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51.
This study was carried out to determine if the desiccation-tolerant fernPolypodium virgimanum L. ecologically resembles lower plants by absorbing atmospheric water through its fronds and actively growing in early spring when the soil along cliff edges is still frozen. Three times between February and April, 1991,P. virginianum clonal mats were treated with deuterium-labelled water. Following each application, fronds were collected over several days and analyzed for the presence of deuterium. Two treatment groups plus a control were used: fronds were sprayed directly while covering the soil, or the roots were watered while protecting the fronds. The control mats were left untreated. Soil, air, and frond temperatures, plus photosynthesis and frond conductance were monitored throughout the study period. At subfreezing temperatures in February, no labelled water was taken up from the soil and no photosynthesis took place. Small amounts of label were absorbed from the soil in March during freeze-thaw cycles when rates of photosynthesis and stomatal conductance were both low. Large amounts of label were taken up from the soil in April when the soil was fully thawed and gas exchange was at normal seasonal levels. Label was not absorbed directly through the fronds when the plants were actively growing. Despite the desiccation tolerance ofP. virginianum, the timing and patterns of its water uptake and gas exchange in early spring resemble those found in higher vascular plants, not poikilohydric lower plants.  相似文献   
52.
An argument is presented that the spontaneous mutation rate, the core of evolution theory, may be dictated by the deuterium/hydrogen (D/H) abundance ratio. This argument is supported by quantum mechanical calculations of the zero-point energy reduction for DNA base pairs upon deuterium substitution for hydrogen and recent experiments that show that the rate of catalytic dsDNA unwinding is dependent on the stability of the dsDNA.  相似文献   
53.
CDK2AP1 (cyclin-dependent kinase 2-associated protein 1), corresponding to the gene doc-1 (deleted in oral cancer 1), is a tumor suppressor protein. The doc-1 gene is absent or down-regulated in hamster oral cancer cells and in many other cancer cell types. The ubiquitously expressed CDK2AP1 protein is the only known specific inhibitor of CDK2, making it an important component of cell cycle regulation during G(1)-to-S phase transition. Here, we report the solution structure of CDK2AP1 by combined methods of solution state NMR and amide hydrogen/deuterium exchange measurements with mass spectrometry. The homodimeric structure of CDK2AP1 includes an intrinsically disordered 60-residue N-terminal region and a four-helix bundle dimeric structure with reduced Cys-105 in the C-terminal region. The Cys-105 residues are, however, poised for disulfide bond formation. CDK2AP1 is phosphorylated at a conserved Ser-46 site in the N-terminal "intrinsically disordered" region by IκB kinase ε.  相似文献   
54.
Intramolecular allosteric interactions responsible for actin conformational regulation are largely unknown. Previous work demonstrated that replacing yeast actin Val-76 with muscle actin Ile caused decreased nucleotide exchange. Residue 76 abuts Trp-79 in a six-residue linear array beginning with Lys-118 on the surface and ending with His-73 in the nucleotide cleft. To test if altering the degree of packing of these two residues would affect actin dynamics, we constructed V76I, W79F, and W79Y single mutants as well as the Ile-76/Phe-79 and Ile-76/Tyr-79 double mutants. Tyr or Phe should decrease crowding and increase protein flexibility. Subsequent introduction of Ile should restore packing and dampen changes. All mutants showed decreased growth in liquid medium. W79Y alone was severely osmosensitive and exhibited vacuole abnormalities. Both properties were rescued by Ile-76. Phe-79 or Tyr decreased the thermostability of actin and increased its nucleotide exchange rate. These effects, generally greater for Tyr than for Phe, were reversed by introduction of Ile-76. HD exchange showed that the mutations caused propagated conformational changes to all four subdomains. Based on results from phosphate release and light-scattering assays, single mutations affected polymerization in the order of Ile, Phe, and Tyr from least to most. Introduction of Ile-76 partially rescued the polymerization defects caused by either Tyr-79 or Phe-79. Thus, alterations in crowding of the 76–79 residue pair can strongly affect actin conformation and behavior, and these results support the theory that the amino acid array in which they are located may play a central role in actin regulation.  相似文献   
55.
56.
The effect of incorporation of 3-43 mol% sterol on the lipid order and bilayer rigidity has been investigated for model membranes of dimyristoylphosphatidylcholine or dipalmitoylphosphatidylcholine. 2H NMR spectra and spin-lattice relaxation rates were measured for macroscopically aligned bilayers. The characteristics of spectra obtained at temperatures between 0-60 °C are interpreted in terms of a two-phase coexistence of the liquid disordered and the liquid ordered phases and the data is found to be in agreement with the phase diagram published by Vist and Davis (Biochemistry 29 (1990), pp. 451-464). The bending modulus of the bilayers was calculated from plots of relaxation rate vs. the square of the order parameter at 44 °C. Clear differences were obtained in the efficiency of the sterols to increase the stiffness of the bilayers. These differences are correlated to the ability of the sterols to induce the liquid ordered phase in binary as well as in ternary systems; the only exception being ergosterol, which was found to be unable to induce lo phases and also had a relatively weak effect on the bilayer stiffness in contrast to earlier reports.  相似文献   
57.
beta-Phenylethylamine (PE) hydrochloride injected intraperitoneally into rats was distributed evenly throughout the various regions of rat brain. Similarly, when a mixture of PE and alpha, alpha, beta, beta-deuterated PE [( 2H4]PE) was injected, no regional differences were observed in the ratios of the amounts of [2H4]PE and PE present; however, significantly more [2H4]PE than PE was present, although a 1:1 mixture had been administered. Further experiments in which the amounts of [2H4]PE and PE in whole rat brain, liver, and plasma were quantified confirmed this finding. The maximum [2H4]PE-to-PE ratios observed were 67 in whole brain 1 h after injection and 8 in liver and in plasma 45 min after injection. The whole brain [2H4]PE-to-PE ratios were decreased by pargyline pretreatment. Subsequent experiments showed that more alpha, alpha-[2H2]PE than PE was present in whole brain, liver, and plasma of rats injected with an equimolar mixture of alpha, alpha-[2H2]PE and PE. In contrast, beta, beta-[2H2]PE was not enriched in comparison to PE under the same experimental conditions. We concluded that the basis for the enrichment of [2H4]PE and alpha, alpha-[2H2]PE compared to PE was due to protection of the deuterated analogs from the actions of monoamine oxidase and perhaps aldehyde dehydrogenase; this protection led to pronounced deuterium substitution effects in vivo especially in the brain.  相似文献   
58.
The appearance of ESR signals from Photosystem I (PS I) electron acceptors A1 and A0 in water or deuterium oxide suspension was followed using a low-temperature photoaccumulation technique. In deuterated samples the A1 signal was narrowed by a factor of 0.66 compared with the control. This effect was fully reversible upon resuspension of treated samples in H2O. The narrow ESR signal from deuterated A1 had similar power saturation characteristics to the normal signal; however, a signal from a second component resolved by deuteration was saturated at higher microwave powers than the control. The power saturation behaviour of A1 in un-modified reaction centres indicated that it is an anionic semiquinone in a ‘protic’ environment. Deuteration reversibly modified the relative extents of reduction of iron sulphur electron acceptors A and B such that centre B became the more stable electron acceptor. The g-value and line-width of iron sulphur centre X was not modified by deuteration although it appeared to become more efficiently reduced. These results are discussed in the light of current evidence from optical, electron spin polarisation and extraction experiments that suggest that A1 is a quinone, probably vitamin K-1.  相似文献   
59.
A facile six-step synthesis of 2,2,3,4,4-d5-androsterone-beta-D-glucuronide (1) starting from epiandrosterone (2) in 63% yield is described and compared with several alternative synthetic pathways. Compound 1 can be used as an internal standard in screening procedures for anabolic steroids to monitor the hydrolysis step of the steroid glucuronides prior to gas chromatography-mass spectrometry (GC-MS) analysis. Thus, a time consuming solid-phase extraction step to remove possible hydrolysis inhibitors can be omitted.  相似文献   
60.
Protonated molecular peptide ions and their product ions generated by tandem mass spectrometry appear as isotopologue clusters due to the natural isotopic variations of carbon, hydrogen, nitrogen, oxygen, and sulfur. Quantitation of the isotopic composition of peptides can be employed in experiments involving isotope effects, isotope exchange, and isotopic labeling by chemical reactions and in studies of metabolism by stable isotope incorporation. Both ion trap and quadrupole-time of flight mass spectrometry are shown to be capable of determining the isotopic composition of peptide product ions obtained by tandem mass spectrometry with both precision and accuracy. Tandem mass spectra of clusters of isotopologue ions obtained in profile mode are fit by nonlinear least squares to a series of Gaussian peaks which quantify the Mn/M0 values which define the isotopologue distribution (ID). To determine the isotopic composition of product ions from their ID, a new algorithm that predicts the Mn/M0 ratios and obviates the need to determine the intensity of all of the ions of an ID is developed. Consequently a precise and accurate determination of the isotopic composition of a product ion may be obtained from only the initial values of the ID, however, the entire isotopologue cluster must be isolated prior to fragmentation. Following optimization of the molecular ion isolation width, fragmentation energy, and detector sensitivity, the presence of isotopic excess (2H, 13C, 15N, 18O) is readily determined within 1%. The ability to determine the isotopic composition of sequential product ions permits the isotopic composition of individual amino acid residues in the precursor ion to be determined.  相似文献   
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