全文获取类型
收费全文 | 861篇 |
免费 | 13篇 |
国内免费 | 24篇 |
出版年
2023年 | 9篇 |
2021年 | 8篇 |
2020年 | 6篇 |
2019年 | 16篇 |
2018年 | 12篇 |
2017年 | 13篇 |
2016年 | 10篇 |
2015年 | 7篇 |
2014年 | 30篇 |
2013年 | 51篇 |
2012年 | 19篇 |
2011年 | 33篇 |
2010年 | 19篇 |
2009年 | 35篇 |
2008年 | 45篇 |
2007年 | 33篇 |
2006年 | 24篇 |
2005年 | 29篇 |
2004年 | 25篇 |
2003年 | 26篇 |
2002年 | 23篇 |
2001年 | 15篇 |
2000年 | 12篇 |
1999年 | 20篇 |
1998年 | 16篇 |
1997年 | 14篇 |
1996年 | 23篇 |
1995年 | 18篇 |
1994年 | 23篇 |
1993年 | 17篇 |
1992年 | 11篇 |
1991年 | 14篇 |
1990年 | 11篇 |
1989年 | 11篇 |
1988年 | 14篇 |
1987年 | 12篇 |
1986年 | 11篇 |
1985年 | 26篇 |
1984年 | 22篇 |
1983年 | 8篇 |
1982年 | 13篇 |
1981年 | 22篇 |
1980年 | 9篇 |
1979年 | 8篇 |
1978年 | 9篇 |
1977年 | 11篇 |
1976年 | 6篇 |
1975年 | 15篇 |
1974年 | 17篇 |
1973年 | 9篇 |
排序方式: 共有898条查询结果,搜索用时 15 毫秒
71.
The objective of this research was to evaluate the effect of enzymatically synthesized maltotriose fatty acid monoesters (Ferrer, M., et al. 2000 Tetrahedron
56, 4053–4061) on Aroclor 1242 solubilization and biodegradation. Three forms of the surfactant, laurate, palmitate and stearate monoester, were tested. Potential enhancement of solubilization of hydrophobic substances mediated by these non-ionic surfactants was exploited in this study. A polychlorinated biphenyl (PCB) degrading organism, Burkholderia cepacia LB400, was also selected. It was found that all surfactants were effective in solubilizing Aroclor 1242 but the rate of Aroclor 1242 biodegradation proceeded rapidly only in the presence of 6-O-palmitoylmaltotriose. For example, the addition of 48 mg 6-O-palmitoylmaltotriose/l increased the apparent solubility from 140 to 305 g/l. As a result, only 8% of the Aroclor remained at the end of 24 h incubation. In contrast, 49.2% of the Aroclor 1242 remained in the absence of surfactant. It appears that maltotriose fatty acid monoesters can significantly increase the bioavailability, and thereby accelerate the biodegradation of highly chlorinated PCBs, particularly Aroclor 1242, by Burkholderia cepacia LB400. The possibility of obtaining these biodegradable surfactants with high yield, easy recovery and high purity by using a new enzymatic methodology, makes maltotriose esters available for bioremediation purposes. 相似文献
72.
Chen F D'Auria JC Tholl D Ross JR Gershenzon J Noel JP Pichersky E 《The Plant journal : for cell and molecular biology》2003,36(5):577-588
Emission of methylsalicylate (MeSA), and occasionally of methylbenzoate (MeBA), from Arabidopsis thaliana leaves was detected following the application of some forms of both biotic and abiotic stresses to the plant. Maximal emission of MeSA was observed following alamethicin treatment of leaves. A gene (AtBSMT1) encoding a protein with both benzoic acid (BA) and salicylic acid (SA) carboxyl methyltransferase activities was identified using a biochemical genomics approach. Its ortholog (AlBSMT1) in A. lyrata, a close relative of A. thaliana, was also isolated. The AtBSMT1 protein utilizes SA more efficiently than BA, whereas AlBSMT1 catalyzes the methylation of SA less effectively than that of BA. The AtBSMT1 and AlBSMT1 genes showed expression in leaves under normal growth conditions and were more highly expressed in the flowers. In A. thaliana leaves, the expression of AtBSMT1 was induced by alamethicin, Plutella xylostella herbivory, uprooting, physical wounding, and methyl jasmonate. SA was not an effective inducer. Using a beta-glucuronidase (GUS) reporter approach, the promoter activity of AtBSMT1 was localized to the sepals of flowers, and also to leaf trichomes and hydathodes. Upon thrip damage to leaves, AtBSMT1 promoter activity was induced specifically around the lesions. 相似文献
73.
Miroslav Vacek Marie Zarevúcka Zdeněk Wimmer Karel Stránský Kateřina Demnerová Marie-Dominique Legoy 《Biotechnology letters》2000,22(19):1565-1570
Selective enzymic esterification of free fatty acids, obtained from blackcurrant oil by chemical saponification, with n-butanol using four immobilized lipases under microwave irradiation and under classical heating was studied. A positive effect of microwave irradiation on chemical yields of the products of the enzymic reactions and specificity of lipases were observed in comparison with a controlled heating in an incubator equipped with shaking (classical heating) applied during the identical enzyme-mediated processes. The maximum quantity of -linolenic acid (30%) was obtained with Lipozyme used as biocatalyst of the reaction under microwave irradiation. The maximum quantity of butyl -linolenate (20%) was obtained by a Pseudomonas cepacia lipase catalyzed esterification under classical heating. 相似文献
74.
Virginia Ladeda Julio A.Aguirre Ghiso Elisa Bal de Kier Joffé 《Experimental cell research》1998,242(2):515
The cell surface glycoprotein CD44 is proposed as a main participant in cell adhesion and migration. We studied the function, expression, and distribution of CD44 in the invasive and metastatic F3II murine carcinoma cell line during adhesion, spreading, migration, and invasion. A mAb anti-CD44 (KM 201) dramatically blocked F3II cell adhesion on both plastic and hyaluronic acid coatings, as well as spreading on uncoated plastic surfaces (P< 0.01). KM201 mAb significantly inhibited F3II cell migration and invasion in Transwell chambers. Immunocytochemistry of spreading cells revealed that CD44 distributed in bands on the cell surface, particularly in the tip of leading edges and in the perinuclear zones of the cell membrane. CD44 antigen was never detected in filopodia or lamellipodia nor in focal adhesion-like structures, but was also detectable as strong interlamellar bands. Fully spread cells showed a decreased CD44 signal compared to cells in early stages of spreading. This decrease correlated with a reduced expression of CD44 as detected by Western blot. We also investigated the signals that may regulate CD44 expression in F3II cells. Treatment of F3II cells, with phorbol myristate acetate (PMA) or phosphatidic acid (PA, the product of PLD-dependent hydrolysis of phosphatidylcholine), significantly enhanced CD44 expression. Conversely, the treatment of F3II cells with H7, a specific PKC inhibitor, or propranolol, which blocks PA conversion to DAG, significantly decreased CD44 expression levels. These results suggest the involvement of PKC and PLD pathways in CD44 expression. These results demonstrate that CD44 plays an important role during F3II cells adhesion, spreading, migration, and invasion. In addition we provide information linking the PLD- and PKC-dependent pathways with the regulation of CD44 expression. 相似文献
75.
David J. Harvey 《Molecular biotechnology》1998,10(3):251-260
The position of unsaturation, chain branching, and other structural features of fatty acids are not often apparent from the
mass spectra of common derivatives such as methyl esters because of factors such as charge location at the carboxy termiunus
and migration of double bonds. The spectra of picolinyl esters, on the other hand, contain fragment ions that provide this
information. The esters are synthesized by reaction of the acids with thionyl chloride to form the acid chloride that is reacted
with 3-pyridylcarbinol to give the ester. Under electron impact conditions in the mass spectrometer, an electron is removed
from the nitrogen of the pyridine ring and a hydrogen atom is abstracted from the alkyl chain to this electron-deficient site.
This process produces a radical site in the chain that initiates chain cleavage. Hydrogen atoms can be removed from any position
of the chain with varying probability, depending on the chain structure. Thus, diagnostic ions are produced from each type
of fatty acid whose masses and relative abundances reflect the structure of the alkyl chain and any substituents. Patterns
of fragmentation for straight-chain, branched-chain, unsaturated and cyclic fatty acids are described together with those
containing hydroxy-, epoxy-, keto-, and ether groups. 相似文献
76.
Adosh Mehta Annette M. Oeser Michael G. Carlson 《Journal of chromatography. B, Analytical technologies in the biomedical and life sciences》1998,719(1-2)
We report a rapid and sensitive method for separation and quantitation of free fatty acids (FFAs) in human plasma using high-performance liquid chromatography (HPLC). Two established techniques of lipid extraction were investigated and modified to achieve maximal FFA recovery in a reasonably short time period. A modified Dole extraction method exhibited greater recovery (90%) and short processing times (30 min) compared to the method of Miles et al. Reversed-phase HPLC using UV detection was used for plasma FFA separation and quantitation. Two phenacyl ester derivatives, phenacyl bromide and p-bromophenacyl bromide, were investigated in order to achieve optimal separation of individual plasma FFAs (saturated and unsaturated) with desirable detection limits. Different chromatographic parameters including column temperature, column type and elution profiles (isocratic and gradient) were tested to achieve optimal separation and recovery of fatty acids. Phenacyl bromide esters of plasma fatty acids were best resolved using an octadecylsilyl column with endcapped silanol groups. An isocratic elution method using acetonitrile–water (83:17) at 2 ml/min with UV detection at 242 nm and a column temperature of 45°C was found to optimally resolve the six major free fatty acids present in human plasma (myristic [14:0], palmitic [16:0], palmitoleic [16:1], stearic [18:0], oleic [18:1] and linoleic [18:2]), with a run time of less than 35 min and detection limits in the nmol range. The entire process including plasma extraction, pre-column derivatization, and HPLC quantitation can be completed in 90 min with plasma samples as small as 50 μl. Over a wide physiological range, plasma FFA concentrations determined using our HPLC method agree closely with measurements using established TLC–GC methods (r2≥0.95). In addition, by measuring [14C] or [3H] radioactivity in eluent fractions following HPLC separation of plasma FFA, this method can also quantitate rates of FFA turnover in vivo in human metabolic studies employing isotopic tracers of one or more fatty acids. 相似文献
77.
78.
79.
Sawsan El-Masry Fikria A. Darwish Amina Abou-Donia Mohamed A. Abou-Karam Michael Grenz 《Phytochemistry》1985,24(5):999-1001
The investigation of the aerial parts of Centaurea glomerata afforded four new sesquiterpene lactones of the germacradienolide type and the lignan lactone arctigenin. Their structures were elucidated by spectroscopic methods. 相似文献
80.
Luis J. Lpez Giraldo Mickaël Laguerre Jrme Lecomte Maria-Cruz Figueroa-Espinoza Nathalie Barouh Bruno Bara Pierre Villeneuve 《Enzyme and microbial technology》2007,41(6-7):721-726
Chlorogenic acid (5-caffeoylquinic acid or 5-CQA) is an hydrophilic phenolic compound with antioxidant properties. Because of its high polarity, its antioxidant properties may be altered when formulated in oil based food or cosmetic preparations. Therefore, there is an interest in trying to enhance its hydrophobicity by grafting of an aliphatic chain. Such lipophilization reactions can be generally achieved through enzymatic catalysis. Our study consisted in synthesizing fatty cholorogenate esters in a two steps reaction. Firstly, 5-CQA was chemically esterified by methanol using an Amberlite IR120 H resin to obtain methyl chlorogenate that is more soluble in the fatty alcohols than 5-CQA. Secondly, this chlorogenate intermediate was transesterified with fatty alcohols of various chain lengths (C4, C8, C12, or C16) in the presence of Candida antarctica B lipase. Under optimal reaction conditions (aw = 0.05; 5% (w/w) of biocatalyst), the transesterification rates were until two-fold higher than in the direct lipase-catalyzed esterification of chlorogenic acid by the same alcohols. The two-step reaction overall yield was between 61 and 93% depending on the alcohol chain length, whereas it was 40–60% for the direct esterification with the same alcohols. 相似文献