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981.
Chamaegigas intrepidus Dinter is a poikilohydric aquatic plant that lives in rock pools on granite outcrops in central Namibia. The pools are filled
with water only intermittently during the wet season, and the plants may pass through up to 20 rehydration/dehydration cycles
during the summer rains. The potential nitrogen sources for the rehydrated plants are ammonium, which is only present at 10–20
μm, amino acids, particularly glycine, and urea, which is generally present at 20–30 μm. We show that urea can be utilised by plants in the field through the presence of urease in the sediments of the rock pools.
Urease activity is higher in non-submerged than in submerged sediments, and it can survive 6 months of complete dryness at
temperatures up to 60°C. Experiments with [14C]urea under laboratory conditions show that the roots of C. intrepidus are unable to take up urea; while 15N-nuclear magnetic resonance experiments show that [15N]urea is only metabolised to labelled glutamine and glutamate after ammonium has been released by the action of urease. Thus
urease plays a vital role in allowing urea to be utilised as a major N source in this nutrient-limited aquatic ecosystem.
Received: 23 April 1999 / Accepted: 8 November 1999 相似文献
982.
Mária Bučková Gabriela Vizárová Alexandra Šimonovičová Mária Chalanyová 《Acta Physiologiae Plantarum》2000,22(2):179-184
The typical soil micromycetes Aspergillus niger and Cladosporium cladosporioides from the family moniliaceae were investigated with emphasis on production of ABA into the culture medium. The both fungi
were cultivated in a static liquid Czapek — Dox medium and agar Czapek — Dox medium. Aspergillus niger and Cladosporium cladosporioides showed ability to produce ABA. Analytical detection of ABA from the culture medium was performed by TLC combinated with biotest
and HPLC with spectroscopy. 相似文献
983.
日本续断中新五糖皂甙的结构及其核磁共振光谱 总被引:1,自引:0,他引:1
从日本续断(Dipsacus japonicus Miq.)根的乙醇提取物中分得一个新的三萜皂甙,其结构被鉴定为3.-O-〖β-D-吡喃葡萄糖(1→)〗〖α-吡喃鼠李糖(1→3)〗-β-D-吡喃葡萄39糖(1→3)-α-L吡喃鼠李糖(1→2)-α-吡喃阿拉伯糖-齐墩酸。采用一维SEMDY谱和转坐标NOE差谱核磁共振新技术相结合的方法,对糖体间和9糖体与甙元间的连接顺连接位置进行了研究。该方法的结果 相似文献
984.
M. Giorgetti I. Ascone M. Berrettoni P. Conti S. Zamponi R. Marassi 《Journal of biological inorganic chemistry》2000,5(2):156-166
An in situ X-ray absorption spectroscopy (XAS) spectroelectrochemical study of aquocobalamin (system B12a-B12r-B12s) has been carried out in aqueous solutions buffered at different pH values. To the best of our knowledge, this is the first
structural study of aquocobalamin at room temperature under controlled oxidation conditions. Most of the previous work was
in fact performed using frozen samples chemically treated to produce the species. The spectroelectrochemical approach offers
several advantages: (1) the reduction products may be studied without poisoning the system with chemical reductive reagents
and (2) any possible variation of the oxidation state owing to the electrons produced by the incident beam is avoided as the
electrode, under potentiostatic control, acts as a scavenger. The spectroelectrochemical approach, together with more careful
data analysis, has led to an improved interpretation of the XAS data. These conditions were not met in previous works where
the oxidation state was not controlled and multiple scattering contributions were not taken into account. The general shape
of the XAS spectra of the different species is not greatly affected by pH. A signature for the base-off square-planar coordination
has been evidenced for the Co(II) compound at basic pH. A new signature for Co(I), indicating square-planar coordination,
has been identified on the experimental spectra and simulated in theoretical X-ray absorption near-edge structure (XANES)
studies. The flexibility of the electrochemical approach, that permits to unambiguously establish the formal oxidation state,
has led to very reliable values for energy shift and peak intensity variations. The experimental XANES and extended X-ray
absorption fine structure (EXAFS) spectra with a very good signal-to-noise ratio have been processed using the GNXAS package
that takes into account multiple scattering contributions. EXAFS and XANES independent analysis result in the same structural
model. The reduction from Co(III) to Co(II) produces the most significant structural changes: the cobalt coordination number
decreases from six to five, and the edge position shifts by 2.4±0.3 eV. In addition, the XANES spectra are strongly modified.
The reduction from Co(II) to Co(I) produces mainly electronic effects with no apparent change of the coordination number.
A discussion of the limits and potentialities of EXAFS in this type of study has also been included.
Received: 26 July 1999 / Accepted: 22 October 1999 相似文献
985.
R. G. Ovodova V. V. Golovchenko A. S. Shashkov S. V. Popov Yu. S. Ovodov 《Russian Journal of Bioorganic Chemistry》2000,26(10):669-676
A pectic polysaccharide, lemnan, was isolated from freshly collected duckweedLemna minor L. Its sugar chain was shown to be mainly composed of the residues ofD-galacturonic acid (64%), galactose, arabinose, xylose, andD-apiose, a branched chain sugar. The high content ofD-apiose (25%) indicated that lemnan is an apiogalacturonan type pectin similar to zosteran, a pectic polysaccharide from a
sea phanerogam of the Zosteraceae family. The results of partial acidic hydrolysis, pectinase digestion, and NMR studies of
lemnan demonstrated that its macromolecule contains regions of the linear α-1,4-D-galacturonan and branched apiogalacturonan. The side chains of apiogalacturonan were found to be formed of single and 1,5-linked
residues ofD-apiofuranose attached to 2- and 3-positions of theD-galacturonic acid residues of the apiogalacturonan backbone. Lemnan was shown to exhibit an immunomodulatory effect activating
the system of phagocytosis. 相似文献
986.
Using surface-enhanced infrared absorption (SEIRA) spectroscopy of dry films of colloidal gold (CG) bioconjugates with protein A, it is shown that certain characteristic bands of the protein (e.g., amide I, amide II and some other vibration modes) are essentially affected by the metal surface. Thus, the method may be used for controlling the quality of such bioconjugates. Moreover, it is demonstrated that the biospecific reaction of protein A attached to CG particles with human immunoglobulin G (IgG) results in further essential changes in SEIRA spectra, providing a means for an easy and rapid IR spectroscopic detection of biospecific immunochemical interactions (i.e., spectroimmunochemistry). The results obtained can form a basis for developing test systems for detecting various biospecific interactions. 相似文献
987.
Krishnendu Pal 《Channels (Austin, Tex.)》2016,10(6):478-497
Inactivation path of voltage gated sodium channel has been studied here under various voltage protocols as it is the main governing factor for the periodic occurrence and shape of the action potential. These voltage protocols actually serve as non-equilibrium response spectroscopic tools to study the ion channel in non-equilibrium environment. In contrast to a lot of effort in finding the crystal structure based molecular mechanism of closed-state(CSI) and open-state inactivation(OSI); here our approach is to understand the dynamical characterization of inactivation. The kinetic flux as well as energetic contribution of the closed and open- state inactivation path is compared here for voltage protocols, namely constant, pulsed and oscillating. The non-equilibrium thermodynamic quantities used in response to these voltage protocols serve as improved characterization tools for theoretical understanding which not only agrees with the previously known kinetic measurements but also predict the energetically optimum processes to sustain the auto-regulatory mechanism of action potential and the consequent inactivation steps needed. The time dependent voltage pattern governs the population of the conformational states which when couple with characteristic rate parameters, the CSI and OSI selectivity arise dynamically to control the inactivation path. Using constant, pulsed and continuous oscillating voltage protocols we have shown that during depolarization the OSI path is more favored path of inactivation however, in the hyper-polarized situation the CSI is favored. It is also shown that the re-factorisation of inactivated sodium channel to resting state occurs via CSI path. Here we have shown how the subtle energetic and entropic cost due to the change in the depolarization magnitude determines the optimum path of inactivation. It is shown that an efficient CSI and OSI dynamical profile in principle can characterize the open-state drug blocking phenomena. 相似文献
988.
谭翠燕 徐春和 沈均人 SAKUMA Shinsuke YAMAMOTO Yasushi BALNY Claude 阮康成 《Acta biochimica et biophysica Sinica》2003,(7)
利用荧光光谱学等方法结合高压力技术研究了光合作用系统II中的一个外周蛋白——— 2 3kD(以P2 3k表示 )蛋白的去折叠。热力学研究表明 ,在 2 0℃、180MPa(1MPa =10 .0大气压 )可使该蛋白质完全去折叠 ,而在3℃ ,16 0MPa即可使该蛋白质完全去折叠 ,这是迄今为止有关研究中最易被高压力去折叠的一个蛋白质。在2 0℃ ,该蛋白质在常压下去折叠反应的标准自由能与标准体积变化分别为 2 3.4 5kJ mol和 - 15 0 .3ml mol;动力学研究揭示该蛋白质的折叠反应的活化体积ΔV f 为正值 (84 .1ml mol) ,而去折叠反应的活化体积ΔV u 为负值(- 6 6 .2ml mol)。在常压下 ,折叠和去折叠反应的速度常数 (K0f,K0u)分别为 1.87s- 1 和 1.3× 10 - 4s- 1 ,这些结果为解释该蛋白质易被压力去折叠提供了线索 相似文献
989.
Fengyu Zhang Scott H. Silver Nakita K. Noel Florian Ullrich Barry P. Rand Antoine Kahn 《Liver Transplantation》2020,10(26)
In this essay, a case study is presented on the electronic structure of several metal halide perovskites (MHP) using Kelvin probe (KP)‐based surface photovoltage (SPV) measurements and ultraviolet photoemission spectroscopy (UPS) to demonstrate the advantages, but also the pitfalls, of using these techniques to characterize the surfaces of these materials. The first part addresses the loss of halide species from perovskite surfaces upon supragap illumination in vacuum. This has the potential to cause both a long‐term alteration of the sample work function and a modification of the KP tip during SPV measurements. If undetected, this leads to a misinterpretation of the MHP surface potential. The second part illustrates the difficulties in determining the valence band maximum (VBM) of MHP surfaces with UPS and stresses the importance of taking into account the low density of states at the VBM edge. Given this circumstance, specific care must be taken to eliminate measurement artifacts in order to ascertain the presence or absence of low densities of electronic gap states above the VBM. This essay also highlights issues such as film degradation, nonequilibrium situations (e.g., SPV), and satellite emissions, which occur during photoemission spectroscopy. 相似文献
990.
Salvatore Chiantia Grzegorz Chwastek Zaiguo Li Petra Schwille 《生物化学与生物物理学报:生物膜》2008,1778(5):1356-1364
Ceramide-induced alterations in the lateral organization of membrane proteins can be involved in several biological contexts, ranging from apoptosis to viral infections. In order to investigate such alterations in a simple model, we used a combined approach of atomic force microscopy, scanning fluorescence correlation spectroscopy and confocal fluorescence imaging to study the partitioning of different membrane components in sphingomyelin/dioleoyl-phosphatidylcholine/cholesterol/ceramide supported bilayers. Such model membranes exhibit coexistence of liquid-disordered, liquid-ordered (raft-like) and ceramide-rich lipid phases. Our results show that components with poor affinity toward the liquid-ordered phase, such as several fluorescent lipid analogues or the synaptic protein Synaptobrevin 2, are excluded from ceramide-rich domains. Conversely, we show for the first time that the raft-associated protein placental alkaline phosphatase (GPI-PLAP) and the ganglioside GM1 are enriched in such domains, while exhibiting a strong decrease in lateral diffusion. Analogue modulation of the local concentration and dynamics of membrane proteins/receptors by ceramide can be of crucial importance for the biological functions of cell membranes. 相似文献