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81.
The synthesis of haemoglobin A (HbA) is exquisitely coordinated during erythrocyte development to prevent damaging effects from individual alpha- and beta-subunits. The alpha-haemoglobin-stabilizing protein (AHSP) binds alpha-haemoglobin (alphaHb), inhibits the ability of alphaHb to generate reactive oxygen species and prevents its precipitation on exposure to oxidant stress. The structure of AHSP bound to ferrous alphaHb is thought to represent a transitional complex through which alphaHb is converted to a non-reactive, hexacoordinate ferric form. Here we report the crystal structure of this ferric alphaHb-AHSP complex at 2.4 A resolution. Our findings reveal a striking bis-histidyl configuration in which both the proximal and the distal histidines coordinate the haem iron atom. To attain this unusual conformation, segments of alphaHb undergo drastic structural rearrangements, including the repositioning of several alpha-helices. Moreover, conversion to the ferric bis-histidine configuration strongly and specifically inhibits redox chemistry catalysis and haem loss from alphaHb. The observed structural changes, which impair the chemical reactivity of haem iron, explain how AHSP stabilizes alphaHb and prevents its damaging effects in cells. 相似文献
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One of the current debates in action research concerns the quality of these practices. Up to now, many contributions have focused on defining specific criteria based on action research epistemology.
This article sustains (1) that prior to dealing with these questions, it is necessary to define for what purpose and for whom
we are making the evaluation; (2) that this leads us to make a distinction between different evaluation models; and (3) that
the quality strategies and criteria will be different for each model. In particular, the article confronts an academic evaluation
model as a form of external control over the quality of action research and an internal, participatory evaluation model as
a quality strategy aimed at establishing feedback for the process. Final considerations are given about the implications of
both models for academia. 相似文献
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We examined abundance and flight periodicity of 3 predators of bark beetles (Coleoptera: Curculionidae, Scolytinae), Temnochila chlorodia (Mannerheim) (Coleoptera: Trogositidae), Enoclerus sphegeus (Fabricius) (Coleoptera: Cleridae), and E. lecontei (Wolcott) (Coleoptera: Cleridae), across an elevational gradient of ponderosa pine ( Pinus ponderosa Lawson) forests in north central Arizona. Predator populations were estimated at 10 sites in each of 3 elevation bands (low: 1600–1736 m; mid: 2058–2230 m; high: 2505–2651 m) for 3 years (2004–2006) using pheromone-baited funnel traps targeting 3 primary bark beetle species. We also investigated how predator abundance and flight seasonality related to those of 5 bark beetle species: Ips pini (Say), I. lecontei Swaine, Dendroctonus frontalis Zimmermann, D. brevicomis LeConte, and D. adjunctus Blandford. Temnochila chlorodia was most abundant in the low- and mid-elevation bands, whereas E. sphegeus was most abundant in the high-elevation band. Enoclerus lecontei showed no consistent elevational trend in abundance. Within each elevation band, changes in annual abundance of pooled predator species tracked shifts in abundance of pooled bark beetle species. In general, predator flight initiation coincided with or closely followed bark beetle flight initiation in the spring, but predator flight terminated before flight activity ended for most bark beetle species in the fall. In addition, the ratio of prey to predators was lowest in the summer and highest in the fall. This suggests that all bark beetle species examined may be provided temporal escape from their predators in the fall. For all 3 predator species, the pheromone-baited trap targeting D. brevicomis was less attractive than the pheromone-baited traps targeting I. pini and I. lecontei. 相似文献
88.
This paper provides a first indication that this is true for a system comprised of a static structure described by hyperbolic
partial differential equations and is subjected to an external random input force. The system deforms the randomness of an
input force sequence in proportion to its algorithmic complexity. The authors demonstrate this by numerical analysis of a
one-dimensional vibrating elastic solid (the system) on which we apply a maximally-random force sequence (input). The level
of complexity of the system is controlled via external parameters. The output response is the field of displacements observed
at several positions on the body. The algorithmic complexity and stochasticity of the resulting output displacement sequence
is measured and compared against the complexity of the system. The results show that the higher the system complexity, the
more random-deficient the output sequence. 相似文献
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Structural basis for the activation of anthrax adenylyl cyclase exotoxin by calmodulin. 总被引:18,自引:0,他引:18
Chester L Drum Shui-Zhong Yan Joel Bard Yue-Quan Shen Dan Lu Sandriyana Soelaiman Zenon Grabarek Andrew Bohm Wei-Jen Tang 《Nature》2002,415(6870):396-402
Oedema factor, a calmodulin-activated adenylyl cyclase, is important in the pathogenesis of anthrax. Here we report the X-ray structures of oedema factor with and without bound calmodulin. Oedema factor shares no significant structural homology with mammalian adenylyl cyclases or other proteins. In the active site, 3'-deoxy-ATP and a single metal ion are well positioned for catalysis with histidine 351 as the catalytic base. This mechanism differs from the mechanism of two-metal-ion catalysis proposed for mammalian adenylyl cyclases. Four discrete regions of oedema factor form a surface that recognizes an extended conformation of calmodulin, which is very different from the collapsed conformation observed in other structures of calmodulin bound to effector peptides. On calmodulin binding, an oedema factor helical domain of relative molecular mass 15,000 undergoes a 15 A translation and a 30 degrees rotation away from the oedema factor catalytic core, which stabilizes a disordered loop and leads to enzyme activation. These allosteric changes provide the first molecular details of how calmodulin modulates one of its targets. 相似文献