全文获取类型
收费全文 | 8035篇 |
免费 | 557篇 |
国内免费 | 2篇 |
出版年
2023年 | 35篇 |
2022年 | 77篇 |
2021年 | 141篇 |
2020年 | 80篇 |
2019年 | 115篇 |
2018年 | 141篇 |
2017年 | 140篇 |
2016年 | 237篇 |
2015年 | 388篇 |
2014年 | 444篇 |
2013年 | 491篇 |
2012年 | 730篇 |
2011年 | 703篇 |
2010年 | 448篇 |
2009年 | 410篇 |
2008年 | 577篇 |
2007年 | 484篇 |
2006年 | 470篇 |
2005年 | 374篇 |
2004年 | 397篇 |
2003年 | 380篇 |
2002年 | 356篇 |
2001年 | 63篇 |
2000年 | 57篇 |
1999年 | 76篇 |
1998年 | 83篇 |
1997年 | 59篇 |
1996年 | 59篇 |
1995年 | 41篇 |
1994年 | 43篇 |
1993年 | 51篇 |
1992年 | 30篇 |
1991年 | 35篇 |
1990年 | 23篇 |
1989年 | 26篇 |
1988年 | 16篇 |
1987年 | 22篇 |
1986年 | 19篇 |
1985年 | 19篇 |
1984年 | 22篇 |
1983年 | 31篇 |
1982年 | 19篇 |
1981年 | 19篇 |
1980年 | 16篇 |
1979年 | 10篇 |
1977年 | 13篇 |
1976年 | 11篇 |
1974年 | 13篇 |
1972年 | 7篇 |
1971年 | 8篇 |
排序方式: 共有8594条查询结果,搜索用时 15 毫秒
51.
Stuart L. Myers Richard Turnage Kevin Kadesky Lori Bartula Angela Riva Barbara Kalley-Taylor 《Prostaglandins & other lipid mediators》1995,50(1)
This study examines the hypothesis that PAF stimulates release of PGI2 from inflamed rabbit gallbladder explant cell cultures. New Zealand white rabbits underwent bile duct ligation for 72 h (72 h BDL), or sham operation, Sham and 72 h BDL gallbladder explants were placed in culture, and the cells grown to 75% confluence. The cells were exposed to increasing concentrations of PAF for 60 min. The media analyzed for eicosanoid release by EIA and the cells analyzed for cyclooxygenase and prostacyclin synthase content by immunoblot analysis. PAF increased release of 6-keto-PGF1α from the 72 h BDL gallbladder cell cultures in a dose-related manner which was inhibited by indomethacin preincubation by 90%. The increased 72 h BDL cell release of 6-keto-PGF1α was not associated with changes in the content of cyclooxygenase or prostacyclin synthase. PAF did not alter eicosanoid release from sham control cell cultures. These data suggest that PAF can only up-regulate endogenous 6-keto-PGF1α release from the 72 h BDL cells that had been previously stimulated by inflammation. PAF may thus contribute to gallbladder distention and injury by chronic stimulation of inflamed gallbladder PGI2 release. 相似文献
52.
53.
Joanne Mcandrew Philip S. Rudland Angela M. Platt-Higgins John A. Smith 《The Histochemical journal》1994,26(4):355-366
Summary Immunoreactive alpha-transforming growth factor (-TGF) was shown by immunocytochemistry to be present in the rat mammary gland at various stages of development, the staining being most intense in mature myoepithelial cells. -TGF was also detected in the secretions of the mammary glands of pregnant and lactating rats. -TGF in the extracts of rat mammary glands at each stage of development, and in several rat mammary cell lines and in culture medium in which they had been grown, was shown by Western blotting to consist primarily of a protein of molecular weight 50 kDa. The amount of this protein was greater in the mammary gland of the lactating rat than in resting or involuting glands. -TGF was also found in some, but not all, human breast carcinomas, and in benign hyperplastic breast diseases. 相似文献
54.
Biological Trace Element Research - The results of a research in progress at the Istituto di Fisica Generale Applicata—University of Milan—on natural and anthropogenic elements'... 相似文献
55.
56.
R. Doll R. Peto K. Wheatley R. Gray I. Sutherland 《BMJ (Clinical research ed.)》1994,309(6959):901-911
OBJECTIVE--To assess the hazards associated with long term use of tobacco. DESIGN--Prospective study of mortality in relation to smoking habits assessed in 1951 and again from time to time thereafter, with causes sought of deaths over 40 years (to 1991). Continuation of a study that was last reported after 20 years'' follow up (1951-71). SUBJECTS--34,439 British male doctors who replied to a postal questionnaire in 1951, of whom 10,000 had died during the first 20 years and another 10,000 have died during the second 20 years. RESULTS--Excess mortality associated with smoking was about twice as extreme during the second half of the study as it had been during the first half. The death rate ratios during 1971-91 (comparing continuing cigarette smokers with life-long non-smokers) were approximately threefold at ages 45-64 and twofold at ages 65-84. The excess mortality was chiefly from diseases that can be caused by smoking. Positive associations with smoking were confirmed for death from cancers of the mouth, oesophagus, pharynx, larynx, lung, pancreas, and bladder; from chronic obstructive pulmonary disease and other respiratory diseases; from vascular diseases; from peptic ulcer; and (perhaps because of confounding by personality and alcohol use) from cirrhosis, suicide, and poisoning. A negative association was confirmed with death from Parkinson''s disease. Those who stopped smoking before middle age subsequently avoided almost all of the excess risk that they would otherwise have suffered, but even those who stopped smoking in middle age were subsequently at substantially less risk than those who continued to smoke. CONCLUSION--Results from the first 20 years of this study, and of other studies at that time, substantially underestimated the hazards of long term use of tobacco. It now seems that about half of all regular cigarette smokers will eventually be killed by their habit. 相似文献
57.
John C. Spurlino Angela M. Smallwood Dennis D. Carlton Tracey M. Banks Karen J. Vavra Jeffrey S. Johnson Ewell R. Cook Joseph Falvo Robert C. Wahl Tricia A. Pulvino John J. Wendoloski Douglas L. Smith 《Proteins》1994,19(2):98-109
The X-ray crystal structure of a 19 kDa active fragment of human fibroblast collagenase has been determined by the multiple isomorphous replacement method and refined at 1.56 Å resolution to an R-factor of 17.4%. The current structure includes a bound hydroxamate inhibitor, 88 waters and three metal atoms (two zincs and a calcium). The overall topology of the enzyme, comprised of a five stranded β-sheet and three α-helices, is similar to the thermolysin-like metalloproteinases. There are some important differences between the collagenase and thermolysin families of enzymes. The active site zinc ligands are all histidines (His-218, His-222, and His-228). The presence of a second zinc ion in a structural role is a unique feature of the matrix metalloproteinases. The binding properties of the active site cleft are more dependent on the main chain conformation of the enzyme (and substrate) compared with thermolysin. A mechanism of action for peptide cleavage similar to that of thermolysin is proposed for fibroblast collagenase. © 1994 Wiley-Liss, Inc. 相似文献
58.
Summary We have examined the 13C and 13C chemical shifts of a number of proteins and found that their values at the N-terminal end of a helix provide a good predictor for the presence of a capping box. A capping box consists of a hydrogen-bonded cycle of four amino acids in which the side chain of the N-cap residue forms a hydrogen bond with the backbone amide of the N3 residue, whose side chain in turn may accept a hydrogen bond from the amide of the N-cap residue. The N-cap residue exhibits characteristic values for its backbone torsion angles, with and clustering around 94±15° and 167±5°, respectively. This is manifested by a 1–2 ppm upfield shift of the 13C resonance and a 1–4 ppm downfield shift of the 13C resonance, relative to their random coil values, and is mainly associated with the unusually large value of . The residues following the N-cap residue exhibit downfield shifts of 1–3 ppm for the 13C resonances and small upfield shifts for the 13C ones, typical of an -helix. 相似文献
59.
Dipl. Geol. Jens Hefter Dipl. Geol. Volker Thiel Dr. Angela Jenisch Dr. Ursula Galling Dr. Stephan Kempe Dr. habil. Walter Michaelis 《Facies》1993,29(1):93-105
Summary Biomarker investigations were applied to the hydrocarbon fractions of three Recent (cyanobacterial mat, Lake Van microbialite
and Lake Satonda microbialite) and two Late Jurassic carbonate samples obtained from sponge bioherms. The relative concentrations
ofn-alkanes, monomethyl alkanes, acyclic isoprenoids, steroids and hopanoids in these samples are studied and their probable
biological precursors are discussed. Normal alkanes with carbon chain lengths ranging from C15 to C34 and monomethyl alkanes ranging from C17 to C21 with a varying methyl branching pattern are found. The major hydrocarbons are low molecular weight (LMW)n-alkanes (C15–C21) with a slight to strong predominance ofn-heptadecane (C17). High molecular weight (HMW)n-alkanes occur in low to moderate relative concentrations showing a preference of odd-carbon numbered compounds with a maximum
at C29. Within the acyclic isoprenoids, pristane, phytane/phytene, pentamethyl-eicosane, squalane and lycopane could be identified.
Polycyclic terpenoids of the sterane and/or hopane type are present in all carbonate samples. The carbon numbers of these
components range from 27 to 29 and 27 to 32, respectively. These organic compounds identified can be attributed to various
source organisms such as cyanobacteria, archaebacteria, algae and vascular plants. All hydrocarbon fractions of the samples
are characterized by moderate to high relative concentrations of compounds derived from cyanobacteria, signifying the role
of these organisms as contributors to the Recent as well as to the Late Jurassic carbonate deposits. 相似文献
60.
Angela M. Smith Andrew J. Dowd Mary Heffernan Colin D. Robertson John P. Dalton 《International journal for parasitology》1993,23(8):977-983
Adult Fasciola hepatica secrete a cysteine proteinase capable of cleaving host IgG close to the papain cleaving site. The proteinase was separated by size permeation chromatography. Gelatinsubstrate polyacrylamide gel electrophoresis analysis revealed that the proteinase migrates as 6 proteolytic bands in the apparent molecular size range 60–90 kDa. Based on pH profiles of activity, inhibition studies using diethylpyrocarbonate and the diazomethylketone Z-phe-ala-CHN2, and characterising the substrate specificity of the enzymes using fluorogenic peptide substrates we have shown that the 60–90-kDa proteinases are cathepsin L-Iike proteinases. 相似文献