全文获取类型
收费全文 | 1802篇 |
免费 | 142篇 |
国内免费 | 84篇 |
出版年
2024年 | 3篇 |
2023年 | 58篇 |
2022年 | 62篇 |
2021年 | 119篇 |
2020年 | 77篇 |
2019年 | 68篇 |
2018年 | 70篇 |
2017年 | 57篇 |
2016年 | 40篇 |
2015年 | 53篇 |
2014年 | 71篇 |
2013年 | 126篇 |
2012年 | 50篇 |
2011年 | 63篇 |
2010年 | 65篇 |
2009年 | 64篇 |
2008年 | 60篇 |
2007年 | 71篇 |
2006年 | 69篇 |
2005年 | 71篇 |
2004年 | 55篇 |
2003年 | 64篇 |
2002年 | 56篇 |
2001年 | 41篇 |
2000年 | 27篇 |
1999年 | 33篇 |
1998年 | 34篇 |
1997年 | 42篇 |
1996年 | 33篇 |
1995年 | 36篇 |
1994年 | 31篇 |
1993年 | 29篇 |
1992年 | 22篇 |
1991年 | 17篇 |
1990年 | 10篇 |
1989年 | 22篇 |
1988年 | 16篇 |
1987年 | 8篇 |
1986年 | 12篇 |
1985年 | 9篇 |
1984年 | 19篇 |
1983年 | 13篇 |
1982年 | 19篇 |
1981年 | 17篇 |
1980年 | 12篇 |
1979年 | 21篇 |
1978年 | 5篇 |
1977年 | 4篇 |
1972年 | 1篇 |
1950年 | 1篇 |
排序方式: 共有2028条查询结果,搜索用时 15 毫秒
11.
12.
Single-channel conductance fluctuations are analysed for gramicidin A incorporated into binary-mixed black lipid membranes of charged phosphatidic acid and neutral lecithin in different molar ratios. At very low Ca++ concentrations in the electrolyte (i.e. in the presence of EDTA) homogeneous lipid mixtures are identified through their conductance and life time probability distributions for integral gramicidin pores. As for the pure lipid components, the conductance histograms each show a single maximum with regular width and for all channels a single mean lifetime is found.For Ca++-levels (10-6–10-5
M) that are close to the critical demixing concentration (10-4
M) unusually broad conductance distributions and reduced lifetimes are found provided the PC content, x, of the membrane is close to the critical mixture (x
crit0.5). We interpret this as a first example of the coupling of a membrane function (the transport of ions) to a lipid matrix with locally fluctuating composition close to a critical demixing point.For
the conductance histogram of gramicidin A in an equimolar mixture of PA and PC shows two well-separated maxima. A correlation analysis between conductance and lifetime of the single pores shows that the two channel populations also differ significantly in their mean channel lifetime, *. This finding is interpreted as being direct evidence for Ca++-induced lateral phase separation in black lipid membranes, as has been postulated recently.Abbreviations used HEPES
N-2-hydroxyethyl-piperazine-N-2-ethane-sulfonic acid
- EDTA
ethylenediaminetetraacetic acid 相似文献
13.
4-Trifluoroacetamidoaniline was reacted with reducing oligosaccharides in the presence of sodium cyanoborohydride to give aminoalditol derivatives, useful for linkage to proteins or solid matrices. A mixture of reducing oligosaccharides, difficult to separate by HPLC, was treated in the same way. The resulting derivatives were easily separated by HPLC.Abbreviations TFAN
4-trifluoroacetamidoaniline
- LcOse4
lacto-N-tetraose
- IV2Fuc-LcOse4
lacto-N-fucopentaose l
- III4Fuc-LcOse4
lacto-N-fucopentaose II
- III3Fuc-nLcOse4
lacto-N-fucopentaose III
- IV2Fuc, III4Fuc-LcOse4
lacto-N-difucohexaose I
- II6Galß1-4GlcNAc-LcOse4
lacto-N-hexaose
- II3NeuAc-Lac
3-sialyllactose
- GlcNAcß1-4GlcNAcß1-4GlcNAc
chitotriose
- GalNac1-3|Fuc1-2|Galß1-4Glc
A-tetrasaccharide 相似文献
14.
Hyalomma anatolicum anatolicum ticks infected with Theileria annulata were partially fed on rabbits and then ground up with tissue culture medium. The ground up ticks were treated by centrifugation at 100 g, filtration through membranes of 8 μm pore diameter and centrifugation on a discontinuous density gradient of Percoll. Counts of sporozoites and tick debris were made from Giemsa stained slides of samples at each stage of the separation. Debris was removed during light centrifugation and filtration at a greater rate than sporozoites. After filtration approximately 41% of the original sporozoites remained in the suspension. After density gradient centrifugation most sporozoites were found in a distinct zone, at approx. 1·08 g/cm3 density, separate from most dense debris and light debris and soluble contaminants. After this final centrifugation approximately 24% of the original sporozoites remained in the recovered suspension. 相似文献
15.
Horse liver alcohol dehydrogenase was immobilized on glycerylpropyl-silica (10 micron, 1000-A pores) activated with 2,2,2-trifluoroethanesulfonyl chloride (tresyl chloride). The coupling and activity yield was almost 100%. The coenzyme-binding sites were equivalent and virtually unaffected by the immobilization process, as judged from Scatchard plots and active-site titrations. The silica-bound enzyme, packed in steel columns, was integrated with HPLC equipment and then successfully used for chromatography of adenine nucleosides, adenine nucleotides, and triazine dyes. Dissociation constants were calculated from chromatographic data and found to correspond well with literature values. The dissociation constants for a number of nucleotide derivatives with potential application in affinity chromatography were also determined. The spaces were found to affect the binding strength of the nucleotides in a qualitatively predictable way. Theoretical plate heights were calculated and found to be in the range 0.01 to 0.1 cm. Attempts to correlate peak widths with the rate constants for the binary complexes involved were only partially successful. 相似文献
16.
17.
The enantioseparation of the sherry aroma components 5-oxo-4-hydroxyhexanoic acid γ-lactone (solerone) and 4,5-dihydroxyhexanoic acid γ-lactone (solerole) is achieved, using Chiraspher (Merck) as the chiral HPLC phase and the optical purity ascertained directly by HRGC with heptakis(3-O-acetyl-2,6-di-O-pentyl)-β-cyclodextrin (Lipodex D) as the chiral stationary phase. The absolute configurations of 4,5-dihydroxyhexanoic acid γ-lactones are assigned by 1H-NMR spectral data of diastereomeric α-methoxy-α-trifluoromethylphenylacetic acid (MTPA) esters, according to Mosher's model. Sensory qualities of the isomers are given. 相似文献
18.
HPLC and 1H-NMR methods for the quantitation of the (R)-enantiomer in (?)-(S)-timolol maleate were developed and validated. The HPLC method requires a 25 cm × 4.6 mm 5 μm Chiracel OD-H (cellulose tris-3,5-dimethylphenylcarbamate) column, a mobile phase of 0.2% (v/v) diethylamine and 4% (v/v) isopropanol in hexane at a flow rate of 1 ml/min and UV detection at 297 nm. A system suitability test was devised to verify the separation of the (R)- and (S)-enantiomers of timolol from other drug-related impurities. The NMR method requires the use of a high-field NMR spectrometer (>360 MHz) and a chiral solvating agent, (?)-(R)-2,2,2-trifluoro-1-(9-anthrylethanol) (R-TFAE). The limits of quantitation were 0.05% and 0.2% (m/m) for HPLC and NMR, respectively. The methods were applied to the determination of the (R)-enantiomer in eight lots of raw material. The results for the two methods were in very good agreement, with results ranging from 0.1 to 4.1% (m/m) by HPLC and none detected to 4.3% (m/m) by NMR. The USP method for specific rotation was found to be unsuitable for detecting the presence of low levels of the (R)-enantiomer in (?)-(S)-timolol maleate. © 1994 Wiley-Liss, Inc. 相似文献
19.
20.
An ultrafiltration membrane bioreactor for the lipolysis of olive oil in reversed micellar media 总被引:2,自引:0,他引:2
The enzymatic hydrolysis of olive oil using Chromobacterium viscosum lipase B encapsulated in reversed micelles of dioctyl sodium sulfosuccinate (AOT) in isooctane was investigated in an ultrafiltration ceramic membrane reactor of tubular type, operating in a batch mode. Water concentration was found to be a critical parameter in the enzyme kinetics and hydrolysis yield of the reaction. The size of micelles, recirculation rate, and substrate concentration were found to be the major factors affecting the separation process. A correlation that enables the prediction of final conversion degrees in this bioreactor from the initial reaction conditions was established. (c) 1993 Wiley & Sons, Inc. 相似文献