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排序方式: 共有8131条查询结果,搜索用时 15 毫秒
11.
Leif Schrder Christian Schmitz Peter Bachert 《Journal of magnetic resonance (San Diego, Calif. : 1997)》2004,171(2):87
Proton NMR resonances of the endogenous metabolites creatine and phosphocreatine ((P)Cr), taurine (Tau), and carnosine (Cs, β-alanyl-l-histidine) were studied with regard to residual dipolar couplings and molecular mobility. We present an analysis of the direct 1H–1H interaction that provides information on motional reorientation of subgroups in these molecules in vivo. For this purpose, localized 1H NMR experiments were performed on m. gastrocnemius of healthy volunteers using a 1.5-T clinical whole-body MR scanner. We evaluated the observable dipolar coupling strength SD0 (S = order parameter) of the (P)Cr-methyl triplet and the Tau-methylene doublet by means of the apparent line splitting. These were compared to the dipolar coupling strength of the (P)Cr-methylene doublet. In contrast to the aliphatic protons of (P)Cr and Tau, the aromatic H2 (δ = 8 ppm) and H4 (δ = 7 ppm) protons of the imidazole ring of Cs exhibit second-order spectra at 1.5 T. This effect is the consequence of incomplete transition from Zeeman to Paschen-Back regime and allows a determination of SD0 from H2 and H4 of Cs as an alternative to evaluating the multiplet splitting which can be measured directly in high-resolution 1H NMR spectra. Experimental data showed striking differences in the mobility of the metabolites when the dipolar coupling constant D0 (calculated with the internuclear distance known from molecular geometry in the case of complete absence of molecular dynamics and motion) is used for comparison. The aliphatic signals involve very small order parameters S ≈ (1.4 − 3) × 10−4 indicating rapid reorientation of the corresponding subgroups in these metabolites. In contrast, analysis of the Cs resonances yielded S ≈ (113 − 137) × 10−4. Thus, the immobilization of the Cs imidazole ring owing to an anisotropic cellular substructure in human m. gastrocnemius is much more effective than for (P)Cr and Tau subgroups. Furthermore, 1H NMR experiments on aqueous model solutions of histidine and N-acetyl-l-aspartate (NAA) enabled the assignment of an additional signal component at δ = 8 ppm of Cs in vivo to the amide group at the peptide bond. The visibility of this proton could result from hydrogen bonding which would agree with the anticipated stronger motional restriction of Cs. Referring to the observation that all dipolar-coupled multiplets resolved in localized in vivo 1H NMR spectra of human m. gastrocnemius collapse simultaneously when the fibre structure is tilted towards the magic angle (θ ≈ 55°), a common model for molecular confinement in muscle tissue is proposed on the basis of an interaction of the studied metabolites with myocellular membrane phospholipids. 相似文献
12.
Microencapsulation of DNA Within alginate microspheres and crosslinked chitosan membranes for in vivo application 总被引:4,自引:0,他引:4
T. Alexakis D. K. Boadi D. Quong A. Groboillot I. O’Neill D. Poncelet R. J. Neufeld 《Applied biochemistry and biotechnology》1995,50(1):93-106
Calf thymus DNA was microencapsulated within crosslinked chitosan membranes, or immobilized within chitosan-coated alginate
microspheres. Microcapsules were prepared by interfacial polymerization of chitosan, and alginate microspheres formed by emulsification/
internal gelation. Diameters ranged from 20 to 500 Μm, depending on the formulation conditions. Encapsulated DNA was quantifiedin situ by direct spectrophotometry (260 nm) and ethidium bromide fluorimetry, and compared to DNA measurements on the fractions
following disruption and dissolution of the microspheres. Approximately 84% of the DNA was released upon core dissolution
and membrane disruption, with 12% membrane bound. The yield of encapsulation was 96%. Leakage of DNA from intact microspheres/capsules
was not observed. DNA microcapsules and microspheres were recovered intact from rat feces following gavage and gastrointestinal
transit. Higher recoveries (60%) and reduced shrinkage during transit were obtained with the alginate microspheres. DNA was
recovered and purified from the microcapsules and microspheres by chromatography and differential precipitation with ethanol.
This is the first report of microcapsules or microspheres containing biologically active material (DNA) being passed through
the gastrointestinal tract, with the potential for substantial recovery. 相似文献
13.
Summary A rapid, robust and reproducible method providing excellent separation performance and simplicity using a 0.5% MC-4000 methyl
cellulosic sieving medium in DB-1 coated capillaries has been developed. The method is suitable for qualitative comparison
of DNA restriction profiles for fragments in the size range 100–1000 base pairs (bp). Efficiencies up to 8.5 million plates/m
(1057 bp fragment) were recorded. Peak resolution of 6 bp (291/297 bp, 335/341 bp) and 4 bp (238/242 bp, 341/345 bp) was achieved.
In addition, 1 bp partial resolution of 123/124 bp and 298/297 bp was obtained. Run-to-run (n=15), day-to-day (n=4), and capillary-to-capillary
(n=3) variations of 0.1–0.2% RSD, 0.3–0.5% RSD, and 0.1–0.3% RSD, respectively, were observed. The MC-4000 sieving matrix
was found to be better than hydroxypropyl methyl cellulose and hydroxypropyl cellulose, in terms of both performance and stability
in the DB-1 coated capillaries. The efficiency and resolution in DB-WAX capillaries were inferior to those obtained in DB-1
capillaries. The commercially available DB-1 capillaries were stable for months in the sieving medium at pH 8.3 and could
be regenerated to provide high efficiency after accidental current breaks. 相似文献
14.
15.
We study the elastic responses of double-(ds) and single-stranded (ss) DNA at external force fields. A double-strand-polymer
elastic model is constructed and solved by path integral methods and Monte Carlo simulations to understand the entropic elasticity,
cooperative extensibility, and supercoiling property of dsDNA. The good agreement with experiments indicates that short-ranged
base-pair stacking interaction is crucial for the stability and the high deformability of dsDNA. Hairpin-coil transition in
ssDNA is studied with generating function method. A threshold force is needed to pull the ssDNA hairpin patterns, stabilized
by base pairing and base-pair stacking, into random coils. This phase transition is predicted to be of first order for stacking
potential higher than some critical level, in accordance with experimental observations. 相似文献
16.
17.
Ken Tachibana Ken Takeda Masahiko Shiraishi 《Proceedings of the Japan Academy. Series B, Physical and biological sciences》2004,80(7):342
It is well-recognized that DNA methylation and histone modifications play critical roles in epigenetic regulation of gene activity through the alteration of chromatin structure. Recent studies have shown that in a subset of cancer cells, the silencing of the human E-cadherin (CDH1) gene is associated with hypermethylation of the CpG island. However, the associated molecular mechanism remains unclear. To understand the mechanism, we have investigated the alteration of CpG island methylation and histone modifications during the reactivation of the CDH1 gene by treatment with 5-aza-2′-deoxycytidine (5-aza-dC). Although the CDH1 gene expression was recovered by treatment with 5-aza-dC in a liver cancer cell line Li21, the methylation status of the entire CpG island and acetylation and methylation status of associated histones were not significantly altered. These results demonstrate that the silenced CDH1 gene can be reactivated without apparent alteration of histone modification or CpG island methylation. 相似文献
18.
Blundell Tom L. Bolanos-Garcia Victor Chirgadze Dimitri Y. Harmer Nicholas J. Lo Thomas Pellegrini Luca Sibanda B. Lynn 《Structural chemistry》2002,13(3-4):405-412
Signaling in living systems needs to achieve high specificity, to be reversible, and to achieve high signal to noise. Signaling mediated by multiprotein systems has evolved that avoids the requirement for high-affinity binary complexes that would be difficult to reverse and which, in the overcrowded cell, would lead to excessive noise in the system. Symmetrical structures are only occasionally formed. When they are, it is principally to colocate components, for example, the tyrosyl kinases of growth factors, where dimers form. Symmetry is, however, often broken, presumably to create more sensitivity and specificity in the signaling system by assembling other components, into higher-order multiprotein systems. The binding of a single heparin to two 1:1 FGF:FGFR complexes is an example, as is the binding of a single ligase to the Xrcc4 dimer, perhaps so creating a further DNA-binding site. 相似文献
19.
《Electroanalysis》2006,18(6):613-620
The interaction of malachite green (MG) with double‐stranded DNA (dsDNA) in pH 7.0 Britton–Robinson (B–R) buffer solution was investigated by electrochemical and spectrophotometric methods. Within the potential scan range of ?1.0 to +1.5 V (vs. SCE), MG has two oxidative peaks at 0.547 V and 0.833 V and one reductive peak at 0.362 V on cyclic voltammogram at the scan rate of 0.20 V/s. After the addition of dsDNA into the MG solution, the oxidative peak current at 0.547 V decreases obviously. The electrochemical parameters, such as the charge transfer coefficient (α), the surface reaction rate constant (ks) and the diffusion coefficient (D), were calculated and compared between in the absence and presence of dsDNA. The results show that these parameters of MG after adding dsDNA have greatly changed, which indicates that an electrochemical active complex was formed. The interaction mechanisms of MG with dsDNA are discussed in some details from the electrochemistry and UV‐vis spectrophotometry. The reduction of the peak current of MG after adding dsDNA was further used for the quantification of dsDNA by differential pulse voltammetry (DPV). The linear range for dsDNA is in the range of 10.0–100.0 μg/mL with the linear regression equation as Δip (μA)=0.065+0.0096 C (μg/mL) and the detection limit of 6.0 μg/mL (3σ). The influences of coexisting substances were investigated and artificial samples were determined with satisfactory results. 相似文献
20.
Michael Smith 《Angewandte Chemie (International ed. in English)》1994,33(12):1214-1221
Most of the significant work has been summarized in a number of reviews and articles. In these there was, of necessity, a good deal of simplification and omission of detail…. With the passage of time, even I find myself accepting such simplified accounts. F Sanger[1] 相似文献