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排序方式: 共有181条查询结果,搜索用时 15 毫秒
61.
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63.
Georgieva DN Genov N Nikolov P Aleksiev B Rajashankar KR Voelter W Betzel C 《Spectrochimica acta. Part A, Molecular and biomolecular spectroscopy》2003,59(3):617-627
The neurotoxic complex Vipoxin is the lethal component of the venom of Vipera ammodytes meridionalis, the most toxic snake in Europe. It is a complex between a toxic phospholipase A2 (PLA2) and a non-toxic and catalytically inactive protein, stabilizing the enzyme and reducing the activity and toxicity. Structure-function relationships in this complex were studied by spectroscopic methods. A good correlation between the ionization behaviour and accessible surface area (ASA) of the tyrosyl residues was observed. In the toxic PLA2 subunit phenolic groups participate in H-bonding network that stabilizes the catalytically and pharmacologically active conformation. The tryptophan fluorescence decay of Vipoxin is well fitted by two exponentials with lifetimes of 0.1 (54%) and 2.5 (46%) ns. W20P, W31P and W31I are located in the interface between the two subunits and participate in hydrophobic interactions stabilizing the complex. Dissociation of the complex leads to a transition of the tryptophans from hydrophobic to hydrophilic environment, which influences mainly tau2. The longer lifetime is more sensitive to the polarity of the environment. Circular dichroism measurements demonstrate that the two components of the neurotoxin preserve their secondary structure after dissociation of the complex. The results of the spectroscopic studies are in accordance with a mechanism of blockade of transmission across the neuromuscular junctions of the breathing muscles by interaction of a dissociated toxic PLA2 with a membrane. The loss of toxicity is connected with slight changes in the secondary structure of PLA2. CD studies also show a substantial contribution of disulfide bonds to the stability of the neurotoxic complex and its components. 相似文献
64.
The acid—base equilibria in the mixed solvent, 80% dimethyl sulfoxide—water, have been investigated by potentiometric measurements with a glass—silver—silver chloride electrode couple. The response of the glass electrode is quick and reproducible over the whole pH scale. The autoprotolysis constant (itKSH) of the mixed solvent has been determined; the value obtained, pKSH = 18.40, agrees well with reported data. The pKa values of the following aliphatic monocarboxylic acids are reported: formic, acetic, propionic, n-butyric, iso-butyric, n-valeric, iso-valeric, capric, pentadecanoic, stearic, monochloroacetic. The mixed solvent offers better titration conditions than water for the determination of these acids. 相似文献
65.
The acid—base constants of ten nitrophenol, sulfonphthalein and phthalein indicators in 80% dimethyl sulfoxide—20% water are reported. The indicators were selected so that their transitional intervals cover a large part of the pH-scale. The relationship between the constants in the mixed solvent and in water is linear: (pKHI)SH = 1.48 (pKHI)H2O— 1.68. 相似文献
66.
Some drawbacks of the phosphate modifier such as reagent blank contribution and background absorption in electrothermal atomic absorption spectrometric determination of cadmium and lead are substantially alleviated by application of small amounts of phosphate, approximately 0.2 mumol (25 mug of NH(4)H(2)PO(4) or (NH(4))(2)HPO(4)), on the integrated platform of transversely heated graphite atomizer pre-treated with 2.7 mumol of Zr (250 mug) or W (500 mug) and 0.1 mumol of Ir (20 mug). Pyrolysis temperatures for Cd and Pb are up to 900 and 1100 degrees C for aqueous solutions and within 400-600 degrees C (Cd) and 750-850 degrees C (Pb) for biological fluids (urine, blood) and tissues (hair, liver, muscle) solubilized with tetraethylammonium hydroxide. The thermally stabilized phosphate on Zr-Ir or W-Ir treated platforms serves as a permanent modifier in analyses of environmental waters by multiple hot injections of large sample aliquots. Applications to water and biological certified reference materials are tabulated and show good agreement with certified values. Characteristic masses are 0.7-1.0 pg for Cd and 26-31 pg for Pb. 相似文献
67.
The kinetics of thermal decomposition of 13 selenites has been studied under isothermal heating conditions with a derivatographer. Values of the activation energy and frequency factor in the Arrenius equation are found. It is found that between these parameters and the radius of selenite cations a close relationship exists: the greater the cation radius, the higher the thermal stability of selenites. The obtained results of the change in these parameters are interpreted by the theory of polarization interactions between ions and by the general Klopman and Hudson’s perturbation theory for ionic orbitals. 相似文献
68.
Georgieva R Moya S Donath E Bäumler H 《Langmuir : the ACS journal of surfaces and colloids》2004,20(5):1895-1900
The permeability of ions and small polar molecules through polyelectrolyte multilayer capsules templated on red blood cells was studied by means of confocal microscopy and electrorotation. Capsules were obtained by removing the cell after polyelectrolyte multilayer formation by means of NaOCl treatment. This procedure results in cross-linking of poly(allylamine hydrochloride) (PAH) molecules and destroying poly(styrene sulfonate) (PSS) within the multilayer. Capsules are obtained being remarkably different from layer-by-layer (LbL) capsules. These capsules are rather permeable for low as well as for high molecular weight species. However, upon adsorption of extra polyelectrolyte layers the permeability decreased remarkably. The assembly of six supplementary layers of PAH and PSS rendered the capsule almost impermeable for fluorescein. Resealing by supplementary layers is a potential means for filling and release control. By means of electrorotation measurements, it was shown that the capsule walls obtained isolating properties in electrolyte solutions. Conclusions are drawn concerning the mechanism of permeability through cell templated polyelectrolyte multilayer capsules. 相似文献
69.
L. T. Vlaev V. G. Georgieva S. D. Genieva 《Journal of Thermal Analysis and Calorimetry》2007,88(3):805-812
The kinetics of dehydration and decomposition of VOSO4·2H2O, VOSO4 and VOSeO3·H2O was studied under non-isothermal heating on a derivatograph. The stages and products of the thermal decomposition were determined.
It was proved that VOSO4·2H2O decomposes to V2O5 while VOSeO3·H2O − to V2O4. A number of kinetic models and calculation procedures were used to determine the values of the kinetic parameters characterizing
the process. The parameters calculated were compared and analyzed. IR-spectra of the initial substances and the solid residue
after decomposition are presented. 相似文献
70.
Adsorptive accumulation of the Pd(II) complex with dimethylglyoxime was evaluated for stripping voltammetry with respect
to different parameters. The sensitivity of the method and the linearity between the peak current and the concentration of
Pd(II) depends on the ionic strength, the electrode area, the preconcentration time, the transport rate to the electrode,
and the potential scan rate. The most appropriate medium was 0.1 mol/L acetate buffer between pH 3.5 and 4. Using 2 min of
preconcentration at a 2.6 mm2 electrode and the differential pulse mode, a detection limit of 0.05 μg/L Pd was achieved for liquid samples and 50 ng/g
for solid samples. Different aqueous and solid samples were analysed and the recovery from biological and inorganic materials
investigated.
Received: 29 February 1996/Revised: 29 July 1996/Accepted: 1 August 1996 相似文献