全文获取类型
收费全文 | 3447篇 |
免费 | 215篇 |
国内免费 | 1篇 |
专业分类
3663篇 |
出版年
2023年 | 35篇 |
2022年 | 59篇 |
2021年 | 105篇 |
2020年 | 67篇 |
2019年 | 87篇 |
2018年 | 102篇 |
2017年 | 81篇 |
2016年 | 116篇 |
2015年 | 178篇 |
2014年 | 226篇 |
2013年 | 216篇 |
2012年 | 289篇 |
2011年 | 234篇 |
2010年 | 152篇 |
2009年 | 122篇 |
2008年 | 170篇 |
2007年 | 173篇 |
2006年 | 143篇 |
2005年 | 131篇 |
2004年 | 126篇 |
2003年 | 121篇 |
2002年 | 111篇 |
2001年 | 32篇 |
2000年 | 16篇 |
1999年 | 14篇 |
1998年 | 37篇 |
1997年 | 14篇 |
1996年 | 15篇 |
1995年 | 26篇 |
1994年 | 15篇 |
1993年 | 15篇 |
1992年 | 25篇 |
1991年 | 16篇 |
1990年 | 15篇 |
1989年 | 16篇 |
1988年 | 16篇 |
1987年 | 13篇 |
1986年 | 10篇 |
1985年 | 16篇 |
1984年 | 12篇 |
1981年 | 15篇 |
1980年 | 12篇 |
1979年 | 8篇 |
1978年 | 10篇 |
1976年 | 9篇 |
1973年 | 15篇 |
1971年 | 9篇 |
1966年 | 7篇 |
1935年 | 7篇 |
1917年 | 7篇 |
排序方式: 共有3663条查询结果,搜索用时 15 毫秒
961.
Svenja Kiesel Denise W?tzlich Christiane Lange Edward Reijerse Markus J. Br?cker Wolfhart Rüdiger Wolfgang Lubitz Hugo Scheer Jürgen Moser Dieter Jahn 《The Journal of biological chemistry》2015,290(2):1141-1154
Bacteriochlorophyll a biosynthesis requires the stereo- and regiospecific two electron reduction of the C7-C8 double bond of chlorophyllide a by the nitrogenase-like multisubunit metalloenzyme, chlorophyllide a oxidoreductase (COR). ATP-dependent COR catalysis requires interaction of the protein subcomplex (BchX)2 with the catalytic (BchY/BchZ)2 protein to facilitate substrate reduction via two redox active iron-sulfur centers. The ternary COR enzyme holocomplex comprising subunits BchX, BchY, and BchZ from the purple bacterium Roseobacter denitrificans was trapped in the presence of the ATP transition state analog ADP·AlF4−. Electron paramagnetic resonance experiments revealed a [4Fe-4S] cluster of subcomplex (BchX)2. A second [4Fe-4S] cluster was identified on (BchY/BchZ)2. Mutagenesis experiments indicated that the latter is ligated by four cysteines, which is in contrast to the three cysteine/one aspartate ligation pattern of the closely related dark-operative protochlorophyllide a oxidoreductase (DPOR). In subsequent mutagenesis experiments a DPOR-like aspartate ligation pattern was implemented for the catalytic [4Fe-4S] cluster of COR. Artificial cluster formation for this inactive COR variant was demonstrated spectroscopically. A series of chemically modified substrate molecules with altered substituents on the individual pyrrole rings and the isocyclic ring were tested as COR substrates. The COR enzyme was still able to reduce the B ring of substrates carrying modified substituents on ring systems A, C, and E. However, substrates with a modification of the distantly located propionate side chain were not accepted. A tentative substrate binding mode was concluded in analogy to the related DPOR system. 相似文献
962.
T?res Theorell Anna Nyberg Constanze Leineweber Linda L. Magnusson Hanson Gabriel Oxenstierna Hugo Westerlund 《PloS one》2012,7(9)
Background
The way in which leadership is experienced in different socioeconomic strata is of interest per se, as well as how it relates to employee mental health.Methods
Three waves of SLOSH (Swedish Longitudinal Occupational Survey of Health, a questionnaire survey on a sample of the Swedish working population) were used, 2006, 2008 and 2010 (n = 5141). The leadership variables were: “Non-listening leadership” (one question: “Does your manager listen to you?” - four response categories), “Self centered leadership” (sum of three five-graded questions – “non-participating”, “asocial” and “loner”). The socioeconomic factors were education and income. Emotional exhaustion and depressive symptoms were used as indicators of mental health.Results
Non-listening leadership was associated with low income and low education whereas self-centered leadership showed a weaker relationship with education and no association at all with income. Both leadership variables were significantly associated with emotional exhaustion and depressive symptoms. “Self centered” as well as “non-listening” leadership in 2006 significantly predicted employee depressive symptoms in 2008 after adjustment for demographic variables. These predictions became non-significant when adjustment was made for job conditions (demands and decision latitude) in the “non-listening” leadership analyses, whereas predictions of depressive symptoms remained significant after these adjustments in the “self-centered leadership” analyses.Conclusions
Our results show that the leadership variables are associated with socioeconomic status and employee mental health. “Non-listening” scores were more sensitive to societal change and more strongly related to socioeconomic factors and job conditions than “self-centered” scores. 相似文献963.
Toxic cyanobacteria blooms are increasingly frequent and object of greater concern due to its ecological and health impacts.
One important lack in the toxic cyanobacteria research field is to understand which parameters influence most and how they
operate to regulate the overall levels of cyanotoxins in a body of water. MC concentration is believed to be influenced by
changes in several seasonal environmental factors that influence the succession of toxic cyanobacteria. In the last years,
qPCR (quantitative polymerase chain reaction) has been applied to determine the seasonal and temporal shifts in the proportions
of MC-producing and non-MC-producing subpopulations by quantifying both mcy genotypes and total population numbers. We discuss the most prominent and recent studies using qPCR to address hepatotoxic
cyanobacteria population dynamics and evaluate how they helped understanding the factors promoting the growth of toxic strains
in situ and the succession of hepatotoxin-producing genera in natural populations. 相似文献
964.
(Co)variance components, genetic parameters and annual trends for calf weights in a Brahman herd kept on floodable savanna 总被引:1,自引:0,他引:1
Plasse D Verde O Arango J Camaripano L Fossi H Romero R Rodriguez CM Rumbos JL 《Genetics and molecular research : GMR》2002,1(4):282-297
(Co)variance components and genetic parameters were estimated for body weights of an elite Brahman herd under a designed, supervised management and genetic program, including strategic artificial insemination (AI). Restricted maximum likelihood methods were used with a univariate animal model for birth weight (BW) and a bivariate model for weaning weight (205-day weight, 205W) and 18-month weight (548-day weight, 548W). Models included random animal direct and maternal genetic effects, maternal permanent environmental effect (c2), and sex-year-month of birth-age of dam and genetic group (identified and unidentified paternity), as fixed effects. Analysis A1 included all calves and analysis A2 included only those with identified sires. Of the 8,066 calves born, 36% were progeny of AI, 11% from single sire and 53% from multi-sire herds. They were born from 1985 to 1998, from 2559 dams and 146 sires (78 identified). Estimates of direct, maternal and total heritabilities from A1 for BW, 205W and 548W were: 0.23, 0.07 and 0.30; 0.08, 0.14 and 0.16; 0.16, 0.04 and 0.28, respectively. Corresponding estimates of direct maternal genetic correlations were 0.22, 0.07 and 0.86, and c2 estimates were 0.04, 0.14 and 0.04, respectively. Estimates of direct and maternal genetic, and permanent environmental correlations between 205W and 548W were: 0.66, 0.70 and 1.00. Variances and genetic parameters from A1 and A2 were, in general, very similar. Estimates of phenotypic, and direct and maternal genetic trends per year from A1 were: 0.393, 0.004 and 0.003 kg (BW), 3.367, 0.142 and 0.115 kg (205W), 1.813, 0.263 and 0.095 kg (548W). Estimates of direct and maternal genetic trends from A2 were: 0.033 and -0.002 kg (BW); 0.186 and 0.276 kg (205W); 0.471 and 0.136 kg (548W). The modern selection methods that have been used recently should be continued, with emphasis on the improvement of cow efficiency for sustainable beef production on floodable savanna combined with improved pasture. 相似文献
965.
C. Darrin Hulsey Benjamin P. Keck Hugo Alamillo Brian C. O'Meara 《Molecular ecology resources》2013,13(3):347-353
Resolving the evolutionary history of rapidly diversifying lineages like the Lake Malawi Cichlid Flock demands powerful phylogenetic tools. Although this clade of over 500 species of fish likely diversified in less than two million years, the availability of extensive sequence data sets, such as complete mitochondrial genomes, could help resolve evolutionary patterns in this group. Using a large number of newly developed primers, we generated whole mitochondrial genome sequences for 14 Lake Malawi cichlids. We compared sequence divergence across protein‐coding regions of the mitochondrial genome and also compared divergence in the mitochondrial loci to divergence at two nuclear protein‐coding loci, Mitfb and Dlx2. Despite the widespread sharing of haplotypes of identical sequences at individual loci, the combined use of all protein‐coding mitochondrial loci provided a bifurcating phylogenetic hypothesis for the exemplars of major lineages within the Lake Malawi cichlid radiation. The primers presented here could have substantial utility for evolutionary analyses of mitochondrial evolution and hybridization within this diverse clade. 相似文献
966.
Alfonso C. Hernández-Romieu Carlos del Rio Juan Eugenio Hernández-ávila Hugo Lopez-Gatell José Antonio Izazola-Licea Patricia Uribe Zú?iga Mauricio Hernández-ávila 《PloS one》2016,11(3)
In Mexico, public health services have provided universal access to antiretroviral therapy (ART) since 2004. For individuals receiving HIV care in public healthcare facilities, the data are limited regarding CD4 T-lymphocyte counts (CD4e) at the time of entry into care. Relevant population-based estimates of CD4e are needed to inform strategies to maximize the impact of Mexico’s national ART program, and may be applicable to other countries implementing universal HIV treatment programs. For this study, we retrospectively analyzed the CD4e of persons living with HIV and receiving care at state public health facilities from 2007 to 2014, comparing CD4e by demographic characteristics and the marginalization index of the state where treatment was provided, and assessing trends in CD4e over time. Our sample included 66,947 individuals who entered into HIV care between 2007 and 2014, of whom 79% were male. During the study period, the male-to-female ratio increased from 3.0 to 4.3, reflecting the country''s HIV epidemic; the median age at entry decreased from 34 years to 32 years. Overall, 48.6% of individuals entered care with a CD4≤200 cells/μl, ranging from 42.2% in states with a very low marginalization index to 52.8% in states with a high marginalization index, and from 38.9% among individuals aged 18–29 to 56.5% among those older than 50. The adjusted geometric mean (95% confidence interval) CD4e increased among males from 135 (131,142) cells/μl in 2007 to 148 (143,155) cells/μl in 2014 (p-value<0.0001); no change was observed among women, with a geometric mean of 178 (171,186) and 171 (165,183) in 2007 and 2014, respectively. There have been important gains in access to HIV care and treatment; however, late entry into care remains an important barrier in achieving optimal outcomes of ART in Mexico. The geographic, socioeconomic, and demographic differences observed reflect important inequities in timely access to HIV prevention, care, and treatment services, and highlight the need to develop contextual and culturally appropriate prevention and HIV testing strategies and linkage programs. 相似文献
967.
968.
Thomas Altmann Gisela Felix Alison Jessop Annette Kauschmann Ursula Uwer Hugo Pe?a-Cortés Lothar Willmitzer 《Molecular & general genetics : MGG》1995,247(5):646-652
Using a two-component Ac/Ds system consisting of a stabilized Ac element (Acc1) and a non-autonomous element (DsA), 650 families of plants carrying independent germinal DsA excisions/transpositions were isolated. Progenies of 559 of these Acc1/DsA families, together with 43 families of plants selected for excision/transposition of wild-type (wt)Ac, were subjected to a broad screening program for mutants exhibiting visible alterations. This resulted in the identification of 48 mutants showing a wide variety of mutant phenotypes, including embryo lethality (24 mutants), chlorophyll defects (5 mutants), defective seedlings (2 mutants), reduced fertility (5 mutants), reduced size (3 mutants), altered leaf morphology (2 mutants), dark green, unexpanded rosette leaves (3 mutants), and aberrant flower or shoot morphology (4 mutants). To test whether these mutants were due to transposon insertions, a series of Southern blot experiments was performed on 28 families, comparing in each case several mutant plants with others showing the wild-type phenotype. A preliminary analysis revealed in 4 of the 28 families analyzed a common, novel DsA fragment in all mutant plants, which was present only in heterozygous plants with wt phenotype, as expected for DsA insertion mutations. These four mutants included two showing embryo lethality, one with dark green, unexpanded rosette leaves and stunted inflorescences, and one with curly growth of stems, leaves and siliques. Further evidence for DsA insertion mutations was obtained for one embryo lethal mutant and for the stunted mutant, while in case of the second embryo lethal mutant, the DsA insertion could be separated from the mutant locus by genetic recombination. 相似文献
969.
Dynamics on human Toll‐like receptor 4 complexation to MD‐2: The coreceptor stabilizing function 下载免费PDF全文
The interaction between human Toll‐like receptor 4 (hTLR4) and its coreceptor, myeloid differentiation factor 2 (MD‐2), is important in Gram‐negative bacteria lipopolysaccharide (LPS) recognition. In this process, MD‐2 recognizes LPS and promotes the dimerization of the complex hTLR4–MD‐2–LPS, triggering an intracellular immune signaling. In this study, we employed distinct computational methods to explore the dynamical properties of the hTLR4–MD‐2 complex and investigated the implications of the coreceptor complexation to the structural biology of hTLR4. We characterized both global and local dynamics of free and MD‐2 complexed hTLR4, in both (hTLR4–MD‐2)1 and (hTLR4–MD‐2)2 states. Both molecular dynamics and normal mode analysis reveled a stabilization of the terminal regions of hTLR4 upon complexation to MD‐2. We are able to identify conserved important residues involved on the hTLR4–MD‐2 interaction dynamics and disclose C‐terminal motions that may be associated to the signaling process upon oligomerization. Proteins 2015; 83:373–382. © 2014 Wiley Periodicals, Inc. 相似文献
970.
Gerardo Valerio-Alfaro Hugo S. García Héctor Luna Raymundo Cruz Almanza 《Biotechnology letters》2000,22(7):575-578
Candida antarctica lipase catalyzed the aminolysis of 2-hydroxy esters with amines in organic solvents to yield the corresponding 2-hydroxy amides. The reactions proceeded at 28–30 °C in dioxane for 6 h with 3 mM substrates with yields ranging between 45% (w/w) (for branched substrates) to 88% (w/w) (for linear substrates). Although the reaction was not enantioselective, because of its simplicity it represents an alternative method for the synthesis of functionalised amides. 相似文献