全文获取类型
收费全文 | 6060篇 |
免费 | 574篇 |
国内免费 | 96篇 |
出版年
2024年 | 17篇 |
2023年 | 78篇 |
2022年 | 94篇 |
2021年 | 118篇 |
2020年 | 171篇 |
2019年 | 211篇 |
2018年 | 240篇 |
2017年 | 164篇 |
2016年 | 167篇 |
2015年 | 181篇 |
2014年 | 277篇 |
2013年 | 373篇 |
2012年 | 129篇 |
2011年 | 299篇 |
2010年 | 330篇 |
2009年 | 349篇 |
2008年 | 454篇 |
2007年 | 406篇 |
2006年 | 372篇 |
2005年 | 347篇 |
2004年 | 275篇 |
2003年 | 263篇 |
2002年 | 199篇 |
2001年 | 101篇 |
2000年 | 91篇 |
1999年 | 99篇 |
1998年 | 117篇 |
1997年 | 89篇 |
1996年 | 68篇 |
1995年 | 86篇 |
1994年 | 63篇 |
1993年 | 60篇 |
1992年 | 45篇 |
1991年 | 40篇 |
1990年 | 32篇 |
1989年 | 31篇 |
1988年 | 29篇 |
1987年 | 24篇 |
1986年 | 16篇 |
1985年 | 24篇 |
1984年 | 59篇 |
1983年 | 34篇 |
1982年 | 38篇 |
1981年 | 24篇 |
1980年 | 24篇 |
1979年 | 11篇 |
1978年 | 3篇 |
1977年 | 4篇 |
1972年 | 1篇 |
1958年 | 1篇 |
排序方式: 共有6730条查询结果,搜索用时 171 毫秒
51.
M. Engelhard K. D. Kohl K. H. Müller B. Hess J. Heidemeier M. Fischer F. Parak 《European biophysics journal : EBJ》1990,19(1):11-18
Bacteriorhodopsin (bR), converted by deionization to the blue form was reconstituted to the active purple membrane by the addition of Fe2+ or Fe3+ ions. 57Fe Mossbauer spectra of these samples were measured at different pH values (pH 3.9, pH 5.0 and pH 7.0) and at temperatures ranging from 4 K to 300 K. The hyperfine parameters reveal two iron environments with oxygen atoms in the neighbourhood of iron. Iron type 1 is in the 3+ high spin state. It is bound to acid side chains of the protein and/or the phosphate groups of the lipids. Iron type 2 is in the 2+ high spin state and is linked to carboxy groups of the protein in a rather unspecific way. Dynamics as measured by Mossbauer spectroscopy show that the purple membrane becomes flexible only above 220 K. At the interface between membrane and bulk water the mobility is comparable to that of proteins with hydrophilic surfaces. The photocycle of Fe 3+-bR is slowed down compared to native bR. 3–5 Fe3+/bR are sufficient to inhibit the photocycle turnover by one order of magnitude. This specific effect is also found with Cr3+, though it is less pronounced. Mössbauer spectra of Fe3+-bR at 4 K reveal that iron nuclei are spin-coupled, indicating their close spatial proximity. It is proposed that iron trinuclear clusters interact with the proton uptake site of bR.
Offprint requests to: M. Engelhard 相似文献
52.
U. Sonnewald N. Westergaard †P. Jones †A. Taylor †H. S. Bachelard A. Schousboe 《Journal of neurochemistry》1996,67(6):2566-2572
Abstract: Metabolism of [U-13 C5 ]glutamine was studied in primary cultures of cerebral cortical astrocytes in the presence or absence of extracellular glutamate. Perchloric acid extracts of the cells as well as redissolved lyophilized media were subjected to nuclear magnetic resonance and mass spectrometry to identify 13 C-labeled metabolites. Label from glutamine was found in glutamate and to a lesser extent in lactate and alanine. In the presence of unlabeled glutamate, label was also observed in aspartate. It could be clearly demonstrated that some [U-13 C5 ]glutamine is metabolized through the tricarboxylic acid cycle, although to a much smaller extent than previously shown for [U-13 C5 ]glutamate. Lactate formation from tricarboxylic acid cycle intermediates has previously been demonstrated. It has, however, not been demonstrated that pyruvate, formed from glutamate or glutamine, may reenter the tricarboxylic acid cycle after conversion to acetyl-CoA. The present work demonstrates that this pathway is active, because [4,5-13 C2 ]glutamate was observed in astrocytes incubated with [U-13 C5 ]glutamine in the additional presence of unlabeled glutamate. Furthermore, using mass spectrometry, mono-labeled alanine, glutamate, and glutamine were detected. This isotopomer could be derived via the action of pyruvate carboxylase using 13 CO2 produced within the mitochondria or from labeled intermediates that had stayed in the tricarboxylic acid cycle for more than one turn. 相似文献
53.
Gordon C. K. Roberts 《Neurochemical research》1996,21(9):1117-1124
NMR spectroscopy has proved to be a valuable tool in the study of the interactions between enzymes and their substrates. The
kinds of structural and dynamic information which can be obtained are illustrated by studies of three enzymes involved in
drug metabolism. Cytochromes P450 play a crucial role in metabolism of a wide range of exogenous chemicals. NMR has been used to measure distances from the
haem iron of the cytochrome to protons of the bound substrate, leading to detailed structural models for the enzyme-substrate
complexes. The other two enzymes, chloramphenicol acetyltransferase and β-lactamase, are responsible for bacterial resistance
to specific antibiotics. In chloramphenicol acetyltransferase, NMR has been used to determine the conformation of coenzyme
A bound to the enzyme, while in the case of β-lactamase the pK of a specific lysine residue at the active site has been determined,
providing valuable information on the catalytic mechanism.
Special issue dedicated to Dr. Herman Bachelard. 相似文献
54.
Bernard L. Strehler 《Photosynthesis research》1996,48(1-2):11-18
The circumstances that led to the discovery that plants luminesce after they are illuminated are described, as are other discoveries that would not have been possible were it not for the fortuitous association I had with my dear and most admirable friend, W.A. Arnold, to whom this special issue is dedicated. 相似文献
55.
Antonio Cupane Eugenio Vitrano Pal Ormos G.Ulrich Nienhaus 《Biophysical chemistry》1996,60(3):111-117
We have measured the Soret band of the photoproduct obtained by complete photolysis of sperm whale carbonmonoxymyoglobin at 10 K. The experimental spectrum has been modeled with an analytical expression that takes into account the homogeneous bandwidth, the coupling of the electronic transition with both high and low frequency vibrational modes, and the effects of static conformational heterogeneity. The comparison with deoxymyoglobin at low temperature reveals three main differences. In the photoproduct, the Soret band is shifted to red. The band is less asymmetric, and an enhanced coupling to the heme vibrational mode at 674 cm−1 is observed. These differences reflect incomplete relaxation of the active site after ligand dissociation. The smaller band asymmetry of the photoproduct can be explained by a smaller displacement of the iron atom from the mean porphyrin plane, in quantitative agreement with the X-ray structure analysis. The enhanced vibrational coupling is attributed to a subtle heme distortion from the planar geometry that is barely detectable in the X-ray structure. 相似文献
56.
P. J. Cáceres C. A. Faúndez B. Matsuhiro J. A. Vasquez 《Journal of applied phycology》1996,8(6):523-527
The second-derivative mode of the Fourier transform I.R. spectra of dried algal material has been applied to distinguish the carrageenans-producingStenogramme interrupta from the isomorphous speciesRhodymenia howeana. Spectra of the tetrasporophyteS. interrupta showed bands assigned to a -carrageenan type polysaccharide, while the gametophytic and cystocarpic plants showed the characteristic absorptions of -and -carrageenans. Results were confirmed by hot water extraction of samples of the three nuclear phases ofS. interrupta and characterization of the extracts by chemical analysis.Author for correspondence 相似文献
57.
Ahsan Ullah Khan 《Luminescence》1995,10(6):329-333
Simple acidification of aqueous alkaline peroxynitrite quantitatively generates singlet (1Δg) molecular oxygen, detected and quantitated spectroscopically (1270 nm). This observation provides a chemical basis for physiological cytotoxicity of ONOO? generated in the diffusion - controlled reaction of cellular NO? and O. The experiments consist of (i) chemical generation of ONOO? from NO? gas and KO2 powder in alkaline aqueous solution; (ii) absorption spectral identification of ONOO? in the near-UV with maximum at 302 nm; (iii) spectroscopic identification of 1O2 by its emission band at 1200–1340 nm with maximum at 1275 nm; and (iv) quantitation of 1O2 generated in ONOO?/H+ reaction by comparison of the chemiluminescence intensity at 1270 nm with that from H2O2/OCl? reaction that generates 1O2 with unit efficiency at alkaline pH. 1O2 was generated with unit efficiency with respect to ONOO? concentration by the ONOO?/H+ reaction. 相似文献
58.
MANUEL SÁNCHEZ-MORENO CARMEN FERNANDEZ-BECERRA EMILIO ENTRALA FRED R. OPPERDOES MICHEL DOLLET ANTONIO OSUNA 《The Journal of eukaryotic microbiology》1995,42(3):314-320
ABSTRACT. We describe the in vitro culture of Phytomonas species isolated from Euphorbia characias . The best choice between tested media was SDM-79, in which promastigotes, after 6 days of culture, reached cell densities as high as 4 × 107 cells/ml. Cells growing in LIT or MTL medium showed longer division times and lower cell densities. We succeeded in obtaining Phytomonas sp. amastigote and spheromastigote forms in modified GRACE's medium, yielding transformation rates of up to 70%. Electron microscopy studies were performed in order to characterize the ultrastructural features of these forms obtained in vitro. On the other hand, metabolic studies based on qualitative (nuclear magnetic resonance spectroscopy) and quantitative metabolic methods (enzymatic assays) showed that promastigote forms secreted mainly ethanol, acetate, glycine, glycerol, piruvate and succinate in SDM-79 medium, whereas the major metabolites found after transformation in modified Grace's medium were ethanol, acetate, glycine, piruvate and smaller amounts of glycerol. 相似文献
59.
H. Beving L. E. G. Eriksson C. L. Davey D. B. Kell 《European biophysics journal : EBJ》1994,23(3):207-215
The dielectric properties of human erythrocytes (red blood cells) suspended in whole blood and in isotonic media at various volume fractions (haematocrits) have been studied in the frequency range 0.2–10 MHz, in which the so-called-dispersion due to the Maxwell-Wagner effect is known to occur. The capacitance and conductance at 25 °C were measured by an instrument interfaced to a computer. The rectangular sample cavity (1 ml volume) contained four pure gold electrode pins, and the sample could be circulated by a roller pump. The frequency-dependence of the permittivity and conductivity were fitted by non-linear least squares regression. Corrections were applied for non-linearity in the dielectric increment at high haematocrit, and for electrode polarisation when diluting the blood in saline. Data were interpreted in terms of a simple equivalent resistor-capacitor circuit. From the measured haematological values the specific membrane capacitance (Cm) and the conductivities internal and external to the cells (i and o respectively) were estimated. The conductivities behaved in a predictable manner with a mean of 0.458 S · m–1 (s.d. ± 0.044) for i, whereas the value of Cm (and indeed the actual capacitance of the suspension) was dependent on the amount of plasma present. Hence, in stationary normal (anticoagulated) whole blood samples, Cm was as high as 2.98 F · cm–2 (s.d. ± 0.40), in contrast to about 0.9 F · cm–2 in blood diluted more than two-fold (to less than 20% hct) in isotonic media. The high value remained when the diluent was plasma. The Cm value returned to a high value when washed erythrocytes were reconstituted with plasma, provided that this was present at above a critical or threshold concentration of about 30 vol % in the medium, irrespective of the haematocrit in the range studied (15–44%). The Cm remained low in serum. When added to washed cells in saline, purified fibrinogen had no effect. However, high Cm values were obtained by fibrinogen supplementation to serum and diluted plasma. Applying moderate flow to whole blood approximately halved its high Cm value in an exponential manner with flow rate, whilst the Cm of washed cells (31–67% hct) slightly increased, and converged to the value for whole blood under flow. We interpret the highapparent Cm value in stationary samples to be a result of rapid cell aggregation in the presence of plasma, where rouleaux formation takes place before visible sedimentation sets in. 相似文献
60.
Takayoshi Yoshida Hiroshi Watari 《European journal of applied physiology and occupational physiology》1994,69(6):465-473
To investigate the splitting of the inorganic phosphate (Pi) peak during exercise and recovery, a time-resolved 31phosphorus nuclear magnetic resonance spectroscopy (31P-MRS) technique was used. Seven healthy young sedentary male subjects performed knee flexion exercise in the prone position inside a 2.1-T magnet, with the surface coil for 31P-MRS being placed on the biceps femoris muscle. After a 1-min warm-up without loading, the exercise intensity was increased by 0.41 W at 15-s intervals until exhaustion, followed by a 5-min recovery period. The 31P-MRS were recorded every 5 s during the rest-exercise-recovery sequence. Computer-aided contour analysis and pixel imaging of the Pi and phosphocreatine peaks were performed. Five of the seven subjects showed two distinct Pi peaks during exercise, suggesting two different pH distributions in exercising muscle (high pH and low pH region). In these five subjects, the high-pH increased rapidly just after the onset of exercise, while the low-pH peak increased gradually approximately 60 s after the onset of exercise. During recovery, the disappearance of the high-pH peak was more rapid than that of the low-pH peak. These findings suggest that our method 31P-MRS provides a simple approach for studying the kinetics of the Pi peak and intramuscular pH during exercise and recovery. 相似文献