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31.
Alba Díaz-Rodríguez 《Tetrahedron letters》2005,46(35):5835-5838
Regioselective syntheses of several O-crotonyl 2′-deoxynucleoside derivatives have been efficiently achieved using a biocatalytic methodology. While Candida antarctica lipase B (CAL-B) afforded the 5′-O-acylated compounds, immobilized lipase from Pseudomonas cepacia (PSL-C) provided the 3′-O-crotonylated analogs. Since classical chemical approaches did not work appropriately due to side isomerization reactions, a mixture of both lipases was used to achieve a useful synthetic route toward diacylated nucleosides. 相似文献
32.
Suárez M Martínez-Alvarez R Martín N Verdecia Y Ochoa E Alba L Seoane C Kayali N 《Rapid communications in mass spectrometry : RCM》2002,16(8):749-754
A variety of 5-alkoxycarbonyl-4-aryl-6-methyl-3,4-dihydro-2(1H)-pyridones and hexahydrofuro[3,4-b]-2(1H)-pyridones have been investigated by electron impact (EI) and electrospray ionisation (ESI) techniques. Sequential product ion fragmentation (MS(n)) was performed to elucidate the degradation pathways for these compounds. Comparisons are made between positive and negative even-electron ions from ESI spectra and the molecular radical cations obtained under EI conditions. The data collected in this paper provide information on the strong impact that different substituents have on the ion fragmentation process. 相似文献
33.
Silica from leached chrysotile fibers (SILO) was silanized with trialkoxyaminosilanes to yield inorganic–organic hybrids designated SILx (x=1–3). The greatest amounts of the immobilized agents were quantified as 2.14, 1.90, and 2.18 mmol g−1 on SIL1, SIL2, and SIL3 for –(CH2)3NH2,–(CH2)3NH(CH2)2NH2, and –(CH2)3NH(CH2)2NH(CH2)2NH2 groups attached to the inorganic support. The infrared spectra for all modified silicas showed the absence of the Si–OH deformation mode, originally found at 950 cm−1, and the appearance of asymmetric and symmetric C–H stretching bands at 2950 and 2840 cm−1. Other important bands associated with the organic moieties were assigned to νas(NH) at 3478 and νsym(NH) at 3418 cm−1. The NMR spectrum of the solid precursor material suggested two different kinds of silicon atoms: silanol and siloxane groups, between −90 and 110 ppm; however, additional species of silicon that contain the organic moieties bonded to silicon at −58 and −66 ppm appeared after chemical modification. These modified silicas showed a high adsorption capacity for cobalt and copper cations in aqueous solution, in contrast to the original SILO matrix, confirming the unequivocal anchoring of silylating agents on the silica surface. 相似文献
34.
Multiaxial mechanical behavior of aramid fibers and identification of skin/core structure from single fiber transverse compression testing
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Judith Wollbrett‐Blitz Sébastien Joannès Rémi Bruant Christophe Le Clerc Marc Romero De La Osa Anthony Bunsell Alba Marcellan 《Journal of Polymer Science.Polymer Physics》2016,54(3):374-384
The transverse and longitudinal mechanical properties of aramid fibers like Kevlar? 29 (K29) fibers are strongly linked to their highly oriented structure. Mechanical characterization at the single fiber scale is challenging especially when the diameter is as small as 15 µm. Longitudinal tensile tests on single K29 fibers and single fiber transverse compression test (SFTCT) have been developed. Our approach consists of coupling morphological observations and mechanical experiments with SFTCT analysis by comparing analytical solutions and finite element modeling. New insights on the analysis of the transverse direction response are highlighted. Systematic loading/unloading compression tests enable to experimentally determine a transverse elastic limit. Taking account of the strong anisotropy of the fiber, the transverse mechanical response sheds light on a skin/core architecture. More importantly, results suggest that the skin of the fiber, typically representing a shell of one micrometer in thickness, has a transverse apparent modulus of 0.2 GPa. That is around more than fifteen times lower than the transverse modulus of 3.0 GPa in the core. By comparison, the measured longitudinal modulus is about 84 GPa. The stress distribution in the fiber is explored and the critical areas for damage initiation are discussed. © 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2016 , 54, 374–384 相似文献
35.
C.M. da Fonseca Dan A. Mazilu Irina Mazilu H. Thomas Williams 《Applied Mathematics Letters》2013,26(12):1206-1211
A straightforward model for deposition and evaporation on discrete cells of a finite array of any dimension leads to a matrix equation involving a Sylvester–Kac type matrix. The eigenvalues and eigenvectors of the general matrix are determined for an arbitrary number of cells. A variety of models to which this solution may be applied are discussed. 相似文献
36.
37.
Matúš Durec Francesco Zaccaria Dr. Célia Fonseca Guerra Prof. Dr. Radek Marek 《Chemistry (Weinheim an der Bergstrasse, Germany)》2016,22(31):10912-10922
Repetitive guanine‐rich nucleic acid sequences play a crucial role in maintaining genome stability and the cell life cycle and represent potential targets for regulatory drugs. Recently, it has been demonstrated that guanine‐based ligands with a porphyrin core can be used as markers of G‐quadruplex assemblies in cell tissues. Herein, model systems of guanine‐based ligands are explored by DFT methods. The energies of formation of modified guanine tetrads and those of modified tetrads stacked on the top of natural guanine tetrads have been calculated. The interaction energy has been decomposed into contributions from hydrogen bonding, stacking, and ion coordination and a twist–rise potential energy scan has been performed to find the individual local minima. Energy decomposition analysis reveals the impact of various substituents (F, Cl, Br, I, Me, NMe2) on individual energy terms. In addition, cooperative reinforcement in forming the modified and stacked tetrads, as well as the frontier orbitals participating in the hydrogen‐bonding framework involving the HOMO–LUMO gap between the occupied σHOMO on the proton‐accepting C=O and =N? groups and unoccupied σLUMO on the N?H groups, has been studied. The investigated systems are demonstrated to have a potential in ligand development, mainly due to stacking enhancement compared with natural guanine, which is used as a reference. 相似文献
38.
In this work we present an optical lattice setup to realize a full Dirac Hamiltonian in 2+1 dimensions. We show how all possible external potentials coupled to the Dirac field can arise from perturbations of the existing couplings of the honeycomb lattice pattern. This greatly simplifies the proposed implementations, requiring only spatial modulations of the intensity of the laser beams to induce complex non-Abelian potentials. We finally suggest several experiments to observe the properties of the quantum field theory in the setup. 相似文献
39.
Carlos M. da Fonseca 《Linear and Multilinear Algebra》2019,67(8):1713-1714
40.
Prof. Dr. Javier A. Cabeza Dr. Pablo García-Álvarez Prof. Dr. Mar Gómez-Gallego Laura González-Álvarez Alba D. Merinero Prof. Dr. Miguel A. Sierra 《Chemistry (Weinheim an der Bergstrasse, Germany)》2019,25(36):8635-8642
The reactivity of amidinatotetrylenes of the type E(tBu2bzm)R1 (E=Si, Ge; tBu2bzm=N,N′-bis(tertbutyl)benzamidinate; R1=alkyl or aryl) with the chromium Fischer alkynylcarbene complexes [Cr{C(OEt)C2R2}(CO)5] (R2=Ph; ferrocenyl, Fc) has been studied. At room temperature, two different reaction pathways have been identified: (a) attack of the amidinatotetrylene to the alkynyl C2 atom (γ-attack), which leads to σ-allenyl complexes in which the original Ccarbene atom maintains its attachment to the Cr(CO)5 and OEt groups (compounds 3 ), and (b) attack of the amidinatotetrylene to the Ccarbene atom (α-attack), which ends in σ-allenyl complexes in which the original Ccarbene atom is not attached to the metal atom and has been inserted into an E−N bond of the amidinatotetrylene forming an E-C-N-C-N five-membered ring (compounds 4 ). It has been found that compounds 3 are thermodynamically less stable than their corresponding 4 isomers and that some of the former (E=Ge; R1=CH2SiMe3) can be transformed into the latter upon heating. At high temperatures (>70 °C) the reactions involving bulky amidinatotetrylenes (R1=Mes, tBu) end in the carbene-substitution products [Cr{E(tBu2bzm)R1}(CO)5]. 相似文献