全文获取类型
收费全文 | 796篇 |
免费 | 34篇 |
专业分类
830篇 |
出版年
2024年 | 3篇 |
2023年 | 13篇 |
2022年 | 32篇 |
2021年 | 37篇 |
2020年 | 21篇 |
2019年 | 25篇 |
2018年 | 33篇 |
2017年 | 10篇 |
2016年 | 46篇 |
2015年 | 32篇 |
2014年 | 41篇 |
2013年 | 48篇 |
2012年 | 61篇 |
2011年 | 54篇 |
2010年 | 33篇 |
2009年 | 31篇 |
2008年 | 44篇 |
2007年 | 40篇 |
2006年 | 26篇 |
2005年 | 33篇 |
2004年 | 22篇 |
2003年 | 21篇 |
2002年 | 19篇 |
2001年 | 6篇 |
2000年 | 7篇 |
1999年 | 10篇 |
1998年 | 6篇 |
1996年 | 3篇 |
1995年 | 2篇 |
1994年 | 6篇 |
1993年 | 4篇 |
1992年 | 4篇 |
1991年 | 7篇 |
1990年 | 11篇 |
1989年 | 2篇 |
1988年 | 4篇 |
1987年 | 2篇 |
1985年 | 2篇 |
1984年 | 3篇 |
1982年 | 2篇 |
1981年 | 2篇 |
1979年 | 4篇 |
1978年 | 4篇 |
1975年 | 3篇 |
1974年 | 1篇 |
1973年 | 1篇 |
1972年 | 1篇 |
1971年 | 4篇 |
1970年 | 1篇 |
1967年 | 1篇 |
排序方式: 共有830条查询结果,搜索用时 15 毫秒
1.
Pascal Demange Salomé Wendenbaum Caroline Linget Christian Mertz Manh Thong Cung Anne Dell Mohamed A. Abdallah 《Biometals》1990,3(3-4):155-170
Summary In iron-deficient conditions,Pseudomonas aeruginosa ATCC 15692 synthesizes two major siderophores, pyoverdins Pa and pyoverdin Pa B. Two other compounds, pyoverdin Pa A (occurring from hydrolysis of pyoverdin Pa during the culture) and pyoverdin Pa C (occurring artifactually during the purification procedure) were also isolated. All these compounds possess the same partly cyclic peptide chain
wherel-Orn(OH · HCO) isN
-formyl,N
-hydroxy-l-ornithine. The chain is bound to a chromophore derived from 2,3-diamino-6,7-dihydroxyquinoline and having the (S) configuration. The four pyoverdins differ only in the acyl substituent bound to the nitrogen atom bound to carbon C3 of the chromophore. This is succinamide (pyoverdin Pa), succinic acid (pyoverdin Pa A), methyl succinate (pyoverdin Pa C) and 2-oxoglutaric acid (pyoverdin Pa B). The complete1H- and13CNMR assignments, using two-dimensional total correlation NMR spectroscopy (TOCSY) and rotating-frame Overhauser enhancement spectroscopy (ROESY) procedures, as well as1H-13C correlations, are reported. The complete sequence of the peptide using CH-NH correlations was achieved by NMR and confirmed the partly cyclic structure earlier reported using fast-atom-bombardment mass spectrometry (FAB-MS) on the siderophores and their dansylated fragments [Briskot G, Taraz K, Budzikiewicz H (1989)Liebigs Ann Chem: 375–384]. The use of these NMR procedures appears to be a tool of choice and a complementary approach to FAB-MS in the structure determination of some complex pyoverdins.Abbreviations Ser
serine
- Arg
arginine
- Thr
ethreonine
- Lys
lysine
- OHOrn
N
-hydroxyornithine
- Chr
chromophore 相似文献
2.
Meloidogyne mayaguensis n. sp. is described and illustrated from specimens obtained from galled roots of eggplant, Solanum melongena L., from Puerto Rico. The perineal pattern of females is round to ovoid with fine, widely spaced striae. It has occasional breaks of striation laterally and a circular tail tip area lacking striae. The stylet, 15.8 μm long, has reniform knobs that merge gradually with the stylet shaft. Males have a high, rectangular, smooth head region, not set off from the body contour. The labial disc is continuous with the medial lips which do not slope posteriorly. The styler, 22.9 μm long, has large rounded backward sloping knobs; the shaft is of uneven diameter. Mean body length of second-stage juveniles is 453.6 μm. The truncate head region is not annulated, and the rounded, slightly raised labial disc and the crescentic medial lips form dumbbell-shaped lip structures. The stylet, 11.6 μm long, has rounded, posteriorly sloping knobs. The slender tail, 54.4 μm long, gradually tapers to a bluntly pointed tip. Tomato, tobacco, pepper, and watermelon are good hosts; cotton and peanut are not hosts. M. mayaguensis n. sp. reproduces by mitotic parthenogenesis and has a somatic chromosome number of 2n = 44-45. The enzyme patterns are unique among Meloidogyne species. 相似文献
3.
A purification procedure for rat brain phosphatidylinositol synthetase (PI synthetase; CDP-1,2-diacyl-sn-glycerol:myo-inositol 3-phosphatidyltransferase; EC 2.7.8.11) is described. The enzyme was purified 200-250-fold from the homogenate by solubilization with Triton X-100 from microsomal membranes and affinity chromatography on CDP-diacylglycerol-Sepharose. Elution of enzyme activity required the presence of Triton X-100, CDP-diacylglycerol, and either phosphatidylcholine or asolectin. The product that was obtained in 5-10% yield from whole brain and in 70% yield from the microsomal fraction contained three protein bands as determined by sodium dodecylsulfate-polyacrylamide gel electrophoresis (SDS-PAGE). The final preparation contained levels of CDP-diacylglycerol hydrolase and CDP-diacylglycerol: sn-glycero-3-phosphate 3-phosphatidyltransferase activities that were less than 1% of PI synthetase activity. The purified enzyme displayed a pH optimum of 8.5-9.0, required either Mg2+ or Mn2+ and exhibited a Km of 4.6 mM for myo-inositol. 相似文献
4.
Zusammenfassung Innerhalb eines nachgewiesenermaßen mitCryptococcus neoformans (Cr. n.) infizierten Stadtgebietes wurden 170 Erdproben aufCr. n. untersucht. Während aus 6 von 50 Erdproben von Lokalisationen mit TaubenbesiedelungCr. n. nachgewiesen wurde, gelang dies bei keiner von 120 Proben ohne Taubenbesiedelung.Diese Engebnisse werden so gedeutet, daß zumindest unter den hiesigen BedingungenCr. n. als primärer Erdbodensaprophyt praktisch nicht vorkommt. Die Besiedelung von Taubenkot mitCr. n. geschieht vielmehr bereits im Darmkanalspontan infizierter Tauben, oder durch Kontakt mit infiziertem Kot. Hierbei gelangensekundär auch Kryptokokken in den Erdboden.Für künftige epidemiologische Untersuchungen wird eine Nachweismethodik empfohlen, die den Direktausstrich des in phys.-Kochsalzlösung suspendierten Untersuchungsmaterials auf Negersaat-Kreatinin-Antibiotika-Diphenylagar, seine Anreicherung in diphenylhaltiger (0,002 %) Bierwürze und den Tierversuch an der weißen Maus umfaßt. Mit dieser Methodik ist beim Vorhandensein von mindestens 10Cr. n. -Zellen in 1 g untersuchtem Ausgangsmaterial der Nachweis des Erregers zu erwarten.Halbquantitative Antrocknungsversuche vonCr. n. (ein stark und ein nahezu unbekapselter Stamm, zwei verschiedene Antrocknungsmethoden) an sterilen Seesand zeigten, daß selbst die unter direkter Einwirkung von Sonnenlicht angetrockneten Kryptokokken weitgehend überlebten.Die Direktkultur war dem Anreicherungsverfahren nahezu gleichwertig, während sich der Tierversuch im Vergleich dazu als weit weniger leistungsfähig erwies. Unter Hinweis auf frühere Untersuchungen wird betont, daß auf den Tierversuch dennoch nicht verzichtet werden sollte.
Die Untersuchungen wurden dankenswerterweise durch Mittel des Bundesministers für Ernährung, Landwirtschaft und Forsten gefördert. 相似文献
Within the area of a city, well known to be infected withCryptococcus neoformans, 170 soil samples have been for this fungus investigated. While six out of 50 soil samples coming from pigeon locations were positive for the fungus, none of the other 120 soil samples became positive from areas without pigeon locations. For future epidemiologic investigations a special method is suggested. The direct culture of the material was almost equivalent with the method of enrichment, while the animal experiment in comparison was less efficient. It is very probable, thatCryptococcus neoformans is no primary germ of the soil. Soil becomes secondarily contaminated by pigeons and their excrements.
Die Untersuchungen wurden dankenswerterweise durch Mittel des Bundesministers für Ernährung, Landwirtschaft und Forsten gefördert. 相似文献
5.
The continuous culturing of Trypanosoma acomys in the presence of a murine areolar-adipose cell line (A9) was possible for the 1st time. The trypanosomes were cultured at 37 degrees C with A9 in DMEM supplemented with 20% heat inactivated fetal bovine serum, using an initial inoculum from primary cultures of lung or blood clots from infected spiny mice. The cultures were maintained for 115 days and underwent 15 passages before termination and cryopreservation. Using this culture system T. acomys subcultures were initiated from 3 different initial inocula (3 x 10(4), 1.5 x 10(5) and 7.4 x 10(5) parasites/ml) and growth curves revealed that the lowest inoculum gave the best growth pattern. This inoculum yielded a population doubling time of less than 12 h for 4 days, a high peak density of 7 x 10(6) parasites/ml and the most gradual decline compared to the other 2 inocula. Rosetting epimastigotes and nests of amastigotes were observed in close association with the feeder layer cells. Epimastigotes were the most predominant form in culture supernatants but other morphological forms observed included trypomastigotes and sphaeromastigotes. 相似文献
6.
7.
Pierluigi Gariboldi Francesca Pelizzoni Marco Tat Luisella Verotta Nadia El-Sebakhy Aya M. Asaad Rokia M. Abdallah Soad M. Toaima 《Phytochemistry》1995,40(6):1755-1760
Three new cycloartane glycosides, trigonoside I, II and III, and the known astragalosides I and II were isolated from the roots of Astragalus trigonus. The structures of the new glycosides were totally elucidated by high field (600 MHz) NMR analyses as cycloastragenol-6-O-β-xylopyranoside, cycloastragenol-3-O-[-l-arabinopyranosyl(1 → 2)-β-d-xylopyranosyl]-6-O-β- d-xylopyranoside and cycloastragenol-3-O-[-l-arabinopyranosyl(1 → 2)-β-d-(3-O-acetyl)-xylopyranosyl]-6-O-β-d-xylopyranoside. 相似文献
8.
E A Abdallah M E Eldefrawi A T Eldefrawi 《Archives of insect biochemistry and physiology》1991,17(2-3):107-118
Muscarinic receptors in brain membranes from honey bees, houseflies, and the American cockroach were identified by their specific binding of the non-selective muscarinic receptor antagonist [3H]quinuclidinyl benzilate ([3H]QNB) and the displacement of this binding by agonists as well as subtype-selective antagonists, using filtration assays. The binding parameters, obtained from Scatchard analysis, indicated that insect muscarinic receptors, like those of mammalian brains, had high affinities for [3H]QNB (KD = 0.47 nM in honey bees, 0.17 nM in houseflies and 0.13 nM in the cockroach). However, the receptor concentration was low (108, 64.7, and 108 fmol/mg protein for the three species, respectively). The association and dissociation rates of [3H]QNB binding to honey bee brain membranes, sensitivity of [3H]QNB binding to muscarinic agonists, and high affinity for atropine were also features generally similar to muscarinic receptors of mammalian brains. In order to further characterize the three insect brain muscarinic receptors, the displacement of [3H]QNB binding by subtype-selective antagonists was studied. The rank order of potency of pirenzepine (PZ), the M1 selective antagonist, 11-[2-[dimethylamino)-methyl)1-piperidinyl)acetyl)-5,11- dihydro-6H-pyrido(2,3-b)-(1,4)-benzodiazepin-6 one (AF-DX 116), the M2-selective antagonist, and 4-DAMP (4-diphenylacetoxy-N-methylpiperidine methiodide) the M3-selective antagonist, was also the same as that of mammalian brains, i.e., 4-DAMP greater than PZ greater than AF-DX 116. The three insect brain receptors had 27-50-fold lower affinity for PZ (Ki 484-900 nM) than did the mammalian brain receptor (Ki 16 nM), but similar to that reported for the muscarinic receptor subtype cloned from Drosophila. Also, the affinity of insect receptors for 4-DAMP (Ki 18.9-56.6 nM) was much lower than that of the M3 receptor, which predominates in rat submaxillary gland (Ki of 0.37 nM on [3H]QNB binding). These drug specificities of muscarinic receptors of brains from three insect species suggest that insect brains may be predominantly of a unique subtype that is close to, though significantly different from, the mammalian M3 subtype. 相似文献
9.
We studied the effects of acute and chronic in vivo inhibition of acetylcholinesterase on both the density and function of brain muscarinic cholinergic receptors. Adult male rats were treated either once or multiple times over a period of 10 days with the irreversible acetylcholinesterase inhibitor diisopropylfluorophosphate (DFP). The concentration and affinity of muscarinic receptors in various brain regions were determined using radioligand binding techniques. Acute DFP treatment resulted in a significant reduction in receptor number only in the brain stem, while chronic treatment caused receptor down-regulation in the brain stem, cerebral cortex, and striatum. There was no change in ligand affinity in any of the brain regions. In sharp contrast, muscarinic receptor function was fully preserved, in terms of coupling of the receptors to increased phosphoinositide hydrolysis in the cerebral cortex, hippocampus, and striatum, or inhibition of cyclic AMP formation in the cerebral cortex or striatum. Therefore, there is a marked lack or correlation between DFP-induced muscarinic receptor down-regulation and receptor desensitization. 相似文献
10.
Elsayed A. M. Abdallah David A. Jett Mohyee E. Eldefrawi Amira T. Eldefrawi 《Journal of biochemical and molecular toxicology》1992,7(2):125-132
The effects of the organophosphorus anticholinesterase paraoxon on the binding of radioactive ligands to the M3 subtype of the muscarinic receptor and receptor-coupled synthesis of second messengers in intact rat submaxillary gland (SMG) cells were investigated. The binding of [3H]quinuclidinyl benzilate ([3H]QNB) was most sensitive to atropine and the M3-specific antagonist 4-DAMP followed by pirenzepine and least sensitive to the cardioselective M2 antagonist AFDX116. This, and the binding characteristics of [3H]4-DAMP, confirmed that the muscarinic receptors in this preparation are of the M3 subtype. Activation of these muscarinic receptors by carbamylcholine (CBC) produced both stimulation of phosphoinositide (PI) hydrolysis and inhibition of cAMP synthesis, suggesting that this receptor subtype couples to both effector systems. Paraoxon (100 μM) reduced Bmax of [3H]4-DAMP binding from 27 ± 4 to 13 ± 3 fmol/mg protein with nonsignificant change in affinity, suggesting noncompetitive inhibition of binding by paraoxon. Like the agonist CBC, paraoxon inhibited the forskolininduced cAMP formation in SMG cells with an EC50 of 200 nM, but paraoxon was > 500 fold more potent than CBC. However, while the inhibition by CBC was counteracted by 2 μM atropine, that by paraoxon was unaffected by up to 100 μM atropine. It suggested that this effect of paraoxon was not via binding to the muscarinic receptor. Paraoxon did not affect β-adrenoreceptor function in the preparation, since it did not affect the 10 μM isoproterenol-induced cAMP synthesis, which was inhibited totally by 10 μM propranolol and partially by CBC. Paraoxon had a small but significant effect on CBC-stimulated PI metabolism in the SMG cells. It is suggested that paraoxon binds to two different sites in these SMG cells. One is an allosteric site on the M3 muscarinic receptor which affects ligand binding and may modulate receptor function. The other site may be on the Gi proteinadenylyl cyclase system, and produces CBC-like action, that is, inhibition of the forskolin-stimulated [3H]cAMP synthesis, and is unaffected by atropine inhibition of the muscarinic receptor. This adds to the complexity of paraoxon actions on muscarinic receptors and their effector systems. 相似文献