全文获取类型
收费全文 | 41305篇 |
免费 | 3064篇 |
国内免费 | 7篇 |
专业分类
44376篇 |
出版年
2023年 | 208篇 |
2022年 | 342篇 |
2021年 | 642篇 |
2020年 | 478篇 |
2019年 | 518篇 |
2018年 | 1277篇 |
2017年 | 1077篇 |
2016年 | 1285篇 |
2015年 | 1872篇 |
2014年 | 1844篇 |
2013年 | 2525篇 |
2012年 | 2977篇 |
2011年 | 2829篇 |
2010年 | 1772篇 |
2009年 | 1531篇 |
2008年 | 2283篇 |
2007年 | 2239篇 |
2006年 | 2192篇 |
2005年 | 1917篇 |
2004年 | 1882篇 |
2003年 | 1701篇 |
2002年 | 1583篇 |
2001年 | 850篇 |
2000年 | 788篇 |
1999年 | 717篇 |
1998年 | 415篇 |
1997年 | 316篇 |
1996年 | 305篇 |
1995年 | 301篇 |
1994年 | 239篇 |
1993年 | 222篇 |
1992年 | 411篇 |
1991年 | 412篇 |
1990年 | 344篇 |
1989年 | 309篇 |
1988年 | 300篇 |
1987年 | 257篇 |
1986年 | 249篇 |
1985年 | 247篇 |
1984年 | 197篇 |
1983年 | 154篇 |
1982年 | 138篇 |
1981年 | 170篇 |
1980年 | 138篇 |
1979年 | 185篇 |
1978年 | 157篇 |
1977年 | 133篇 |
1976年 | 139篇 |
1975年 | 126篇 |
1974年 | 131篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
121.
OBJECTIVE: To investigate whether low serum cholesterol concentration or changing serum cholesterol concentration is associated with risk of suicide in men. DESIGN: Cohort study with annual repeat measurements of serum cholesterol concentration (for up to four years). SETTING: Paris, France. SUBJECTS: 6393 working men, aged 43-52 in 1967-72, who had at least three measurements of serum cholesterol concentration. MAIN OUTCOME MEASURES: Individual change over time in serum cholesterol concentration (estimated using within person linear regression method); death from suicide during average of 17 years'' follow up after last examination. RESULTS: 32 men committed suicide during follow up. After adjustment for age and other factors, relative risk of suicide for men with low average serum cholesterol concentration (< 4.78 mmol/l) compared with those with average serum cholesterol concentration of 4.78-6.21 mmol/l was 3.16 (95% confidence interval 1.38 to 7.22, P = 0.007). Men whose serum cholesterol concentration decreased by more than 0.13 mmol/l a year had multivariate adjusted relative risk of 2.17 (0.97 to 4.84, P = 0.056) compared with those whose cholesterol remained stable (change of < or = 0.13 mmol/l a year). CONCLUSION: Both low serum cholesterol concentration and declining cholesterol concentration were associated with increased risk of death from suicide in men. Although there is some evidence in favour of a concomitant rather than a causal effect for interpreting these associations, long term surveillance of subjects included in trials of lipid lowering treatments seems warranted. 相似文献
122.
Christophe Demaison Denis David Franck Letourneur Jacques Thèze Sentob Saragosti Moncef Zouali 《Immunogenetics》1995,42(5):342-352
Using CD19 B-cell selection and polymerase chain reaction-amplified cDNA libraries, we analyzed the peripheral immunoglobulin heavy chain variable repertoire of three healthy adult donors. Here we report that most of the CD19+ circulating B cells expressed unmutated V
H-D-JH rearrangements. By specific V
H family hybridization, we show that V
H gene family utilization in the periphery roughly corresponds to the complexity of these families in the germline and appears to be relatively constant among the analyzed subjects. However, sequence data of clones picked at random from one IgM cDNA library reveals that in spite of this random utilization, the V
H gene expression in naive circulating B cells is highly biased towards the expression of a limited set of V
H genes. As previously reported by others, this restricted mechanism is also found for the D and J
H segments.The nucleotide sequence data reported in this paper have been submitted to the GenBank/EMBL nucleotide sequence database and have been assigned the accession numbers Z47213-Z47243 and Z47349 相似文献
123.
José de JesÚs Garcia M. Tuena de Gómez-Puyou A. Gómez-Puyou 《Journal of bioenergetics and biomembranes》1995,27(1):127-136
The effect of trifluoperazine (TFP) on the ATPase activity of soluble and paniculate F1ATPase and on ATP synthesis driven by succinate oxidation in submitochondrial particles from bovine heart was studied at pH 7.4 and 8.8. At the two pH. TFP inhibited ATP hydrolysis. Inorganic phosphate protected against the inhibiting action of TFP. The results on the effect of various concentrations of phosphate in the reversal of the action of TFP on hydrolysis at pH 7.4 and 8.8 showed that H2PO
4
–
is the species that competes with TFP. The effect of TFP on oxidative phosphorylation was studied at concentrations that do not produce uncoupling or affect the aerobic oxidation of succinate (<15M). TFP inhibited oxidative phosphorylation to a higher extent at pH 8.8 than at pH 7.4; this was through a diminution in theV
max, and an increase in theK
m for phosphate. Data on phosphate uptake during oxidative phosphorylation at several pH showed that H2PO
4
–
is the true substrate for oxidative phosphorylation. Thus, in both synthesis and hydrolysis of ATP, TFP and H2PO
4
–
interact with a common site. However, there is a difference in the sensitivity to TFP of ATP synthesis and hydrolysis; this is more noticeable at pH 8.8, i.e. ATPase activity of soluble F1 remains at about 40% of the activity of the control in a concentration range of TFP of 40–100M, whereas in oxidative phosphorylation 14M TFP produces a 60% inhibition of phosphate uptake. 相似文献
124.
Júlia Tandori László Nagy Ágnes Puskás Magdolna Droppa Gábor Horváth Péter Maróti 《Photosynthesis research》1995,45(2):135-146
A spontaneous mutant (R/89) of photosynthetic purple bacterium Rhodobacter sphaeroides R-26 was selected for resistance to 200 M atrazin. It showed increased resistance to interquinone electron transfer inhibitors of o-phenanthroline (resistance factor, RF=20) in UQo reconstituted isolated reaction centers and terbutryne in reaction centers (RF=55) and in chromatophores (RF=85). The amino acid sequence of the QB binding protein of the photosynthetic reaction center (the L subunit) was determined by sequencing the corresponding pufL gene and a single mutation was found (IleL229 Met). The changed amino acid of the mutant strain is in van der Waals contact with the secondary quinone QB. The binding and redox properties of QB in the mutant were characterized by kinetic (charge recombination) and multiple turnover (cytochrome oxidation and semiquinone oscillation) assays of the reaction center. The free energy for stabilization of QAQB
– with respect to QA
–QB was GAB=–60 meV and 0 meV in reaction centers and GAB=–85 meV and –46 meV in chromatophores of R-26 and R/89 strains at pH 8, respectively. The dissociation constants of the quinone UQo and semiquinone UQo
– in reaction centers from R-26 and R/89 showed significant and different pH dependence. The observed changes in binding and redox properties of quinones are interpreted in terms of differential effects (electrostatics and mesomerism) of mutation on the oxidized and reduced states of QB.Abbreviations BChl
bacteriochlorophyll
- Ile
isoleucine
- Met
methionin
- P
primary donor
- QA
primary quinone acceptor
- QB
secondary quinone acceptor
- RC
reaction center protein
- UQo
2,3-dimethoxy-5-methyl benzoquinone
- UQ10
ubiquinone 50
This work is dedicated to the memory of Randall Ross Stein (1954–1994) and is, in a small way, a testament to the impact which Randy's ideas have had on the development of the field of competitive herbicide binding. 相似文献
125.
126.
András Hrabák Tamás Bajor Garry J. Southan Andrew L. Salzman Csaba Szabó 《Life sciences》1997,60(26):111-PL401
Novel, non-arginine based compounds have been identified as potent inhibitors of nitric oxide synthase (NOS). Members of the isothiourea and mercapto-alkylguanidine classes have generated much interest, as some members of these classes show selectivity towards the inducible isoform of NOS (iNOS), which plays a role in inflammation and shock. Here we compared the effect of a number of these compounds as well as L-arginine based NOS inhibitor reference compounds on macrophage-derived and liver arginase and macrophage iNOS activities. From the nonarginine based NOS inhibitors studied only S-aminoethyl-isothiourea (AETU) caused a slight inhibition of arginase activity. This inhibition was kinetically competitive and due to the rearrangement of AETU to mercapto-ethylguanidine (MEG). The weak inhibitory effect of non-arginine based iNOS inhibitors on arginase activity further supports the view that such compounds may be of practical use for inhibition of NO production in cells simultaneously expressing iNOS and arginase. 相似文献
127.
Twenty-five accessions belonging to the genus Cytisus and allies (Genisteae: Leguminosae), potentially useful for agrosylvopastoral systems in the Mediterranean basin, were analyzed using isoenzymes. The aim was to assess the taxonomic usefulness of these characters at the specific and generic levels, and its value as an aid for the taxonomy. A high level of infrageneric and infraspecific variability was detected. Numerical analysis showed that isoenzymes are useful for the characterization both at the specific and generic levels. The obtained dendrogram supports the generic arrangement proposed by Bisby [(1981) Advances in Legume Systematics, Vol. 1, pp. 409–425], therefore helping to elucidate the difficult taxonomy of the studied species. 相似文献
128.
pAMβ1 resolvase has an atypical recombination site and requires a histone-like protein HU 总被引:1,自引:1,他引:0
The broad-host-range plasmid pAMβ1 from Gram-positive bacteria encodes a resolvase, designated Resβ, which shares homology with the proteins of the resolvase—invertase family. Here we report the purification and in vitro characterization of Resβ. This resolvase is particular in two aspects: it has an atypical binding site and requires a cofactor to promote resolution in vitro . Resβ binds to two regions within its resolution site res . One contains two inverted repeats (R1 and R2), the other contains only one repeat (R3). The cofactor required for resolution in vitro is present in crude extracts of both Bacillus subtilis and Escherichia coli and can be substituted by the E. coli histone-like protein HU. The possible mode of action of HU in the resolution process is discussed. 相似文献
129.
130.
Protein kinase(s) in bovine brain coated vesicles 总被引:2,自引:0,他引:2
Purified bovine brain coated vesicles contain protein kinase activity which phosphorylates 165, 54 and 50 kDa protein substrates. These phosphorylations do not seem to be induced by a unique protein kinase: indeed, the three substrates present different localizations in coated vesicles, the phosphorylation sites are either serine or threonine residues and vanadate and ATP[gamma S] have different effects on 32P incorporation in the substrates. Comparison of the coated vesicle protein and phosphorylation patterns from different tissues and animal origins shows that only the 50 kDa protein phosphorylation is always observed, compared to the great diversity in other minor phosphorylations which are observed or not in the various coated vesicles. The possible presence of a 50 kDa phosphoprotein phosphatase is also discussed. It is suggested that the 50 kDa protein with its connected specific kinase and phosphatase seems to constitute a regulatory system present in coated vesicles. 相似文献