全文获取类型
收费全文 | 3109篇 |
免费 | 152篇 |
国内免费 | 1篇 |
专业分类
3262篇 |
出版年
2024年 | 4篇 |
2023年 | 18篇 |
2022年 | 50篇 |
2021年 | 67篇 |
2020年 | 45篇 |
2019年 | 59篇 |
2018年 | 71篇 |
2017年 | 70篇 |
2016年 | 125篇 |
2015年 | 170篇 |
2014年 | 223篇 |
2013年 | 219篇 |
2012年 | 270篇 |
2011年 | 255篇 |
2010年 | 172篇 |
2009年 | 142篇 |
2008年 | 195篇 |
2007年 | 171篇 |
2006年 | 128篇 |
2005年 | 124篇 |
2004年 | 125篇 |
2003年 | 93篇 |
2002年 | 84篇 |
2001年 | 75篇 |
2000年 | 68篇 |
1999年 | 43篇 |
1998年 | 16篇 |
1997年 | 14篇 |
1996年 | 14篇 |
1995年 | 7篇 |
1994年 | 4篇 |
1993年 | 5篇 |
1992年 | 15篇 |
1991年 | 12篇 |
1990年 | 11篇 |
1989年 | 14篇 |
1988年 | 9篇 |
1987年 | 6篇 |
1986年 | 6篇 |
1985年 | 9篇 |
1984年 | 5篇 |
1983年 | 4篇 |
1982年 | 6篇 |
1979年 | 9篇 |
1978年 | 4篇 |
1975年 | 4篇 |
1974年 | 5篇 |
1973年 | 4篇 |
1972年 | 3篇 |
1968年 | 2篇 |
排序方式: 共有3262条查询结果,搜索用时 15 毫秒
1.
2.
To determine whether the bicyclic monoterpene olefins (-)-alpha-pinene and (-)-beta-pinene arise biosynthetically from the same monoterpene cyclase by alternate deprotonations of a common carbocationic intermediate, the product distributions arising from the acyclic precursor [10-2H3,1-3H]geranyl pyrophosphate were compared with those resulting from incubation of [1-3H]geranyl pyrophosphate with (-)-pinene cyclase from Salvia officinalis. Alteration in proportions of the olefinic products generated by the partially purified pinene cyclase resulted from the suppression of the formation of (-)-beta-pinene (C10 deprotonation) by a primary deuterium isotope effect with a compensating stimulation of the formation of (-)-alpha-pinene (C4 deprotonation). (-)-Pinene cyclase as well as (+)-pinene cyclase also exhibited a decrease in the proportion of the acyclic olefin myrcene generated from the deuteriated substrate, accompanied by a corresponding increase in the commitment to cyclized products. The observation of isotopically sensitive branching, in conjunction with quantitation of the magnitude of the secondary deuterium isotope effect on the overall rate of product formation by the (+)- and (-)-pinene cyclases as well as two other monoterpene cyclases from the same tissue, supports the biosynthetic origin of (-)-alpha-pinene and (-)-beta-pinene by alternative deprotonations of a common enzymatic intermediate. A biogenetic scheme consistent with these results is presented, and alternate proposals for the origin of the pinenes are addressed. 相似文献
3.
Ryan K. Cheu Andrew T. Gustin Christina Lee Luca Schifanella Charlene J. Miller Avie Ha Casey Kim Violeta J. Rodriguez Margaret Fischl Adam D. Burgener Kelly B. Arnold Maria L. Alcaide Nichole R. Klatt 《PLoS pathogens》2020,16(12)
Despite the efficacy of antiretroviral-based pre-exposure prophylactics (PrEP) in men who have sex with men, studies in women have produced widely varying outcomes. Recent evidence demonstrates that vaginal microbial communities are associated with increased HIV acquisition risk and may impact PrEP efficacy. Here, we investigate the mechanisms underlying how vaginal bacteria alter PrEP drug levels and impact HIV infection rates ex vivo. Using cervicovaginal lavages (CVLs) from women with or without bacterial vaginosis (BV), we identified microbial metabolism of PrEP drugs in BV samples through LC-MS/MS analysis of soluble drug levels and metabolite formation in dual T-cell cultures. CVL samples were assessed for microbiome analysis using sequencing of bacterial 16S rRNA genes. We also observed non-Lactobacillus bacteria that are associated with BV may potentially impact PrEP efficacy through increased HIV infection rates in co-cultures containing Lactobacillus or BV bacteria, PrEP drugs, CEM-GFP cells, and HIV-1LAI virus. Finally, we used these data to develop a novel predictive mathematical simulation modeling system to predict these drug interactions for future trials. These studies demonstrate how dysbiotic vaginal microbiota may impact PrEP drugs and provides evidence linking vaginal bacteria to PrEP efficacy in women. 相似文献
4.
To improve turfgrasses using genetic engineering, we have developed a transformation system in turf-type tall fescue, one of the most important turfgrass species. Embryogenic cell cultures were established after callus induction from embryos of mature seed. The agarose-bead method with nurse cells was used to culture protoplasts and plants were regenerated from protoplasts of tall fescue cultured cells. To develop transgenic tall fescue plants, the hygromycin resistance gene and the -glucuronidase gene were introduced into the tall fescue protoplasts by electroporation. A high concentration (200 mg/l) of hygromycin was required to select transformed cells because of the high level of endogenous resistance to the antibiotic in tall fescue. Most of the transformed cells exhibited GUS activity and several plants were regenerated from these cells. The presence of introduced genes was confirmed by Southern blot hybridization of PCR amplified DNA from transgenic plants.Abbreviations
Adh
alcohol dehydrogenase
- BAP
benzylaminopurine
- bp
base pair(s)
- GUS
-glucuronidase
- Kb
kilobase(s)
- MS
Murashige and Skoog's medium
- PCR
polymerase chain reaction 相似文献
5.
Inhibition of cathepsin L-induced degradation of epidermal growth factor receptors by c-Ha-ras gene products 总被引:1,自引:0,他引:1
T Hiwasa S Sakiyama S Yokoyama J M Ha J Fujita S Noguchi Y Bando E Kominami N Katunuma 《Biochemical and biophysical research communications》1988,151(1):78-85
The inhibitory activities of c-Ha-ras gene products (p21s) toward several cysteine proteinases have been investigated. The activity of cathepsin L was inhibited by p21s most effectively while those of cathepsin B and papain were slightly inhibited by p21s. p21s did not show any inhibitory activity toward cathepsin H. In order to connect the protease-inhibitor activity of p21s with cell growth, the degradation of epidermal growth factor receptors (EGF-receptors) was investigated. EGF-receptors were preferentially cleaved by cathepsin L but not by cathepsin B or H. The cleavage of EGF-receptors by cathepsin L was inhibited by p21s dose-dependently. These results raise the possibility that p21s can suppress the degradation of growth-related proteins such as EGF-receptors and thereby affect cell growth. 相似文献
6.
Equilibrium binding of insulin to isolated rat epididymal fat cells was investigated. A temperature of 15 degrees C was chosen for the study to minimize lysosomal degradation of insulin. Indeed, medium insulin lost only 1% of its precipitability in trichloroacetic acid during the 4-h incubation required to approach equilibrium. Binding was measured by a method that did not perturb the equilibrium of the system. A new formalism for analyzing binding data in general was introduced. A correction for trapping of insulin in the interstitial space of cell pellets was both necessary and sufficient to derive specific binding data from raw observations. Thus, so-called "nonspecific binding" was unmasked as a misnomer, and the expression "correction for trapping" was proposed as a substitute. Equations for one and two independent classes of binding sites were fit to the data by the method of maximum likelihood, and the best fit was selected based on Akaike's information criterion, as adapted for a constant fractional error. More than 99.7% of the binding sites were found to be describable by a simple binding isotherm with Kd,app = 8.8 multiplied by over divided by 1.3 nM. Less than 0.3% sites had a higher affinity (Kd approximately equal to 8 multiplied by over divided by 3 pM). There were 99,000 x/divided by 1.6 binding sites/cell. These equilibrium parameters are in agreement with values derived from a kinetic analysis, presented in the subsequent paper (Lipkin, E. W., Teller, D. C., and de Ha?n, C. (1986) J. Biol. Chem. 260, 1702-1711). 相似文献
7.
Native and non-native conformation-specific antibodies directed to the loop region of hen egg-white lysozyme 总被引:1,自引:0,他引:1
Two types of antibodies were differentiated in conventional guinea pig anti-hen egg-white lysozyme (HEL) antisera. The specificities of both antibodies were directed to the loop I region (mainly directed to Cys64--Cys80 loop) but the antibodies were distinct in respect of reactivities with native HEL. One type of antibody reacted with HEL and loop-peptides of HEL but not with the completely reduced and carboxymethylated form of loop-peptides (native conformation specific antibody: NC-Ab). On the other hand, the other type of antibody did not react with HEL but reacted with loop-peptides and also with the completely reduced and carboxymethylated form of loop-peptides (non-native conformation specific antibody: NNC-Ab). The percentage of NNC-Ab in loop I reactive antibody fraction from pooled guinea pig anti-HEL antisera obtained by two different immunization methods was about 25%. Since the affinities of the NNC-Ab to loop-related peptides were higher by one order of magnitude than those of the NC-Ab to the same peptides, care is necessary in evaluating antigenic determinants in native protein. The immunization of guinea pigs with Ploop I . II [sequence 57-107 (Cys64-Cys80, Cys76-Cys94)] evoked an antibody population having specificity similar to but not identical with that of the NNC-Ab type anti-loop I antibody in conventional anti-HEL antisera. 相似文献
8.
Carcinoembryonic Antigen (CEA) was measured in 59 consecutive fine needle aspirates (FNAs) of the lung from 58 patients to determine if the CEA content would enhance the sensitivity of the cytologic diagnosis. Twenty-eight males and 30 females with tumors 1-40 cm in diameter were studied. Final diagnoses were correlated with the clinical history, radiologic studies, tissue (when available) and follow-up. Image-guided FNAs were performed by radiologists using a 22-gauge Chiba needle and 20-mL syringe with one to four passes per specimen. Cytologic examination included rapid assessment in the radiology suite and a final diagnosis in 24 hours. CEA was measured by enzyme immunoassay using monoclonal antibody. Nine benign aspirates and 50 malignant aspirates were diagnosed. The sensitivity of cytology was 86% and specificity, 100%. Using 5 ng/mL as the cutoff, the sensitivity of CEA for malignant aspirates was 50% and specificity, 90%. The combined sensitivity of CEA and cytology was 95%. The mean CEA in malignant aspirates was 131 ng/mL and in benign aspirates, 2.41. The highest mean CEA was seen in adenocarcinoma, 402.6 ng/mL. Lower CEA content was seen in epidermoid carcinoma (58.6 ng/mL), large cell carcinoma (8.09), oat cell carcinoma, metastatic carcinoma of the kidney and breast, thymoma and lymphoma (each less than 1 ng/mL). Elevated CEA alone was diagnostic in two aspirates of bronchioloalveolar carcinoma; carcinoma with an unknown primary source, three; and large cell carcinoma, one. The adjunctive use of CEA in FNAs of the lung enhances the sensitivity of the cytologic diagnosis. 相似文献
9.
10.
Nguyen Dinh Luyen Le Mai Huong Nguyen Thi Thu Ha Nguyen Thanh Tra Le Thi Tu Anh Nguyen Van Tuyen Katalin Posta Ninh The Son Hai Pham-The 《化学与生物多样性》2023,20(2):e202200456
The current report describes the chemical investigation and biological activity of extracts produced by three fungal strains Fusarium oxysporum, Penicillium simplicissimum, and Fusarium proliferatum isolated from the roots of Piper nigrum L. growing in Vietnam. These fungi were namely determined by morphological and DNA analyses. GC/MS identification revealed that the EtOAc extracts of these fungi were associated with the presence of saturated and unsaturated fatty acids. These EtOAc extracts showed cytotoxicity towards cancer cell lines HepG2, inhibited various microbacterial organisms, especially fungus Aspergillus niger and yeast Candida albicans (the MIC values of 50–100 μg/mL). In α-glucosidase inhibitory assay, they induced the IC50 values of 1.00-2.53 μg/mL were better than positive control acarbose (169.80 μg/mL). The EtOAc extract of F. oxysporum also showed strong anti-inflammatory activity against NO production and PGE-2 level. Four major compounds linoleic acid (37.346 %), oleic acid (27.520 %), palmitic acid (25.547 %), and stearic acid (7.030 %) from the EtOAc extract of F. oxysporum were selective in molecular docking study, by which linoleic and oleic acids showed higher binding affinity towards α-glucosidase than palmitic and stearic acids. In subsequent docking assay with inducible nitric oxide synthase (iNOS), palmitic acid, oleic acid and linoleic acid could be moderate inhibitors. 相似文献