全文获取类型
收费全文 | 3162篇 |
免费 | 357篇 |
国内免费 | 1篇 |
出版年
2023年 | 28篇 |
2022年 | 40篇 |
2021年 | 93篇 |
2020年 | 53篇 |
2019年 | 60篇 |
2018年 | 78篇 |
2017年 | 57篇 |
2016年 | 92篇 |
2015年 | 156篇 |
2014年 | 167篇 |
2013年 | 187篇 |
2012年 | 261篇 |
2011年 | 236篇 |
2010年 | 122篇 |
2009年 | 112篇 |
2008年 | 143篇 |
2007年 | 157篇 |
2006年 | 145篇 |
2005年 | 141篇 |
2004年 | 116篇 |
2003年 | 113篇 |
2002年 | 101篇 |
2001年 | 39篇 |
2000年 | 35篇 |
1999年 | 47篇 |
1998年 | 35篇 |
1997年 | 26篇 |
1996年 | 39篇 |
1995年 | 28篇 |
1994年 | 17篇 |
1993年 | 23篇 |
1992年 | 31篇 |
1991年 | 22篇 |
1990年 | 37篇 |
1989年 | 38篇 |
1988年 | 32篇 |
1987年 | 22篇 |
1986年 | 17篇 |
1985年 | 21篇 |
1984年 | 27篇 |
1983年 | 19篇 |
1981年 | 16篇 |
1979年 | 26篇 |
1978年 | 16篇 |
1977年 | 13篇 |
1974年 | 13篇 |
1973年 | 19篇 |
1972年 | 17篇 |
1970年 | 17篇 |
1969年 | 15篇 |
排序方式: 共有3520条查询结果,搜索用时 15 毫秒
131.
132.
Glutamate-l-semialdehyde (GSA) aminotransferase catalyses the final step in the C5 pathway converting glutamate to the tetrapyrrole precursor δ-aminolaevulinic acid. This enzyme is sensitive to gabaculine (2,3-dihydro-3-amino benzoic acid) and to 4-amino-5-fluoropentanoic acid (AFPA), which are irreversible, mechanism-based inhibitors of pyridoxal phosphatedependent enzymes. Spontaneous mutants of Synechococcus PCC6301 resistant to these inhibitors contain altered enzyme that displays corresponding resistance to high concentrations of the inhibitor. The enzyme from strain GR6, resistant to both inhibitors, contains a three-amino-acid deletion at positions 5–7 and a Met248 → Ile substitution. The enzyme from strain K40 resistant to AFPA but not to gabaculine, contains a Ser163 → Thr substitution. GSA aminotransferases containing either the deletion or the substitution that are characteristic of the GR6 mutant were produced in Escherichia coli using the expression vector pMalc2. These engineered mutant enzymes were characterized in terms of their catalytic parameters and sensitivities to gabaculine and AFPA. Furthermore, maltose binding protein/aminotransferase fusion proteins were characterized spectrophotometrically to monitor the interaction of bound cofactor with diamino- and dioxocompounds related to the substrate and both inhibitors. Results were compared with those for similarly produced recombinant wild-type, K40 and GR6 GSA aminotransferases. The engineered products with either the N-terminal deletion or the Met248 → Ile substitution displayed catalytic efficiencies that were intermediate between the wild-type and GR6 or K40 enzymes. However, with respect to their absorption spectra, sensitivity to inhibitors and the reactivity of bound cofactor, they were essentially wild-type. These in vitro studies demonstrate that both changes in enzyme structure are necessary to obtain the distinctive properties of the GR6 aminotransferase, including resistance to high concentrations of gabaculine and AFPA. 相似文献
133.
Nicholas T. Loux David S. Brown Claudia R. Chafin Jerry D. Allison Sayed M. Hassan 《Chemical Speciation and Bioavailability》2013,25(3):111-125
AbstractCoupled geochemical speciation/transport models are being developed to assess potential transport of metal contaminants in the subsurface environment. In a test of the geochemical speciation portion of the effort, MINTEQA2 model predictions are compared with laboratory data concerning the pH dependent partitioning behavior of eight cationic contaminants (Ba, Be, Cd, Cu, Ni, Pb, Tl and Zn) on a sandy aquifer material in an oxidized environment. MINTEQA2 contains provisions for describing potential attenuation due to both mineral phase precipitation processes and adsorption processes resulting from amorphous iron oxides in aquifer materials (MIT Diffuse Layer Model). In the comparison, several trends were discerned. (1) Adsorptive processes tend to better describe the pH-dependent partitioning behavior of transition metals (especially Pb, Zn and Ni). (2) Cd behavior is better described by precipitation as a cadmium carbonate phase. (3) Cu behavior is not reasonably described by the model. (4) Ba and Be comparisons are poor (although presumably their partitioning behavior results from adsorptive and/or pH sensitive solid solution processes). (5) unlike the other elements, the behavior of Tl is relatively insensitive to pH. 相似文献
134.
135.
Gancho Slavov Paul Robson Elaine Jensen Edward Hodgson Kerrie Farrar Gordon Allison Sarah Hawkins Sian Thomas‐Jones Xue‐Feng Ma Lin Huang Timothy Swaller Richard Flavell John Clifton‐Brown Iain Donnison 《Global Change Biology Bioenergy》2013,5(5):562-571
Species and hybrids of Miscanthus are a promising energy crop, but their outcrossing mating systems and perennial life cycles are serious challenges for breeding programs. One approach to accelerating the domestication of Miscanthus is to harness the tremendous genetic variation that is present within this genus using phenotypic data from extensive field trials, high‐density genotyping and sequencing technologies, and rapidly developing statistical methods of relating phenotype to genotype. The success of this approach, however, hinges on detailed knowledge about the population genetic structure of the germplasm used in the breeding program. We therefore used data for 120 single‐nucleotide polymorphism and 52 simple sequence repeat markers to depict patterns of putatively neutral population structure among 244 Miscanthus genotypes grown in a field trial near Aberystwyth (UK) and delineate a population of 145 M . sinensis genotypes that will be used for association mapping and genomic selection. Comparative multivariate analyses of molecular marker and phenotypic data for 17 traits related to phenology, morphology/biomass, and cell wall composition revealed significant geographic patterns in this population. A longitudinal cline accounted for a substantial proportion of molecular marker variation (R2 = 0.60, P = 3.4 × 10?15). In contrast, genetic variation for phenotypic traits tended to follow latitudinal and altitudinal gradients, with several traits appearing to have been affected by divergent selection (i.e., QST >> FST). These contrasting geographic trends are unusual relative to other plants and provide opportunities for powerful studies of phenotype–genotype associations and the evolutionary history of M. sinensis. 相似文献
136.
Gabriel Gomez Jianwu Pei Waithaka Mwangi L. Garry Adams Allison Rice-Ficht Thomas A. Ficht 《PloS one》2013,8(3)
Brucella is the etiologic agent of brucellosis, one of the most common and widely distributed zoonotic diseases. Its highly infectious nature, the insidious, systemic, chronic, debilitating aspects of the disease and the lack of an approved vaccine for human use in the United States are features that make Brucella a viable threat to public health. One of the main impediments to vaccine development is identification of suitable antigens. In order to identify antigens that could potentially be used in a vaccine formulation, we describe a multi-step antigen selection approach. We initially used an algorithm (Vaxign) to predict ORF encoding outer membrane proteins with antigenic determinants. Differential gene expression during acute infection and published evidence for a role in virulence were used as criteria for down-selection of the candidate antigens that resulted from in silico prediction. This approach resulted in the identification of nine Brucella melitensis outer membrane proteins, 5 of which were recombinantly expressed and used for validation. Omp22 and Hia had the highest in silico scores for adhesin probability and also conferred invasive capacity to E. coli overexpressing recombinant proteins. With the exception of FlgK in the goat, all proteins reacted to pooled sera from exposed goats, mice, and humans. BtuB, Hia and FlgK stimulated a mixed Th1–Th2 response in splenocytes from immunized mice while BtuB and Hia elicited NO release from splenocytes of S19 immunized mice. The results support the applicability of the current approach to the identification of antigens with immunogenic and invasive properties. Studies to assess immunogenicity and protective efficacy of individual proteins in the mouse are currently underway. 相似文献
137.
Andrew R. Davis Daniel Coleman Allison Broad Maria Byrne Symon A. Dworjanyn Rachel Przeslawski 《PloS one》2013,8(2)
Climate change and ocean acidification will expose marine organisms to synchronous multiple stressors, with early life stages being potentially most vulnerable to changing environmental conditions. We simultaneously exposed encapsulated molluscan embryos to three abiotic stressors—acidified conditions, elevated temperate, and solar UV radiation in large outdoor water tables in a multifactorial design. Solar UV radiation was modified with plastic filters, while levels of the other factors reflected IPCC predictions for near-future change. We quantified mortality and the rate of embryonic development for a mid-shore littorinid, Bembicium nanum, and low-shore opisthobranch, Dolabrifera brazieri. Outcomes were consistent for these model species with embryos faring significantly better at 26°C than 22°C. Mortality sharply increased at the lowest temperature (22°C) and lowest pH (7.6) examined, producing a significant interaction. Under these conditions mortality approached 100% for each species, representing a 2- to 4-fold increase in mortality relative to warm (26°C) non-acidified conditions. Predictably, development was more rapid at the highest temperature but this again interacted with acidified conditions. Development was slowed under acidified conditions at the lowest temperature. The presence of UV radiation had minimal impact on the outcomes, only slowing development for the littorinid and not interacting with the other factors. Our findings suggest that a warming ocean, at least to a threshold, may compensate for the effects of decreasing pH for some species. It also appears that stressors will interact in complex and unpredictable ways in a changing climate. 相似文献
138.
139.
Melissa A. Napolitano Sharon Hayes Gary G. Bennett Allison K. Ives Gary D. Foster 《Obesity (Silver Spring, Md.)》2013,21(1):25-31
Objective:
Between 31 and 35% of the college‐aged population is overweight or obese, yet few weight loss trials for this population have been conducted. This study examined the feasibility, acceptability, and initial efficacy of a technology‐based 8‐week weight loss intervention among college students.Design and Methods:
Students (N = 52) were randomly assigned to one of the three arms: Facebook (n = 17); Facebook Plus text messaging and personalized feedback (n = 18); Waiting List control (n = 17), with assessments at 4 weeks and 8 weeks (post‐treatment). Participants were 20.47 ± 2.19 years old, 86.45 ± 17.11 kg, with a body mass index of 31.36 ± 5.3 kg/m2. Participants were primarily female (86.5%), and the sample was racially diverse (57.7% Caucasian, 30.8% African American, 5.8% Hispanic, and 5.7% other races).Results:
The primary outcome was weight loss after 8 weeks (post‐treatment); 96.0% of the participants completed this assessment. At 8 weeks, the Facebook Plus group had significantly greater weight loss (?2.4 ± 2.5 kg) than the Facebook (?0.63 ± 2.4 kg) and Waiting List (?0.24 ± 2.6 kg) (both Ps < 0.05). Weight change at 8 weeks was not significantly different between the Facebook and Waiting List groups.Conclusions:
Results show preliminary efficacy and acceptability of the two active intervention arms (97.0% found the program helpful, 81.3% found the videos/handouts helpful, and 100% would recommend the program to others). Results indicate the potential for an innovative weight loss intervention that uses technology platforms (Facebook and text messaging) that are frequently used and already integrated into the cultural life of college students.140.