全文获取类型
收费全文 | 4183篇 |
免费 | 165篇 |
国内免费 | 37篇 |
专业分类
化学 | 2668篇 |
晶体学 | 9篇 |
力学 | 169篇 |
数学 | 928篇 |
物理学 | 611篇 |
出版年
2023年 | 22篇 |
2022年 | 32篇 |
2021年 | 92篇 |
2020年 | 102篇 |
2019年 | 90篇 |
2018年 | 89篇 |
2017年 | 66篇 |
2016年 | 179篇 |
2015年 | 148篇 |
2014年 | 180篇 |
2013年 | 275篇 |
2012年 | 314篇 |
2011年 | 323篇 |
2010年 | 157篇 |
2009年 | 197篇 |
2008年 | 270篇 |
2007年 | 259篇 |
2006年 | 217篇 |
2005年 | 201篇 |
2004年 | 205篇 |
2003年 | 145篇 |
2002年 | 103篇 |
2001年 | 47篇 |
2000年 | 39篇 |
1999年 | 35篇 |
1998年 | 39篇 |
1997年 | 37篇 |
1996年 | 49篇 |
1995年 | 22篇 |
1994年 | 28篇 |
1993年 | 16篇 |
1992年 | 30篇 |
1991年 | 19篇 |
1990年 | 23篇 |
1989年 | 21篇 |
1988年 | 21篇 |
1987年 | 17篇 |
1986年 | 15篇 |
1985年 | 34篇 |
1984年 | 25篇 |
1983年 | 25篇 |
1982年 | 29篇 |
1981年 | 15篇 |
1980年 | 18篇 |
1979年 | 11篇 |
1978年 | 19篇 |
1977年 | 13篇 |
1976年 | 14篇 |
1974年 | 11篇 |
1967年 | 4篇 |
排序方式: 共有4385条查询结果,搜索用时 15 毫秒
991.
Rachel Crespo-Otero Yoana Pérez-Badell Juan Alexander Padrón-García Luis Alberto Montero-Cabrera 《Theoretical chemistry accounts》2007,118(3):649-663
Association between NO and each of the 20 amino acids and their related organic functional groups was studied by exploring
the configuration space of the potential energy of association surface by using the multiple minima hypersurface procedure.
AM1 semiempirical Hamiltonian was used in order to explore such complex hypersurfaces of biological molecular interactions
at finite computational times. An appropriate test with a set of NO and small molecule complexes obtained at the MP2/6-311++g(2d,2p)
level of theory was also carried out. Stabilization energies of larger models were also evaluated at the conventional PBE1PBE/6-31g(d,p)
DFT level. NO–aminoacid hypersurface explorations yielded that interactions of NO with NH group together with the C=O belonging
to the backbone appeared predominant in all cases. Models of polar aminoacids and NO also show stable interactions with the
lateral chains. Interactions with charged amino acids were found as the most stable and Lys was, undoubtedly, the preferred
association. The study of these kinds of interactions must take into account the deepest and other minima because the entropy
of association plays an important role.
Contribution to the Serafin Fraja Memorial Issue.
Dedicated to Prof. Serafín Fraga, an unforgettable friend. 相似文献
992.
Carmona D Sáez J Granados H Pérez E Blair S Angulo A Figadere B 《Natural product research》2003,17(4):275-280
Dichloromethane extracts of both the roots and the leaves of Raimondia cf. monoica showed in vitro antiplasmodial and leishmanicidal activities against Plasmodium falciparum and Leishmania panamensis, respectively. Three 6-substituted 5,6-dihydro-2H-pyran-2-ones were isolated. (1) and (2) were identified as (6S)-(5'-oxohepten-1'E,3'E-dienyl)-5,6-dihydro-2H-pyran-2-one (1) and (6R)-(5'-oxohepten-1'Z,3'E-dienyl)-5,6-dihydro-2H-pyran-2-one (2), respectively. (-)-Arentilactone (3) was also isolated. The structure of the new compound (1) was determined by spectroscopic methods; additional spectroscopic data for (2) are reported for the first time. 相似文献
993.
Alberto L. Mancinelli Oliver Hobert † George Nikas 《Photochemistry and photobiology》1992,56(5):585-591
The spectrophotometric assay of phytochrome in vivo in etiolated plant material was used to determine the effects of changes in reflected light on the state of the photoreceptor in etiolated seedlings exposed simultaneously to direct and reflected light. Changes in reflected light that were small in terms of the total (direct + reflected) radiation incident on the seedlings produced detectable changes in the state of phytochrome in vivo. The contribution of reflected light to the state of phytochrome in vertical organs was greater than expected from its low contribution to total incident light. These data from laboratory studies complement and are consistent with results of field studies on the effects of light reflected from neighboring vegetation on plant growth under natural radiation conditions. 相似文献
994.
Solid-phase synthesis of fullerene-peptides 总被引:1,自引:0,他引:1
Pantarotto D Bianco A Pellarini F Tossi A Giangaspero A Zelezetsky I Briand JP Prato M 《Journal of the American Chemical Society》2002,124(42):12543-12549
The solid-phase synthesis of peptides (SPPS) containing [60]fullerene-functionalized amino acids is reported. A new amino acid, fulleropyrrolidino-glutamic acid (Fgu), is used for the SPPS of a series of analogues of different length based on the natural Leu(5)-Enkephalin and on cationic antimicrobial peptides. These fullero-peptides were prepared on different solid supports to analyze the influence of the resin on the synthesis. Optimized protocols for the coupling and deprotection procedures were determined allowing the synthesis of highly pure peptides in sufficient quantities for evaluation of biological activities. In particular, to avoid side reactions of the fullerene moiety with bases and nucleophiles, the removal of the protecting groups was performed under inert conditions (nitrogen or argon in the dark). We have encountered serious problems with the recovery of the crude compounds, especially when Fgu was inserted in the proximity of the resin core as fullero-peptides tend to remain embedded inside the resin. Eventually, all of the fullero-peptides were easily purified, and the cationic peptides were tested for their antimicrobial activities. They displayed a specific activity against the Gram-positive bacterium S. aureus and also lysed erythrocytes. The availability of a fullero-amino acid easily useable in the SPPS of fullero-peptides may thus open the way to the synthesis of new types of biologically active oligomers. 相似文献
995.
Caballero A Tormos R Espinosa A Velasco MD Tárraga A Miranda MA Molina P 《Organic letters》2004,6(24):4599-4602
A new 1-(9-anthryl)-4-ferrocenyl-2-aza-1,3-butadiene showing selective fluorescence enhancement upon binding to lithium cations over other alkaline cations in aqueous environment has been conveniently synthesized and characterized. [structure: see text] 相似文献
996.
Andrea M. Gasco Clara Cena Antonella Di Stilo Giuseppe Ermondi Claudio Medana Alberto Gasco 《Helvetica chimica acta》1996,79(7):1803-1817
The synthesis, structural characterization, and NO donor properties of a series of terfuroxan (= terfurazan trioxide) derivatives 1a–h and 2a–j are reported (Schemes 1 and 2). Structural assignments were confirmed principally by mass and 13C- and 1H-NMR spectroscopy. The extent and the initial rate of NO release in the presence of thiol cofactor was evaluated for each derivative. Vasodilator effects of all the terfuroxan derivatives were evaluated on endothelium-denuded strips of rat aorta precontracted with noradrenaline. Concentration-response curves were also evaluated in the presence of 10 m?M oxyhemoglobin (HbO2), a well known NO scavenger. The whole series displays high vasodilating potency; the most active terfuroxans ( 1a, b, g and 2i ) are 5–10 times as potent as glyceryl trinitrate taken as reference (see Table 3). The potency decrease observed in the presence of HbO2 agrees with the involvement of NO in the vasorelaxing action. The 4,3′:4′,4″ connection (series 1 ; furoxan numbering) gives rise to the most potent compounds. The conformational factors seem to play important roles in the activity. No clear relationship between physico-chemical properties of the substituents and potencies of derivative emerges. 相似文献
997.
THE PHOTODYNAMIC EFFECT OF ROSE BENGAL ON PROTEINS OF THE MITOCHONDRIAL INNER MEMBRANE 总被引:2,自引:0,他引:2
Cecilia Giulivi Monica Sarcansky Esteban Rosenfeld Alberto Boveris 《Photochemistry and photobiology》1990,52(4):745-751
Photodynamic action promoted by Rose Bengal was evaluated in solutions of unsaturated fatty acids or histidine, and on beef heart submitochondrial particles. Rose Bengal-promoted photooxidation of histidine was mainly due to the opening up of the imidazole ring by singlet oxygen. Photosensitization of polyunsaturated fatty acids (PUFA) resulted in oxygen consumption and thiobarbituric acid-reactive substances (TBARS) formation, the extent of which was linearly related to the increasing degree of unsaturation. Photosensitization of submitochondrial particles caused oxygen consumption and TBARS production. These processes involved two different reaction components: during the first, most of the mitochondrial proteins were inactivated, the most sensitive being succinate dehydrogenase and cytochrome c. The values for the rate ratios of [TBARS] formation/[O2] consumption for the first and second phase were 0.36 and 1.32%, respectively, pointing to a larger contribution of lipid peroxidation during the second phase. The calculation of the rate constants for reaction of singlet oxygen with mitochondrial proteins suggests that singlet oxygen is more reactive towards proteins than to PUFA. The biological role of this selectivity is discussed in terms of the mitochondria as one of the first targets for photosensitized reactions. 相似文献
998.
Alberto R. Dias 《Journal of organometallic chemistry》2005,690(7):1840-1844
Metal-η5-cyclopentadienyl (M-Cp) and metal-η5-pyrrolyl (M-pyr) bond dissociation enthalpies in group 4 complexes were determined from DFT/B3LYP calculations with a VTZP basis set. Thermochemical cycles involving calculated enthalpies of ligand exchange reactions and experimental values of ligand electron affinities and M-Cl bond dissociation enthalpies were applied to [M(η5-X)Cl3] piano stool complexes (M = Ti, Zr, Hf; X = pyr, Cp), allowing a comparative study of those metal-ligand bond strengths. The results indicate that both ligands establish weaker bonds with Ti than with the heavier elements, Zr and Hf. Very similar bond dissociation enthalpies were obtained for pyrrolyl and cyclopentadienyl (within 1 kcal mol−1), suggesting that the well known difference in reactivity between those families of complexes should derive from kinetic rather than thermodynamic causes. 相似文献
999.
Ma Prado García-Aparicio Ignacio Ballesteros Alberto González José Miguel Oliva Mercedes Ballesteros Ma José Negro 《Applied biochemistry and biotechnology》2006,129(1-3):278-288
The influence of the liquid fraction (prehydrolysate) generated during steam-explosion pretreatment (210°C, 15 min) of barley
straw on the enzymatic hydrolysis was determined. Prehydrolysate was analyzed for degradation compounds and sugars' content
and used as a medium for enzymatic hydrolysis tests after pH adjusting to 4.8. Our results show that the presence of the compounds
contained in the prehydrolysate strongly affects the hydrolysis step (a 25% decrease in cellulose conversion compared with
control). Sugars are shown to be more potent inhibitors of enzymatic hydrolysis than degradation products. 相似文献
1000.
Analytical results are reported for the determination of inorganic species in water and sediments sampled in the Lamone and Marzeno rivers ecosystem. The species determined are Cd, Co, Cr, Cu, Fe, Mn, Ni, Pb, Zn, Hg, F-, Cl- Br-, NO3-, SO4--, Na+, K+, Mg++, Ca++, NH4+ in superficial water, and Cd, Co, Cr, Cu, Fe, Mn, Ni, Pb, Zn, Hg in sediments. For all the elements, in addition to detection limits, precision and accuracy are given: the former, expressed as relative standard deviation (s(r)), and the latter, expressed as relative error (e), were good, being in all cases lower than 6%. 相似文献