全文获取类型
收费全文 | 2840篇 |
免费 | 178篇 |
国内免费 | 153篇 |
出版年
2023年 | 41篇 |
2022年 | 28篇 |
2021年 | 67篇 |
2020年 | 72篇 |
2019年 | 112篇 |
2018年 | 86篇 |
2017年 | 60篇 |
2016年 | 49篇 |
2015年 | 78篇 |
2014年 | 133篇 |
2013年 | 152篇 |
2012年 | 119篇 |
2011年 | 120篇 |
2010年 | 90篇 |
2009年 | 136篇 |
2008年 | 127篇 |
2007年 | 147篇 |
2006年 | 151篇 |
2005年 | 131篇 |
2004年 | 105篇 |
2003年 | 106篇 |
2002年 | 90篇 |
2001年 | 67篇 |
2000年 | 58篇 |
1999年 | 65篇 |
1998年 | 62篇 |
1997年 | 52篇 |
1996年 | 64篇 |
1995年 | 54篇 |
1994年 | 54篇 |
1993年 | 72篇 |
1992年 | 46篇 |
1991年 | 37篇 |
1990年 | 34篇 |
1989年 | 32篇 |
1988年 | 29篇 |
1987年 | 24篇 |
1986年 | 27篇 |
1985年 | 23篇 |
1984年 | 47篇 |
1983年 | 16篇 |
1982年 | 16篇 |
1981年 | 20篇 |
1980年 | 20篇 |
1979年 | 16篇 |
1978年 | 6篇 |
1977年 | 8篇 |
1976年 | 6篇 |
1975年 | 5篇 |
1972年 | 5篇 |
排序方式: 共有3171条查询结果,搜索用时 43 毫秒
101.
Addi R. Fadel Dan Q. Jin Gaetano T. Montelione Ronald M. Levy 《Journal of biomolecular NMR》1995,6(2):221-226
Summary Order parameters for the backbone N–H and C–H bond vectors have been calculated from a 150 ps molecular dynamics (MD) simulation of human type- transforming growth factor in H2O solvent. Two kinds of crankshaft motions of the polypeptide backbone are observed in this MD trajectory. The first involves small-amplitude rocking of the rigid peptide bond due to correlated changes in the backbone dihedral angles i–1 and i. These high-frequency librational crankshaft motions are correlated with systematically smaller values of motional order parameters for backbone N–H bond vectors compared to C–H bond vectors. In addition, infrequent crankshaft flips of the peptide bond from one local minimum to another are observed for several amino acid residues. These MD simulations demonstrate that comparisons of N–H and C–H order parameters provide a useful approach for identifying crank-shaft librational motions in proteins. 相似文献
102.
Jonathan H. Davis 《Journal of biomolecular NMR》1995,5(4):433-437
Summary 2D 15N-1H correlation spectra are ideal for measuring backbone amide populations to determine amide exchange protection factors in studies of protein folding or other structural features. Most protein NMR spectroscopists use HSQC, which has been shown to be generally superior to HMQC in both resolution and sensitivity. The refocused HSQC experiment is intrinsically less sensitive than the regular HSQC, due to T2 relaxation during the refocusing delays. However, we show here that, when high 15N resolution is needed, an optimized refocused HSQC sequence that utilizes a semi-constant time evolution period and pulsed field gradients has better signal-to-noise ratio and resolution, and integrates more accurately, than a similar HSQC. The differences are demonstrated on a 20 kDa protein. The technique can also be applied to 3D NOESY experiments to eliminate strong NH2 geminal peaks and their truncation artefacts at a modest cost in sensitivity. 相似文献
103.
Rasmus H. Fogh Dick Schipper Rolf Boelens Robert Kaptein 《Journal of biomolecular NMR》1995,5(3):259-270
Summary The 1H, 13C and 15N NMR resonances of serine protease PB92 have been assigned using 3D tripleresonance NMR techniques. With a molecular weight of 27 kDa (269 residues) this protein is one of the largest monomeric proteins assigned so far. The side-chain assignments were based mainly on 3D H(C)CH and 3D (H)CCH COSY and TOCSY experiments. The set of assignments encompasses all backbone carbonyl and CHn carbons, all amide (NH and NH2) nitrogens and 99.2% of the amide and CHn protons. The secondary structure and general topology appear to be identical to those found in the crystal structure of serine protease PB92 [Van der Laan et al. (1992) Protein Eng., 5, 405–411], as judged by chemical shift deviations from random coil values, NH exchange data and analysis of NOEs between backbone NH groups.Abbreviations 2D/3D/4D
two-/three-/four-dimensional
- HSQC
heteronuclear single-quantum coherence
- HMQC
heteronuclear multiple-quantum coherence
- COSY
correlation spectroscopy
- TOCSY
total correlation spectroscopy
- NOE
nuclear Overhauser enhancement (connectivity)
- NOESY
2D NOE spectroscopy
Experiment nomenclature (H(C)CH, etc.) follows the conventions used elsewhere [e.g. Ikura et al. (1990) Biochemistry, 29, 4659–4667]. 相似文献
104.
F Zeytin C Mougin P Brazeau N Ling 《Biochemical and biophysical research communications》1984,120(2):540-547
The effects of forskolin and cholera toxin on the regulation of cAMP release were studied in a neurotensin-secreting rat C-cell line. The interaction of these agents with norepinephrine, a potent neurotensin secretagogue, was also investigated. Forskolin stimulated cAMP release 10(2)-10(3) fold while it increased neurotensin release 2-3 fold. Cholera toxin caused a 10(2)-10(3) fold increase in cAMP release and had no effect on neurotensin release. We conclude that the 44-2 C-cells provide a new model for studying the regulation of the concomitant (via forskolin) or independent (via cholera toxin) secretion of cyclic AMP and/or neurotensin. 相似文献
105.
Effect of plant genotype and nitrogen fertilizer on symbiotic nitrogen fixation by soybean cultivars
Summary Isotopic as well as non-isotopic methods were used to assess symbiotic nitrogen fixation within eight soybean [Glycine max (L.) Merr.] cultivars grown at 20 and 100 kg N/ha levels of nitrogen fertilizer under field conditions.The15N methodology revealed large differences between soybean cultivars in their abilities to support nitrogen fixation. In almost all cases, the application of 100 kg N/ha resulted in lower N2 fixed in soybean than at 20 kg N/ha in the first year of the study. However, N2 fixed in one cultivar, Dunadja, was not significantly affected by the higher rate of N fertilizer application. These results were confirmed by measurements of acetylene reduction activity, nodule dry weight and N2 fixed as measured by the difference method. Further proof of differences in N2 fixed within soybean cultivars and the ability of Dunadja to fix similar amounts of N2 at 20 and 100 kg N/ha was obtained during a second year experiment. Dunadja yield was affected by N fertilizer and produced larger yield at 100 kg N/ha than at 20 kg N/ha. This type of cultivar could be particularly useful in situations where soil N levels are high or where there is need to apply high amounts of N fertilizer.The present study reveals the great variability between legume germplasms in the ability to fix N2 at different inorganic N levels, and also the potential that exists in breeding for nitrogen fixation associative traits. The15N methodology offers a unique tool to evaluate germplasms directly in the field for their N2 fixation abilities at different N fertilizer levels. 相似文献
106.
Dynamics of soil microbial biomass N under zero and shallow tillage for spring wheat,using15N urea 总被引:2,自引:0,他引:2
Summary Field studies to determine the effect of zero and shallow (10 cm) cultivation on microbial biomass were conducted on several Chernozemic soils in western Canada. Using the CHCl3 fumigation method, the distribution of microbial biomass N and the immobilization and subsequent release of added15N (15N-urea) from the microbial biomass were determined in the A horizon, at the 0 to 5 and 5 to 10 cm depth, during the growing season for spring wheat.Temporal variation in microbial biomass N, associated with the development of the rhizosphere, was characterized by an increase between Feekes stage 1 and 5 or 10 and decrease at Feekes stage 11.4. Over the long term, the variation in biomass N between tillage systems corresponded with crop residue distribution. Immobilization of fertilizer N was related to the increase in biomass N from Feekes stage 1, which in turn, was associated with the incorporation of recent crop residues or levels of labile organic matter in the surface soil. The study demonstrated the relatively rapid remineralization of immobilized fertilizer N under field conditions and emphasized the role of the microbial biomass N as both a sink and source of mineral N. 相似文献
107.
R. A. Virginia L. M. Baird J. S. La Favre W. M. Jarrell B. A. Bryan G. Shearer 《Plant and Soil》1984,79(2):273-284
Summary The relative nitrogen fixation efficiencies (RE 1-[H2 evolved÷C2H2 reduced]·100) of four mesquite (Prosopis glandulosa var.torreyana) rhizobia (Strains WR 1001, WR 1002, L5, L9) and a cowpea rhizobia (Strain 176A32) on mesquite were evaluated in a glasshouse experiment. Plant yield, shoot N accumulation, and the natural15N abundance (15N) of nodule tissue were determined. Strain WR 1002 failed to nodulate mesquite and strain L5 produced ineffective nodules. Among the three effective strains (WR 1001, L9, 176A32) the cowpea strain (176A32) and strain L9 had significantly higher RE than strain WR 1001. Differences in RE, however, were not accompanied by significantly higher plant yield and shoot N accumulation. The difference in15N abundance between foliar tissue and nodules (nodules minus leaves) was 0.47 15N for the ineffective L5 nodules, while for the effective WR 1001, L9, and 176A32 nodules, respectively, this difference was 8.35, 7.81, and 8.35 15N. This indicates a similar relationship between N2-fixing effectiveness and natural15N enrichment of nodules that was previously observed in soybeans (Glycine max, L. Merr.). Strains WR 1001 and L9 produced elongate, indeterminate nodules typical for mesquite. The ineffective L5 nodules had few infected cells and an abundance of cortical amyloplasts. Mesquite nodules produced by the cowpea strain were spherical and were somewhat more similar in internal morphology to determinate nodules typical of cowpea than indeterminate nodules normally associated with mesquite. 相似文献
108.
Summary A field experiment on N turnover in rotations of groundnut, sorghum and pigeonpea crops was conducted in an Indian Alfisol during 1978–80.15N-labelled urea N was applied @ 40 kg ha–1 in 1978. In the first year, the groundnut utilized 19.5% of the applied labelled N, sorghum 25.5%, and pigeon pea 10.3%. More fertilizer N was removed through weeding than by crop uptake in sorghum and pigeon pea. The fertilizer N left behind in soil upto 40 cm depth was 44.4% in groundnut plots, 35.1% in sorghum plots and 40.1% in pigeon pea plots.The uptake in 1979 of the residual fertilizer N in the soil was 8.9% in sorghum, 8.3% in groundnut and 2.8% in pigeon pea. In 1980, it declined to less than 2% for pigeon pea and groundnut and to about 4% for sorghum.A balance sheet drawn at the end of each rotation showed that about 51.3% of the applied labelled N was accounted for in groundnut-sorghum-pigeon pea rotation, 70.9% in sorghumpigeon pea-groundnut, and 43.5% in pigeon pea-groundnut-sorghum. 相似文献
109.
Summary Small differences in N2 fixation by nodulated soybeans (Glycine max. (L.) Merr.), inoculated with various strains ofRhizobium japonicum, were assessed in field experiments using15N methodology, and compared with yields of plant dry matter and total N. Percentage of plant-N derived from atmospheric N2 and from fertilizer, and values of %15N atom excess had lower coefficients of variation than did total N and dry matter yield. Nevertheless the precision of estimates of kg N/ha fixed were sufficient to differentiate only the extremes of the range of strains tested, and there were discrepancies between ranking of strains based on % N derived from fertilizer and on total N yield. 相似文献
110.
Summary Field experiments were carried out using15N-labelled calcium nitrate, to investigate the relative uptake by barley of fertilizer-N and soil-N. On imperfectly drained till soils uptake of soil-N increased with increasing rate of fertilizer, but remained constant on a brown sand, possibly due to more efficient root exploration in the latter soil. In four out of five seasons, late uptake of soil-derived N was a major feature, and uptake from ploughed soil as compared with uptake from direct-drilled soil was correlated with seasonal rainfall patterns. Significantly higher quantities of both fertilizer- and soil-derived N were taken up by winter barley than by spring barley, reflecting the longer growth period and higher dry matter yield from the former crop. 相似文献