首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   1353篇
  免费   48篇
  国内免费   7篇
  2024年   2篇
  2023年   15篇
  2022年   53篇
  2021年   98篇
  2020年   27篇
  2019年   46篇
  2018年   41篇
  2017年   33篇
  2016年   55篇
  2015年   59篇
  2014年   80篇
  2013年   108篇
  2012年   119篇
  2011年   99篇
  2010年   44篇
  2009年   57篇
  2008年   62篇
  2007年   67篇
  2006年   55篇
  2005年   66篇
  2004年   38篇
  2003年   38篇
  2002年   23篇
  2001年   9篇
  2000年   11篇
  1999年   7篇
  1998年   4篇
  1997年   8篇
  1996年   7篇
  1995年   12篇
  1994年   7篇
  1993年   8篇
  1992年   3篇
  1991年   3篇
  1990年   2篇
  1989年   4篇
  1988年   3篇
  1987年   5篇
  1986年   3篇
  1985年   1篇
  1984年   3篇
  1983年   3篇
  1981年   5篇
  1980年   5篇
  1979年   1篇
  1978年   2篇
  1977年   1篇
  1976年   3篇
  1973年   1篇
  1970年   2篇
排序方式: 共有1408条查询结果,搜索用时 171 毫秒
991.
A new lignan characterised as (-)-4-hydroxy-2,6-di-(4′-hydroxy-3′-methoxy)phenyl-3,7-dioxabicyclo-(3.3.0)octane along with n-10-nonacosanol, scopoletin, syringic acid, β-sitosterol and its glucoside, has been isolated from the aerial parts of Lonicera hypoleuca. The stereochemistry of the lignan has been established by its spectroscopic analysis and those of its derivatives, and by its conversion to (+)-pinoresinol. β-Sitosterol-β-D-glucoside displayed good spasmolytic activity.  相似文献   
992.
Inherited cataract is a clinically and genetically heterogeneous disease that most often presents as a congenital autosomal dominant trait. Here we report linkage of a three-generation family of Pakistani origin with autosomal dominant cataract "zonular nuclear" pulverulent type (CZNP) on chromosome 1q21.1. Genome wide-linkage analysis excluded all the known cataract loci except on chromosome 1q. Significantly positive 2-point lod score values (Z=3.01 at θ=0) were obtained for markers D1S305 and D1S2721, which are known to flank the gene for connexin 50 (Cx50) or gap junction protein alpha-8 (Gja8). Previously a mutation in this gene has been reported in a British family with zonular pulverulent cataract (CZP).Here we describe a second mutation (E48K) in connexin 50 that confirms the involvement of this gene in cataractogenesis. Electronic Publication  相似文献   
993.
Nanoemulsion-based delivery systems are finding increasing use in food, pharmaceutical, agrochemical, and personal care applications due to their ability to increase the stability and/or activity of lipophilic functional components. In this study, a low-energy homogenization method (spontaneous emulsification) was used to encapsulate β-carotene in nanoemulsions. The main objective was to optimize lipid phase composition to form stable nanoemulsions that would effectively enhance β-carotene bioavailability. Lipid phase composition was varied by mixing long chain triglycerides (LCT) with medium chain triglycerides (MCT) or flavor oil (orange oil). LCT was added to promote bioaccessibility, whereas MCT or orange oil was added to facilitate nanoemulsion formation. Our hypothesis was that an optimum level of LCT is required to form stable nanoemulsions with good bioaccessibility characteristics. Stable nanoemulsions could be formed at LCT-to-orange oil ratios of 1:1 (d 32 = 109 nm) and at LCT-to-MCT ratios of 1:2 (d 32 = 145 nm). Thus, higher LCT loading capacities and smaller droplet sizes could be obtained using orange oil. The influence of oil composition on the potential gastrointestinal fate of the nanoemulsions was studied using a simulated gastrointestinal tract (GIT) consisting of mouth, stomach, and small intestine phases. The transformation and bioaccessibility of β-carotene in the GIT was highly dependent on lipid phase composition. In particular, β-carotene bioaccessibility increased with increasing LCT level due to greater solubilization in mixed micelles. These results are useful for optimizing the design of nanoemulsion-based delivery systems for encapsulation and release of lipophilic nutraceuticals and pharmaceuticals.  相似文献   
994.
Alkaline protease from alkaliphilic Bacillus sp. NPST-AK15 was immobilized onto functionalized and non-functionalized rattle-type magnetic core@mesoporous shell silica (RT-MCMSS) nanoparticles by physical adsorption and covalent attachment. However, the covalent attachment approach was superior for NPST-AK15 protease immobilization onto the activated RT-MCMSS-NH2 nanoparticles and was used for further studies. In comparison to free protease, the immobilized enzyme exhibited a shift in the optimal temperature and pH from 60 to 65 °C and pH 10.5–11.0, respectively. While free protease was completely inactivated after treatment for 1 h at 60 °C, the immobilized enzyme maintained 66.5 % of its initial activity at similar conditions. The immobilized protease showed higher k cat and K m , than the soluble enzyme by about 1.3-, and 1.2-fold, respectively. In addition, the results revealed significant improvement of NPST-AK15 protease stability in variety of organic solvents, surfactants, and commercial laundry detergents, upon immobilization onto activated RT-MCMSS-NH2 nanoparticles. Importantly, the immobilized protease maintained significant catalytic efficiency for ten consecutive reaction cycles, and was separated easily from the reaction mixture using an external magnetic field. To the best of our knowledge this is the first report about protease immobilization onto rattle-type magnetic core@mesoporous shell silica nanoparticles that also defied activity-stability tradeoff. The results clearly suggest that the developed immobilized enzyme system is a promising nanobiocatalyst for various bioprocess applications requiring a protease.  相似文献   
995.
996.
Tau protein from Alzheimer disease (AD) brain is hyperphosphorylated by both proline-dependent protein kinases (PDPKs) and non-PDPKs. It is presently unclear how PDPKs and non-PDPKs interact in tau hyperphosphorylation. Previously we have shown that non-PDPKs can positively modulate the activity of a PDPK (GSK-3) in tau phosphorylation (Singh et al. (1995) FEBS Lett. 358, 267-272). In this study we have investigated whether (A) non-PDPKs can also modulate the activity of the PDPK, cdk5, (B) a PDPK can modulate the activities of another PDPK, as well as non-PDPKs. We found that, like GSK-3, the activity of cdk5 is stimulated if tau were first prephosphorylated by any of several non-PDPKs (A-kinase, C-kinase, CK-1, CaM-kinase II). Prephosphorylation of tau by cdk5 stimulated both the rate and extent of a subsequent phosphorylation catalyzed by GSK-3. Under these conditions thr 231 phosphorylation was especially enhanced (9-fold). No significant stimulation of phosphorylation was obser ved when the order of these kinases was reversed (i.e. GSK-3 followed by cdk5). By contrast, prephosphorylation of tau by cdk5 served to inhibit subsequent phosphorylation catalyzed by C-kinase and CK-1, but not by A-kinase or CaM-kinase II. Our results suggest that in tau hyperphosphorylation in AD brain, cdk5-catalyzed phosphorylation may serve to up-regulate the activity of GSK-3 and down-regulate the activities of C-kinase and CK-1. (Mol Cell Biochem 167: 99-105, 1997)  相似文献   
997.
To determine the conditions of cholesterol assimilation, various strains of Bifidobacterium species were cultured in the presence of cholesterol and bile salts. During culturing, Bifidobacterium breve ATCC 15700 assimilates cholesterol in the presence of oxgall at pH values lower than 6. This strain was selected to study the influence of conjugated (taurocholic acid) and deconjugated (cholic acid) bile salts on cholesterol assimilation. B. breve ATCC 15700 assimilated cholesterol (up to 51%) when cultures were undertaken in the presence of taurocholic acid, whereas less than 13% of the initial amount of cholesterol was measured in the cells in the presence of cholic acid. Cultured in the presence of six individual di- or trihydroxyconjugated bile salts, bifidobacteria strains assimilated cholesterol. This assimilation appeared to be more important in the presence of trihydroxyconjugated bile salts (tauro- and glycocholic acids). It is concluded that trihydroxyconjugated bile salts are involved in the assimilation of cholesterol by bifidobacteria. Received: 20 June 1996 / Accepted: 19 July 1996  相似文献   
998.
Summary Feasibility for bacterial leaching of two different copper ores from Pakistan was investigated with locally isolated, acidophilic thiobacilli in shake flask culture at 28±2°C. After a lag period of 16 d,Thiobacillus thiooxidans solubilized up to 19% of copper present in chalcopyrite ore in 14 d before entering stationary phase. In oxidized copper ore amended with chalcopyrite ore and supplemented with sulphur to provide an energy source forT. ferrooxidans, the pH dropped as a result of bacterial growth and was accompanied by copper solubilization. The highest copper release, 61% of total, occurred in flasks having oxidized copper ore, sulphur and calcopyrite (111, by weight) and inoculated withT. ferrooxidans. Supplementation used in these tests resulted in nearly six times more solubilization as compared to tests conducted without supplementation.
Resumen Se ha estudiado la posibilidad de realizar un lixiviado mediante bacterias de dos minerales de cobre del Pakistan. Las bacterias utilizadas son cepas, aísladas localmente, de tiobacilos acidófilos que se hacen crecer en un medio líquido en agitación a 28±2°C. Después de una fase lag de 16 dThiobacillus thiooxidans solubilizó hasta 19% del cobre presente en la calcopirita en 14 d, antes de alcanzar la fase estacionaria. El mineral de cobre oxidado se modificó con calcopirita y se suplementó con azufre afín de sumministrar una fuente de energía paraT. ferrooxidans, a resultas del crecimiento bacteriano el pH disminuyó solubilizándose cobre. La mayor cantidad de cobre sulubilizado se cencontró en frascos con mineral de cobre oxidado, azufre y calcopirita (111 en peso) e inoculados conT. ferrooxidans. La adición de suplementos incrementó 6 veces la solubilización comparada con la obtenida en ensayos sin adición de suplementos.

Résumé On a testé la faisabilité de la lixiviation bactérienne de deux minerais différents de cuivre du Pakistan au moyen de thiobacilli acidophiles,isolés localement, en culture en flaçons agités à 28 ±2°C. Après une période de latence de 16 jours,Thiobacillus thiooxidans a solubilisé jusqu'à 19% du cuivre présent dans le minerai de chalcopyrite en 14 jours avant d'entrer en phase stationnaire de croissance. Dans un minerai de cuivre amendé par le minerai de chalcopyrite et additionné de soufre de manière à fournir une source d'énergie pourT. ferrooxidans, le pH a baissé comme suite de la croissance bactérienne, ce qui a entraîné la solubilisation du cuivre. La libération la plus élevée de cuivre, 61% au total, a eu lieu dans les flaçons qui avaient à oxyder le minerai de cuivre, le soufre et la chalcopyrite oans les proportions de 111 en poids, et qui avaient été inoculés parT. ferrooxydans. La supplémentation effectuée dans ces tests a eu pour effet de dissoudre six fois plus de cuivre que dans les tests conduits sans supplémentation.
  相似文献   
999.
Summary A bacterium isolated from an oil spillage sample and identified asPseudomonas aeruginosa degraded hexadecane and heptadecane by 47% and 58% with 9% and 12% of total carbon from the respective substrates being liberated as CO2. With octadecane and nonadecane as substrates, 73% and 60% were biodegraded while 27% and 25% of total carbon was evolved as CO2, respectively. Production of biosurfactant by this bacterium was studied using hexadecane 5% (v/v) as substrate. The surface tension of spent culture medium, as well as the supernatant, was 30 mN/m compared to 71 mN/m for water. When the supernatant was mixed with hexadecane (15, v/v) a stable emulsion was formed which deteriorated only by 10% after one month.
Dégradation de différents hydrocarbures et production de biosurfactants parPseudomonas aeruginosa isolé d'eaux côtières
Résumé Une bactérie, isolée d'un échantillon d'hydrocarbures épandus, et identifiée commePseudomonas aeruginosa, dégrade l'hexaet l'heptadécane à raison respective de 47 et de 58% du carbone total avec 9 et 12% des substrats respectifs libérés sous la forme de CO2. Avec l'octa- et le nonadécane comme substrats, la biodégradation atteint respectivement 73 et 60% avec 27 et 25% du carbone total transformé en CO2. On a étudié la production de biosurfactant par cette bactérie en utilisant l'hexadécane à 5% (v/v) comme substrat. La tension superficielle de la liqueur mixte, comme celle de son surnageant atteint 30 mN/m par comparaison à 71 mN/m pour l'eau. Lorsque le surnageant est mélangé à l'hexadécane (15, v/v), on forme une émulsion stable qui ne se détériore que de 10% au bout d'un mois.


This work was carried out at the Biotechnology Laboratories of the Nuclear Institute for Agriculture & Biology (NIAB), Faisalabad.  相似文献   
1000.

Background

Little is known about Emergency Medical Services (EMS) use and pre-hospital triage of patients with acute ST-elevation myocardial infarction (STEMI) in Arabian Gulf countries.

Methods

Clinical arrival and acute care within 24 h of STEMI symptom onset were compared between patients transferred by EMS (Red Crescent and Inter-Hospital) and those transferred by non-EMS means. Data were retrieved from a prospective registry of 36 hospitals in 6 Arabian Gulf countries, from January 2014 to January 2015.

Results

We enrolled 2,928 patients; mean age, 52.7 (SD ±11.8) years; 90% men; and 61.7% non-Arabian Gulf citizens. Only 753 patients (25.7%) used EMS; which was mostly via Inter-Hospital EMS (22%) rather than direct transfer from the scene to the hospital by the Red Crescent (3.7%). Compared to the non-EMS group, the EMS group was more likely to arrive initially at a primary or secondary health care facility; thus, they had longer median symptom-onset-to-emergency department arrival times (218 vs. 158 min; p˂.001); they were more likely to receive primary percutaneous coronary interventions (62% vs. 40.5%, p = 0.02); they had shorter door-to-needle times (38 vs. 42 min; p = .04); and shorter door-to-balloon times (47 vs. 83 min; p˂.001). High EMS use was independently predicted mostly by primary/secondary school educational levels and low or moderate socioeconomic status. Low EMS use was predicted by a history of angina and history of percutaneous coronary intervention. The groups had similar in-hospital deaths and outcomes.

Conclusion

Most acute STEMI patients in the Arabian Gulf region did not use EMS services. Improving Red Crescent infrastructure, establishing integrated STEMI networks, and launching educational public campaigns are top health care system priorities.  相似文献   
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号