全文获取类型
收费全文 | 3754篇 |
免费 | 214篇 |
专业分类
3968篇 |
出版年
2022年 | 38篇 |
2021年 | 70篇 |
2020年 | 37篇 |
2019年 | 52篇 |
2018年 | 63篇 |
2017年 | 70篇 |
2016年 | 91篇 |
2015年 | 133篇 |
2014年 | 140篇 |
2013年 | 183篇 |
2012年 | 266篇 |
2011年 | 228篇 |
2010年 | 153篇 |
2009年 | 124篇 |
2008年 | 171篇 |
2007年 | 151篇 |
2006年 | 129篇 |
2005年 | 137篇 |
2004年 | 122篇 |
2003年 | 92篇 |
2002年 | 107篇 |
2001年 | 90篇 |
2000年 | 63篇 |
1999年 | 71篇 |
1998年 | 30篇 |
1997年 | 29篇 |
1996年 | 27篇 |
1995年 | 27篇 |
1992年 | 50篇 |
1991年 | 61篇 |
1990年 | 47篇 |
1989年 | 55篇 |
1988年 | 39篇 |
1987年 | 48篇 |
1986年 | 31篇 |
1985年 | 74篇 |
1984年 | 46篇 |
1983年 | 51篇 |
1982年 | 24篇 |
1981年 | 29篇 |
1980年 | 41篇 |
1979年 | 53篇 |
1978年 | 33篇 |
1976年 | 25篇 |
1975年 | 27篇 |
1974年 | 32篇 |
1973年 | 28篇 |
1971年 | 34篇 |
1970年 | 29篇 |
1969年 | 33篇 |
排序方式: 共有3968条查询结果,搜索用时 15 毫秒
81.
A new steryl ester isolated from the aerial parts of Lepidium sativumfy1>cf1>, has been identified as stigmast-5-en-3,β27-diol 27-benzoate on the basis of spectral data analyses and chemical reactions. 相似文献
82.
Lytic phages infect their bacterial hosts, use the host machinery to replicate, and finally lyse and kill their hosts, releasing progeny phages. Various mathematical models have been developed that describe these phage-host viral dynamics. The aim of this study was to determine which of these models best describes the viral dynamics of lytic RNA phage MS2 and its host Escherichia coli C-3000. Experimental data consisted of uninfected and infected bacterial cell densities, free phage density, and substrate concentration. Parameters of various models were either determined directly through other experimental techniques or estimated using regression analysis of the experimental data. The models were evaluated using a Bayesian-based model discrimination technique. Through model discrimination it was shown that phage-resistant cells inhibited the growth of phage population. It was also shown that the uninfected bacterial population was a quasispecies consisting of phage-sensitive and phage-resistant bacterial cells. When there was a phage attack the phage-sensitive cells died out and the phage-resistant cells were selected for and became the dominant strain of the bacterial population. 相似文献
83.
An efficient protocol was developed for qualitative screening of phosphate-solubilizing bacteria, based upon visual observation.
Our results indicate that, by using our formulation containing bromophenol blue, it is possible to quickly screen on a qualitative
basis the phosphate-solubilizing bacteria. Qualitative analysis of the phosphate solubilized by various groups correlated
well with grouping based upon quantitative analysis of bacteria isolated from soil, effect of carbon, nitrogen, salts, and
phosphate solubilization-defective transposon mutants. However, unlike quantitative analysis methods that involve time-consuming
biochemical procedures, the time for screening phosphate-solubilizing bacteria is significantly reduced by using our simple
protocol. Therefore, it is envisaged that usage of this formulation based upon qualitative analysis will be salutary for the
quick screening of phosphate-solubilizing bacteria. Our results indicate that the formulation can also be used as a quality
control test for expeditiously screening the commercial bioinoculant preparations, based on phosphate solubilizers.
Received: 17 November 2000 / Accepted: 22 December 2000 相似文献
84.
Methyl mercury uptake in free cells and different immobilizates of the cyanobacteriumNostoc calcicola has been examined. The general growth of the immobilized cyanobacterial cells could be negatively correlated with methyl mercury uptake. Alginate spheres proved most efficient in terms of uptake rate (0.48 nmol mg protein–1 min–1, 10 min) and total bioaccumulation (10.71 nmol mg protein–1, 1 h) with a bioconcentration factor of 3.3×103. Alginate biofilms showed a faster methyl mercury accumulation rate (0.83 nmol mg protein–1 min–1, 10 min) with a saturation of 10.28 nmol mg protein–1 reached within only 30 min (bioconcentration factor, 3.1×103). Foam preparations with a slow initial uptake approximated biofilms but were characterized by a lower bioconcentration factor (2.8×103). Free cells, in comparison, maintained the initial slow rate of uptake (0.62 nmol mg protein–1 min–1, 10 min), saturating at 30 min (8.81 nmol mg protein–1), and the resultant lowest bioconcentration factor (2.7×103). Cell ageing (30 days) brought a drastic reduction (3-fold) in organomercury uptake by free cells while alginate spheres maintained the same potential. Foam preparations of the same age showed a significant improvement in methyl mercury uptake followed by only a marginal decline in alginate biofilms. Data are discussed in the light of the physiological efficiency and longevity of immobilized cells. 相似文献
85.
Russell Vassell Yong He Prasad Vennakalanti Antu K. Dey Min Zhuang Wei Wang Yide Sun Zohar Biron-Sorek Indresh K. Srivastava Celia C. LaBranche David C. Montefiori Susan W. Barnett Carol D. Weiss 《PloS one》2015,10(6)
The membrane proximal external region (MPER) of the gp41 subunit of the HIV-1 envelope glycoprotein (Env) contains determinants for broadly neutralizing antibodies and has remained an important focus of vaccine design. However, creating an immunogen that elicits broadly neutralizing antibodies to this region has proven difficult in part due to the relative inaccessibility of the MPER in the native conformation of Env. Here, we describe the antigenicity and immunogenicity of a panel of oligomeric gp41 immunogens designed to model a fusion-intermediate conformation of Env in order to enhance MPER exposure in a relevant conformation. The immunogens contain segments of the gp41 N- and C-heptad repeats to mimic a trapped intermediate, followed by the MPER, with variations that include different N-heptad lengths, insertion of extra epitopes, and varying C-termini. These well-characterized immunogens were evaluated in two different immunization protocols involving gp41 and gp140 proteins, gp41 and gp160 DNA primes, and different immunization schedules and adjuvants. We found that the immunogens designed to reduce extension of helical structure into the MPER elicited the highest MPER antibody binding titers, but these antibodies lacked neutralizing activity. The gp41 protein immunogens also elicited higher MPER titers than the gp140 protein immunogen. In prime-boost studies, the best MPER responses were seen in the groups that received DNA priming with gp41 vectors followed by gp41 protein boosts. Finally, although titers to the entire protein immunogen were similar in the two immunization protocols, MPER-specific titers differed, suggesting that the immunization route, schedule, dose, or adjuvant may differentially influence MPER immunogenicity. These findings inform the design of future MPER immunogens and immunization protocols. 相似文献
86.
Govind Ragupathi Payal Damani Geeta Srivastava Om Srivastava Steven J. Sucheck Yoshi Ichikawa Philip O. Livingston 《Cancer immunology, immunotherapy : CII》2009,58(9):1397-1405
Sialyl Lewisa (sLea), also termed CA19-9 antigen, is recognized by murine mAb19-9 and is expressed on the cancer cell surface as a glycolipid
and as an O-linked glycoprotein. It is highly expressed in a variety of gastrointestinal epithelial malignancies including
colon cancer and pancreatic cancer, and in breast cancer and small cell lung cancer, but has a limited expression on normal
tissues. sLea is known to be the ligand for endothelial cell selectins suggesting a role for sLea in cancer metastases and adhesion. For these reasons, sLea may be a good target for antibody mediated immunotherapy including monoclonal antibodies and tumor vaccines. However, sLea is structurally similar to sLex and other blood group related carbohydrates which are widely expressed on polymorphonucleocytes and other circulating cells,
raising concern that immunization against sLea will induce antibodies reactive with these more widely expressed autoantigens. We have shown previously both in mice and
in patients that conjugation of a variety of carbohydrate cancer antigen to keyhole limpet hemocyanin (KLH) and administration
of this conjugate mixed with saponin adjuvants QS-21 or GPI-0100 are the most effective methods for induction of antibodies
against these cancer antigens. We describe here for the first time the total synthesis of pentenyl glycoside of sLea hexasaccharide and its conjugation to KLH to construct a sLea-KLH conjugate. Groups of five mice were vaccinated subcutaneously four times over 6 weeks. Sera were tested against sLea-HSA by ELISA and against sLea positive human cell lines adenocarcinoma SW626 and small cell lung cancer (SCLC) DMS79 by FACS. As expected, mice immunized
with unconjugated sLea plus GPI-0100 or unconjugated sLea mixed with KLH plus GPI-0100 failed to produce antibodies against sLea. However, mice immunized with sLea-KLH conjugate without GPI-0100 produced low levels of antibodies and mice immunized with sLea-KLH plus GPI-0100 produced significantly higher titer IgG and IgM antibodies against sLea by ELISA. These antibodies were highly reactive by FACS and mediated potent complement mediated cytotoxicity against sLea positive SW626 and DMS79 cells. They showed no detectable cross reactivity against a series of other blood group-related
antigens, including Ley, Lex, and sLex by dot blot immune staining. This vaccine is ready for testing as an active immunotherapy for treating sLea positive cancer in clinical settings.
Govind Ragupathi and Philip O. Livingston are paid consultants and shareholders in MabVax Therapeutics, Inc., San Diego, CA
92121. The sLea vaccine is licensed to MabVax. 相似文献
87.
Various parameters such as solvent selection, concentration, soaking time, and temperature were tested in a single bioreactor
in order to determine optimum extraction conditions of glucoamylase, when produced simultaneously with protease by Aspergillus awamari nakazawa MTCC 6652. Optimum conditions were achieved in a 10% glycerol solution soaked for 2 h at 40°C, followed by concentration
of extracted glucoamylase (9,157 U/gds) by acetone precipitation (1:2, v/v), which yielded 51.9% recovery. Ion exchange chromatography
and gel filtration showed specific activities of 270.5 and 337.5 U/mg, respectively, while SDS-PAGE and zymogram analysis
of glucoamylase indicated the presence of three starch-hydrolyzing isoforms with molecular weights of approximately 109.6,
87.1, and 59.4 kDa, respectively 相似文献
88.
A suitable method for extraction of floridoside phosphate synthase (FPS, UDP-galactose: sn-3-glycerol phosphate: 1→2′α-D-galactosyl transferase)from Porphyra perforata J. Ag. was developed. Two assay methods for enzyme activity were utilized, one measuring the amount of floridoside formed by using gas-liquid chromatography, the other measuring the sn-3-glycerol phosphate-dependent formation of UDP; both assays gave similar results. FPS is a soluble protein, and FPS activity in the extract as determined by the amount of product formed in vitro compared well with the in vivo rate of floridoside synthesis (4–7 μMmol product formed·h?1·g?1 fresh wt). The rate of product formation in vitro was linear up to 45 min and proportional to protein concentration in the assay mixture. The temperature optimum was 30–35° C. FPS was active over a range of pH values from 7.0–8.5. It was stable in concentrated solutions in the presence of 0.3 M ammonium sulfate, but activity was lost in diluted solution (protein concentration below 0.2 mg·mL?1) or below 0.2 M ion strength. The data suggest that FPS may be an oligomeric protein which occurs free in the cytoplasm or loosely bound to a membrane. It may also be a regulatory protein controlling the overall rate of synthesis of floridoside in vivo. 相似文献
89.
In the traditional Indian system of medicine, Ayurveda, several spices and herbs are claimed to possess medicinal properties, such as being antithrombotic, antiatherosclerotic, hypolipidemic, anti-inflammatory etc. Earlier we have reported that extracts from several spices behave as antiaggregatory agents and inhibit eicosanoid synthesis. Similar studies with extracts prepared from cumin (Cuminum cyminum) and turmeric (Curcuma longa) were undertaken. Ethereal extract of both cumin and turmeric inhibited arachidonate-induced platelet aggregation. Extracts from these spices inhibited thromboxane B2 production from exogenous (14C) arachidonic acid (AA) in washed platelets; a simultaneous increase in the formation of lipoxygenase-derived products was observed. Less TxB2 was produced in blood samples treated with turmeric extract when they were allowed to clot. Turmeric extract inhibited incorporation of (14C)AA into platelet phospholipids and deacylation of AA-labelled phospholipids on stimulation with calcium ionophore A23187. Cumin extract was devoid of such effects. Extracts from the two spices reduced the formation of (14C)TxB2 from AA-labelled platelets when they were challenged with A23187. The anti-inflammatory property of turmeric may, in part, be explained by its effect on eicosanoid biosynthesis. 相似文献
90.
Substance P (SP) is an important neuropeptide involved in pain transmission and induction of inflammation. Its antagonists are being extensively investigated for their non-narcotic analgesic and anti-inflammatory activity. With a view towards better understanding the structural requirements of these analogs for efficient interaction with the SP receptor, the conformation of three SP antagonists [D-Arg1, D-Trp7,9, Leu11]-SP, [D-Arg1, D-Pro2, D-Trp7,9, Leu11]-SP and [D-Pro2, D-Trp7,9]-SP has been studied by CD, NMR and molecular dynamics (MD) simulations. All three peptides exhibit a high dependence of structure on the solvent. The molecules tend to adopt beta-turns in solvents like DMSO and H2O and form helices in a hydrophobic environment. A direct relation between the helix forming potential of these antagonists with their receptor binding potency has been observed. 相似文献