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91.
Howard C. Haas Monis J. Manning Martin H. Mach 《Journal of polymer science. Part A, Polymer chemistry》1970,8(7):1725-1730
Differential thermal analysis has been used to study the fusion of aqueous thermally reversible gels of gelatin and polyacrylylglycinamide (PAG). In the case of gelatin gels, endotherms close to the melting point are readily observed and these are sometimes preceeded by a small exothermic heat of gel reorganization. Calculations are presented to show that breaking of the gelatin gel network requires only a small fraction of the observed endothermic heat of fusion and that most of the heat is required for melting larger crystallites within gelatin aggregates and for perhaps a helix → coil transition. Failure to observe endotherms by DTA over the known temperature range of fusion of PAG gels is consistent with prior measurements and conclusions. The noncrystallinity of PAG gels and soluble aggregates together with a heat of crosslinking of only ?5 to ?10 kcal/mole of crosslinks places the heat of fusion of PAG gels outside the lower limits of DTA sensitivity. 相似文献
92.
Summary A method is described for the determination of alkaloids in morning glory leaves by means of spectrophotofluorimetry. The total alkaloidal contents found in different batches of leaves ranged from 0.027 to 0.04%.
For part XIV see Mikrochim. Acta [Wien]1976 I, 227. 相似文献
Zusammenfassung Ein Verfahren zur Bestimmung der Gesamtalkaloide in den Blättern vonIpomoea violacea wurde angegeben. Spektralfluoreszenzmessungen ergaben für verschiedene Chargen solcher Blätter Gehalte von 0,027 bis 0,04%.
For part XIV see Mikrochim. Acta [Wien]1976 I, 227. 相似文献
93.
Khademhosseini A Yeh J Eng G Karp J Kaji H Borenstein J Farokhzad OC Langer R 《Lab on a chip》2005,5(12):1380-1386
We present a soft lithographic method to fabricate multiphenotype cell arrays by capturing cells within an array of reversibly sealed microfluidic channels. The technique uses reversible sealing of elastomeric polydimethylsiloxane (PDMS) molds on surfaces to sequentially deliver various fluids or cells onto specific locations on a substrate. Microwells on the substrate were used to capture and immobilize cells within low shear stress regions inside channels. By using an array of channels it was possible to deposit multiple cell types, such as hepatocytes, fibroblasts, and embryonic stem cells, on the substrates. Upon formation of the cell arrays on the substrate, the PDMS mold could be removed, generating a multiphenotype array of cells. In addition, the orthogonal alignment and subsequent attachment of a secondary array of channels on the patterned substrates could be used to deliver fluids to the patterned cells. The ability to position many cell types on particular regions within a two dimensional substrate could potentially lead to improved high-throughput methods applicable to drug screening and tissue engineering. 相似文献
94.
Chen W Wu H Bernard D Metcalf MD Deschamps JR Flippen-Anderson JL MacKerell AD Coop A 《The Journal of organic chemistry》2003,68(5):1929-1932
Treatment of 5-trimethylsilylthebaine with L-Selectride gave rise to a rearrangement to 10-trimethylsilylbractazonine through migration of the phenyl group, whereas treatment of thebaine with strong Lewis acids is known to lead to a similar rearrangement through migration of the alkyl bridge to give, after reduction, (+)-neodihydrothebaine. It is suggested that the rearrangement of the alkyl group of thebaine is favored due to the formation of a tertiary benzylic cation. However, for 5-trimethylsilylthebaine, the lithium ion of L-Selectride acts as the Lewis acid and the beta-silyl effect dominates in the stabilization of any positive charge. This rearrangement provides a clear example of the greater relative migratory aptitude of phenyl groups over alkyl groups, and provides an efficient synthesis of (+)-bractazonine from thebaine. 相似文献
95.
The reaction of [SiMe2(C5H4)2][(C5Me5)ZrMe2]2 with H2S at 110°C proceeds with the formation of [SiMe2(C5H4)2][(C5Me5)Zr(-S)]2]2. The molecular structure of this dinuclear zirconocenophane compound features a non-planar four-membered Zr2(-S)2
2 ring with two symmetrically-bridging sulfido groups. The bridging dimethylsilyl-bridged bis(cyclopentadienyl) ligand adopts a skewed orientation to accommodate the long Zr ... Zr separation of 3.611(1) Å. This compound crystallizes in the centrosymmetric space group P1 with refined lattice parameters ofa = 10.905(1) Å,b = 10.939(2) Å,c = 14.621(2) Å, = 106.32(1)°, = 91.25(1)°, = 111.25(1)°,V = 1545.1(8) Å3, calc. = 1.512g/cm3,Z=2. Full-matrix refinement converged with final discrepancy indices of R(F
o = 0.045 andR(F
o) = 0.079 with GOF= 1.60 for 4403 data withF
o > 3(F
o).This paper is dedicated to Professor Larry Dahl, a valued friend and mentor, on the occasion of his 65th birthday and in recognition of his significant fundamental contributions to the field of metal cluster chemistry. May the Goddess Fortuna continue to bring distinction and honor to him and his research endeavors. 相似文献
96.
FrankH. Allen Raju Mondal NigelA. Pitchford JudithA.K. Howard 《Helvetica chimica acta》2003,86(4):1129-1139
Symmetry‐modified principal‐component analysis has been used to visualise the geometrical distortions of three‐coordinate metal centres observed in crystal structures retrieved from the Cambridge Structural Database. It is shown that compounds of Cu, Ag, Hg, Zn, and Au dominate the dataset, and exhibit distortions away from the trigonal planar archetype towards a) T‐shaped and b) Y‐shaped geometries. A small number of compounds, principally of Cu and Ag, also show distortions towards trigonal pyramidal geometries. The interconversions from Y‐shaped geometries, through the trigonal planar form to T‐shaped geometries are clearly mapped by the PC analysis. For HgII complexes, it is possible to interpret the transition from T‐shaped geometries to the trigonal planar form in terms of a reaction pathway for ligand addition to linear L1? HgII? L2 species. 相似文献
97.
Comparative properties of siloxane vs phosphonate monolayers on a key titanium alloy 总被引:2,自引:0,他引:2
Silverman BM Wieghaus KA Schwartz J 《Langmuir : the ACS journal of surfaces and colloids》2005,21(1):225-228
A direct comparison of surface loading, interface shear strength, and interface hydrolytic stability was made between a phosphonate and two siloxane monolayers formed on the native oxide surface of Ti-6Al-4V. Surface loading for the phosphonate was ca. four times greater (on a nanomole/area basis) than for the siloxanes; mechanical strengths per surface-bound molecule were comparable, but the hydrolytic stability (pH 7.5) of the siloxanes was poor. These results suggest that phosphonate monolayer interfaces are more desirable than comparable siloxane ones for applications where such interfaces contact even slightly alkaline water. 相似文献
98.
The time evolution of silica nanoparticles in solutions of tetrapropylammonium (TPA) has been studied using a combination of small-angle scattering, conductivity, and pH measurements to provide the first comprehensive analysis of nanoparticle structural and compositional changes at elevated temperatures. We have found that silica-TPA nanoparticles subjected to hydrothermal treatment (70-90 degrees C) grow via an Ostwald ripening mechanism with growth rates that depend on both pH and temperature. Small-angle X-ray (SAXS) and neutron (SANS) scattering confirm that the core-shell structure of the particles, initially present at room temperature, is maintained during heating, but an evolution toward sphericity is evidenced especially at high values of pH. SAXS absolute intensity calculations were utilized to calculate the changes in nanoparticle composition and concentration over time. These changes along with the conductivity and pH measurements and SANS contrast matching studies indicate that, upon heating, TPA becomes embedded in the core of nanoparticles giving rise to more zeolitic-looking nanomaterials. 相似文献
99.
E.Gordon Muller Jeffrey L. Petersen Lawrence F. Dahl 《Journal of organometallic chemistry》1976,111(1):91-112
Our preparation of Ti(h5?C5H5)2 S5 by the reaction of elemental sulfur with Ti(h5?C5H5)2(CO)2 in hexane and of V(h5?C5H5)2S5? H2O by the reacti of V(h5?C5H5)2 Cl2 with Na2S5 in THF and structural analyses by single crystal X-ray diffraction (together with infrared, solution EPR, and temperaturedependent magnetic susceptibility measurements) represent an extension of our previous work on M(h5?C5H5)2 (SC6H5)2 (M = Ti, V). The crystallographic results provide further support of our previous conclusions that the Ballhausen—Dahl model is not valid for M(h5?C5H5)2L2 systems. The structuralfeatures of the chair-like titanium and vanadium pentasulfide molecules are compared to the corresponding phenylmercapto analogs and to the chair-like cyclohexasulfur molecule in rhombohedral sulfur. Ti(h5?C5H5)2S5 was isolated as a mixture of monoclinic and orthorhombic Crystalline phases-which were both characterized by preliminary X-ray data. A complete Structural determination and refinement of the monoclinic phase, which contains two independent molecules in a cell of dimensions a 22.843(2), b 7.958(1), c 14.465(1) Å, β 90.074(4)° and symmetry P21/c, yielded R1 5.3 % and R2 5.9 % for 2168 independent diffractometry-collected data with I≥ 2.5o(I). V(h5?C5H5)2S5- H2O contains four V(h5?C5H5)2S5 molecules and two water molecules of hydration (of crystallographic site symmetry C2-2) in onorthorhombic unit cell of symmetry P21212 and of dimensions a 13.491(1), b 12.748(1), c 7.715(1) Å. Least-squares refinement of 750 diffractometry data with I≥2.0σ(I) gave R1 2.4% and R2 3.0% Both of these compounds were independently synthesized and Spectroscopically characterized by Köpf and co-workers, and-a Complete X-ray diffraction study was performed by Epstein and Bernal on a different monoclinic phase of Ti(h5?C5H5)2S5 (isolated by-Köpf). An extraction of V(h5?C5H5)2S5 with re fluxing benzene under nitrogen atmosphere in a Soxhlet apparatus led to the formation of the previously reported [V2 (h5?C5H5)2S5]n compound which was characterized by physical measurements including a preliminary X-ray diffraction study. 相似文献
100.
Boyd DR Sharma ND King AW Shepherd SD Allen CC Holt RA Luckarift HR Dalton H 《Organic & biomolecular chemistry》2004,2(4):554-561
Direct and indirect evidence, of unexpected stereoselective reductase-catalysed deoxygenations of sulfoxides, was found. The deoxygenations proceeded simultaneously, with the expected dioxygenase-catalysed asymmetric sulfoxidation of sulfides, during some biotransformations with the aerobic bacterium Pseudomonas putida UV4. Stereoselective reductase-catalysed asymmetric deoxygenation of racemic alkylaryl, dialkyl and phenolic sulfoxides was observed, without evidence of the reverse sulfoxidation reaction, using anaerobic bacterial strains. A purified dimethyl sulfoxide reductase, obtained from the intact cells of the anaerobic bacterium Citrobacter braakii DMSO 11, yielded, from the corresponding racemates, enantiopure alkylaryl sulfoxide and thiosulfinate samples. 相似文献