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991.
Discontinuous molecular dynamics simulations, together with the protein intermediate resolution model, an intermediate-resolution model of proteins, are used to carry out several microsecond-long simulations and study folding transition and stability of alpha-de novo-designed proteins in slit nanopores. Both attractive and repulsive interaction potentials between the proteins and the pore walls are considered. Near the folding temperature T(f) and in the presence of the attractive potential, the proteins undergo a repeating sequence of folding/partially folding/unfolding transitions, with T(f) decreasing with decreasing pore sizes. The unfolded states may even be completely adsorbed on the pore's walls with a negative potential energy. In such pores the energetic effects dominate the entropic effects. As a result, the unfolded state is stabilized, with a folding temperature T(f) which is lower than its value in the bulk and that, compared with the bulk, the folding rate decreases. The opposite is true in the presence of a repulsive interaction potential between the proteins and the walls. Moreover, for short proteins in very tight pores with attractive walls, there exists an unfolded state with only one alpha-helical hydrogen bond and an energy nearly equal to that of the folded state. The proteins have, however, high entropies, implying that they cannot fold onto their native structure, whereas in the presence of repulsive walls the proteins do attain their native structure. There is a pronounced asymmetry between the two termini of the protein with respect to their interaction with the pore walls. The effect of a variety of factors, including the pore size and the proteins' length, as well as the temperature, is studied in detail. 相似文献
992.
Muhammad N. Akhtar Muhammad Atiqullah Akhlaq A. Moman Atieh H. Abu‐Raqabah Neaz Ahmed 《大分子反应工程》2008,2(4):339-349
This study investigated the effects of several organotin silica surface modifiers on the ethylene polymerization performance of (nBuCp)2ZrCl2‐based supported catalysts in which MAO and metallocene were sequentially loaded. Each organotin compound acted as a spacer, increasing the catalyst activity. However, the catalyst activity and of the resulting polyethylenes varied as follows: Activity and fractional Sn+ charge: nBuSn(OH)2Cl > MeSnCl3 > nBuSnCl3 > Reference catalyst; and, : Reference catalyst > nBuSnCl3 > MeSnCl3 > nBuSn(OH)2Cl. The above catalyst activity rating was explained considering the influence of the Lewis acidity, that is, the fractional Sn+ charge of the organotin modifiers on the generation, concentration, and electron density at the active [(nBuCp)2ZrMe]+ cation. All the catalysts showed fairly stable kinetic profiles and produced narrow molecular weight distribution resins; 2.8 ≤ PDI ≤ 3.
993.
Sonia Zulfiqar Muhammad Ilyas Sarwar 《Journal of inclusion phenomena and macrocyclic chemistry》2008,62(3-4):353-361
Aramid–organoclay nanocomposites were fabricated through solution intercalation technique. Montmorillonite was modified with p-amino benzoic acid in order to have compatibility with the matrix. The effect of clay dispersion and the interaction between clay and polyamide chains on the properties of nanocomposites were investigated using X-ray diffraction (XRD), transmission electron microscopy (TEM), tensile testing of thin films, differential scanning calorimetry (DSC), thermogravimetric analysis (TGA) and water uptake measurements. Excessive clay dispersion was achieved even on the addition of high proportions of clay. The structural investigations confirmed the formation of delaminated nanostructures at low clay contents and disordered intercalated morphology at higher clay loadings. The tensile behavior and thermal stability significantly amplified while permeability reduced with increasing dispersibility of organoclay in the polyamide matrix. 相似文献
994.
Saima Shabbir Sonia Zulfiqar Muhammad Ishaq Muhammad Ilyas Sarwar 《Colloid and polymer science》2008,286(6-7):673-681
Blends containing PVC and aramid (Ar) matrices were probed for their miscibility. In this respect, Ar chains were synthesized by aromatic diamine and diacid chloride in amide solvent. The Ar thus synthesized was characterized through Fourier transform infrared (FTIR) spectroscopy and molecular weight determination. Blend system Ar/PVC was investigated over a range of Ar/PVC ratios. Their mechanical profiles in terms of maximum stress, maximum strain, toughness, and initial moduli have been explored. Thermal properties and morphology of the blends were estimated using thermogravimetric analysis (TGA), differential scanning calorimetry (DSC), and scanning electron microscopy (SEM). A good correlation was observed between thermal, mechanical, and morphological properties of the blends. The presence of hydrogen bonding among polymers was evaluated through FTIR spectroscopy, which is believed to be responsible for the blend miscibility. Optimal thermal and mechanical profiles were depicted by the blend containing 40-wt% PVC in the Ar matrix. 相似文献
995.
Syed Qaiser Shah Muhammad Rafiullah Khan 《Journal of Radioanalytical and Nuclear Chemistry》2011,288(2):485-490
The uranium ingot casting process is one of the steps which consolidate uranium deposits produced by electrorefiner in an
ingot form in a pryprocessing technique. Since molten uranium metal reacts with a graphite crucible when the uranium is being
dissolved, a graphite crucible cannot be used. Accordingly, a ceramic material must be selected which does not react with
the dissolving uranium and this must be used as a coating material on the graphite crucible surface. As to this research,
a reactivity experiments were performed between the coating layer and uranium by applying a thermal spray coating to the graphite
material with alumina and YSZ ceramic material. As shown in the experimental result, the YSZ coating layer showed a stronger
adhesive property on the side where there is no Ni–Al binding material. Moreover, no reaction was apparent between the YSZ
coating layer and the uranium. Accordingly, the YSZ material and the process of thermal spray coating are considered to solve
the reactive problem between uranium and a graphite crucible. 相似文献
996.
Abdul Shakoor Tasneem Zahra Rizvi Hafiz Umer Farooq Najmul Hassan Abdul Majid Muhammad Saeed 《Polymer Science Series B》2011,53(9-10):540-545
Polypyrrole was synthesized and doped with dodecylbenzenesulphonic acid; the latter was confirmed by means of FTIR spectroscopy. The percentage of crystallinity of synthesized polymers was estimated from X-rays diffraction studies. The formation of flaky structure in doped polypyrrole was observed by means of SEM. DC conductivity was found to be influenced by dopant dodecylbenzenesulphonic acid. Temperature dependant DC shows three dimensional variable ranges hopping (3D VRH). Activation energy, density of states and hopping length were calculated and found to be influenced by adding dopant to polypyrrole. The doped polypyrrole was found to be more thermally stable as compared to that of pristine polypyrrole. 相似文献
997.
A new compound, jolynamine (1), was isolated from the marine brown alga Jolyna laminarioides collected from the coast of Karachi, Pakistan. In addition, four known compounds, namely saringosterol (2), loliolide (3), methyl-4-hydroxybenzoate (4) and propyl-4-hydroxybenzoate (5), were isolated for the first time from the marine brown alga Iyengaria stellata, and two known compounds, namely 3,4,5-trimethylaniline (6) and harmine (7), were isolated for the first time from the marine brown alga Melanothamnus afaqhusainii. Compound 6 is synthetically known but was isolated for the first time from a natural source. The structures of these compounds were elucidated with the help of powerful spectroscopic techniques. Furthermore, the methanolic extracts of both algae showed anti-microbial activities against various bacteria and fungi. 相似文献
998.
Leflunomide is a leading drug for the treatment of rheumatoid arthritis. The principle aim of this study was to develop and validate an RP‐HPLC method for the determination of leflunomide in bulk and pharmaceutical dosage form using diclofenac sodium as an internal standard. For this purpose, chromatography was accomplished on a Purospher Start, C18 (5 (m, 12.5 cm×0.46 cm) column at ambient temperature. Methanol:water (80:20, V/V) solvent system was selected as mobile phase, the pH of which was adjusted to 3.4 by ortho‐phosphoric acid and delivered at a flow rate of 1.2 mL·min−1. Seperation of leflunomide and diclofenac sodium was carried out on a Purospher Start, C18 equipped with a UV‐visible detector at 248 nm. The suitability of the method for the quantitative determination of the drugs is proven by validation in accordance with the requirements laid down by the International Conference on Harmonization (ICH) guidelines. The method was accurate (99.55%–100.03%), specific, linear (R2>0.999) and precise (intra‐day precision 0.023%–0.93% and inter‐day precision 0.26%–0.944%) in the range of 0.5–20 (g·mL−1. The minimum limit of detection and quantification in pharmaceutical formulation were 0.05 and 0.15 (g·mL−1, respectively. Thus the proposed method is simple, accurate, reproducible and suitable for the routine analysis of leflunomide in pharmaceutical formulations and was applied to study in vitro drug‐metal interactions. 相似文献
999.
Mohammed Shahid Ali Aamer Roshanali Khatri Muhammad Imran Munir Mohsin Ghori 《Chromatographia》2009,70(3-4):539-544
A stability-indicating hydrophilic interaction liquid chromatography (HILIC) method has been developed and validated for the quantitative determination of Brimonidine tartrate (BT) formulated as an ophthalmic solution. Isocratic separation was achieved using an acetonitrile-buffer mixture (92:8, v/v) at pH 7.1 on an unmodified silica column (250 × 4.6 mm, 5 μm). The drug was subjected to oxidative, hydrolytic, photolytic and thermal stress conditions and complete separation was achieved for the parent compound and degradation products. The influence of acetonitrile, pH and ionic strength of the buffer was studied. Linearity range and recoveries for BT were 100–400 μg mL?1 and 100.12%, respectively. The method was validated for BT and indicated that the method was sufficiently sensitive with a limit of detection at 0.005 μg mL?1 and a limit of quantitation at 0.02 μg mL?1, respectively. 相似文献
1000.
Abdul Niaz Jan Fischer Jiří Barek Bogdan Yosypchuk Sirajuddin Muhammad Iqbal Bhanger 《Electroanalysis》2009,21(16):1786-1791
A differential pulse voltammetric (DPV) method was developed for the determination of 4‐nitrophenol (4‐NP) at a newly developed silver amalgam paste electrode (AgA‐PE) in Britton–Robinson buffer pH 3.0. The electrode is based on a disposable plastic pipette tip filled with paste amalgam based on a mixture of mercury and fine silver powder (9 : 1, w/w). The experimental parameters, such as pH of Britton–Robinson buffer and activation and regeneration potential of the electrode surface were optimized. The reduction peak current dependences were linear for the concentration of 4‐NP from 0.2 to 100 μM. The method showed reproducible results with RSD (n=45) of 1.7%. The limit of determination (LOD) was 0.3 μM. The method was successfully applied for the direct determination of 4‐NP in drinking water. 相似文献