全文获取类型
收费全文 | 2962篇 |
免费 | 99篇 |
国内免费 | 9篇 |
专业分类
化学 | 1976篇 |
晶体学 | 26篇 |
力学 | 85篇 |
数学 | 368篇 |
物理学 | 615篇 |
出版年
2023年 | 26篇 |
2022年 | 26篇 |
2021年 | 94篇 |
2020年 | 72篇 |
2019年 | 64篇 |
2018年 | 71篇 |
2017年 | 52篇 |
2016年 | 132篇 |
2015年 | 93篇 |
2014年 | 111篇 |
2013年 | 177篇 |
2012年 | 169篇 |
2011年 | 217篇 |
2010年 | 148篇 |
2009年 | 118篇 |
2008年 | 166篇 |
2007年 | 146篇 |
2006年 | 128篇 |
2005年 | 98篇 |
2004年 | 106篇 |
2003年 | 81篇 |
2002年 | 78篇 |
2001年 | 50篇 |
2000年 | 40篇 |
1999年 | 25篇 |
1998年 | 25篇 |
1997年 | 28篇 |
1996年 | 28篇 |
1995年 | 18篇 |
1994年 | 23篇 |
1993年 | 24篇 |
1992年 | 30篇 |
1991年 | 23篇 |
1990年 | 9篇 |
1989年 | 8篇 |
1988年 | 9篇 |
1987年 | 10篇 |
1986年 | 9篇 |
1985年 | 13篇 |
1984年 | 12篇 |
1983年 | 11篇 |
1981年 | 16篇 |
1980年 | 8篇 |
1979年 | 9篇 |
1978年 | 7篇 |
1976年 | 10篇 |
1972年 | 11篇 |
1918年 | 8篇 |
1901年 | 11篇 |
1885年 | 9篇 |
排序方式: 共有3070条查询结果,搜索用时 31 毫秒
21.
Alternatives based on polynomial expansions of gaussian basis set exponents are introduced and evaluated. The formulas presented here outperform methods based upon the even-tempered formula or combinations of it. They closely match the performance of other methods based upon larger polynomial expansions of the logarithm of the exponents using the same or one less parameter per orbital angular symmetry. 相似文献
22.
Ricardo de Souza Pereira Nivaldo AntÔnio Parizotto Vitor Baranauskas 《Applied biochemistry and biotechnology》1996,59(2):135-143
Different strains of baker’s yeast(Saccharomyces cerevisiae) were imaged with an atomic force microscope (AFM). The images of uncoated and nonfixed samples were reproducible with high-constrast
and nanometer-resolution. Molecules from the polysaccharide surface of the cell wall were pictured and the distance of atoms
was measured. The preparation of samples was easy, suggesting that AFM is a useful tool in this type of analyses. 相似文献
23.
G. Carelli N. Ioli A. Moretti D. Pereira F. Strumia R. Densing 《Applied physics. B, Lasers and optics》1988,45(2):97-100
FIR laser lines close to the N+
3
P
2–3
P
1 transition at 122 m can be used as a local oscillator for heterodyne detectors. The frequencies of some candidate laser lines from CH2F2,13CH3OH, CD3OD, and CD3OH have been measured and the output power of the most interesting lines has been optimized. 相似文献
24.
Eliana M. Alhadeff Andrea M. Salgado Nei Pereira Jr. Belkis Valdman 《Applied biochemistry and biotechnology》2004,113(1-3):125-136
An automated flow injection analysis (FIA) system for quantifying ethanol was developed using alcohol oxidase, horseradish
peroxidase, 4-aminophenazone, and phenol. A colorimetric detection method was developed using two different methods of analysis,
with free and immobilized enzymes. The system with free enzymes permitted analysis of standard ethanol solution in a range
of 0.05–1.0 g of ethanol/L without external dilution, a sampling frequency of 15 analyses/h, and relative SD of 3.5%. A new
system was designed consisting of a microreactor with a 0.91-mL internal volume filled with alcohol oxidase immobilized on
glass beads and an addition of free peroxidase, adapted in an FIA line, for continued reuse. This integrated biosensor-FIA
system is being used for quality control of biofuels, gasohol, and hydrated ethanol. The FIA system integrated with the microreactor
showed a calibration curve in the range of 0.05–1.5 g of ethanol/L, and good results were obtained compared with the ethanol
content measured by high-performance liquid chromatography and gas chromatography standard methods. 相似文献
25.
The effect of ohmic and conventional heat processing of different food products on their chemical and physical parameters
was studied. Depending on the food being analysed, parameters such as pH, total solids, ash, titratable acidity, ascorbic
acid, total sugars, total fatty acids, total phenolic compounds, and anthocyanins content were determined before and after
ohmic and conventional pasteurization techniques and the results were compared using one-way analysis of variance. In goat
milk samples treated by ohmic technology the pH value (6.58) and total fatty acids content in milk fat (86.5 mass %) were
comparable to those found in milk treated by conventional process, however, ohmically treated samples presented a lower content
of lactic acid, 0.13 %. In cloudberry jam samples treated by ohmic technology the results of some of the main parameters tested,
such as total sugar content 46.1 mass %, ascorbic acid content 2.83 mass %, and titratable acidity 6.01 mass % (as citric
acid) did not show significant differences when compared with samples treated by conventional technology.
Presented at the 33rd International Conference of the Slovak Society of Chemical Engineering, Tatranské Matliare, 22–26 May
2006. 相似文献
26.
Elaine C. Petronilho Murilo M. Pedrote Mayra A. Marques Yulli M. Passos Michelle F. Mota Benjamin Jakobus Gileno dos Santos de Sousa Filipe Pereira da Costa Adriani L. Felix Giulia D. S. Ferretti Fernando P. Almeida Yraima Cordeiro Tuane C. R. G. Vieira Guilherme A. P. de Oliveira Jerson L. Silva 《Chemical science》2021,12(21):7334
Mutant p53 tends to form aggregates with amyloid properties, especially amyloid oligomers inside the nucleus, which are believed to cause oncogenic gain-of-function (GoF). The mechanism of the formation of the aggregates in the nucleus remains uncertain. The present study demonstrated that the DNA-binding domain of p53 (p53C) underwent phase separation (PS) on the pathway to aggregation under various conditions. p53C phase separated in the presence of the crowding agent polyethylene glycol (PEG). Similarly, mutant p53C (M237I and R249S) underwent PS; however, the process evolved to a solid-like phase transition faster than that in the case of wild-type p53C. The data obtained by microscopy of live cells indicated that transfection of mutant full-length p53 into the cells tended to result in PS and phase transition (PT) in the nuclear compartments, which are likely the cause of the GoF effects. Fluorescence recovery after photobleaching (FRAP) experiments revealed liquid characteristics of the condensates in the nucleus. Mutant p53 tended to undergo gel- and solid-like phase transitions in the nucleus and in nuclear bodies demonstrated by slow and incomplete recovery of fluorescence after photobleaching. Polyanions, such as heparin and RNA, were able to modulate PS and PT in vitro. Heparin apparently stabilized the condensates in a gel-like state, and RNA apparently induced a solid-like state of the protein even in the absence of PEG. Conditions that destabilize p53C into a molten globule conformation also produced liquid droplets in the absence of crowding. The disordered transactivation domain (TAD) modulated both phase separation and amyloid aggregation. In summary, our data provide mechanistic insight into the formation of p53 condensates and conditions that may result in the formation of aggregated structures, such as mutant amyloid oligomers, in cancer. The pathway of mutant p53 from liquid droplets to gel-like and solid-like (amyloid) species may be a suitable target for anticancer therapy.Mutant p53 tends to form aggregates with amyloid properties, especially amyloid oligomers inside the nucleus, which are believed to cause oncogenic gain-of-function (GoF). 相似文献
27.
2'-azido-2'-deoxyribonucleoside 5'-diphosphates are mechanism-based inhibitors of Ribonucleotide Reductase. Considerable effort has been made to elucidate their mechanism of inhibition, which is still controversial and not fully understood. Previous studies have detected the formation of a radical intermediate when the inhibitors interact with the enzyme, and several authors have proposed possible structures for this radical. We have conducted a theoretical study of the possible reactions involved, which allowed us to identify the structure of the new radical among the several proposals. A new reactional path is also proposed that is the most kinetically favored to yield this radical and ultimately inactivate the enzyme. The energetic involved in this mechanism, both for radical formation and radical decay, as well as the calculated Hyperfine Coupling Constants for the radical intermediate, are in agreement with the correspondent experimental values. This mechanistic alternative is fully coherent with remaining experimental data. 相似文献
28.
Abstract— Since Hayflick's pioneering work in the early sixties, human diploid fibroblasts have become a widely accepted in vitro model system. Recently, Bayreuther and co-workers extended this experimental approach showing that fibroblasts in culture resemble, in their design, the hemopoietic stem-cell differentiation system. They found that the chemical agent mitomycin C accelerates the differentiation pathway from mitotic to postmitotic fibroblasts. We measured the response of endogenous glutathione levels after UVA irradiation (320-400 nm) in mitotic and mitomycin C-induced postmitotic human skin fibroblasts and foreskin-derived keratinocytes. The initial levels in mitotic foreskin derived human fibroblasts were 14.4 nmol glutathione per mg protein, whereas a 30% higher value was obtained in matching foreskin-derived keratinocytes. Similiar elevated levels of this important intracellular free radical scavenging system were found in fibroblasts of a donor suffering from xeroderma pigmentosum. Furthermore, three to four times higher levels of glutathione in mitomycin C-treated mitotic fibroblasts have been determined. In mitotic skin fibroblasts, UVA irradiation resulted in a depletion of glutathione up to 90% following a fluence of 1.0 MJ/m2UVA radiation. Higher initial glutathione levels were found in keratinocytes and mitomycin C-treated skin fibroblasts. In these fibroblasts a 70% depletion was detected and a much lower depletion (10-20%) was seen in some keratinocyte cell lines following fluences up to 1.0 MJ/m2. The depletion in skin fibroblasts was retained after 24 h following a fluence of 0.75 MJ/m2UVA light. In view of the fact that glutathione has been shown to be involved in a variety of metabolic processes and plays a role in cellular protection against UVA radiation, our results imply that the fibroblast differentiation system is a very useful tool to unravel the complex mechanism of UVA-induced oxidative stress. 相似文献
29.
In the crystalline N,N′-dimethylated uracil derivatives 2a , b , the kinetically stabilized enol group forms an H-bond with O? C(4), as demonstrated by increased shielding of specifically labelled 2a and 2b in the 17O-NMR spectra (Δδ(17O)(C(4)—O) ? ?30 ppm); absence of dilution and solvent effects show that the H-bridge is intra-molecular, forming an eight-membered chelate ring. The (apparent) shielding effect Δδ(17O) in 2a, b is larger than that in salicylamide. The strong H-bond explains why the enols 2 , in spite of the absence of steric hindrance, are kinetically stabilized. 相似文献
30.
Hélder A. Santos Elisabete S. Ferreira Elisa J. Pereira Carlos M. Pereira Prof. Kyösti Kontturi Prof. Fernando Silva Prof. 《Chemphyschem》2007,8(10):1540-1547
The interaction between glucose oxidase (GOx) and phospholipid monolayers is studied at the 1,2-dichloroethane/water interface by electrochemical impedance spectroscopy. Electrochemical experiments show that the presence of GOx induces changes in the capacitance curves at both negative and positive potentials, which are successfully explained by a theoretical model based on the solution of the Poisson-Boltzmann equation. These changes are ascribed to a reduced partition coefficient of GOx and an increase of the permittivity of the lipid hydrocarbon domain. Our results show that the presence of lipid molecules enhances the adsorption of GOx molecules at the liquid/liquid interface. At low lipid concentrations, the adsorption of GOx is probably the first step preceding its penetration into the lipid monolayer. The experimental results indicate that GOx penetrates better and forms more stable monolayers for lipids with longer hydrophobic tails. At high GOx concentrations, the formation of multilayers is observed. The phenomenon described here is strongly dependent on 1) the GOx and lipid concentrations, 2) the nature of the lipid, and 3) the potential drop across the interface. 相似文献