全文获取类型
收费全文 | 2232篇 |
免费 | 90篇 |
国内免费 | 6篇 |
专业分类
化学 | 1823篇 |
晶体学 | 30篇 |
力学 | 25篇 |
数学 | 95篇 |
物理学 | 355篇 |
出版年
2022年 | 13篇 |
2021年 | 20篇 |
2020年 | 26篇 |
2019年 | 43篇 |
2018年 | 20篇 |
2017年 | 15篇 |
2016年 | 48篇 |
2015年 | 36篇 |
2014年 | 68篇 |
2013年 | 87篇 |
2012年 | 111篇 |
2011年 | 134篇 |
2010年 | 87篇 |
2009年 | 59篇 |
2008年 | 143篇 |
2007年 | 137篇 |
2006年 | 139篇 |
2005年 | 145篇 |
2004年 | 136篇 |
2003年 | 117篇 |
2002年 | 95篇 |
2001年 | 50篇 |
2000年 | 45篇 |
1999年 | 31篇 |
1998年 | 32篇 |
1997年 | 32篇 |
1996年 | 24篇 |
1995年 | 23篇 |
1994年 | 12篇 |
1993年 | 13篇 |
1992年 | 37篇 |
1991年 | 23篇 |
1990年 | 18篇 |
1989年 | 6篇 |
1988年 | 15篇 |
1987年 | 9篇 |
1986年 | 8篇 |
1985年 | 34篇 |
1984年 | 31篇 |
1983年 | 16篇 |
1982年 | 21篇 |
1981年 | 24篇 |
1980年 | 15篇 |
1979年 | 25篇 |
1978年 | 14篇 |
1977年 | 15篇 |
1976年 | 10篇 |
1975年 | 13篇 |
1974年 | 11篇 |
1973年 | 19篇 |
排序方式: 共有2328条查询结果,搜索用时 15 毫秒
971.
Amphidinols are a unique dinoflagellate metabolite with potent antifungal activity. We examined membrane permeabilizing action by amphidinol analogues with structural variations in polyhydroxy and polyene moieties. Consequently, the polyene and polyhydroxy moieties turned out to play important roles in binding to lipid bilayer membrane and in forming ion-permeable pore/lesion across membrane, respectively. NMR-constrained modeling experiments have revealed for the fist time that amphidinols in membrane generally take a hairpin configuration, which plausibly accounts for their potent antifungal and other membrane permeabilizing activities. 相似文献
972.
973.
Through mixing of porous polystyrene particles (Amberlite XAD-4), non-ionic surfactants, and surfactant-conjugated substrates (affinity ligand) in an aqueous solution led to the formation of a novel medium (affinity admicelle) for protein separation. The ligand (CB-Triton) was synthesized by mixing a triazine dye (Cibacron Blue 3GA (CB)) and a polyoxyethylene-type non-ionic surfactant (Triton X-100) in weakly alkaline solutions. Triton X-100 and CB-Triton were competitively sorbed onto XAD-4. Albumin (bovine serum), alcohol dehydrogenase (yeast), and lysozyme (chicken egg) having specific interaction to CB were collected onto the affinity admicelle. On the other hand, the collection of ovalubmin (chicken egg white), having no binding ability to CB, was negligibly small. Lysozyme in 100 microl of chicken egg white, diluted with 900 microl of 10 mM Tris-HCl (pH 7.4), was successfully collected on 18 mg of CB-Triton admicelles and, then, it was eluted with 1 ml of aqueous solution of 100 mM phosphate (pH 7.4). The recovery based on the activity for the lysis of micrococcus and the concentration factor were 60% and 40 (n = 3), respectively. 相似文献
974.
A new pyrrolophenanthridone alkaloid, criasiaticidine A (1), was isolated from the bulbs of Crinum asiaticum var. japonicum, together with pratorimine (2), lycorine (3) and 4'-hydroxy-7-methoxyflavan (4). The structure of the new alkaloid was determined to be 4,5-etheno-9,10-dihydroxy-6-phenanthridone by spectroscopic means. The cytotoxicity of the isolated compounds 1-4 was evaluated in vitro against Meth-A (mouse sarcoma) and Lewis lung carcinoma (mouse lung carcinoma) tumor cell lines. Furthermore, 3 was examined for in vivo antitumor activity with LLC tumor cells. 相似文献
975.
The oxidative coupling reaction of benzenes with alkenes was successfully achieved by the Pd(OAc)(2)/molybdovanadophosphoric acid (HPMoV)/O(2) system. For example, the reaction of benzene with ethyl acrylate by the above catalytic system in acetic acid afforded ethyl cinnamate as a major product in satisfactory yield. 相似文献
976.
Ohashi Y Itoh Y Kubota M Hamada K Ohashi M Hirano T Niwa H 《European journal of mass spectrometry (Chichester, England)》2004,10(2):269-278
Galbeta1-4GlcNAc-6,6'-disulfate and 2'-epimer corresponding to Galbeta1- 4ManNAc-6,6'-disulfate were distinguished by mass spectrometry by utilizing fast atom bombardment (FAB), electrospray ionization (ESI) and matrix-assisted laser desorption/ionization (MALDI) methods. As for the steric information, negative-ion ESI mass spectrometry/mass spectrometry (MS/MS) provides the most extensive data, but FAB MS/MS also reveals detailed structural information of interest in our case, where MALDI MS is not yet fully equipped with post-source decay. 相似文献
977.
Okamoto S Kasahara M Kamiya A Nakahira Y Ohmori M 《Photochemistry and photobiology》2004,80(3):429-433
In the filamentous, nitrogen-fixing cyanobacterium Anabaena sp. PCC7120, red light (630 nm) decreased, whereas far-red light (720 nm) increased cellular adenosine 3',5'-cyclic monophosphate (cAMP) content. To find a red and far-red light photoreceptor that triggers the cAMP signal cascade, we disrupted 10 open reading frame having putative chromophore-binding GAF domains. The response of the cellular cAMP concentration to red and far-red light in each open reading frame disruptant was determined. It was found that only the mutant of the gene all2699 failed to respond to far-red light. The open reading frame named as aphC encoded a protein with 920 amino acids including GAF domains similar to those involved in Cph2, a photoreceptor of Synechocystis sp. PCC6803. To determine which adenylate cyclase (AC) is responsible for far-red light signal, we disrupted all AC genes and found that CyaC was the candidate. The enzymatic activity of CyaC might be controlled by a far-red light photoreceptor through the phosphotransfer reaction. The site-specific mutant of the Asp59 residue of the receiver (R1) domain of CyaC lost its light-response capability. It was suggested that the far-red light signal was received by AphC and then transferred to the N-terminal response regulator domain of CyaC. Then its catalytic activity was stimulated, which increased the cellular cAMP concentration and drove the subsequent signal transduction cascade. 相似文献
978.
Tamura O Mitsuya T Huang X Tsutsumi Y Hattori S Ishibashi H 《The Journal of organic chemistry》2005,70(26):10720-10725
[reaction: see text] Intramolecular cycloaddition of novel 1,3-dipoles, N-boranonitrones, was examined. Treatment of O-tert-butyldimethylsilyloximes 9-12 having olefin moieties with 2 equiv of BF3 x OEt2 generated N-boranonitrones, which underwent intramolecular cycloaddition to afford N-nonsubstituted cycloadducts 16 (and/or 18) after extractive workup. Despite the Lewis-acidic conditions, the olefin geometry of the substrates was retained in the cycloadducts in the present cycloaddition. The electronic nature of the N-boranonitrones appeared to be electrophilic. In the case of substrate 11c, having an electron-donating methyl group at an internal position of the olefin moiety, the cycloaddition gave the bridged cycloadduct 18b. The cycloaddition proceeded at relatively low temperature, and the diastereoselectivity was high. 相似文献
979.
[structure: see text] Four new dolabellane-type diterpene alkaloids, nigellamines A(1) (1), A(2) (2), B(1) (3), and B(2) (4), were isolated from the seeds of Nigella sativa. Their absolute stereostructures were determined on the basis of chemical and physicochemical evidence. Nigellamines A(1) (1), B(1) (3), and B(2) (4) were found to show potent lipid metabolism promoting activity in primary cultured mouse hepatocytes, and their activities were equivalent to that of a PPAR-alpha agonist, clofibrate. 相似文献
980.
Grossman RB Comesse S Rasne RM Hattori K Delong MN 《The Journal of organic chemistry》2003,68(3):871-874
Hexamethylphosphorous triamide (HMPT) and other phosphoramidites and phosphites have been found to be efficient catalysts for the Michael reaction of alkenones and alkynones with malonates, alpha-cyano esters, beta-keto esters, and nitro compounds. The relatively nontoxic, easily hydrolyzed HMPT catalyzes the Michael reaction within seconds at room temperature in the absence of a solvent, and the reaction is worked up simply by removing the catalyst in vacuo. The Michael reactions of alkynones, unlike those of alkenones, are shown to be irreversible. The implications for asymmetric catalysis are discussed. 相似文献