全文获取类型
收费全文 | 8040篇 |
免费 | 554篇 |
国内免费 | 2篇 |
出版年
2023年 | 70篇 |
2022年 | 125篇 |
2021年 | 245篇 |
2020年 | 166篇 |
2019年 | 247篇 |
2018年 | 270篇 |
2017年 | 231篇 |
2016年 | 317篇 |
2015年 | 463篇 |
2014年 | 446篇 |
2013年 | 586篇 |
2012年 | 625篇 |
2011年 | 690篇 |
2010年 | 424篇 |
2009年 | 349篇 |
2008年 | 464篇 |
2007年 | 455篇 |
2006年 | 392篇 |
2005年 | 334篇 |
2004年 | 301篇 |
2003年 | 285篇 |
2002年 | 245篇 |
2001年 | 86篇 |
2000年 | 66篇 |
1999年 | 68篇 |
1998年 | 53篇 |
1997年 | 55篇 |
1996年 | 42篇 |
1995年 | 37篇 |
1994年 | 23篇 |
1993年 | 27篇 |
1992年 | 42篇 |
1991年 | 28篇 |
1990年 | 33篇 |
1989年 | 17篇 |
1988年 | 37篇 |
1987年 | 18篇 |
1986年 | 15篇 |
1985年 | 11篇 |
1984年 | 23篇 |
1983年 | 18篇 |
1982年 | 20篇 |
1981年 | 19篇 |
1980年 | 19篇 |
1979年 | 13篇 |
1978年 | 13篇 |
1977年 | 16篇 |
1976年 | 16篇 |
1975年 | 10篇 |
1974年 | 8篇 |
排序方式: 共有8596条查询结果,搜索用时 15 毫秒
31.
I Moura A R Lino J J Moura A V Xavier G Fauque H D Peck J LeGall 《Biochemical and biophysical research communications》1986,141(3):1032-1041
Two new low molecular weight proteins with sulfite reductase activity, isolated from Methanosarcina barkeri (DSM 800) and Desulfuromonas acetoxidans (strain 5071), were studied by EPR and optical spectroscopic techniques. Both proteins have visible spectra similar to that of the low-spin sulfite reductase of Desulfovibrio vulgaris strain Hildenborough and no band at 715 nm, characteristic of high-spin Fe3+ complexes in isobacteriochlorins is observed. EPR shows that as isolated the siroheme is in a low-spin ferric state (S = 1/2) with g-values at 2.40, 2.30 and 1.88 for the Methanosarcina barkeri enzyme and g-values at 2.44, 2.33 and 1.81 for the Desulfuromonas acetoxidans enzyme. Chemical analysis shows that both proteins contain one siroheme and one [Fe4S4] center per polypeptidic chain. These results suggest that the low molecular weight, low-spin non-heme iron siroheme proteins represent a new homologous class of sulfite reductases common to anaerobic microorganisms. 相似文献
32.
Pedro Labarca Daniel Wolff Ubaldo Soto Cecilia Necochea Federico Leighton 《The Journal of membrane biology》1986,94(3):285-291
Summary Fusion of a highly purified fraction of rat liver peroxisomal membranes to planar lipid bilayers incorporates large, cation-selective voltage-dependent pores. TheP
K/P
Cl ratio of these pores, estimated in KCl gradients, is close to 4. The pores display several conductance states and spend most of the time open at voltages near 0 mV, closing at more positive and negative voltages. At voltages near 0 mV the most frequent open state has a conductance of 2.4 nS in 0.3m KCl. At voltages more positive and more negative than 10 mV the most frequent open state displays a conductance of 1.2 nS in 0.3m KCl. With these results pore diameters of 3 and 1.5 nm, respectively, can be estimated. We suggest that these pores might account for the unusually high permeability of peroxisomes to low molecular weight solutes. Fusion also incorporates a perfectly anion-selective, two-open states channel with conductances of 50 and 100 pS in 0.1m KCl. 相似文献
33.
Local immunity was evaluated in 47 patients affected by recidivant vaginal candidiasis and 33 control women. IgG, IgA, IgM and secretory component (SC) were determined by single radial immunodiffusion in samples of cervicovaginal secretion. IgG in dosable levels was detected in 17/47 samples (36.2%) and IgA in 15/47 patients (31.9%) whereas in the controls, the incidence was 31/33 (93.9%) for IgG and 24/33 (72.7%) for IgA. The difference was significative (P< 0.001) for both immunoglobulins. Significant differences were not obtained for IgM. The SC was detected in 4/47 cervicovaginal secretions of patients affected by candidiasis (8.5%) whereas in the control samples the incidence was 21/33 (63.6%) (P<0.001). In only 2/15 patients with dosable levels of IgA (13%) the secretory nature of this immunoglobulin could be shown by its reaction with anti-SC serum. In the control group, secretory IgA was detected in 19/24 cases (79%) (P< 0.001). Serum immunoglobulins levels were normal. The lack of secretory IgA and SC in the secretion could be related to the adherence capacity of the Candida albicans to epithelial cells. 相似文献
34.
Restriction cleavage maps of the DNAs of Streptococcus pneumoniae bacteriophages containing protein covalently bound to their 5'' ends 总被引:6,自引:0,他引:6
Rubens López Concepción Ronda Pedro García Cristina Escarmís Ernesto García 《Molecular & general genetics : MGG》1984,197(1):67-74
Summary Several pneumococcal bacteriophages showing a morphology similar to that previously described for Cp-1 (Ronda et al. 1981) have been isolated and purified from throat samples taken from healthy children. Three of these phages (Cp-5, Cp-7 and Cp-9) have been studied in detail and compared to Cp-1. The four phages differed in several respects, e.g. size, structural polypeptides, restriction enzyme cleavage patterns, etc. The DNA of Cp-5, Cp-7 and Cp-9 showed protease-sensitive transfecting activity. This, together with the results obtained by electrophoretic analyses as well as by isotopic labelling of these DNAs with [-32P] ATP and polynucleotide kinase indicated that all these new phages have a protein covalently linked to the 5 ends of their DNAs as in the case of Cp-1 (García et al. 1983). Restriction enzyme cleavage maps of Cp-1, Cp-5, Cp-7 and Cp-9 have been constructed. 相似文献
35.
A new experimental model, the latissimus dorsi flap of the rabbit, was studied. This was found to be a relatively inexpensive research model. Its use is advocated for composite tissue transfer as transposition, island, or free myocutaneous flaps. 相似文献
36.
37.
Twenty-two patients with chronic productive bronchitis or bronchiectasis were treated by direct instillations of normal saline and N-acetylcysteine into the trachea through a percutaneous catheter following a period of conventional routine therapy.The instillation, using either normal saline or varying concentrations of N-acetylcysteine did not produce significant change in alveolar gas exchange as reflected by measurement of arterial PaCO2 and the alveolar arterial gradient for oxygen during and after the introduction of the medication into the bronchial tree. Studies were carried out after patients had been stabilized breathing pure oxygen on an IPPB machine for 30 minutes.Evaluation of the treatment by means of pulmonary function tests demonstrated significant improvement in overall function following therapy.The results indicate that the technique of tracheobronchial lavage is physiologically benign and that overall improvement in pulmonary function can be obtained by this means in cases of the type described in this study. 相似文献
38.
M Teixeira I Moura A V Xavier J J Moura J LeGall D V DerVartanian H D Peck B H Huynh 《The Journal of biological chemistry》1989,264(28):16435-16450
The hydrogenase (EC 1.2.2.1) of Desulfovibrio gigas is a complex enzyme containing one nickel center, one [3Fe-4S] and two [4Fe-4S] clusters. Redox intermediates of this enzyme were generated under hydrogen (the natural substrate) using a redox-titration technique and were studied by EPR and M?ssbauer spectroscopy. In the oxidized states, the two [4Fe-4S]2+ clusters exhibit a broad quadrupole doublet with parameters (apparent delta EQ = 1.10 mm/s and delta = 0.35 mm/s) typical for this type of cluster. Upon reduction, the two [4Fe-4S]1+ clusters are spectroscopically distinguishable, allowing the determination of their midpoint redox potentials. The cluster with higher midpoint potential (-290 +/- 20 mV) was labeled Fe-S center I and the other with lower potential (-340 +/- 20 mV), Fe-S center II. Both reduced clusters show atypical magnetic hyperfine coupling constants, suggesting structural differences from the clusters of bacterial ferredoxins. Also, an unusually broad EPR signal, labeled Fe-S signal B', extending from approximately 150 to approximately 450 mT was observed concomitantly with the reduction of the [4Fe-4S] clusters. The following two EPR signals observed at the weak-field region were tentatively attributed to the reduced [3Fe-4S] cluster: (i) a signal with crossover point at g approximately 12, labeled the g = 12 signal, and (ii) a broad signal at the very weak-field region (approximately 3 mT), labeled the Fe-S signal B. The midpoint redox potential associated with the appearance of the g = 12 signal was determined to be -70 +/- 10 mV. At potentials below -250 mV, the g = 12 signal began to decrease in intensity, and simultaneously, the Fe-S signal B appeared. The transformation of the g = 12 signal into the Fe-S signal B was found to parallel the reduction of the two [4Fe-4S] clusters indicating that the [3Fe-4S]o cluster is sensitive to the redox state of the [4Fe-4S] clusters. Detailed redox profiles for the previously reported Ni-signal C and the g = 2.21 signal were obtained in this study, and evidence was found to indicate that these two signals represent two different oxidation states of the enzyme. Finally, the mechanistic implications of our results are discussed. 相似文献
39.
EPR studies with 77Se-enriched (NiFeSe) hydrogenase of Desulfovibrio baculatus. Evidence for a selenium ligand to the active site nickel 总被引:5,自引:0,他引:5
S H He M Teixeira J LeGall D S Patil I Moura J J Moura D V DerVartanian B H Huynh H D Peck 《The Journal of biological chemistry》1989,264(5):2678-2682
The periplasmic hydrogenase containing equivalent amounts of nickel and selenium plus non-heme iron [NiFeSe) hydrogenase) has been purified from cells of the sulfate reducing bacterium Desulfovibrio baculatus (DSM 1748) grown on a lactate/sulfate medium containing natural Se isotopes and the nuclear isotope, 77Se. Both the 77Se-enriched and unenriched hydrogenases were shown to be free of other hydrogenases and characterized with regard to their Se contents. EPR studies of the reduced nickel signal generated by redox titrations of the enriched and unenriched (NiFeSe) hydrogenases demonstrated that the gx = 2.23 and gy = 2.17 resonances are appreciably broadened by the spin of the 77Se nucleus (I = 1/2). This observation demonstrates unambiguously that the unpaired electron is shared by the Ni and Se atoms and that Se serves as a ligand to the nickel redox center of the (NiFeSe) hydrogenase. 相似文献
40.
Guy G. Orgambide Saleela Philip-Hollingsworth Pedro F. Mateos Rawle I. Hollingsworth Frank B. Dazzo 《Plant and Soil》1996,186(1):93-98
Axenic seedling bioassays were performed on white clover, vetch, and alfalfa to assess the variety and dose responses of biological activities exhibited by membrane chitolipooligosaccharides (CLOSs) from wild type Rhizobium leguminosarum bv. trifolii ANU843. Subnanomolar concentrations of CLOSs induced deformation of root hairs (Had) and increased the number of foci of cortical cell divisions (Ccd) in white clover, some of which developed into nodule meristems. In contrast, ANU843 CLOSs were unable to induce Had in alfalfa and required a 104-fold higher threshold concentration to induce this response in vetch. Also, ANU843 CLOSs were not mitogenic on either of these non-host legumes. In addition, CLOS action also increased chitinase activity in white clover root exudate. Thus, the membrane CLOSs from wild type R. leguminosarum bv. trifolii are fully capable of eliciting various symbiosis-related responses in white clover in the same concentration range as extracellular CLOSs of other rhizobia on their respective legume hosts. These results and our earlier studies indicate that membrane CLOSs represent one of many different classes of bioactive metabolites made by R. leguminosarum bv. trifolii which elicit more intense symbiosis-related responses in white clover than in other legumes. Therefore, CLOSs evidently play an important role in symbiotic development, but they may not be the sole determinant of host-range in the Rhizobium-clover symbiosis.Abbreviations Ccd
cortical cell division
- CLOS
chitolipooligosaccharide
- Had
root hair deformation 相似文献