全文获取类型
收费全文 | 1979篇 |
免费 | 27篇 |
国内免费 | 53篇 |
出版年
2023年 | 15篇 |
2022年 | 6篇 |
2021年 | 17篇 |
2020年 | 24篇 |
2019年 | 17篇 |
2018年 | 15篇 |
2017年 | 15篇 |
2016年 | 25篇 |
2015年 | 36篇 |
2014年 | 58篇 |
2013年 | 82篇 |
2012年 | 39篇 |
2011年 | 183篇 |
2010年 | 112篇 |
2009年 | 203篇 |
2008年 | 159篇 |
2007年 | 176篇 |
2006年 | 108篇 |
2005年 | 126篇 |
2004年 | 121篇 |
2003年 | 75篇 |
2002年 | 64篇 |
2001年 | 31篇 |
2000年 | 25篇 |
1999年 | 32篇 |
1998年 | 27篇 |
1997年 | 32篇 |
1996年 | 18篇 |
1995年 | 22篇 |
1994年 | 10篇 |
1993年 | 24篇 |
1992年 | 9篇 |
1991年 | 13篇 |
1990年 | 19篇 |
1989年 | 8篇 |
1988年 | 9篇 |
1987年 | 10篇 |
1986年 | 11篇 |
1985年 | 9篇 |
1984年 | 11篇 |
1983年 | 13篇 |
1982年 | 15篇 |
1981年 | 11篇 |
1980年 | 16篇 |
1979年 | 8篇 |
排序方式: 共有2059条查询结果,搜索用时 15 毫秒
971.
Alessio Terenzi Antonio Palumbo Piccionello Annalisa Guarcello 《Inorganica chimica acta》2011,373(1):62-67
The synthesis and structural characterization of NiII, CuII and ZnII complexes of two chelating 1,2,4-oxadiazole ligands, namely 3,5-bis(2′-pyridyl)-1,2,4-oxadiazole (bipyOXA) and 3-(2′-pyridyl)5-(phenyl)-1,2,4-oxadiazole (pyOXA), is here reported. The formed hexacoordinated metal complexes are [M(bipyOXA)2(H2O)2](ClO4)2 and [M(pyOXA)2(ClO4)2], respectively (M = Ni, Cu, Zn). X-ray crystallography, 1H and 13C NMR spectroscopy and C, N, H elemental analysis data concord in attributing them an octahedral coordination geometry. The two coordinated pyOXA ligands assume a trans coplanar disposition, while the two bipyOXA ligands are not. The latter result is a possible consequence of the formation of H-bonds between the coordinated water molecules and the nitrogen atom of the pyridine in position 5 of the oxadiazole ring. The expected splitting of the d metal orbitals in an octahedral ligand field explains the observed paramagnetism of the d8 and d9 electron configuration of the nickel(II) and copper(II) complexes, respectively, as determined by the broadening of their NMR spectra. 相似文献
972.
Palanisamy Uma Maheswari Manuel Quesada Francesco Buda Anthony L. Spek Jan Reedijk 《Inorganica chimica acta》2011,374(1):406-414
A square-planar compound [Cu(pyrimol)Cl] (pyrimol = 4-methyl-2-N-(2-pyridylmethylene)aminophenolate) abbreviated as CuL-Cl) is described as a biomimetic model of the enzyme galactose oxidase (GOase). This copper(II) compound is capable of stoichiometric aerobic oxidation of activated primary alcohols in acetonitrile/water to the corresponding aldehydes. It can be obtained either from Hpyrimol (HL) or its reduced/hydrogenated form Hpyramol (4-methyl-2-N-(2-pyridylmethyl)aminophenol; H2L) readily converting to pyrimol (L−) on coordination to the copper(II) ion. Crystalline CuL-Cl and its bromide derivative exhibit a perfect square-planar geometry with Cu-O(phenolate) bond lengths of 1.944(2) and 1.938(2) Å. The cyclic voltammogram of CuL-Cl exhibits an irreversible anodic wave at +0.50 and +0.57 V versus ferrocene/ferrocenium (Fc/Fc+) in dry dichloromethane and acetonitrile, respectively, corresponding to oxidation of the phenolate ligand to the corresponding phenoxyl radical. In the strongly donating acetonitrile the oxidation path involves reversible solvent coordination at the Cu(II) centre. The presence of the dominant CuII-L chromophore in the electrochemically and chemically oxidised species is evident from a new fairly intense electronic absorption at 400-480 nm ascribed to a several electronic transitions having a mixed π → π∗(L) intraligand and Cu-Cl → L charge transfer character. The EPR signal of CuL-Cl disappears on oxidation due to strong intramolecular antiferromagnetic exchange coupling between the phenoxyl radical ligand (L) and the copper(II) centre, giving rise to a singlet ground state (S = 0). The key step in the mechanism of the primary alcohol oxidation by CuL-Cl is probably the α-hydrogen abstraction from the equatorially bound alcoholate by the phenoxyl moiety in the oxidised pyrimol ligand, Cu-L, through a five-membered cyclic transition state. 相似文献
973.
J. Cambrollé E. Mateos-NaranjoS. Redondo-Gómez T. LuqueM.E. Figueroa 《Environmental and Experimental Botany》2011,71(1):57-64
Glaucium flavum Crantz. is found in an anthropized coastal grassland at the joint estuary of the Tinto and Odiel rivers (SW Spain), growing under the influence of high levels of copper contamination derived from nearby petrochemical industries, with no obvious adverse affects on the performance of the plant. In addition, this species exhibits a series of ecological characteristics which may render it appropriate for use in the phytoremediation of contaminated areas. Nonetheless, the response of G. flavum to elevated copper concentrations has not been studied. A greenhouse experiment was conducted to investigate the effects of a range of Cu concentrations (0 to 47 mmol l−1) on the growth, reproduction and photosynthetic performance of G. flavum, by measuring relative growth rate, fruit and seed production, chlorophyll fluorescence parameters, gas exchange and photosynthetic pigment concentrations. We also determined total copper, nitrogen, phosphorous, sulphur, calcium and magnesium concentrations. G. flavum survived with concentrations of up to 730 mg Cu kg−1 DW in the leaves, when treated with 30 mmol Cu l−1 (2000 mg l−1). Quantum efficiency of PSII, net photosynthesis rate, as well as leaf Ca and Mg concentrations were all negatively affected by Cu concentrations greater than 9 mmol l−1 in the nutrient solution. Our results indicate that the reduction in photosynthetic performance may be attributed to the adverse effect of excess Cu on the photosynthetic apparatus of the plant, both directly, via a decrease in pigment concentrations, and indirectly, via interference of Cu with Ca ions of PSII. Growth and seed production were only slightly affected by leaf tissue concentrations as high as 230 mg Cu kg−1 dry mass, which suggests that this species could play an important role in phytoremediation of Cu-contaminated soils. 相似文献
974.
Emanuela VurroRoberta Ruotolo Simone OttonelloLisa Elviri Monica MaffiniGiuseppina Falasca Letizia ZanellaMaria Maddalena Altamura Luigi Sanità di Toppi 《Environmental and Experimental Botany》2011,72(1):26-33
Cuscuta sp., known with the common name of “dodder”, is an obligate parasite capable of invading stems and leaves of a wide range of host plants. Dodder stem usually coils counterclockwise around the host and, within a few days, develops haustorial structures at each point of contact. As soon as dodder haustoria reach host vascular bundles, they start tapping water, photosynthates and minerals. Metal ions such as zinc (Zn) and copper (Cu) are essential for dodder growth and metabolism, although an exceedingly high (over-homeostatic) supply of these micronutrients can result in growth inhibition and cellular toxicity. Even more so, non-essential metals such as cadmium (Cd), if transferred from the host to the parasite, need to be neutralized by timely detoxification mechanisms. In this work, we showed that Cuscuta campestris Yuncker establishes effective haustorial connections with leaf petioles and blades of Daucus carota L. (carrot), with the consequent transfer of Cd and essential metals (such as Zn and Cu) from the host vascular bundles to the parasite. Following up to this point, we detected the presence in the parasite of significant amounts of glutathione and phytochelatins, even in the absence of Cd exposure. This suggests that thiol peptides in dodder might be particularly important for Zn and Cu homeostasis as well as for Cd detoxification. Finally, we demonstrated that dodder is capable of synthesizing phytochelatins on its own, rather than massively importing them from the host, and also provided evidence for the existence of an endogenous, constitutively expressed, dodder's phytochelatin synthase. 相似文献
975.
Lodyga-Chruścińska E Ołdziej S Sochacka E Korzycka K Chruściński L Micera G Sanna D Turek M Pawlak J 《Journal of inorganic biochemistry》2011,105(9):1212-1219
Copper(II) complexes with a new chelator-type nucleoside-histidine modified 2′-deoxyriboadenosine (N-[(9-β-D-2′-deoxyribofuranosylpurin-6-yl)-carbamoyl]histidine) were studied by potentiometric and spectroscopic (UV-visible, CD, EPR) techniques, in conjunction with computer modeling optimization. The ligand can act as bidentate or tridentate depending on pH range. In acidic pH a very stable dimeric complex Cu2L2 predominates with coordination spheres of both metal ions composed of oxygen atoms from carboxylic groups, one oxygen atom from ureido group and two nitrogen atoms derived from purine base and histidine ring. Above pH 5, deprotonation of carbamoyl nitrogens leads to the formation of CuL2, Cu2L2H− 1 and Cu2L2H− 2 species. The CuL2H− 1 and CuL2H− 2 complexes with three or four nitrogens in Cu(II) coordination sphere have been detected in alkaline medium.Our findings suggest that N-[(9-beta-D-2′-deoxyribofuranosylpurin-6-yl)-carbamoyl]histidine chelates copper(II) ions very efficiently. The resulting complex might be used as an alternative base-pairing mode in which hydrogen-bonded base pairs present in natural DNA are replaced by metal-mediated ones. 相似文献
976.
Qiao X Ma ZY Xie CZ Xue F Zhang YW Xu JY Qiang ZY Lou JS Chen GJ Yan SP 《Journal of inorganic biochemistry》2011,105(5):728-737
A new cytotoxic copper(II) complex with Schiff base ligand [CuII(5-Cl-pap)(OAc)(H2O)]·2H2O (1) (5-Cl-pap = N-2-pyridiylmethylidene-2-hydroxy-5-chloro-phenylamine), was synthesized and structurally characterized by X-ray diffraction. Single-crystal analysis revealed that the copper atom shows a 4 + 1 pyramidal coordination, a water oxygen appears in the apical position, and three of the basal positions are occupied by the NNO tridentate ligand and the fourth by an acetate oxygen. The interaction of Schiff base copper(II) complex 1 with DNA was investigated by UV-visible spectra, fluorescence spectra and agarose gel electrophoresis. The apparent binding constant (Kapp) value of 6.40 × 105 M− 1 for 1 with DNA suggests moderate intercalative binding mode. This copper(II) complex displayed efficient oxidative cleavage of supercoiled DNA, which might indicate that the underlying mechanism involve hydroxyl radical, singlet oxygen-like species, and hydrogen peroxide as reactive oxygen species. In addition, our present work showed the antitumor effect of 1 on cell cycle and apoptosis. Flow cytometric analysis revealed that HeLa cells were arrested in the S phase after treatment with 1. Fluorescence microscopic observation indicated that complex 1 can induce apoptosis of HeLa cells, whose process was mediated by intrinsic mitochondrial apoptotic pathway owing to the activation of caspase-9 and caspase-3. 相似文献
977.
Leovac VM Bogdanović GA Jovanović LS Joksović L Marković V Joksović MD Denčić SM Isaković A Marković I Heinemann FW Trifunović S Đalović I 《Journal of inorganic biochemistry》2011,105(11):1413-1421
New polymeric copper(II) complexes with two tridentate ONS thiosemicarbazone ligands containing substituted pyrazolone moiety were synthesized and characterized by means of spectroscopic, electrochemical and crystallographic techniques. While both ligands exist as different tautomers in the solid state and DMSO-d6 solution, Cu(II) ion coordinates the ligands from the same tautomeric form with square-pyramidal geometry around each Cu atom. In the crystal structures, the copper(II) complex cation forms polymeric chains {[Cu(L)Cl]+}n with a bridging chlorine atom. One of the complexes was found to have a significantly higher cytotoxic potential in comparison with cisplatin in inhibition of several cell lines (HL60, REH, C6, L929 and B16). The results obtained on the basis of flow cytometry indicated that apoptosis could be possible mechanism of cell death. 相似文献
978.
979.
Balasubramanian R Kenney GE Rosenzweig AC 《The Journal of biological chemistry》2011,286(43):37313-37319
Methanobactin (Mb), a 1217-Da copper chelator produced by the methanotroph Methylosinus trichosporium OB3b, is hypothesized to mediate copper acquisition from the environment, particularly from insoluble copper mineral sources. Although indirect evidence suggests that Mb provides copper for the regulation and activity of methane monooxygenase enzymes, experimental data for direct uptake of copper loaded Mb (Cu-Mb) are lacking. Uptake of intact Cu-Mb by M. trichosporium OB3b was demonstrated by isotopic and fluorescent labeling experiments. Confocal microscopy data indicate that Cu-Mb is localized in the cytoplasm. Both Cu-Mb and unchelated Cu are taken up by M. trichosporium OB3b, but by different mechanisms. Uptake of unchelated Cu is inhibited by spermine, suggesting a porin-dependent passive transport process. By contrast, uptake of Cu-Mb is inhibited by the uncoupling agents carbonyl cyanide m-chlorophenylhydrazone and methylamine, but not by spermine, consistent with an active transport process. Cu-Mb from M. trichosporium OB3b can also be internalized by other strains of methanotroph, but not by Escherichia coli, suggesting that Cu-Mb uptake is specific to methanotrophic bacteria. These findings are consistent with a key role for Cu-Mb in copper acquisition by methanotrophs and have important implications for further investigation of the copper uptake machinery. 相似文献
980.
Adsorption of copper and zinc by biochars produced from pyrolysis of hardwood and corn straw in aqueous solution 总被引:19,自引:0,他引:19
Xincai Chen Guangcun ChenLinggui Chen Yingxu ChenJohannes Lehmann Murray B. McBride Anthony G. Hay 《Bioresource technology》2011,102(19):8877-8884
Biochars produced by pyrolysis of hardwood at 450 °C (HW450) and corn straw at 600 °C (CS600) were characterized and investigated as adsorbents for the removal of Cu(II) and Zn(II) from aqueous solution. The adsorption data were well described by a Langmuir isotherm, with maximum Cu(II) and Zn(II) adsorption capacities of 12.52 and 11.0 mg/g for CS600, 6.79 and 4.54 mg/g for HW450, respectively. Thermodynamic analysis suggested that the adsorption was an endothermic process and did not occur spontaneously. Although Cu(II) adsorption was only marginally affected by Zn(II), Cu(II) competed with Zn(II) for binding sites at Cu(II) and Zn(II) concentrations ?1.0 mM. Results from this study indicated that plant-residue or agricultural waste derived biochar can act as effective surface sorbent, but their ability to treat mixed waste streams needs to be carefully evaluated on an individual basis. 相似文献