全文获取类型
收费全文 | 4084篇 |
免费 | 106篇 |
国内免费 | 23篇 |
专业分类
化学 | 2929篇 |
晶体学 | 26篇 |
力学 | 89篇 |
数学 | 468篇 |
物理学 | 701篇 |
出版年
2022年 | 27篇 |
2021年 | 36篇 |
2020年 | 51篇 |
2019年 | 44篇 |
2018年 | 26篇 |
2017年 | 32篇 |
2016年 | 82篇 |
2015年 | 78篇 |
2014年 | 84篇 |
2013年 | 173篇 |
2012年 | 204篇 |
2011年 | 279篇 |
2010年 | 108篇 |
2009年 | 109篇 |
2008年 | 202篇 |
2007年 | 243篇 |
2006年 | 248篇 |
2005年 | 240篇 |
2004年 | 202篇 |
2003年 | 144篇 |
2002年 | 172篇 |
2001年 | 76篇 |
2000年 | 72篇 |
1999年 | 51篇 |
1998年 | 53篇 |
1997年 | 44篇 |
1996年 | 72篇 |
1995年 | 57篇 |
1994年 | 45篇 |
1993年 | 48篇 |
1992年 | 53篇 |
1991年 | 39篇 |
1990年 | 32篇 |
1989年 | 34篇 |
1988年 | 41篇 |
1987年 | 38篇 |
1986年 | 21篇 |
1985年 | 47篇 |
1984年 | 41篇 |
1983年 | 41篇 |
1982年 | 57篇 |
1981年 | 46篇 |
1980年 | 47篇 |
1979年 | 38篇 |
1978年 | 28篇 |
1977年 | 31篇 |
1976年 | 34篇 |
1975年 | 35篇 |
1974年 | 28篇 |
1973年 | 25篇 |
排序方式: 共有4213条查询结果,搜索用时 0 毫秒
71.
Carboxylate-substituted radicals from phenylselenide derivatives. Designs on models for coenzyme B12-dependent enzyme-catalyzed rearrangements 总被引:1,自引:0,他引:1
Miranda N Daublain P Horner JH Newcomb M 《Journal of the American Chemical Society》2003,125(18):5260-5261
Laser flash photolysis (266 nm) of alpha- and beta-phenylselenyl esters, carboxylic acids, and carboxylates in aqueous acetonitrile media gave the corresponding radicals by homolytic cleavage of the phenylselenyl groups. In the beta-substituted systems, acid and carboxylate radicals reacted in intramolecular reporter reactions with approximately equal rate constants. For the alpha-substituted systems, an ester- and carboxylic acid-substituted radical reacted in an intramolecular reporter reaction with the same rate constants, but the analogous alpha-carboxylate radical, a radical anion, reacted an order of magnitude less rapidly and with an activation energy that is 3 kcal/mol greater than that found for analogues. A kinetic titration of the equilibrating alpha-acid and alpha-carboxylate radicals gave pKa = 4.6. The results indicate that alpha-ester and alpha-carboxylic acid radicals are unlikely to be appropriate models for alpha-carboxylate radicals, the intermediates formed in a large subset of coenzyme B12-dependent enzyme-catalyzed reactions. 相似文献
72.
James N. Sturgis C. Neil Hunter Robert A. Niederman 《Photochemistry and photobiology》1988,48(2):243-247
Abstract— Membranes from Rhodobacter (formerly Rhodopseudomonas) sphaeroides mutant strains that lack one or more of the bacteriochlorophyll a (BChl)-protein complexes were used to obtain spectra and molar extinction coefficients of the near-IR absorption bands. The strains examined were NF57 which lacks the B875 light-harvesting and reaction center complexes, and M21 which lacks the B800–850light-harvesting complex. The extinction coefficients obtained for the B800, B850 and B875 bands were 226 ± 10, 170 ± 5, and 118 ± 5 m M -1 cm-1 , respectively, in reasonable agreement with values reported for detergent-isolated complexes (Clayton, R. K. and B. J. Clayton, Proc. Natl. Acad. Sci. USA 78 ,5583–5587, 1981). The results also demonstrated that detergent solubilization altered the spectra, causing a band broadening on the blue side of the B875 and B800 peaks. The data obtained from the analysis of the mutant strains were used for deconvolution of the BChl species in membranes of the wild-type. A short BASIC computer program for performing this deconvolution is included. 相似文献
73.
The synthesis and chemical properties of a series of amino acid derivatives containing the thiolsulphonate functionality are described. 相似文献
74.
Xuejun Pan Dan Xie Neil Gilkes David J. Gregg Jack N. Saddler 《Applied biochemistry and biotechnology》2005,124(1-3):1069-1079
Pretreatment of Douglas-fir by steam explosion produces a substrate containing approx 43% lignin. Two strategies were investigated
for reducing the effect of this residual lignin on enzymatic hydrolysis of cellulose: mild alkali extraction and protein addition.
Extraction with cold 1% NaOH reduced the lignin content by only approx 7%, but cellulose to glucose conversion was enhanced
by about 30%. Before alkali extraction, addition of exogenous protein resulted in a significant improvement in cellulose hydrolysis,
but this protein effect was substantially diminished after alkali treatment. Lignin appears to reduce cellulose hydrolysis
by two distinct mechanisms: by forming a physical barrier that prevents enzyme access and by non-productively binding cellulolytic
enzymes. Cold alkali appears to selectively remove a fraction of lignin from steam-exploded Douglas-fir with high affinity
for protein. Corresponding data for mixed softwood pretreated by organosolv extraction indicates that the relative importance
of the two mechanisms by which residual lignin affects hydrolysis is different according to the pre- and post-treatment method
used. 相似文献
75.
76.
Tao Huang Seyyedmohsen Hosseinibarkooie Adam L. Borne Mitchell E. Granade Jeffrey W. Brulet Thurl E. Harris Heather A. Ferris Ku-Lung Hsu 《Chemical science》2021,12(9):3295
Sulfonyl-triazoles are a new class of electrophiles that mediate covalent reaction with tyrosine residues on proteins through sulfur-triazole exchange (SuTEx) chemistry. Recent studies demonstrate the broad utility and tunability of SuTEx chemistry for chemical proteomics and protein ligand discovery. Here, we present a strategy for mapping protein interaction networks of structurally complex binding elements using functionalized SuTEx probes. We show that the triazole leaving group (LG) can serve as a releasable linker for embedding hydrophobic fragments to direct molecular recognition while permitting efficient proteome-wide identification of binding sites in live cells. We synthesized a series of SuTEx probes functionalized with a lipid kinase fragment binder for discovery of ligandable tyrosines residing in catalytic and regulatory domains of protein and metabolic kinases in live cells. We performed competition studies with kinase inhibitors and substrates to demonstrate that probe binding is occurring in an activity-dependent manner. Our functional studies led to discovery of probe-modified sites within the C2 domain that were important for downregulation of protein kinase C-alpha in response to phorbol ester activation. Our proof of concept studies highlight the triazole LG of SuTEx probes as a traceless linker for locating protein binding sites targeted by complex recognition elements in live cells.Sulfonyl-triazole probes modified with a kinase recognition element are developed for live cell activity-based profiling to identify tyrosine sites located in catalytic and regulatory domains that are important for kinase function. 相似文献
77.
Radioimmunoassays were used to investigate the repair of cyclobutane pyrimidine dimers and pyrimidine (6-4)pyrimidone photoproducts ((6-4] photoproducts) in the epidermis of the South American opossum, Monodelphis domestica. In the absence of photoreactivating light, both types of photodamage were excised with similar kinetics, 50% of the damage remaining 8 h after UV irradiation in vivo. Exposure of UV-irradiated skin to photoreactivating light resulted in removal of most of the cyclobutane dimers and an enhanced rate of (6-4) photoproduct repair. Photoenhanced excision repair of non-dimer damage increases the range of biologically effective lesions removed by in vivo photoreactivation. 相似文献
78.
Ab initio molecular orbital calculations using the STO3-21G basis set has been carried out for the cluster series Na
n
+
, Na
n
, and Na
n
–
(wheren=2–7). The basis set is shown to be reliable compared with more extensive basis sets at the Hartree-Fock level. Thirty-one optimized structures are reported and discussed, many of which (especially for the anions) have not been considered. The STO3-21G//STO3-21G calculations suggest that for most of the species the optimum geometries are planar. In particular, the optimized structures for the anionic species should provide a starting point for more sophisticated configuration interaction calculations. 相似文献
79.
The stereochemistry observed in organocuprate SN2′ displacements can be rationalized as a stereoelectronic effect arising from “bidentate” binding involving a d orbital of nueleophilic copper and π* and σ* orbitals of the substrate. The extension of this idea to other reactions of organocuprates including additions to acetylenes and enones is discussed. 相似文献
80.
David L. Mitchell James E. Cleaver Louise H. Lutze 《Photochemistry and photobiology》1994,60(3):221-224
Abstract Direct determination has been made of cyclobutyl pyrimidine dimer induction and excision repair in an episomal SV40 DNA population in vivo . Maintaining SV40-transformed human (GM637) cells in confluent culture results in amplification of a mutant SV40 episome to high copy number. T4 endonuclease V was used to quantify the induction and repair of cyclobutane dimers in the SV40 episome and genomic DNA of the same cells. Differences in both parameters were observed cyclobutane dimers were induced at 1.5–2-fold greater frequency in episomal DNA and excised at a reduced rate compared to genomic DNA in the host cells. 相似文献