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71.
72.
In the G-protein-coupled receptor (GPCR) rhodopsin, the inactivating ligand 11-cis-retinal is bound in the seven-transmembrane helix (TM) bundle and is cis/trans isomerized by light to form active metarhodopsin II. With metarhodopsin II decay, all-trans-retinal is released, and opsin is reloaded with new 11-cis-retinal. Here we present the crystal structure of ligand-free native opsin from bovine retinal rod cells at 2.9 ?ngstr?m (A) resolution. Compared to rhodopsin, opsin shows prominent structural changes in the conserved E(D)RY and NPxxY(x)(5,6)F regions and in TM5-TM7. At the cytoplasmic side, TM6 is tilted outwards by 6-7 A, whereas the helix structure of TM5 is more elongated and close to TM6. These structural changes, some of which were attributed to an active GPCR state, reorganize the empty retinal-binding pocket to disclose two openings that may serve the entry and exit of retinal. The opsin structure sheds new light on ligand binding to GPCRs and on GPCR activation. 相似文献
73.
Bousquet E Dawber M Stucki N Lichtensteiger C Hermet P Gariglio S Triscone JM Ghosez P 《Nature》2008,452(7188):732-736
Ferroelectric thin films and superlattices are currently the subject of intensive research because of the interest they raise for technological applications and also because their properties are of fundamental scientific importance. Ferroelectric superlattices allow the tuning of the ferroelectric properties while maintaining perfect crystal structure and a coherent strain, even throughout relatively thick samples. This tuning is achieved in practice by adjusting both the strain, to enhance the polarization, and the composition, to interpolate between the properties of the combined compounds. Here we show that superlattices with very short periods possess a new form of interface coupling, based on rotational distortions, which gives rise to 'improper' ferroelectricity. These observations suggest an approach, based on interface engineering, to produce artificial materials with unique properties. By considering ferroelectric/paraelectric PbTiO3/SrTiO3 multilayers, we first show from first principles that the ground-state of the system is not purely ferroelectric but also primarily involves antiferrodistortive rotations of the oxygen atoms in a way compatible with improper ferroelectricity. We then demonstrate experimentally that, in contrast to pure PbTiO3 and SrTiO3 compounds, the multilayer system indeed behaves like a prototypical improper ferroelectric and exhibits a very large dielectric constant of epsilon(r) approximately 600, which is also fairly temperature-independent. This behaviour, of practical interest for technological applications, is distinct from that of normal ferroelectrics, for which the dielectric constant is typically large but strongly evolves around the phase transition temperature and also differs from that of previously known improper ferroelectrics that exhibit a temperature-independent but small dielectric constant only. 相似文献
74.
75.
PRDM16 controls a brown fat/skeletal muscle switch 总被引:4,自引:0,他引:4
Seale P Bjork B Yang W Kajimura S Chin S Kuang S Scimè A Devarakonda S Conroe HM Erdjument-Bromage H Tempst P Rudnicki MA Beier DR Spiegelman BM 《Nature》2008,454(7207):961-967
76.
Anthropogenic ocean acidification over the twenty-first century and its impact on calcifying organisms 总被引:15,自引:0,他引:15
Orr JC Fabry VJ Aumont O Bopp L Doney SC Feely RA Gnanadesikan A Gruber N Ishida A Joos F Key RM Lindsay K Maier-Reimer E Matear R Monfray P Mouchet A Najjar RG Plattner GK Rodgers KB Sabine CL Sarmiento JL Schlitzer R Slater RD Totterdell IJ Weirig MF Yamanaka Y Yool A 《Nature》2005,437(7059):681-686
Today's surface ocean is saturated with respect to calcium carbonate, but increasing atmospheric carbon dioxide concentrations are reducing ocean pH and carbonate ion concentrations, and thus the level of calcium carbonate saturation. Experimental evidence suggests that if these trends continue, key marine organisms--such as corals and some plankton--will have difficulty maintaining their external calcium carbonate skeletons. Here we use 13 models of the ocean-carbon cycle to assess calcium carbonate saturation under the IS92a 'business-as-usual' scenario for future emissions of anthropogenic carbon dioxide. In our projections, Southern Ocean surface waters will begin to become undersaturated with respect to aragonite, a metastable form of calcium carbonate, by the year 2050. By 2100, this undersaturation could extend throughout the entire Southern Ocean and into the subarctic Pacific Ocean. When live pteropods were exposed to our predicted level of undersaturation during a two-day shipboard experiment, their aragonite shells showed notable dissolution. Our findings indicate that conditions detrimental to high-latitude ecosystems could develop within decades, not centuries as suggested previously. 相似文献
77.
Mouquet H Scheid JF Zoller MJ Krogsgaard M Ott RG Shukair S Artyomov MN Pietzsch J Connors M Pereyra F Walker BD Ho DD Wilson PC Seaman MS Eisen HN Chakraborty AK Hope TJ Ravetch JV Wardemann H Nussenzweig MC 《Nature》2010,467(7315):591-595
During immune responses, antibodies are selected for their ability to bind to foreign antigens with high affinity, in part by their ability to undergo homotypic bivalent binding. However, this type of binding is not always possible. For example, the small number of gp140 glycoprotein spikes displayed on the surface of the human immunodeficiency virus (HIV) disfavours homotypic bivalent antibody binding. Here we show that during the human antibody response to HIV, somatic mutations that increase antibody affinity also increase breadth and neutralizing potency. Surprisingly, the responding naive and memory B cells produce polyreactive antibodies, which are capable of bivalent heteroligation between one high-affinity anti-HIV-gp140 combining site and a second low-affinity site on another molecular structure on HIV. Although cross-reactivity to self-antigens or polyreactivity is strongly selected against during B-cell development, it is a common serologic feature of certain infections in humans, including HIV, Epstein-Barr virus and hepatitis C virus. Seventy-five per cent of the 134 monoclonal anti-HIV-gp140 antibodies cloned from six patients with high titres of neutralizing antibodies are polyreactive. Despite the low affinity of the polyreactive combining site, heteroligation demonstrably increases the apparent affinity of polyreactive antibodies to HIV. 相似文献
78.
Pressure garments are functional garments designed for medical purposes and arc normally used under prolonged stress. The stress across seams is related to the size of the wearer and the fit and the garment, the more close fitting a garment, the greater stress is put on the seam. Since pressure garments are subjected to appreciable stress during wear, seams with the required strength and extensiblity are particular important.
The seam type, stitch type, stitch density and thread strength are the main factors determining the strength and extensibility of a seam. In this paper, investigation of seam strength and extensibility are made on four types of seams which have been used for making pressure garments. Five different types of stitch densities and two sizes of sewing thread are used on each type of scam to be tested. The seaming conditions of the selected fabric are also examined. 相似文献
79.
Albane le Maire William Bourguet Patrick Balaguer 《Cellular and molecular life sciences : CMLS》2010,67(8):1219-1237
Endocrine-disrupting chemicals (EDCs) represent a broad class of exogenous substances that cause adverse effects in the endocrine
system by interfering with hormone biosynthesis, metabolism, or action. The molecular mechanisms of EDCs involve different
pathways including interactions with nuclear hormone receptors (NHRs) which are primary targets of a large variety of environmental
contaminants. Here, based on the crystal structures currently available in the Protein Data Bank, we review recent studies
showing the many ways in which EDCs interact with NHRs and impact their signaling pathways. Like the estrogenic chemical diethylstilbestrol,
some EDCs mimic the natural hormones through conserved protein–ligand contacts, while others, such as organotins, employ radically
different binding mechanisms. Such structure-based knowledge, in addition to providing a better understanding of EDC activities,
can be used to predict the endocrine-disrupting potential of environmental pollutants and may have applications in drug discovery. 相似文献
80.
Many snakes employ tail movements to attract prey, but this behavior is rare in lizards. Use of caudal movements to distract prey, however, is rare in all squamates. Recently, caudal movements in a Sceloporus occidentalis individual immediately prior to attempted prey capture were documented. We supplement this sole documentation of caudal movements in S. occidentalis with additional field observations. Additionally, video footage of the behavior was recorded on 2 occasions. From these additional observations, we hypothesize that this behavior might serve to distract prey. 相似文献