全文获取类型
收费全文 | 4632篇 |
免费 | 132篇 |
国内免费 | 37篇 |
专业分类
化学 | 3384篇 |
晶体学 | 36篇 |
力学 | 104篇 |
综合类 | 1篇 |
数学 | 611篇 |
物理学 | 665篇 |
出版年
2022年 | 25篇 |
2021年 | 35篇 |
2020年 | 59篇 |
2019年 | 62篇 |
2018年 | 36篇 |
2017年 | 42篇 |
2016年 | 98篇 |
2015年 | 98篇 |
2014年 | 102篇 |
2013年 | 232篇 |
2012年 | 221篇 |
2011年 | 340篇 |
2010年 | 136篇 |
2009年 | 139篇 |
2008年 | 277篇 |
2007年 | 285篇 |
2006年 | 282篇 |
2005年 | 279篇 |
2004年 | 258篇 |
2003年 | 179篇 |
2002年 | 178篇 |
2001年 | 66篇 |
2000年 | 54篇 |
1999年 | 46篇 |
1998年 | 47篇 |
1997年 | 45篇 |
1996年 | 67篇 |
1995年 | 42篇 |
1994年 | 52篇 |
1993年 | 54篇 |
1992年 | 56篇 |
1991年 | 49篇 |
1990年 | 30篇 |
1989年 | 28篇 |
1988年 | 40篇 |
1987年 | 29篇 |
1986年 | 27篇 |
1985年 | 63篇 |
1984年 | 54篇 |
1983年 | 43篇 |
1982年 | 72篇 |
1981年 | 58篇 |
1980年 | 48篇 |
1979年 | 45篇 |
1978年 | 51篇 |
1977年 | 45篇 |
1976年 | 35篇 |
1975年 | 46篇 |
1974年 | 28篇 |
1973年 | 17篇 |
排序方式: 共有4801条查询结果,搜索用时 15 毫秒
91.
Inchul Shin Ian Davis Karinel Nieves-Merced Yifan Wang Stanton McHardy Aimin Liu 《Chemical science》2021,12(11):3984
SfmD is a heme-dependent enzyme in the biosynthetic pathway of saframycin A. Here, we present a 1.78 Å resolution de novo crystal structure of SfmD, which unveils a novel heme cofactor attached to the protein with an unusual HxnHxxxC motif (n ∼ 38). This heme cofactor is unique in two respects. It contains a single thioether bond in a cysteine–vinyl link with Cys317, and the ferric heme has two axial protein ligands, i.e., His274 and His313. We demonstrated that SfmD heme is catalytically active and can utilize dioxygen and ascorbate for a single-oxygen insertion into 3-methyl-l-tyrosine. Catalytic assays using ascorbate derivatives revealed the functional groups of ascorbate essential to its function as a cosubstrate. Abolishing the thioether linkage through mutation of Cys317 resulted in catalytically inactive SfmD variants. EPR and optical data revealed that the heme center undergoes a substantial conformational change with one axial histidine ligand dissociating from the iron ion in response to substrate 3-methyl-l-tyrosine binding or chemical reduction by a reducing agent, such as the cosubstrate ascorbate. The labile axial ligand was identified as His274 through redox-linked structural determinations. Together, identifying an unusual heme cofactor with a previously unknown heme-binding motif for a monooxygenase activity and the structural similarity of SfmD to the members of the heme-based tryptophan dioxygenase superfamily will broaden understanding of heme chemistry.The de novo crystal structure of SfmD reveals a novel c-type heme cofactor for promoting a monooxygenation reaction in the biosynthetic pathway of saframycin A. 相似文献
92.
The apperrance of peaks at non-integral masses in linked scans, where the ratio B/E is kept constant, is shown to be caused by fragmentations of metastable ions occuring in the region between the magnetic and electric sectors. A formula is given for calculating the mass of the parent metastable ion and that of its daughter ion. 相似文献
93.
Let: E M be a fiber bundle and let be an infinitesimal Lie transformation group acting onE. We announce various new results concerning the cohomology of the invariant variational bicomplex (
*,*
(J(E)), dH, dV) and the associated invariant Euler-Lagrange complex. As one application of our general theory, we completely solve the local invariant inverse problem of the calculus of variations for finite-dimensional infinitesimal Lie transformation groups. 相似文献
94.
Vortex shedding from a bluff ring in a linear shear flow has been investigated experimentally. The shedding frequency, measured using a hot wire in various positions within the near wake of the ring, was found to be remarkably insensitive to a mean flow shear parameter, , defined by (d/U
o).U/y, where d is the mean ring diameter and U
o is the undisturbed upstream velocity on the ring axis; even for = 0.41, corresponding to a velocity variation of about ±24% across the outer ring diameter, the Strouhal number was only about 5% lower than in uniform flow. However, the strong axisymmetric shedding which dominates the flow for = 0 is significantly affected by shear. Indeed, spatial correlation measurements demonstrated that the shedding actually becomes anti-symmetric at the highest values of . The implication of these results is that vortex flowmeters constructed using a ring as the shedding body would be relatively unaffected by changes in the upstream mean flow profile. 相似文献
95.
96.
A flashlamp-pumped tunable dye laser has been used to study the photochemical production of Br* (42P1/2) atoms from Br2 and IBr exc 相似文献
97.
98.
Gordon McKay Stephen J. Allen Ian F. McConvey Michael S. Otterburn 《Journal of colloid and interface science》1981,80(2):323-339
A three-step model has been proposed for the adsorption of Astrazone Blue dye (Basic Blue 69) on peat. The initial rate of uptake of dye ions due to physical adsorption and chemisorption (ion exchange) has been correlated using a surface mass transfer coefficient. These coefficients have been determined and expressed in the dimensionless mass transfer form, Sh/Sc0.33, as a function of agitation, initial dye concentration, peat particle size range, dye solution temperature, and mass of peat. 相似文献
99.
1,2-Bis(2,5-diphenylphospholano)methane (Ph-BPM) has been prepared in good yield from 2,5-trans-diphenylphospholane-borane adduct. Rhodium and ruthenium complexes of this ligand have been prepared and their usefulness in asymmetric hydrogenation has been investigated. [Ph-BPM Rh(COD)]BF4 showed high activity and selectivity for itaconate and dehydroamino acid hydrogenation. Ph-BPM RuCl2(DPEN) was effective for imine hydrogenation. 相似文献
100.
Hydroxylamine derivatives of (S)-prolinol have been generated using a Cope elimination. These undergo reverse-Cope elimination onto a pendant double bond to give morpholine N-oxides containing three contiguous chiral centres. 相似文献