全文获取类型
收费全文 | 7736篇 |
免费 | 402篇 |
国内免费 | 400篇 |
出版年
2023年 | 106篇 |
2022年 | 118篇 |
2021年 | 103篇 |
2020年 | 148篇 |
2019年 | 185篇 |
2018年 | 232篇 |
2017年 | 142篇 |
2016年 | 178篇 |
2015年 | 189篇 |
2014年 | 387篇 |
2013年 | 554篇 |
2012年 | 233篇 |
2011年 | 423篇 |
2010年 | 380篇 |
2009年 | 394篇 |
2008年 | 347篇 |
2007年 | 360篇 |
2006年 | 363篇 |
2005年 | 372篇 |
2004年 | 286篇 |
2003年 | 245篇 |
2002年 | 193篇 |
2001年 | 131篇 |
2000年 | 113篇 |
1999年 | 133篇 |
1998年 | 105篇 |
1997年 | 112篇 |
1996年 | 62篇 |
1995年 | 117篇 |
1994年 | 117篇 |
1993年 | 93篇 |
1992年 | 100篇 |
1991年 | 82篇 |
1990年 | 63篇 |
1989年 | 62篇 |
1988年 | 50篇 |
1987年 | 55篇 |
1986年 | 40篇 |
1985年 | 126篇 |
1984年 | 278篇 |
1983年 | 169篇 |
1982年 | 164篇 |
1981年 | 105篇 |
1980年 | 94篇 |
1979年 | 87篇 |
1978年 | 52篇 |
1977年 | 28篇 |
1976年 | 16篇 |
1975年 | 12篇 |
1974年 | 20篇 |
排序方式: 共有8538条查询结果,搜索用时 691 毫秒
41.
Chloroplasts with high rates of photosynthetic O2 evolution (up to 120 mol O2· (mg Chl)-1·h-1 compared with 130 mol O2· (mg Chl)-1·h-1 of whole cells) were isolated from Chlamydomonas reinhardtii cells grown in high and low CO2 concentrations using autolysine-digitonin treatment. At 25° C and pH=7.8, no O2 uptake could be observed in the dark by high- and low-CO2 adapted chloroplasts. Light saturation of photosynthetic net oxygen evolution was reached at 800 mol photons·m-2·s-1 for high- and low-CO2 adapted chloroplasts, a value which was almost identical to that observed for whole cells. Dissolved inorganic carbon (DIC) saturation of photosynthesis was reached between 200–300 M for low-CO2 adapted chloroplasts, whereas high-CO2 adapted chloroplasts were not saturated even at 700 M DIC. The concentrations of DIC required to reach half-saturated rates of net O2 evolution (Km(DIC)) was 31.1 and 156 M DIC for low- and high-CO2 adapted chloroplasts, respectively. These results demonstrate that the CO2 concentration provided during growth influenced the photosynthetic characteristics at the whole cell as well as at the chloroplast level.Abbreviations Chl
chlorophyll
- DIC
dissolved inorganic carbon
- Km(DIC)
coneentration of dissolved inorganic carbon required for the rate of half maximal net O2 evolution
- PFR
photon fluence rate
- SPGM
silicasol-PVP-gradient medium 相似文献
42.
长叶车前花叶病毒上海分离株(HRVsh)单克隆抗体1H2,经纯化后以溴化氰活化法偶联于Sepharose 4B上制成亲和层析柱。HRVsh感染的三生烟提取液,经一次聚乙二醇沉淀初步纯化,悬浮液上亲和层析柱,于磷酸缓冲液中吸附,蒸馏水洗脱。收集的病毒制剂接种心叶烟有感染性,电镜观察见典型的HRVsh粒子,紫外吸收光谱与常规方法提纯的病毒相似,SDS-聚丙烯酰胺凝胶电泳呈一条带。结果表明单克隆抗体亲和层忻得到高度纯化的HRVsh。最后讨论了单克隆抗体亲和层析方法的优点。 相似文献
43.
Simultaneous determination of ascorbic acid and dehydroascorbic acid in cultures of C3H/10T1/2 cells
Luminita L. V. Ibric William F. Benedict Andrew R. Peterson 《In vitro cellular & developmental biology. Plant》1988,24(7):669-676
Summary A reproducible method is described for the separation and quantification of ascorbic acid and dehydroascorbic acid by ion-pairing
reverse-phase high performance liquid chromatography and detection by absorbance at 232 nm. Lowest detectable concentrations
with a linear response of detection were 5 nmol for ascorbic acid and 50 nmol for dehydroascorbic acid. This method was applied
to the analysis of C3H/10T1/2 cells and culture medium after influx or efflux experiments and single or multiple treatments
with ascorbic acid. Subsequent measurement of the radioactivity in the eluted fractions increased the detectability of both
ascorbic acid and dehydroascorbic acid to 10 to 20 pmol.
This research was supported by grant CA 09320 and CA 31574 from the National Cancer Institute, Bethesda, MD, and grant BC441
from The American Cancer Society. 相似文献
44.
T. J. Budd C. D. Dolman A. M. Lawson W. Chai J. Saxton F. W. Hemming 《Glycoconjugate journal》1992,9(5):274-278
N-Acetylneuraminic acid (Neu5Ac) andN-glycoloylneuraminic acid (Neu5Gc) are distributed widely in nature. Using a Carbopac PA-1 anion exchange column, we have determined the ratios of Neu5Ac and Neu5Gc in hydrolysates of platelets and their precursors: a rat promegakaryoblastic (RPM) cell line and a human megakaryoblastic leukemia cell line (MEG-01). The ratio of Neu5Gc:Neu5Ac in cultured RPM cells is 16:1, whereas in platelet rich plasma and cultured MEG-01 cells it is 1:38 and 1:28, respectively. The nature of these sialic acids from RPM cells was verified using thin layer chromatography and liquid secondary ion mass spectrometry. The relevance of increased Neu5Gc levels in early stages of development is discussed.Abbreviations Neu5Ac
N-acetylneuraminic acid
- Neu5Gc
N-glycoloylneuraminic acid
- RPM
rat promegakaryoblast
- MEG-01
human megakaryoblastic leukaemia cell line
- PAD
pulsed amperometric detection
- WGA
wheat germ agglutinin
- FCS
foetal calf serum
- PPEADF
phosphatidylethanolamine dipalmitoyl
- LSIMS
liquid secondary ion mass spectrometry
- HPAEC
high performance anion exchange chromatography
- TBA
thiobarbituric acid 相似文献
45.
Saraswati Manda Nicoll Lerner-Marmarosh Mazzaz Hashmi Leo G. Abood 《Neurochemical research》1992,17(12):1191-1194
The present study, utilizing thioglycolamido as the reactive group, describes the synthesis and pharmacology of a new opioid antagonist affinity ligand, 6-thioglycolamido-6-desoxynaltrexone (TAN) and compares TAN with a related known compound, 6-bromoacetamido-6-desoxynaltrexone (BAN). Both compounds were tested for their reversible and irreversible inhibition of [3H]naloxone binding to calf brain membranes. Reversible binding of BAN and TAN had Ki values of 1×10–9 and 1×10–10 M, respectively as determined by log probit plots. Irreversible binding was determined after extensive washing to remove all non-covalently bound ligand. At a concentration of 5×10–8 and 1×10–8 M for BAN and TAN irreversible binding was inhibited 50% of the maximum value. A study of the time course of irreversible inhibition of [3H]naloxone binding revealed that maximal inhibition occurred within 5 min with a concentration of 1×10–7 M of either agent. TAN but not BAN when administered systematically to mice produced an antinociceptive effect as measured by the writhing test. When administered intracerebraventricularly BAN did not block morphine-induced analgesia for more than 2 hr; whereas, with a single ED50 dose of 20 nmoles of TAN i.c.v. morphine-induced analgesia was almost completely blocked for a period of over 24 hr, as determined by the tail flick test. Although the SH group of TAN were required for the covalent interaction with opioid receptors, the site of TAN's interaction appears to involve other than protein SH groups. 相似文献
46.
47.
The hemolymph of the adult female Manduca sexta was fractionated by cascade-mode multi-affinity chromatography (CASMAC) on a main-line tandem column chain containing Zn(2+)-TED, T-gel, Ni(2+)-DPA, and phenylsepharose and a side-line column containing Zn(2+)-DPA. The technique separated some of the previously described major hemolymph proteins, and yielded a number of fractions with simple composition. Some of these fractions contained only less abundant proteins of Manduca hemolymph. Thus, it appears that CASMAC would be a very useful fractionation technique for purification and characterization of the minor proteins of insect hemolymph. 相似文献
48.
The direct enantiomeric separation of a series of beta-blockers has been carried out on two chiral stationary phases (CSPs) derived from 3,5-dinitrobenzoyl tyrosine: the commercially available ChyRoSine-A and a recent improved version of this CSP. Using supercritical fluid chromatography (SFC), facile separations are achieved (1.1 less than Rs less than 7) within short analysis times. The parameters affecting the enantioselectivity (temperature, pressure, mobile phase nature, solute structure) have been investigated. The optimal mobile phase consists in a mixture of carbon dioxide-methanol-propylamine at 25 degrees C. The solute structure has a great influence on the enantioselectivity. For instance, both amine and hydroxyl protons are necessary for chiral discrimination to occur. Furthermore, the steroselectivity value is directly connected to the amine substituent steric bulkiness. Surprisingly, these solutes are poorly resolved using normal phase liquid chromatography (NPLC). Accordingly, the specific influence of carbon dioxide on the enantiomeric separation of 1,2-amino-alcohols have been investigated using various techniques such as nuclear magnetic resonance (NMR) or molecular modelisation. It has been shown that carbon dioxide acts as a complexing agent toward the amino-alcohol by setting up of a bridge with the hydroxyl and the amine protons of the solute. In that way, the resulting complex possesses lower acido-basic properties and a higher conformational rigidity, responsible for chiral discrimination. 相似文献
49.
The disposition of venlafaxine enantiomers in dogs, rats, and humans receiving venlafaxine. 总被引:4,自引:0,他引:4
A stereospecific high-performance liquid chromatographic (HPLC) method was developed for the quantitation of the enantiomers of venlafaxine, an antidepressant, in dog, rat, and human plasma. The procedure involves derivatization of venlafaxine with the chiral reagent, (+)-S-naproxen chloride, and a postderivatization procedure. The method was linear in the range of 50 to 5,000 ng of each enantiomer per ml of plasma. No interference by endogenous substances or known metabolites of venlafaxine occurred. Studies to characterize the disposition of the enantiomers of venlafaxine were conducted in dog, rat, and human, following oral administration of venlafaxine. The Cmax, area under the curve (AUC) and (S)/(R) concentration ratios of the (R)- and (S)-enantiomers were compared. In rats, the mean plasma ratio of (S)-venlafaxine to that of (R)-venlafaxine over 0.5 to 6.0 h varied from 2.97 to 8.50 with a mean value of 5.51 +/- 2.45. The Cmax, AUC0-infinity, and t 1/2 values of the (R)- and (S)-enantiomers in dogs were not significantly different from one another (P greater than 0.1). The mean ratios [(S)/(R)] of enantiomers of venlafaxine in human over a 2 to 6 h interval ranged from 1.33 to 1.35 with an overall ratio of 1.34 +/- 0.26 (n = 12). These ratios of the enantiomers [(S)/(R)] were not statistically different from unity (P greater than 0.1) indicating that the disposition of venlafaxine enantiomers in humans is not stereoselective and is more similar to that in dogs than that in rats. 相似文献
50.
A solvent mixture containing dioxane, acetonitrile, and hexane was found to be suitable as a mobile phase to resolve oxazepam enantiomers by chiral stationary phase high performance liquid chromatography using covalent Pirkle columns. The resolved oxazepam enantiomers in this solvent mixture had a racemization half-life greater than 3 days at 23°C. When desiccated at 0°C as dried residue, OX enantiomers were stable for at least 50 days with less than 2% racemization. The conditions which stabilized OX enantiomers significantly facilitated the determination of racemization half-lives of OX enantiomers in a variety of aqueous and nonaqueous solvents and at different temperatures. © 1992 Wiley-Liss, Inc. 相似文献