全文获取类型
收费全文 | 6685篇 |
免费 | 644篇 |
国内免费 | 2篇 |
出版年
2021年 | 78篇 |
2019年 | 59篇 |
2018年 | 90篇 |
2017年 | 68篇 |
2016年 | 124篇 |
2015年 | 204篇 |
2014年 | 237篇 |
2013年 | 285篇 |
2012年 | 362篇 |
2011年 | 372篇 |
2010年 | 241篇 |
2009年 | 206篇 |
2008年 | 308篇 |
2007年 | 319篇 |
2006年 | 299篇 |
2005年 | 294篇 |
2004年 | 303篇 |
2003年 | 270篇 |
2002年 | 301篇 |
2001年 | 55篇 |
1999年 | 77篇 |
1998年 | 88篇 |
1997年 | 68篇 |
1996年 | 63篇 |
1995年 | 56篇 |
1994年 | 52篇 |
1993年 | 55篇 |
1992年 | 49篇 |
1991年 | 45篇 |
1990年 | 40篇 |
1989年 | 43篇 |
1988年 | 47篇 |
1987年 | 45篇 |
1986年 | 39篇 |
1985年 | 76篇 |
1984年 | 79篇 |
1983年 | 66篇 |
1982年 | 127篇 |
1981年 | 127篇 |
1980年 | 104篇 |
1979年 | 84篇 |
1978年 | 65篇 |
1977年 | 47篇 |
1976年 | 65篇 |
1975年 | 40篇 |
1974年 | 52篇 |
1973年 | 65篇 |
1971年 | 39篇 |
1970年 | 41篇 |
1960年 | 38篇 |
排序方式: 共有7331条查询结果,搜索用时 31 毫秒
101.
The Delaware Bay is characterized as having greater nutrient and turbidity levels than the Chesapeake Bay. In reference to these differences, a one year study was conducted to identify any similarities and differences in the phytoplankton populations in these estuaries. The results indicated patterns of similarity in the diatom composition, with the total phytoplankton assemblage forming two site groups along a salinity gradient in each bay. These site groups were associated with stations located in the tidal fresh-oligohaline and meso-polyhaline regions of both estuaries. The seasonal concentrations of diatoms and total phytoplankton in both of these regions were higher in the Chesapeake Bay.Subtle differences between the two estuaries include a more diversified and abundant assemblage of neritic phytoplankters (including dinoflagellates) are present in the lower Chesapeake Bay. In contrast, a diatom dominated community is more characteristic of Delaware Bay. It is suggested the entry of neritic species into lower regions of the estuaries was enhanced by the reduced amount of rainfall and flow rates that occurred during the study period. The greater success of neritic species in the Chesapeake Bay is attributed to the lower turbidity of that estuary compared to Delaware Bay. 相似文献
102.
Sharon X. Chen Charles C. Hardin Harold E. Swaisgood 《Journal of Protein Chemistry》1993,12(5):613-625
Incubation of -lactoglobulin with immobilized trypsin at 5–10°C results in a time-dependent release of several fragments of the core domain in yields approaching 15%. Digests were fractionated by ion-exchange chromatography with a Mono Q HR5/5 column and analyzed after disulfide reduction by polyacrylamide gel electrophoresis in sodium dodecylsulfate. Three fragments with approximate molecular weights of 13.8, 9.6, and 6.7 kD were identified. The fraction from ion-exchange chromatography yielding the 6.7 kD fraction after disulfide reduction was further characterized because it was most homogeneous and gave the highest yield. The C-terminal cleavage site of the 6.7 kD core fragment appeared to be Lys100 or Lys101 as determined by C-terminal amino acid analysis. The exact masses, after reduction with dithiothreitol, are 6195 and 6926 as determined by laser desorption mass spectrometry, corresponding to residues 48–101 and 41–100. Prior to reduction, -lactoglobulin C-terminal residues 149–162 are connected to these core domain fragments as shown by C-terminal analysis and mass spectrometry. Structural studies indicate that these 7.9 and 8.6 kD core domain fragments released by immobilized trypsin retain much of their native structure. CD spectra indicate the presence of antiparallel -sheet structure similar to the native protein but the -helix is lost. Spectra in the aromatic region indicate the existence of tertiary structure. Moreover, structural transitions in urea are completely reversible as measured by CD spectra, although the extrapolated G
D
H20
and the urea concentration at the transition midpoint are lower than for the native protein. The core domain fragments also display apH-dependent binding to immobilizedtrans-retinal as does intact protein. A single endotherm is obtained for both core domain fragments and native protein upon differential scanning calorimetry, but again, the domain is less stable as indicated by a transition peak maxima of 56.9°C as compared with 81.1°C for native protein.Abbreviations used: CD, circular dichroism; CPG, controlled pore glass; DSC, differential scanning calorimetry; DTT, dithiothreitol; FPLC, fast flow liquid chromatography; HPLC, high-performance liquid chromatography; PITC, phenylisothiocyanate; SDS-PAGE, sodium dodecyl sulfate-polyacrylamide gel electrophoresis; TEA, triethylamine; UV, ultraviolet. 相似文献
103.
Disa cardinalis and three populations within the D. tripetaloides species complex contain variation in their chloroplast DNA (cpDNA) variability. All four taxa possessed unique cpDNAs and sequence divergence values ranged from 0.34 to 1.03%. A phylogeny of these genomes was reconstructed, along with the genomes of three other species, D. racemosa, D. uniflora and D. venosa, all of which are also section Disa and series RAcemosae, to determine the relationship of these closely related species to the D. tripetaloides complex. A phylogeny of the taxa using morphological data was also reconstructed. Outgroup comparison was made with D. sagittalis, a member of section Coryphaea. Although the molecular and morphological data were not completely congruent, both data types revealed D. cardinalis, rather than D. tripetaloides ssp. aurata, to be more closely allied with D. tripetaloides ssp. tripetaloides, suggesting that D. tripetaloides ssp. aurata should be elevated to species rank. Additionally, the high sequence divergence observed between the Natal and Cape populations, coupled with their geographical isolation and alternate flowering seasons, suggests that these two D. tripetaloides ssp. tripetaloides populations may, in fact, be more appropriately ranked as subspecies. 相似文献
104.
Susanne Popp Anna Jauch Detlev Schindler Michael R. Speicher Christoph Lengauer Helen Donis-Keller Harold C. Riethman Thomas Cremer 《Human genetics》1993,92(6):527-532
The identification of marker chromosomes in clinical and tumor cytogenetics by chromosome banding analysis can create problems. In this study, we present a strategy to define minute chromosomal rearrangements by multicolor fluorescence in situ hybridization (FISH) with whole chromosome painting probes derived from chromosome-specific DNA libraries and Alu-polymerase chain reaction (PCR) products of various region-specific yeast artificial chromosome (YAC) clones. To demonstrate the usefulness of this strategy for the characterization of chromosome rearrangements unidentifiable by banding techniques, an 8p+ marker chromosome with two extra bands present in the karyotype of a child with multiple anomalies, malformations, and severe mental retardation was investigated. A series of seven-color FISH experiments with sets of fluorochrome-labeled DNA library probes from flow-sorted chromosomes demonstrated that the additional segment on 8p+ was derived from chromosome 6. For a more detailed characterization of the marker chromosome, three-color FISH experiments with library probes specific to chromosomes 6 and 8 were performed in combination with newly established telomeric and subtelomeric YAC clones from 6q25, 6p23, and 8p23. These experiments demonstrated a trisomy 6pter6p22 and a monosomy 8pter8p23 in the patient. The present limitations for a broad application of this strategy and its possible improvements are discussed.Dedicated to Professor Dr. U. Wolf on the occasion of his 60th birthday 相似文献
105.
Joe F. Bozeman III Shauhrat S. Chopra Philip James Sajjad Muhammad Hua Cai Kangkang Tong Maya Carrasquillo Harold Rickenbacker Destenie Nock Weslynne Ashton Oliver Heidrich Sybil Derrible Melissa Bilec 《Journal of Industrial Ecology》2023,27(2):382-394
Now is the time to refocus efforts in urban research and design. A changing climate and extreme weather events are presenting unique challenges to urban systems around the world. These challenges illuminate the social barriers that accompany disruptive events such as resource inequities and injustices. In this perspective, we provide three research priorities for just and sustainable urban systems that help to address these matters. The three research priorities are: (1) social equity and justice, (2) circularity, and (3) digital twins. Conceptual context and future research directions are provided for each. For social equity and justice, the future directions are mandatory equity analysis and inclusionary practices, understanding and reconciling historical injustices, and intentional integration with diverse community stakeholders. For circularity applications, they are better metrics for integration, more robust evaluation frameworks, and dynamic modeling at multiple spatial and temporal scales. Future directions for digital twins include developing principles to reduce complexity, integrating model and system components, and reducing barriers to data access. These research priorities are core to meeting several of the United Nations Sustainable Development Goals (i.e., 1—No Poverty, 8—Decent Work and Economic Growth, 10—Reduced Inequalities, and 11—Sustainable Cities and Communities). Useful social and technical matters are discussed throughout, where we highlight the importance of prioritizing localized research efforts, provide guidance for community-engaged research and co-development practices, and explain how these priorities interact to align with the evolving field of industrial ecology. 相似文献
106.
Laurence M. Demers Allan Lipton Harold A. Harvey Kathleen B. Kambic Howard Grossberg Carolyn Brady Richard J. Santen 《The Journal of steroid biochemistry and molecular biology》1993,44(4-6):687-691
The pharmacologic inhibition of aromatase activity has been the focus of clinical trials in patients with advanced stage breast cancer. Recent developments with imidazole compounds that inhibit aromatase activity suggest their clinical use as potent inhibitors of estrogen biosynthesis in postmenopausal breast cancer patients. In this Phase I, open-label, dose-range finding study, we examined the inhibitory potency of CGS 20267 on blood and urine levels of estradiol, estrone and estrone sulfate in 8 patients with metastatic breast cancer. Studies included evaluation of adrenal and thyroid function to look for evidence of general hydroxylase inhibition at dose levels effective for aromatase blockade. Patients were administered CGS 20267 at doses of 0.1 and 0.25 mg, once a day in ascending doses over a 12-week period. Preliminary data reveal that CGS 20267 elicits a striking suppression in plasma estradiol, estrone and estrone sulphate which was observed in some patients as quickly as within 24 h of the first dose. Estrogen suppression of over 90% was achieved within 2 weeks of therapy. No alterations in either baseline or ACTH (cortrosyn) stimulated cortisol and aldosterone levels were observed through the 12 weeks of therapy. In addition, 24 h urine sodium and potassium values were not appreciably altered during therapy. We conclude that CGS 20267 is a potent, specific inhibitor of estrogen biosynthesis in postmenopausal patients with metastatic breast cancer and effectively reduces blood and urine estrogens to undetectable levels. 相似文献
107.
Michael A. Poss Joyce A. Reid Charles A. Free W. Lynn Rogers Helen Weber Denis E. Ryono Tamara Dejneka Jack M. DeForrest Thomas L. Waldron Russell J. Brittain Harold N. Weller Maria P. Cimarusti Edward W. Petrillo 《Bioorganic & medicinal chemistry letters》1993,3(12):2739-2744
The syntheses and pharmacological activity of a series of diol sulfonamides which function as inhibitors of human renin are described. The most potent compound in this series, compound 20 (SQ 33,800), is a subnanomolar inhibitor of human renin (IC50 = 0.35 × 10−9 M). 相似文献
108.
Isolation and characterization of full-length chromosomes from non-culturable plant-pathogenic Mycoplasma-like organisms 总被引:4,自引:0,他引:4
We describe the isolation and characterization of full-length chromosomes from non-culturable plant-pathogenic, mycoplasma-like organisms (MLOs). MLO chromosomes are circular and their sizes (640 to 1185kbp) are heterogeneous. Divergence in the range of chromosome sizes is apparent between MLOs in the two major MLO disease groups, and chromosome size polymorphism occurs among some related agents. MLO chromosome sizes overlap those of culturable mycoplasmas; consequently, small genome size alone cannot explain MLO non-culturability. Hybridization with cloned MLO-specific chromosomal and 16S rRNA probes detected two separate chromosomes in some MLO ‘type’ strains. Large DNA molecules that appear to be MLO megaplasmids were also demonstrated. The ability to characterize full-length chromosomes from virtually any non-culturable prokaryote should greatly facilitate the molecular and genetic analysis of these difficult bacteria. 相似文献
109.
Grégoire Levasseur Marie-Annick Persuy Denise Grebert Jean-Jacques Remy Roland Salesse Edith Pajot-Augy 《European journal of biochemistry》2003,270(13):2905-2912
Primary olfactory neuronal cultures exposed to odorant stimulation have previously exhibited concentration-related effects in terms of intracellular cAMP levels and adenylate cyclase activity [Ronnett, G.V., Parfitt, D.J., Hester, L.D. & Snyder, S.H. (1991) PNAS88, 2366-2369]. Maximal stimulation occurred for intermediate concentrations, whereas AC activity declined for both low and high odorant concentrations. We suspected that this behavior might be ascribed to the intrinsic response of the first molecular species concerned by odorant detection, i.e. the olfactory receptor itself. In order to check this hypothesis, we developed an heterologous expression system in mammalian cells to characterize the functional response of receptors to odorants. Two mammalian olfactory receptors were used to initiate the study, the rat I7 olfactory receptor and the human OR17-40 olfactory receptor. The cellular response of transfected cells to an odorant stimulation was tested by a spectrofluorimetric intracellular calcium assay, and proved in all cases to be dose-dependent for the known ligands of these receptors, with an optimal response for intermediate concentrations. Further experiments were carried out with the rat I7 olfactory receptor, for which the sensitivity to an odorant, indicated by the concentration yielding the optimal calcium response, depended on the carbon chain length of the aldehydic odorant. The response is thus both ligand-specific and dose-dependent. We thus demonstrate that a differential dose-response originates from the olfactory receptor itself, which is thus capable of efficient discrimination between closely related agonists. 相似文献
110.