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51.
[structure: see text] Thiophene dendrons and dendrimers were designed and synthesized using a convergent approach. Metal-mediated coupling reactions were used in the synthesis. A rational approach allowed the formation of alphaalpha, betabeta, and alphabeta linkages between the dendrons and thiophene units. 相似文献
52.
Jason D. Katz 《Tetrahedron》2004,60(43):9559-9568
A highly functionalized 4,5-dihydropyrrole-2-carboxylate is assembled by alkene-enamide ring-closing metathesis. Subsequent intramolecular azomethine imine dipolar cycloaddition provides a triazacyclopenta[cd]pentalene intermediate of potential use in a total synthesis of palau'amine. 相似文献
53.
The adsorption of lipoteichoic acid onto the surface of titanium dioxide is shown to be heterogeneous. 31P CPMAS solid-state NMR reveals two distinct phosphate species. The chemical shift anisotropy, asymmetry parameter, and rotating-frame spin-lattice relaxation suggest that 50% of the phosphates are bound to the surface. The remaining phosphates also exhibit restricted molecular motion, but do not have a direct surface bond. 相似文献
54.
Dong Won Kim Young Shin Jeon Young Kyu Jeong Moo Yul Suh Kih Soo Joe 《Journal of Radioanalytical and Nuclear Chemistry》1995,189(2):219-227
The chromatographic separation of lithium isotopes was investigated by chemical exchange with the recently synthesized polymer-bound dibenzo pyridino diamide azacrown (DBPDA) and reduced dibenzo pyridino diamide azacrown (RDBPDA). Column chromatography was employed for the determination of the effect of solvents and ligand conformation on the separation coefficients. The maximum separation coefficients, , for the DBPDA and RDBPDA at 20.0±0.02°C with acetonitrile as eluent, were found to be 0.034±0.002 and 0.035±0.002, respectively. The isotope separation coefficient and adsorption capability of the lithium ion on the DBPDA and RDBPDA were only slightly dependent on ligand structure, but strongly dependent on the solvent. DBPDA and RDBPDA appeared to have almost the same value for the isotope separation coefficient of lithium. 相似文献
55.
The application of the Clar aromatic sextet valence bond (VB) model to extended, defect-free single-walled carbon nanotubes (CNTs) with roll-up vectors (m, n) provides a real space model of their electronic structure. If m - n = 3k, where k is an integer, then all pi-electrons can be represented by aromatic sextets, and the CNT is fully benzenoid; the converse is also true. Since m - n = 3k is known to be a necessary criterion for conductivity in CNTs, only fully benzenoid CNTs are metallic, and only potentially metallic CNTs are fully benzenoid. This behavior contrasts with that of planar polycyclic aromatic hydrocarbons, in which the fully benzenoid structures are known to have large HOMO-LUMO gaps. For CNTs that are not fully benzenoid, e.g., m - n = 3k + l, where l = 1 or 2 and k is an integer, a seam of double bonds wraps about an otherwise benzenoid CNT at the chiral angle - 60 degrees or the chiral angle, respectively. Nucleus-independent chemical shift calculations on hydrogen-terminated CNT segments support this, and show that the magnetic manifestation of aromatic sextets is not due to electron correlation. The resonance hybrid of the Clar VB structures corresponds to patterns occasionally observed in scanning tunneling microscopy images of CNTs. 相似文献
56.
Oxidative damage to plasmenyl-type lipids contributes to decreased membrane barrier function, loss of membrane structure and formation of nonlamellar defects in membrane bilayers. Previous results from this laboratory have shown that membrane-soluble sensitizers (e.g. zinc phthalocyanine and bacteriochlorophyll a) mediate the photooxidation of palmitoyl plasmenylcholine (1-O-alk-1'-Z-enyl-2-palmitoyl-sn-glycero-3-phosphocholine; PPlsC) vesicles with the subsequent creation of lamellar defect structures, vesicle contents leakage and membrane-membrane fusion. Because plasmalogen lipids are significant components of sarcoplasma and myelin membranes, we sought to characterize the products of their photooxidation. This study focuses on the photooxidation of PPlsC vesicles in the presence of the water-soluble sensitizer, aluminum phthalocyanine tetrasulfonate (AlPcS4(4-)). Attack of photogenerated singlet oxygen on the 1-O-alkenyl ether linkage of PPlsC lipids was expected to generate dioxetane- and ene-type photoproducts. The products formed during continuous aerobic irradiation (28 mW/cm2, (610 nm) of PPlsC vesicles in the presence of AlPcS4(4-) were separated via reverse-phase high-performance liquid chromatography (HPLC) with electrochemical detection (ECD) or evaporative light-scattering detection (ELSD). Photooxidized dipalmitoyl-phosphatidylcholine-cholesterol vesicles (control) were used to optimize the HPLC-ECD conditions, using 7 alpha-hydroperoxy-cholesterol as standard. HPLC-ECD was found to be most sensitive for PPlsC hydroperoxides, whereas HPLC-ELSD was more sensitive for nonhydroperoxide photoproducts. The three major photoproducts formed during vesicle irradiation were isolated via preparative HPLC and then characterized by 1H-nuclear magnetic resonance and mass spectrometry. 1-Formyl-2-palmitoyl-sn-glycero-3-phosphocholine and 1-hydroxy-2-palmitoyl-sn-glycero-3-phosphocholine were identified as dioxetane cleavage products that coeluted at approximately 3 min. The second fraction (retention time [RT] = 48 min) was identified as a PPlsC allylic hydroperoxide. The third photoproduct, eluting at RT = 64 min, is tentatively identified as an oxidation product arising from allylic hydroperoxide degradation via Hock rearrangement or free radical decomposition. 相似文献
57.
TEMPO, 2,2,6,6-tetramethylpiperidinyloxy, was used in a series of protection-deprotection chemical reactions in order to gain single molecule-level control over the extent of styrene line growth on hydrogen-terminated Si(100). The mechanism involves the reaction of TEMPO with the dangling bond at the end of individual styrene lines. The TEMPO cap protects the dangling bond from further reaction with styrene resulting in the termination of line growth. TEMPO is then selectively removed from desired lines, deprotecting the dangling bond, using the scanning tunneling microscope. Further exposure of the surface to styrene ensures that only the deprotected areas continue to grow while the protected lines do not. All lines can then be capped with TEMPO, and this allows for the generation of stable styrene lines of varying lengths. 相似文献
58.
Poirier JS Tripp CP Neivandt DJ 《Langmuir : the ACS journal of surfaces and colloids》2005,21(7):2876-2880
Changes in the structure of a surfactant adsorbed on oxidized silicon arising from interaction with a polyelectrolyte have been studied using polarized infrared attenuated total reflection spectroscopy. Specifically, the cationic surfactant cetyltrimethylammonium bromide (CTAB) was found to form a highly ordered layer on oxidized silicon at a concentration of 5.5 x 10(-5) M and a pH of 9.6. Addition of a solution of the anionic polyelectrolyte poly(styrenesulfonate) to the ordered CTAB layer resulted in a rapid and dramatic decrease in the surface excess of CTAB. Interestingly however, the interfacial order of the residual surfactant was retained for a time period greater than 1 h, before decreasing. Reintroduction of a surfactant solution prior to destabilization of the residual interfacial CTAB resulted in the readsorption of the surfactant, the recovery of the initial equilibrium coverage, and the maintenance of an ordered CTAB conformation. This desorption/readsorption process may be subsequently repeated without destroying the order of the CTAB on the surface. If however sufficient time is allowed for the residual interfacial surfactant to destabilize prior to readdition of CTAB, the degree of surfactant order remains low, despite the rapid reobtainment of a surface excess equal to or greater than that initially measured. These results are interpreted in terms of polymer/surfactant interfacial complexation and the removal of adsorbed surfactant into solution. The ordering behavior of the residual surfactant suggests that CTAB is left on the surface in isolated patches of highly ordered species that maintain their order until two-dimensional diffusion leads to a more homogeneous surfactant surface distribution and hence the loss of conformational order. The degree of orientation order assumed by surfactant readsorbing on a depleted surface appears to be templated by the order of the residual interfacial surfactant, suggestive of a two-dimensional epitaxial growth mechanism for CTAB readsorption. 相似文献
59.
A numerical method to simulate the dynamics of polymer solutions in confined geometries has been implemented and tested. The method combines a fluctuating lattice-Boltzmann model of the solvent [Ladd, Phys. Rev. Lett. 70, 1339 (1993)] with a point-particle model of the polymer chains. A friction term couples the monomers to the fluid [Ahlrichs and Dunweg, J. Chem. Phys. 111, 8225 (1999)], providing both the hydrodynamic interactions between the monomers and the correlated random forces. The coupled equations for particles and fluid are solved on an inertial time scale, which proves to be surprisingly simple and efficient, avoiding the costly linear algebra associated with Brownian dynamics. Complex confined geometries can be represented by a straightforward mapping of the boundary surfaces onto a regular three-dimensional grid. The hydrodynamic interactions between monomers are shown to compare well with solutions of the Stokes equations down to distances of the order of the grid spacing. Numerical results are presented for the radius of gyration, end-to-end distance, and diffusion coefficient of an isolated polymer chain, ranging from 16 to 1024 monomers in length. The simulations are in excellent agreement with renormalization group calculations for an excluded volume chain. We show that hydrodynamic interactions in large polymers can be systematically coarse-grained to substantially reduce the computational cost of the simulation. Finally, we examine the effects of confinement and flow on the polymer distribution and diffusion constant in a narrow channel. Our results support the qualitative conclusions of recent Brownian dynamics simulations of confined polymers [Jendrejack et al., J. Chem. Phys. 119, 1165 (2003) and Jendrejack et al., J. Chem. Phys. 120, 2513 (2004)]. 相似文献
60.
Nelson A Belitsky JM Vidal S Joiner CS Baum LG Stoddart JF 《Journal of the American Chemical Society》2004,126(38):11914-11922
A self-assembled pseudopolyrotaxane consisting of lactoside-displaying cyclodextrin (CD) "beads" threaded onto a linear polyviologen "string" was investigated for its ability to inhibit galectin-1-mediated T-cell agglutination. The CDs of the pseudopolyrotaxane are able to spin around the axis of the polymer chain as well as to move back and forth along its backbone to alter the presentation of its ligand. This supramolecular superstructure incorporates all the advantages of polymeric structures, such as the ability to span large distances, along with a distinctively dynamic presentation of its lactoside ligands to afford a neoglycoconjugate that can adjust to the relative stereochemistries of the lectin's binding sites. The pseudopolyrotaxane exhibited a valency-corrected 10-fold enhancement over native lactose in the agglutination assay, which was greater than the enhancements observed for lactoside-bearing trivalent glycoclusters and a lactoside-bearing chitosan polymer tested using the same assay. The experimental results indicate that supramolecular architectures, such as the pseudopolyrotaxane, provide tools for investigating protein-carbohydrate interactions. 相似文献