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1.
The five cysteines, at positions 82, 191, 192, 270, and 401, of Escherichia coli aspartate aminotransferase (AATase) were, individually and in some combinations, converted to alanine by site-directed mutagenesis (C82A, C191A, C192A, C270A, C401A). Cys-191, which is conserved in all AATase isozymes, was mutated to serine as well (C191S). A quintuple mutant, with all cysteines converted to alanines (Quint), was also constructed. The effects of these single and multiple mutations were examined by steady-state kinetics and urea denaturation. The thermal stabilities of Quint and of the wild-type enzyme (WT) were determined by differential scanning calorimetry. The mutants had kcat values up to 50% greater than that of WT and KMAsp and KM alpha-KG values up to 1.5- and 3.3-fold higher than that of WT. The mutants C82A and C191A exhibit nearly the same CM in urea denaturation experiments as WT, while the other single mutants and Quint are less stable, with CM differences of up to 0.7 M urea. Quint is also less thermostable than WT, with a delta TM of 3.3-4.4 degrees C. Thus the five cysteine replacements yield small, but significant, changes in catalytic and denaturation parameters, but none of the cysteines was found to be essential. The changes manifested in the mutation of the conserved Cys-191 to alanine are no greater than those observed with the four nonconserved cysteines. We consider the evolutionary implications of these findings. 相似文献
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The reactions of aliphatic and aromatic amines with reducing sugars are important in both drug stability and synthesis. The
formation of glycosylamines in solution, the first step in the Maillard reaction, does not typically cause browning but results
in decreased potency and is hence significant from the aspect of drug instability. The purpose of this research was to present
(1) unreported ionic equilibria of model reactant (kynurenine), (2) the analytical methods used to characterize and measure
reaction products, (3) the kinetic scheme used to measure reaction rates and (4) relevant properties of various reducing sugars
that impact the reaction rate in solution. The methods used to identify the reversible formation of two products from the
reaction of kynurenine and monosaccharides included LC mass spectrometry, UV spectroscopy, and 1-D and 2-D 1H–1H COSY NMR spectroscopy. Kinetics was studied using a stability-indicating HPLC method. The results indicated the formation
of α and β glycosylamines by a pseudo first-order reversible reaction scheme in the pH range of 1–6. The forward reaction
was a function of initial glucose concentration but not the reverse reaction. It was concluded that the reaction kinetics
and equilibrium concentrations of the glycosylamines were pH-dependent and also a function of the acyclic content of the reacting
glucose isomer. 相似文献
5.
G. Wersuhn K. Kirsch R. Gienapp 《TAG. Theoretical and applied genetics. Theoretische und angewandte Genetik》1987,74(4):480-482
Summary Culture-derived plants and cell cultures of potato (Solanum tuberosum L.) respond to the application of the herbicides SYS 67 ME (MCPA) and OMNIDEL (Na-2,2-dichloropropionate) in a comparable fashion. By gradually increasing the herbicide concentration, cell lines were developed which tolerated 50 mg/l of ME or 300 mg/l of OMNIDEL. Any further increase in concentration resulted in the death of all cell cultures. From cell cultures that had been able to grow on media supplemented with 30 mg/l of ME, regenerate plants were obtained that were also tolerant to this concentration. This new trait was retained even after repeated vegetative propagation of the plants. 相似文献
6.
The isolation of a fetal rat liver glutathione S-transferase isoenzyme with high glutathione peroxidase activity 总被引:3,自引:0,他引:3
A previously uncharacterized glutathione S-transferase isoenzyme which is absent from normal adult rat livers has been isolated from fetal rat livers. The enzyme was purified using a combination of affinity chromatography, CM-cellulose column chromatography and chromatofocusing. It is composed of two non-identical subunits, namely, subunit Yc (Mr 28,000) and a subunit (Mr 25,500) recently reported by us to be uniquely present in fetal rat livers and which we now refer to as subunit 'Yfetus'. The enzyme which we term glutathione S-transferase YcYfetus has an isoelectric point of approx. 8.65 and has glutathione S-transferase activity towards a number of substrates. The most significant property of the fetal isozyme is its high glutathione peroxidase activity towards the model substrate cumene hydroperoxide. We suggest that this isozyme serves a specific function in protecting fetuses against the possible teratogenic effects of organic peroxides. 相似文献
7.
The C alpha primary hydrogen kinetic isotope effects (C alpha-KIEs) for the reaction of the cytoplasmic isozyme of aspartate aminotransferase (cAATase) with [alpha-2H]-L-aspartate are small and only slightly affected by deuterium oxide solvent (DV = 1.43 +/- 0.03 and DV/KAsp = 1.36 +/- 0.04 in H2O; DV = 1.44 +/- 0.01 and DV/KAsp = 1.61 +/- 0.06 in D2O). The D2O solvent KIEs (SKIEs) are somewhat larger and are essentially independent of deuterium at C alpha (D2OV = 2.21 +/- 0.07 and D2OV/KAsp = 1.70 +/- 0.03 with [alpha-1H]-L-aspartate; D2OV = 2.34 +/- 0.12 and D2OV/KAsp = 1.82 +/- 0.06 with [alpha-2H]-L- aspartate). The C alpha-KIEs on V and on V/KAsp are independent of pH from pH 5.0 to pH 10.0. These results support a rate-determining concerted 1,3 prototropic shift mechanism by the multiple KIE criteria [Hermes, J. D., Roeske, C. A., O'Leary, M. H., & Cleland, W. W. (1982) Biochemistry 21, 5106]. The large C alpha-KIEs for the reaction of mitochondrial AATase (mAATase) with L-glutamate (DV = 1.88 +/- 0.13 and DV/KGlu = 3.80 +/- 0.43 in H2O; DV = 1.57 +/- 0.05 and DV/KGlu = 4.21 +/- 0.19 in D2O) coupled with the relatively small SKIEs (D2OV = 1.58 +/- 0.04 and D2OV/KGlu = 1.25 +/- 0.05 with [alpha-1H]-L-glutamate; D2OV = 1.46 +/- 0.06 and D2OV/KGlu = 1.16 +/- 0.05 with [alpha-2H]-L-glutamate) are most consistent with a two-step mechanism for the 1,3 prototropic shift for this isozyme-substrate pair.(ABSTRACT TRUNCATED AT 250 WORDS) 相似文献
8.
Partitioning of the ketimine (or ketimine + quinonoid) intermediate(s) in the mitochondrial aspartate aminotransferase reactions was investigated by following the rates of loss of 18O from carbonyl-18O-enriched alpha-ketoglutarate together with the rate of L-glutamate formation. The ratio of these rate constants was found to equal 1 at 10 degrees C, implying that the above intermediate(s) face(s) equal barriers with respect to the forward and reverse reactions. This partition ratio of 1 together with that measured from the alpha-amino acid side of the reaction [Julin, D.A., Wiesinger, H., Toney, M. D., & Kirsch, J.F. (1989) Biochemistry (preceding paper in this issue)] suggests that the rate constant for exchange of alpha-ketoglutarate-derived H2(18)O from the ketimine (or ketimine + quinonoid) form(s) of the enzyme with solvent is comparable with that for kcat. 相似文献
9.
Matthias Kirsch Mustafa B. A. Djamgoz Hans-Joachim Wagner 《Cell and tissue research》1990,260(1):123-130
Summary A negative feedback interaction between luminosity type horizonatal cells (HCs) and green-sensitive cones generates the long-wavelength-sensitive depolarizing response in biphasic chromaticity type HCs. This interaction is suppressed in the dark and is potentiated by light adaptation of the retina. HCs are morphologically plastic; during light adaptation, their dendritic terminals within cone pedicles extend, giving rise to spinules. This paper examines whether there is a quantitative correlation between the time course of light-dependent formation of the spinules and enhancement of the feedback interaction. The strength of the feedback interaction in isolated retinac of the roach was determined as the neutral wavelength at which reversal of spectral response polarity occurred in biphasic HCs. A good correlation was found between the neutral wavelength and the spinule/ribbon ratios of retinae. Biphasic HCs were intracellularly stained with horseradish peroxidase and the correlative ultrastructure of the contacted pedicles was examined. Neutral wavelength was found to be correlated with the spinule number, weighted according to the number of synaptic contacts mediating feed-forward transmission. The latter was estimated from the total number of labelled Cb/H2 HC processes (central and lateral) at synaptic triads. A model in which spinules mediate the negative feedback interaction of HCs in the retina of cyprinid fish is presented. 相似文献