Theoretical analysis of centrifugal dewatering of superabsorbent hydrogels using Terzaghi-Voigt combined model

This paper presents results of experimental work carried out to study the centrifugal dewatering behaviors of superabsorbent hydrogels. Taking the creep deformation of the hydrogels into consideration and assuming that the mechanical properties of gel network can be represented by Terzaghi-Voigt combined model, the basic differential equation expressing the centrifugal dewatering of superabsorbent hydrogels is solved. The progress of centrifugal dewatering is represented by an average consolidation ratio U_c as in mechanical expression. The agreement between calculated and experimental U_c is satisfactory when the creep deformation of the material is considered. Consistent with mechanical expression and electro-osmotic dewatering, as the driving force for centrifugal dewatering increases, the modified consolidation coefficient of the hydrogel network C_e also increases. Effectiveness of centrifugal dewatering, expressed as a relative difference between the initial void ratio and final void ratio, increases with the rotational speed. For a given rotational speed, the final void ratio remains constant regardless of the initial weight of the gel. On the other hand, as the rotational speed of the centrifuge gets higher, the final void ratio decreases for all initial weights of the gel investigated in the current study. The magnitude of creep deformation B depends upon the amount of initial weight of the hydrogel. For a given rotational speed of the centrifuge, the magnitude of creep deformation becomes larger as the initial amount of hydrogel is increased.     

Oxidation of Alcohols with 4-Aminopyridinium Chlorochromate Supported on Silica Gel

4-Aminopyridinium chlorochromate supported on silica gel was found to be an efficient reagent for the oxidation of primary and secondary alcohols to the corresponding carbonyl compounds.     

Oxidation of Alcohols with 4-Aminopyridinium Chlorochromate Supported on Silica Gel

Summary. 4-Aminopyridinium chlorochromate supported on silica gel was found to be an efficient reagent for the oxidation of primary and secondary alcohols to the corresponding carbonyl compounds.     

Cyclization of lithiated pyridine and quinoline carboxamides: synthesis of partially saturated pyrrolopyridines and spirocyclic beta-lactams

Lithiation of N-benzyl pyridine and quinoline carboxamides alpha to nitrogen gives anions that undergo intramolecular attack on the pyridine or quinoline ring, either directly or on activation of the ring by N-acylation. The resulting four-, five-, or six-membered-ring-containing compound may be oxidized, protonated, alkylated, or acylated to give a range of polycyclic heterocycles, including pyrrolopyridines, pyrroloquinolines, benzonaphthyridines, and azaspirocyclic beta-lactams. [reaction: see text]     

Di‐2‐pyridyl ketone p‐aminobenzoylhydrazone hydrate

The title compound [systematic name: 4‐amino‐2′‐(di‐2‐pyridyl­methyl­ene)­benzohydrazide hydrate], C₁₈H₁₅N₅O·H₂O, crystallizes in the triclinic space group P. Structural analysis shows one pyridine ring and the p‐amino­benzoylhydrazone moiety to be coplanar and orthogonal to the second pyridine ring. The packing reveals infinite molecular units interlocked via a network of hydrogen bonds.     

A hydration study of (1-->4) and (1-->6) linked alpha-glucans by comparative 10 ns molecular dynamics simulations and 500-MHz NMR

The hydration behavior of two model disaccharides, methyl-alpha-D-maltoside (1) and methyl-alpha-D-isomaltoside (2), has been investigated by a comparative 10 ns molecular dynamics study. The detailed hydration of the two disaccharides was described using three force fields especially developed for modeling of carbohydrates in explicit solvent. To validate the theoretical results the two compounds were synthesized and subjected to 500 MHz NMR spectroscopy, including pulsed field gradient diffusion measurements (1: 4.0. 10(-6) cm(2). s(-1); 2: 4.2. 10(-6) cm(2). s(-1)). In short, the older CHARMM-based force field exhibited a more structured carbohydrate-water interaction leading to better agreement with the diffusional properties of the two compounds, whereas especially the alpha-(1-->6) linkage and the primary hydroxyl groups were inaccurately modeled. In contrast, the new generation of the CHARMM-based force field (CSFF) and the most recent version of the AMBER-based force field (GLYCAM-2000a) exhibited less structured carbohydrate-water interactions with the result that the diffusional properties of the two disaccharides were underestimated, whereas the simulations of the alpha-(1-->6) linkage and the primary hydroxyl groups were significantly improved and in excellent agreement with homo- and heteronuclear coupling constants. The difference between the two classes of force field (more structured and less structured carbohydrate-water interaction) was underlined by calculation of the isotropic hydration as calculated by radial pair distributions. At one extreme, the radial O em leader O pair distribution function yielded a peak density of 2.3 times the bulk density in the first hydration shell when using the older CHARMM force field, whereas the maximum density observed in the GLYCAM force field was calculated to be 1.0, at the other extreme.     

Determination of inorganic tin and organotin compounds by graphite-furnace atomic absorption spectrometry with a new matrix modifier

The determination of total tin in water samples by graphite-furnace atomic absorption spectrometry is described. A matrix modifier containing 0.4% (w/v) potassium dichromate and 0.2% ammonium dihydrogenphosphate in 2% nitric acid is proposed. Interferences from major ions in natural fresh waters decreased and the sensitivity is greatly improved. The procedure is compatible with all the commercial injection devices and requires no pretreatment of the graphite furnace or use of a stabilized temperature platform. The 3σ detection limit is 0.62 ng Sn ml^?1, and calibration is linear up to 30 ng ml^?1 tin.     

Liquid chromatographic determination of S-adenosyl-L-methionine in dietary supplement tablets

A simple and reliable liquid chromatographic method was developed for the determination of S-adenosyl-L-methionine (SAM) in dietary supplement tablets. SAM in products was extracted with a phosphate buffer and separated from the mixture on a reversed-phase C8 column by ion-pair chromatography. A gradient mobile phase containing phosphate buffer, sodium octanesulfonate as the ion-pair reagent, and acetonitrile at a flow rate of 1.2 mL/min was used in the analysis. The UV detection wavelength was set at 257 nm. The calibration curve was linear over a range of 75-375 microg/mL for the SAM active ion with R2 = 0.9999. Replicate tests indicated good reproducibility of the method with a relative standard deviation of 0.9% (n = 8). The multiple extractions and recoveries from fortified products showed the high accuracy of the analysis. The use of the acidic buffer for SAM extraction and elution and the use of a fresh standard for each calibration to counteract the instability of the SAM compound significantly improved the accuracy of the method.     

Rapid, sensitive and selective spectrophotometric determination of phosphate as an ion associate of 12-molybdophosphate with Astra Phloxine

A highly sensitive and selective reaction of 12-molybdophosphate with the polymethine dye Astra Phloxine has been used for the spectrophotometric determination of phosphate. If the concentration of phosphate is less than 1 μmol L⁻¹, supersaturated solutions of ion associate (IA) are stable without the use of a surfactant. Under these conditions, a new band appears in the absorption spectrum at 574 nm. The color of the IA develops immediately after mixing of reagents and remains constant over several hours. The molar absorptivity of the IA is 1.54 × 10⁵ L mol⁻¹ cm⁻¹. The calibration graph is linear between 0.02 and 0.8 μmol L⁻¹ of phosphate. The limit of detection is 7 nmol L⁻¹. Phosphate was determined in pure chemicals and water samples, including Dead Sea water.     

Fusogenic tilted peptides induce nanoscale holes in supported phosphatidylcholine bilayers

Tilted peptides are known to insert in lipid bilayers with an oblique orientation, thereby destabilizing membranes and facilitating membrane fusion processes. Here, we report the first direct visualization of the interaction of tilted peptides with lipid membranes using in situ atomic force microscopy (AFM) imaging. Phase-separated supported dioleoylphosphatidylcholine/dipalmitoylphosphatidylcholine (DOPC/DPPC) bilayers were prepared by fusion of small unilamellar vesicles and imaged in buffer solution, in the absence and in the presence of the simian immunodeficiency virus (SIV) peptide. The SIV peptide was shown to induce the rapid appearance of nanometer scale bilayer holes within the DPPC gel domains, while keeping the domain shape unaltered. We attribute this behavior to a local weakening and destabilization of the DPPC domains due to the oblique insertion of the peptide molecules. These results were directly correlated with the fusogenic activity of the peptide as determined using fluorescently labeled DOPC/DPPC liposomes. By contrast, the nontilted ApoE peptide did not promote liposome fusion and did not induce bilayer holes but caused slight erosion of the DPPC domains. In conclusion, this work provides the first direct evidence for the production of stable, well-defined nanoholes in lipid bilayer domains by the SIV peptide, a behavior that we have shown to be specifically related to the tilted character of the peptide. A molecular mechanism underlying spontaneous insertion of the SIV peptide within lipid bilayers and the subsequent removal of bilayer patches is proposed, and its relevance to membrane fusion processes is discussed.