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1.
Marit Seifert Stephanie Hesse Vasken Kabrelian Dieter Klemm 《Journal of polymer science. Part A, Polymer chemistry》2004,42(3):463-470
For the modification of medically useful biomaterials from bacterially synthesized cellulose, fleeces of Acetobacter xylinum have been produced in the presence of 0.5, 1.0, and 2.0% (m/v) carboxymethylcellulose (CMC), methylcellulose (MC), and poly(vinyl alcohol) (PVA), respectively, in the Hestrin-Schramm culture medium. The incorporation of the water-soluble polymers into cellulose and their influence on the structure, crystal modifications, and material properties are described. With IR and solid-state 13C NMR spectroscopy of the fleeces, the presence of the cellulose ethers and an increase in the amorphous parts of the cellulose modifications (NMR results) have been detected. The incorporation is represented by a higher product yield, too. As demonstrated by scanning electron microscopy, a porelike cellulose network structure forms in the presence of CMC and MC. This modified structure increases the water retention ability (expressed as the water content), the ion absorption capacity, and the remaining nitrogen-containing residues from the culture medium or bacteria cells. The water content of bacterial cellulose (BC) in the never dried state and the freeze-dried, reswollen state can be controlled by the CMC concentration in the culture solution. The freeze-dried, reswollen BC-CMC (2.0%) contains 96% water after centrifugation, whereas standard BC has only 73%. About 98% water is included in a BC-MC composite in the wet state, and about 93% is included in the reswollen state synthesized in the presence of 0.5, 1.0, or 2.0% MC. These biomaterial composites can be stored in the dried state and reswollen before use, reaching a higher water absorption than pure, never dried BC. The copper ion capacity of BC-CMC composites increases proportionally with the added amount of CMC. BC-CMC (0.5%) can absorb 3 times more copper ions than original BC. In the case of 0.5 and 1.0% PVA additions to the culture solution, this polymer cannot be detected in the cellulose fleeces after they are washed. Nevertheless the presence of PVA in the culture medium effects a decreased product yield, a retention of nitrogen-containing residues in the material during purification, a reduced water absorption ability, and a slightly higher copper ion capacity in comparison with original BC. The water content of freeze-dried, reswollen BC-PVA (0.5%) is only 62%. © 2003 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: 463–470, 2004 相似文献
2.
Active flow-separation control is an effective and efficient mean for drag reduction and unsteady load alleviation resulting
from locally or massively separated flow. Such a situation occurs in configurations where the aerodynamic performance is of
secondary importance to functionality. The performance of heavy transport helicopters and aeroplanes, having a large, and
almost flat, aft loading ramp suffer from the poor aerodynamics of the aft body. Hence, a combined experimental and numerical
investigation was undertaken on a generic transport aeroplane/helicopter configuration. The experimental study provided surface
pressures, direct drag measurements, surface and smoke flow visualization. The baseline flow was numerically analyzed, using
finite volume solutions of the RANS equations. The baseline flow around the model was insensitive to the Reynolds number in
the range it was tested. The flow separating from the aft body was characterized by two main sources of drag and unsteadiness.
The first is a separation bubble residing at the lower ramp corner and the second is a pair of vortex systems developing and
separating from the sides of the ramp. As the model incidence is decreased, the pair of vortex systems also penetrates deeper
towards the centerline of the ramp, decreasing the pressure and increasing the drag. As expected, the ramp lower corner bubble
was highly receptive to periodic excitation introduced from four addressable piezo-fluidic actuators situated at the ramp
lower corner. Total drag was reduced by 3–11%, depending on the model incidence. There are indications that the flow in the
wake of the model is also significantly steadier when the bubble at the lower ramp corner is eliminated. The vortex system
is tighter and steadier when the ramp-corner bubble is eliminated. 相似文献
3.
The ternary system CsCl?NaCl?LaCl3 was investigated by means of differential thermal analysis and X-ray powder diffraction analysis. There exists one congruently melting compound, Cs2NaLaCl6, crystallizing with the cubic elpasolite structure. No quasi-binary section exists for the whole system, however three binaries range from the ternary compound Cs2NaLaCl6 to NaCl, CsLa2Cl7 and Cs3LaCl6 resp., dividing the system in three areas of composition: one triangle, Cs3LaCl6?Cs2NaLaCl6?CsLa2Cl7, containing additionally a compound Cs2LaCl5 below 510°C, and the two areas CsCl?NaCl?Cs2NaLaCl6?Cs3LaCl6 and Cs2NaLaCl6?NaCl?LaCl3?CsLa2Cl7, containing a mixed crystal range between LaCl3 and Na3La5Cl18. These areas could be further divided in five triangles, so that the whole system contains six Alkemade triangles. 相似文献
4.
F. Jancik B. Seifert C. K. Laird W. Czysz J. S. Dunnett W. Schmidt und P. Bachhausen 《Fresenius' Journal of Analytical Chemistry》1987,327(7):747-755
Ohne Zusammenfassung 相似文献
5.
Ohne Zusammenfassung 相似文献
6.
7.
Douglas R. Hurst Kristen L. French April J. Angel Angela R. Williams Mary E. Rampey Tina S. Guion Kam W. Chan Camille M. Kassis Shannon L. Studer Martinez Charles F. Beam 《Journal of heterocyclic chemistry》1998,35(6):1357-1359
Several acetoacetanilides were trilithiated in excess lithium diisopropylamide, and the resulting polylithiated intermediates were regioselectively condensed with lithiated methyl salicylates followed by acid cyclization to substituted 4-oxo-N-aryl-4H-1-benzopyran-2-acetamides (benzopyranone-2-acetamides). 相似文献
8.
Stereochemistry of lactide polymerization with chiral catalysts: new opportunities for stereocontrol using polymer exchange mechanisms 总被引:7,自引:0,他引:7
The synthesis of chiral aluminum and yttrium alkoxides and their application for lactide polymerization are reported. The complexes (SalBinap)MOR [4, M = Al, R = (i)Pr; 5, M = Y, R = (CH(2))(2)NMe(2)] are synthesized by reacting the ligand (SalBinap)H(2) [2,2'-[(1,1'-binaphthalene)-2,2'-diylbis(nitrilomethylidyne)]bisphenol] with the appropriate metal trisalkoxide. While enantiomerically pure yttrium complex 5 did not effect stereocontrol in the polymerization of either meso- or rac-lactide, homochiral 4 was found to exhibit excellent stereocontrol in a range of lactide polymerizations. Enantiomerically pure 4 polymerizes meso-lactide to syndiotactic poly(lactic acid) (PLA), while rac-4 polymerizes meso- and rac-lactide to heterotactic and isotactic stereoblock PLA, respectively. On the basis of the absolute stereochemistry of ring-opening of meso-lactide using (R)-4, a polymer exchange mechanism is proposed to account for the PLA microstructures resulting from rac-4. 相似文献
9.
Reduced inorganic phosphorus in the natural environment: significance, speciation and determination 总被引:1,自引:0,他引:1
It is commonly assumed that phosphorus occurs almost exclusively in the environment as fully oxidized phosphate (primarily H2PO4− and HPO42−, where the oxidation state of phosphorus is +V). Recent developments in the field of microbiology and research on the origin of life have suggested a possibly significant role for reduced, inorganic forms of phosphorus in bacterial metabolism and as evolutionary precursors of biological phosphate compounds. Reduced inorganic forms of phosphorus include phosphorus acid (H3PO3, P(+III)), hypophosphorus acid (H3PO2, P(+I)) and various forms of phosphides (P(−III)). Reduced phosphorus has been detected in anaerobic sediments, sewage treatment facilities and in industrial and agricultural processes.Microbiological evidence suggests a significant role for reduced phosphorus species in metabolic processes and raises interesting questions regarding the biogeochemistry of this nutrient in the environment. However, the paucity of data on the presence and cycling of reduced phosphorus compounds in the environment requires attention in order to elucidate the role of these compounds in natural systems. This paper discusses the significance of reduced phosphorus in the natural environment, its speciation and methods of detection. 相似文献
10.
Liang X Parkinson JA Weishäupl M Gould RO Paisey SJ Park HS Hunter TM Blindauer CA Parsons S Sadler PJ 《Journal of the American Chemical Society》2002,124(31):9105-9112
As platforms for the design of metal-based therapeutic and diagnostic agents, macrocycles are rigid enough to provide strong metal binding sites and orient functional groups stereoselectively, yet flexible enough to accommodate structural changes required for induced-fit recognition of biological targets. We consider the recognition of the Zn(II) complex of the bis-tetraazamacrocycle xylyl-bicyclam, a potent anti-HIV agent, by the coreceptor CXCR4, a G-protein-coupled receptor used by HIV for membrane fusion and cell entry. NMR studies show that the macrocycles of Zn(II)(2)-xylyl-bicyclam perchlorate exist in aqueous solution as two major configurations, trans-I (nitrogen chirality R,S,R,S), and trans-III (S,S,R,R). Acetate addition induced a major structural change. X-ray crystallography shows that the acetate complex contains the unusual cis-V cyclam configuration (R,R,R,R and folded) with bidentate coordination of acetate to Zn(II) plus second-coordination-sphere double H-bond formation between diagonal NH protons on the opposite cyclam face and acetate carboxylate oxygens. Detailed 1D and 2D NMR studies show that the major configuration of Zn(II)(2)-xylyl-bicyclam acetate in aqueous solution is cis-V/trans-I. Molecular modeling shows that an analogous cis-V site can be formed when Zn(II)(2)-xylyl-bicyclam binds to CXCR4, involving the carboxylate groups of Asp262 (Zn(II) coordination) and Glu288 (double H-bonding). The second cyclam can adopt the trans-I (or trans-III) configuration with Zn(II) binding to Asp171. These interactions are consistent with the known structure-activity relationships for bicyclam anti-HIV activity and receptor mutation. Consideration of the anti-HIV activity of xylyl-bicyclam complexes of other metal ions suggests that affinity for carboxylates, configurational flexibility, and kinetic factors may all play roles in receptor recognition. For example, Pd(II) cyclam complexes interact only weakly with axial ligands and are inflexible and inactive, whereas Co(III) cyclams bind carboxylates strongly, are configurationally flexible, and yet have low activity. Our findings should aid the design of new generations of active macrocycles including highly specific chemokine receptor antagonists. 相似文献