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131.
Biomolecular structure determination by mass spectrometry 总被引:2,自引:0,他引:2
H R Morris 《Nature》1980,286(5772):447-452
Mass spectometry, conventionally used with smallish molecules (less than 1,000 daltons), has suprisingly emerged as a powerful technique for the determination of the amino acid sequences of proteins and the structure of glycopeptides as well as for the characterization of biologically active materials of unusual structure such as the peptidolipids. Further applications to biomedical problems are foreseen. 相似文献
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Distribution of coitus in the menstrual cycle 总被引:4,自引:0,他引:4
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Head and backbone of the Early Cambrian vertebrate Haikouichthys 总被引:10,自引:0,他引:10
Shu DG Morris SC Han J Zhang ZF Yasui K Janvier P Chen L Zhang XL Liu JN Li Y Liu HQ 《Nature》2003,421(6922):526-529
Agnathan fish hold a key position in vertebrate evolution, especially regarding the origin of the head and neural-crest-derived tissue. In contrast to amphioxus, lampreys and other vertebrates possess a complex brain and placodes that contribute to well-developed eyes, as well as auditory and olfactory systems. These sensory sytems were arguably a trigger to subsequent vertebrate diversifications. However, although they are known from skeletal impressions in younger Palaeozoic agnathans, information about the earliest records of these systems has been largely wanting. Here we report numerous specimens of the Lower Cambrian vertebrate Haikouichthys ercaicunensis, until now only known from the holotype. Haikouichthys shows significant differences from other fossil agnathans: key features include a small lobate extension to the head, with eyes and possible nasal sacs, as well as what may be otic capsules. A notochord with separate vertebral elements is also identifiable. Phylogenetic analysis indicates that this fish lies within the stem-group craniates. Although Haikouichthys somewhat resembles the ammocoete larva of modern lampreys, this is because of shared general craniate characters; adult lampreys and hagfishes (the cyclostomes if monophyletic) are probably derived in many respects. 相似文献
137.
Coureux PD Wells AL Ménétrey J Yengo CM Morris CA Sweeney HL Houdusse A 《Nature》2003,425(6956):419-423
The myosin superfamily of molecular motors use ATP hydrolysis and actin-activated product release to produce directed movement and force. Although this is generally thought to involve movement of a mechanical lever arm attached to a motor core, the structural details of the rearrangement in myosin that drive the lever arm motion on actin attachment are unknown. Motivated by kinetic evidence that the processive unconventional myosin, myosin V, populates a unique state in the absence of nucleotide and actin, we obtained a 2.0 A structure of a myosin V fragment. Here we reveal a conformation of myosin without bound nucleotide. The nucleotide-binding site has adopted new conformations of the nucleotide-binding elements that reduce the affinity for the nucleotide. The major cleft in the molecule has closed, and the lever arm has assumed a position consistent with that in an actomyosin rigor complex. These changes have been accomplished by relative movements of the subdomains of the molecule, and reveal elements of the structural communication between the actin-binding interface and nucleotide-binding site of myosin that underlie the mechanism of chemo-mechanical transduction. 相似文献
138.
Homogeneous catalysts--which exist in the same (usually liquid) phase as reactants and products--are usually more selective than heterogeneous catalysts and far less affected by limitations due to slow transport of reactants and products, but their separation from reaction products can be costly and inefficient. This has stimulated the development of strategies that facilitate the recycling of homogeneous catalysts. Some of these methods exploit the preference of a catalyst for one of two solvents with thermoregulated miscibility; others exploit a dramatic decrease in catalyst solubility as one reagent is consumed or temperature changed after completion of the reaction. Here we describe a tungsten catalyst for the solvent-free hydrosilylation of ketones that retains its activity until essentially all of the liquid substrate is converted to liquid products, which we can then simply decant to separate the catalyst that precipitates from the products of the reaction. We attribute the ability of the catalyst to retain its solubility and hence activity until completion of the reaction to the transient formation of liquid clathrate that contains a few molecules of the substrate per molecule of the otherwise solid catalyst. Insights into the fundamental processes controlling the formation of this liquid clathrate might help to tailor other catalysts and substrates, so as to develop efficient and solvent-free schemes for reactions of practical interest. 相似文献
139.
Paired-associate learning is often used to examine episodic memory in humans. Animal models include the recall of food-cache locations by scrub jays and sequential memory. Here we report a model in which rats encode, during successive sample trials, two paired associates (flavours of food and their spatial locations) and display better-than-chance recall of one item when cued by the other. In a first study, pairings of a particular foodstuff and its location were never repeated, so ensuring unique 'what-where' attributes. Blocking N-methyl-d-aspartate receptors in the hippocampus--crucial for the induction of certain forms of activity-dependent synaptic plasticity--impaired memory encoding but had no effect on recall. Inactivating hippocampal neural activity by blocking alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA) receptors impaired both encoding and recall. In a second study, two paired associates were trained repeatedly over 8 weeks in new pairs, but blocking of hippocampal AMPA receptors did not affect their recall. Thus we conclude that unique what-where paired associates depend on encoding and retrieval within a hippocampal memory space, with consolidation of the memory traces representing repeated paired associates in circuits elsewhere. 相似文献
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