全文获取类型
收费全文 | 230篇 |
免费 | 16篇 |
国内免费 | 1篇 |
出版年
2022年 | 1篇 |
2021年 | 5篇 |
2020年 | 3篇 |
2019年 | 1篇 |
2016年 | 5篇 |
2015年 | 9篇 |
2014年 | 5篇 |
2013年 | 5篇 |
2012年 | 14篇 |
2011年 | 9篇 |
2010年 | 5篇 |
2009年 | 9篇 |
2008年 | 12篇 |
2007年 | 18篇 |
2006年 | 13篇 |
2005年 | 9篇 |
2004年 | 9篇 |
2003年 | 8篇 |
2002年 | 10篇 |
2001年 | 11篇 |
2000年 | 10篇 |
1999年 | 9篇 |
1998年 | 7篇 |
1997年 | 8篇 |
1996年 | 3篇 |
1994年 | 1篇 |
1993年 | 5篇 |
1992年 | 6篇 |
1991年 | 4篇 |
1990年 | 4篇 |
1989年 | 1篇 |
1988年 | 3篇 |
1987年 | 5篇 |
1986年 | 1篇 |
1985年 | 1篇 |
1983年 | 2篇 |
1982年 | 1篇 |
1981年 | 1篇 |
1980年 | 2篇 |
1979年 | 1篇 |
1977年 | 2篇 |
1976年 | 2篇 |
1974年 | 2篇 |
1971年 | 1篇 |
1970年 | 2篇 |
1967年 | 1篇 |
1958年 | 1篇 |
排序方式: 共有247条查询结果,搜索用时 15 毫秒
21.
Five 3-hydroxy-type homoisoflavonoids, 3,5-dihydroxy-7,8-dimethoxy-3-(3',4'-dimethoxybenzyl)-4-chromanone, 3,5-dihydroxy-7-methoxy-3-(3',4'-dimethoxybenzyl)-4-chromanone, 3,5-dihydroxy-7,8-dimethoxy-3-(3'-hydroxy-4'-methoxybenzyl)-4-chromanone, 3,5,6-trihydroxy-7-methoxy-3-(3'-hydroxy-4'-methoxybenzyl)-4-chromanone and 3,5,7-trihydroxy-3-(3'-hydroxy-4'methoxybenzyl)-4-chromanone in addition to the nortriterpenoid, 15-deoxoeucosterol, have been isolated from the dichloromethane extract of the bulbs of Pseudoprospero firmifolium, the sole representative of the tribe Pseudoprospereae of the subfamily Hyacinthoideae of the Hyacinthaceae. 相似文献
22.
Penali L Mulholland DA Tano KD Cheplogoi PK Randrianarivelojosia M 《Parasite (Paris, France)》2007,14(2):161-164
The stem bark of Zanthoxylum rubescens (syn. Fagara rubescens) is used for treating fevers associated with malaria in the Ivory Coast. Three alkaloids: N-nornitidine, 7,9-dimethoxy-2,3-methylenedioxybenzophenanthridine, and bis[6-5,6-dihydrochelerythrinyl)] ether; and two amides: zanthomamide and lemairamide, were isolated from the stem bark of this plant. These compounds were screened in vitro against the chloroquine-sensitive 3D7 strain and the chloroquine-resistant FCM29 strain of P. falciparum. N-nornitidine was found to be inactive. 7,9-dimethoxy-2,3-methylenedioxybenzophenanthridine, lemairamide and zanthomamide showed weak activity with average IC50 values ranging from 45.6 microM to 149.9 microM. Bis[6-15,6-dihydrochelerythrinyl)] ether was the most active of the tested compounds with mean IC50s of 14.9 +/- 1.4 microM in FCM29 strain and 15.3 +/- 3.4 microM in 3D7 strain (approximately 58 to approximately 1130 times less active than chloroquine respectively). The anti-Plasmodium activities of the tested alkaloids of Z. rubescens were low; and do not encourage the use of this plant as antimalarial. 相似文献
23.
Headwater streams are key sites of nutrient and organic matter processing and retention, but little is known about temporal
variability in gross primary production (GPP) and ecosystem respiration (ER) rates as a result of the short duration of most
metabolism measurements in lotic ecosystems. We examined temporal variability and controls on ecosystem metabolism by measuring
daily rates continuously for 2 years in Walker Branch, a first-order deciduous forest stream. Four important scales of temporal
variability in ecosystem metabolism rates were identified: (1) seasonal, (2) day-to-day, (3) episodic (storm-related), and
(4) inter-annual. Seasonal patterns were largely controlled by the leaf phenology and productivity of the deciduous riparian
forest. Walker Branch was strongly net heterotrophic throughout the year with the exception of the open-canopy spring when
GPP and ER rates were co-equal. Day-to-day variability in weather conditions influenced light reaching the streambed, resulting
in high day-to-day variability in GPP particularly during spring (daily light levels explained 84% of the variance in daily
GPP in April). Episodic storms depressed GPP for several days in spring, but increased GPP in autumn by removing leaves shading
the streambed. Storms depressed ER initially, but then stimulated ER to 2–3 times pre-storm levels for several days. Walker
Branch was strongly net heterotrophic in both years of the study, with annual GPP being similar (488 and 519 g O2 m−2 y−1 or 183 and 195 g C m−2 y−1) but annual ER being higher in 2004 than 2005 (−1,645 vs. −1,292 g O2 m−2 y−1 or −617 and −485 g C m−2 y−1). Inter-annual variability in ecosystem metabolism (assessed by comparing 2004 and 2005 rates with previous measurements)
was the result of the storm frequency and timing and the size of the spring macroalgal bloom. Changes in local climate can
have substantial impacts on stream ecosystem metabolism rates and ultimately influence the carbon source and sink properties
of these important ecosystems. 相似文献
24.
An investigation of the seeds of the Madagascan Meliaceae Quivisia papinae has yielded five mexicanolide group limonoids, together with two known mexicanolide limonoids and two known triterpenoids. Quivisianolide A 9 possesses a hitherto unreported 9alpha,11alpha-epoxide ring, quivisianolide B 10 the corresponding delta(9(11)) double bond, and quivisianone 11 is a 17-keto seco-ring D compound. 相似文献
25.
The chloroform and ethyl acetate extracts of the leaves of Teclea natalensis have yielded two furoquinoline alkaloids, 6-[(2,3-epoxy-3-methylbutyl)oxy]-4,7-dimethoxyfuro[2,3-b]quinoline and 4,7-dimethoxy-6-[(3-methyl-2-butenyl)oxy]furo[2,3-b]quinoline, and the known alkaloids 4,7-dimethoxy-8-[(3-methyl-2-butenyl)oxy]furo[2,3-b]quinoline, flindersiamine and dictamnine. 相似文献
26.
Nutrient cycling and energy flow in ecosystems are tightly linked through the metabolic processes of organisms. Greater uptake
of inorganic nutrients is expected to be associated with higher rates of metabolism [gross primary production (GPP) and respiration
(R)], due to assimilatory demand of both autotrophs and heterotrophs. However, relationships between uptake and metabolism
should vary with the relative contribution of autochthonous and allochthonous sources of organic matter. To investigate the
relationship between metabolism and nutrient uptake, we used whole-stream and benthic chamber methods to measure rates of
nitrate–nitrogen (NO3–N) uptake and metabolism in four headwater streams chosen to span a range of light availability and therefore differing rates
of GPP and contributions of autochthonous carbon. We coupled whole-stream metabolism with measures of NO3–N uptake conducted repeatedly over the same stream reach during both day and night, as well as incubating benthic sediments
under both light and dark conditions. NO3–N uptake was generally greater in daylight compared to dark conditions, and although day-night differences in whole-stream
uptake were not significant, light–dark differences in benthic chambers were significant at three of the four sites. Estimates
of N demand indicated that assimilation by photoautotrophs could account for the majority of NO3–N uptake at the two sites with relatively open canopies. Contrary to expectations, photoautotrophs contributed substantially
to NO3–N uptake even at the two closed-canopy sites, which had low values of GPP/R and relied heavily on allochthonous carbon to
fuel R. 相似文献
27.
One of the most controversial questions in enzymology today is whether protein dynamics are significant in enzyme catalysis. A particular issue in these debates is the unusual temperature-dependence of some kinetic isotope effects for enzyme-catalysed reactions. In the present paper, we review our recent model [Glowacki, Harvey and Mulholland (2012) Nat. Chem. 4, 169-176] that is capable of reproducing intriguing temperature-dependences of enzyme reactions involving significant quantum tunnelling. This model relies on treating multiple conformations of the enzyme-substrate complex. The results show that direct 'driving' motions of proteins are not necessary to explain experimental observations, and show that enzyme reactivity can be understood and accounted for in the framework of transition state theory. 相似文献
28.
29.
Soluble epoxide hydrolase (sEH) is an enzyme involved in drug metabolism that catalyzes the hydrolysis of epoxides to form their corresponding diols. sEH has a broad substrate range and shows high regio- and enantioselectivity for nucleophilic ring opening by Asp333. Epoxide hydrolases therefore have potential synthetic applications. We have used combined quantum mechanics/molecular mechanics (QM/MM) umbrella sampling molecular dynamics (MD) simulations (at the AM1/CHARMM22 level) and high-level ab initio (SCS-MP2) QM/MM calculations to analyze the reactions, and determinants of selectivity, for two substrates: trans-stilbene oxide (t-SO) and trans-diphenylpropene oxide (t-DPPO). The calculated free energy barriers from the QM/MM (AM1/CHARMM22) umbrella sampling MD simulations show a lower barrier for phenyl attack in t-DPPO, compared with that for benzylic attack, in agreement with experiment. Activation barriers in agreement with experimental rate constants are obtained only with the highest level of QM theory (SCS-MP2) used. Our results show that the selectivity of the ring-opening reaction is influenced by several factors, including proximity to the nucleophile, electronic stabilization of the transition state, and hydrogen bonding to two active site tyrosine residues. The protonation state of His523 during nucleophilic attack has also been investigated, and our results show that the protonated form is most consistent with experimental findings. The work presented here illustrates how determinants of selectivity can be identified from QM/MM simulations. These insights may also provide useful information for the design of novel catalysts for use in the synthesis of enantiopure compounds. 相似文献
30.
Moses K. Langat Neil R. Crouch Leena Pohjala Päivi Tammela Peter J. Smith Dulcie A. Mulholland 《Phytochemistry letters》2012,5(3):414-418
Two new ent-kauren-19-oic acid derivatives, ent-14S*-hydroxykaur-16-en-19-oic acid and ent-14S*,17-dihydroxykaur-15-en-19-oic acid together with eleven known compounds ent-kaur-16-en-19-oic acid, ent-kaur-16-en-19-al, ent-12β-hydroxykaur-16-en-19-oic acid, ent-12β-acetoxykaur-16-en-19-oic acid, 8R,13R-epoxylabd-14-ene, eudesm-4(15)-ene-1β,6α-diol, (?)-7-epivaleran-4-one, germacra-4(15), 5E,10(14)-trien-9β-ol, acetyl aleuritolic acid, β-amyrin, and stigmasterol were isolated from the stem bark of Croton pseudopulchellus (Euphorbiaceae). Structures were determined using spectroscopic techniques. Ent-14S*-hydroxykaur-16-en-19-oic acid, ent-kaur-16-en-19-oic acid, ent-12β-hydroxykaur-16-en-19-oic acid, ent-12β-acetoxykaur-16-en-19-oic acid and 8R,13R-epoxylabd-14-ene were tested for their effects on Semliki Forest virus replication and for cytotoxicity against human liver tumour cells (Huh-7 strain) but were found to be inactive. Ent-kaur-16-en-19-oic acid, the major constituent, showed weak activity against the Plasmodium falciparum (CQS) D10 strain. 相似文献