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71.
72.
5-Azido neuraminic acid thioglycoside with O-benzyl protecting groups was synthesized. The sialylations of this new donor type showed good alpha-selectivities for certain primary hydroxyls. 相似文献
73.
Expression and self-assembly of norwalk virus capsid protein from venezuelan equine encephalitis virus replicons 总被引:2,自引:0,他引:2 下载免费PDF全文
Baric RS Yount B Lindesmith L Harrington PR Greene SR Tseng FC Davis N Johnston RE Klapper DG Moe CL 《Journal of virology》2002,76(6):3023-3030
The Norwalk virus (NV) capsid protein was expressed using Venezuelan equine encephalitis virus replicon particles (VRP-NV1). VRP-NV1 infection resulted in large numbers of recombinant NV-like particles that were primarily cell associated and were indistinguishable from NV particles produced from baculoviruses. Mutations located in the N-terminal and P1 domains of the NV capsid protein ablated capsid self-assembly in mammalian cells. 相似文献
74.
By monitoring the thermally driven displacements of imbedded polystyrene microspheres via video fluorescence microscopy, we quantified the microstructural and micromechanical heterogeneities of wheat gliadin suspensions. We found that the degree of heterogeneity of the suspensions, as measured by the width and skewness of the microspheres' mean squared displacement (MSD) distribution, increased dramatically over a narrow range of gliadin concentrations. The ensemble-averaged MSD of a 250 mg/mL gliadin suspension exhibited a power-law behavior scaling linearly with time, a behavior similar to that observed for a homogeneous aqueous glycerol solution. However, the MSD distribution was wider and more asymmetric than for glycerol. With increasing concentration of gliadin, the ensemble-averaged MSD rapidly displayed a plateau at small time scales, the MSD distribution became wider and more asymmetric, and the local viscoelastic moduli extracted from multiple-particle-tracking measurements showed an increasingly wide range. 相似文献
75.
76.
Twin-arginine targeting (Tat) protein secretion systems consist of two protein types, members of the TatA and TatC families. Homologues of these proteins are found in many archaea, bacteria, chloroplasts and mitochondria. Every prokaryotic organism with a fully sequenced genome exhibits either neither family member, or between one and three paralogues of these two family members. The Arabidopsis thaliana genome encodes three of each. Although many mitochondrially encoded TatC homologues have been identified, corresponding TatA homologues have not been found in this organelle. Phylogenetic analyses reveal that most prokaryotic Tat systems consist of one TatC homologue and two sequence-divergent TatA homologues (TatA and TatB). When only one TatA homologue is present, TatB is missing, and when three TatA homologues are present, the third one arose by duplication of TatA, not TatB. Further, homologues most resembling TatB are more sequence-divergent than those more closely resembling TatA. In contrast to the TatA family, the TatC family shows phylogenetic clustering in strict accordance with organismal type. These results are discussed in terms of their probable structural, functional and evolutionary significance. 相似文献
77.
Contact between an adherent cell and the extracellular matrix (ECM) promotes the recruitment of structural and signaling molecules to the cytoplasmic domain of integrins, which mediate cell adhesion, cell migration, and cell growth. It is unclear whether the intracellular recruitment of these cytoplasmic molecules enhances the affinity between the ECM and the extracellular domain of the cell surface receptors (integrins). Using soft microneedles coated with Arg-Gly-Asp (RGD) peptides, a sequence commonly shared by ECM proteins, we apply a localized ramp shear stress to the surface of a HeLa cell and measure the cell stiffness and the collective (or apparent) unbinding lifetime of its surface receptors to RGD. These measurements demonstrate that both cell stiffness and the collective cell surface receptor-RGD unbinding lifetime increase with the duration of the pre-shear cell-microneedle contact and with the rate of shear applied to the cell membrane. These parameters are also crucially dependent on the integrity of the actin filament network. Our results are consistent with a model of positive feedback signaling where RGD-mediated initial recruitment of cytoskeletal proteins to the cytoplasmic domain of integrins directly enhances the interaction between the extracellular domain of integrins and the RGD sequence of ECM molecules. 相似文献
78.
This paper introduces the method of live-cell multiple-particle-tracking microrheology (MPTM), which quantifies the local mechanical properties of living cells by monitoring the Brownian motion of individual microinjected fluorescent particles. Particle tracking of carboxylated microspheres imbedded in the cytoplasm produce spatial distributions of cytoplasmic compliances and frequency-dependent viscoelastic moduli. Swiss 3T3 fibroblasts are found to behave like a stiff elastic material when subjected to high rates of deformations and like a soft liquid at low rates of deformations. By analyzing the relative contributions of the subcellular compliances to the mean compliance, we find that the cytoplasm is much more mechanically heterogeneous than reconstituted actin filament networks. Carboxylated microspheres embedded in cytoplasm through endocytosis and amine-modified polystyrene microspheres, which are microinjected or endocytosed, often show directed motion and strong nonspecific interactions with cytoplasmic proteins, which prevents computation of local moduli from the microsphere displacements. Using MPTM, we investigate the mechanical function of alpha-actinin in non-muscle cells: alpha-actinin-microinjected cells are stiffer and yet mechanically more heterogeneous than control cells, in agreement with models of reconstituted cross-linked actin filament networks. MPTM is a new type of functional microscopy that can test the local, rate-dependent mechanical and ultrastructural properties of living cells. 相似文献
79.
In vivo instability of a polycationic vector limits its efficacy after systemic administration. Conjugation of hydrophilic polymers with neutral charge onto polycationic vectors has been used to improve the stability by reducing the interactions between the vectors and the blood components, such as serum albumin. In this study, dextrans of molecular weight 10000 (dex-10000) and 1500 (dex-1500) were used to produce various degrees of grafting on linear and branched polyethylenimines (PEI), and the dextran-grafted polymers were used to prepare DNA-polymer complexes. The changes in size and in zeta-potential and the extent of DNA release after the exposure of the complexes to bovine serum albumin (BSA) were used to evaluate the stability of the complexes prepared at various ratios of DNA to polymer. Only the use of dextran-grafted branched PEI was found to be effective to improve the stability of the complexes in the presence of BSA. Dex-10000 was noted to provide a slightly better shielding than dex-1500 against the aggregation caused by BSA and helped maintain the sizes within 200 nm and the zeta-potentials close to neutral. It is thus concluded that the dextran-grafted branched PEI improved the stability of the DNA-polymer complexes and showed potential to conjugate with ligands for in vivo targeted gene delivery. 相似文献
80.
Activation of NMDA receptor partly involved in beta-bungarotoxin-induced neurotoxicity in cultured primary neurons 总被引:2,自引:0,他引:2
In this study, we demonstrated that a snake presynaptic toxin, beta-bungarotoxin (beta-BuTX), was capable of binding to NMDA receptors of the cultured primary neurons (cerebellar granule neurons, CGNs). We labeled beta-BuTX with fluorescent FITC (FITC-beta-BuTX) and showed that the binding of FITC-beta-BuTX was inhibited by unlabeled beta-BuTX and MK801 (an NMDA receptor antagonist). Meanwhile, the binding of [3H]-MK801 was also reduced by unlabeled MK801 and beta-BuTX. In addition, beta-BuTX produced a very potent neurotoxic effect on mature CGNs with the EC(50) of 3ng/ml (equivalent to 144pM), but was less effective in immature CGNs. We explored the signaling pathway of neuronal death and found that it was apparently due to the excessive production of reactive oxygen species (ROS) induced by beta-BuTX. MK801 and antioxidants (Vitamin C, N-acetylcysteine (NAC), melatonin, epigallocatechin gallate (EGCG), superoxide dismutase (SOD) and catalase) attenuated not only ROS production but also beta-BuTX-neurotoxicity. The downstream signaling of ROS was identified as the activation of caspase-3. Caspase inhibitor (z-DEVD-fmk) and antioxidants depressed both caspase-3 activation and neurotoxicity. Based on these findings and our previous reports, we conclude that the binding and activation of NMDA receptors by beta-BuTX was crucial step to produce the potent neurotoxic effect. The binding of NMDA receptors resulted in excessive Ca(2+) influx, followed by ROS production and activation of caspase-3. This snake toxin is considered not only to be a useful tool for exploring the death-signaling pathway of neurotoxicity, but also provides a model for searching neuroprotective agents. 相似文献