minute irregular octahedron of cBN wafers. When the applied voltage is along [1 1 1] direction of the wafer, due to refraction, the angle between the incident beam direction and the (1 1 1) plane (top or bottom plane) of the wafer should be 25.4°, and the angle between the polarization direction of the polarizer and the plane of incidence should be 50.8° by calculation, respectively. The half-wave voltage of the cBN sample was obtained for the first time, by means of detection of the output optic signals from the modulator with and without an applied electric field on the sample, respectively. Furthermore, the linear EO coefficient was obtained, . The analysis of the experimental resulting error was carried out. 相似文献
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