全文获取类型
收费全文 | 1074篇 |
免费 | 36篇 |
国内免费 | 4篇 |
专业分类
1114篇 |
出版年
2023年 | 2篇 |
2022年 | 7篇 |
2021年 | 5篇 |
2020年 | 6篇 |
2019年 | 17篇 |
2018年 | 27篇 |
2017年 | 7篇 |
2016年 | 18篇 |
2015年 | 24篇 |
2014年 | 45篇 |
2013年 | 85篇 |
2012年 | 27篇 |
2011年 | 64篇 |
2010年 | 77篇 |
2009年 | 74篇 |
2008年 | 75篇 |
2007年 | 83篇 |
2006年 | 58篇 |
2005年 | 79篇 |
2004年 | 42篇 |
2003年 | 22篇 |
2002年 | 25篇 |
2001年 | 11篇 |
2000年 | 10篇 |
1999年 | 12篇 |
1998年 | 15篇 |
1997年 | 19篇 |
1996年 | 10篇 |
1995年 | 14篇 |
1994年 | 16篇 |
1993年 | 16篇 |
1992年 | 6篇 |
1991年 | 9篇 |
1990年 | 4篇 |
1989年 | 6篇 |
1988年 | 5篇 |
1987年 | 7篇 |
1986年 | 3篇 |
1985年 | 2篇 |
1984年 | 16篇 |
1983年 | 10篇 |
1982年 | 13篇 |
1981年 | 8篇 |
1980年 | 6篇 |
1979年 | 8篇 |
1978年 | 7篇 |
1977年 | 2篇 |
1975年 | 3篇 |
1973年 | 3篇 |
1971年 | 2篇 |
排序方式: 共有1114条查询结果,搜索用时 15 毫秒
11.
Maria Antonietta Zoroddu Michelina Fruianu Roberto Dallocchio Andreina Masia 《Biometals》1996,9(1):91-97
Vanadium uptake by whole cells and isolated cell walls of the yeast Saccharomyces cerevisiae was studied. When orthovanadate was added to wild-type S. cerevisiae cells growing in rich medium, growth was inhibited as a function of the VO4
3- concentration and the growth was completely arrested at a concentration of 20 mM of VO4
3- in YEPD. Electron paramagnetic resonance (EPR) spectroscopy was used to obtain structural and dynamic information about the cell-associated paramagnetic vanadyl ion. The presence of EPR signals indicated that vanadate was reduced by whole cells to the vanadyl ion. On the contrary, no EPR signals were detected after interaction of vanadate with isolated cell walls. A mobile and an immobile species associated in cells with small chelates and with macromolecular sites, respectively, were identified. The value of rotational correlation time
r indicated the relative motional freedom at the macromolecular site. A strongly immobilized vanadyl species bound to polar sites mainly through coulombic attractions was detected after interaction of VO2+ ions with isolated cell walls. 相似文献
12.
Duvatrienediol is a diterpene specifically occurred in tobacco plants and thought to be a precursor of tobacco aroma. Green tobacco leaves contained 0.2~1% of duvatrienediol per dry weight and it was corresponded to 30~60% of leaf surface lipid. Leaves on upper stalk position contained more of leaf surface lipid and duvatrienediol. In leaves on each stalk position, leaf surface lipid and duvatrienediol contents increased with leaf growth and decreased by over-maturation. Production of leaf surface lipid and duvatrienediol was affected by soil conditions or applied amount of nitrogen fertilizer. Both leaf surface lipid and duvatrienediol were decreased during curing of tobacco leaves, but the change in the latter was more drastic. Comparing to leaf surface lipid, changes in cytoplasmic lipid were less during growth and senescence of tobacco leaves. 相似文献
13.
《Free radical research》2013,47(5-6):395-407
Spin trapping of short-lived R. radicals is done by use of N-tert-butylhydroxylamine (1) and H2O2. The hydroxylamine is oxidized to the radical t-BuN(O)H (2) which is converted into the spin trap 2-methyl-2-nitrosopropane (3). Simultaneously, hydroxyl radicals. OH are formed from H2O2. The latter radical species abstracts hydrogen atoms from suitable molecules HR to give R. radicals, which are trapped with the formation of aminooxyl radicals, i. e., t-BuN(O)R (4) detectable by EPR spectroscopy. The reaction is enhanced by the presence of iron ions. The cleavage of H2O2 into. OH radicals is considered to involve both a radical-driven (t-BuN(O)H 2) and an iron-driven Fenton reaction. 相似文献
14.
The long-lived, light-induced radical YD• of the Tyr161 residue in the D2 protein of Photosystem II (PSII) is known to magnetically interact with the CaMn4 cluster, situated ∼ 30 Å away. In this study we report a transient step-change increase in YD• EPR intensity upon the application of a single laser flash to S1 state-synchronised PSII-enriched membranes from spinach. This transient effect was observed at room temperature and high applied microwave power (100 mW) in samples containing PpBQ, as well as those containing DCMU. The subsequent decay lifetimes were found to differ depending on the additive used. We propose that this flash-induced signal increase was caused by enhanced spin relaxation of YD• by the OEC in the S2 state, as a consequence of the single laser flash turnover. The post-flash decay reflected S2 → S1 back-turnover, as confirmed by their correlations with independent measurements of S2 multiline EPR signal and flash-induced variable fluorescence decay kinetics under corresponding experimental conditions. This flash-induced effect opens up the possibility to study the kinetic behaviour of S-state transitions at room temperature using YD• as a probe. 相似文献
15.
In Photosystem II preparations at low temperature we were able to generate and trap an intermediate state between the S1 and S2 states of the Kok scheme for photosynthetic oxygen evolution. Illumination of dark-adapted, oxygen-evolving Photosystem II preparations at 140 K produces a 320-G-wide EPR signal centered near g = 4.1 when observed at 10 K. This signal is superimposed on a 5-fold larger and somewhat narrower background signal; hence, it is best observed in difference spectra. Warming of illuminated samples to 190 K in the dark results in the disappearance of the light-induced g = 4.1 feature and the appearance of the multiline EPR signal associated with the S2 state. Low-temperature illumination of samples prepared in the S2 state does not produce the g = 4.1 signal. Inhibition of oxygen evolution by incubation of PS II preparations in 0.8 M NaCl buffer or by the addition of 400 μM NH2OH prevents the formation of the g = 4.1 signal. Samples in which oxygen evolution is inhibited by replacement of Cl? with F? exhibit the g = 4.1 signal when illuminated at 140 K, but subsequent warming to 190 K neither depletes the amplitude of this signal nor produces the multiline signal. The broad signal at g = 4.1 is typical for a spin system in a rhombic environment, suggesting the involvement of non-heme Fe in photosynthetic oxygen evolution. 相似文献
16.
Bhavani Gopalakrishnan Kevin M. Nash Murugesan Velayutham Frederick A. Villamena 《Journal of visualized experiments : JoVE》2012,(66)
Reactive nitrogen/oxygen species (ROS/RNS) at low concentrations play an important role in regulating cell function, signaling, and immune response but in unregulated concentrations are detrimental to cell viability1, 2. While living systems have evolved with endogenous and dietary antioxidant defense mechanisms to regulate ROS generation, ROS are produced continuously as natural by-products of normal metabolism of oxygen and can cause oxidative damage to biomolecules resulting in loss of protein function, DNA cleavage, or lipid peroxidation3, and ultimately to oxidative stress leading to cell injury or death4. Superoxide radical anion (O2•-) is the major precursor of some of the most highly oxidizing species known to exist in biological systems such as peroxynitrite and hydroxyl radical. The generation of O2•- signals the first sign of oxidative burst, and therefore, its detection and/or sequestration in biological systems is important. In this demonstration, O2•- was generated from polymorphonuclear neutrophils (PMNs). Through chemotactic stimulation with phorbol-12-myristate-13-acetate (PMA), PMN generates O2•- via activation of nicotinamide adenine dinucleotide phosphate (NADPH) oxidase5. Nitric oxide (NO) synthase which comes in three isoforms, as inducible-, neuronal- and endothelial-NOS, or iNOS, nNOS or eNOS, respectively, catalyzes the conversion of L- arginine to L-citrulline, using NADPH to produce NO6. Here, we generated NO from endothelial cells. Under oxidative stress conditions, eNOS for example can switch from producing NO to O2•- in a process called uncoupling, which is believed to be caused by oxidation of heme7 or the co-factor, tetrahydrobiopterin (BH4)8.There are only few reliable methods for the detection of free radicals in biological systems but are limited by specificity and sensitivity. Spin trapping is commonly used for the identification of free radicals and involves the addition reaction of a radical to a spin trap forming a persistent spin adduct which can be detected by electron paramagnetic resonance (EPR) spectroscopy. The various radical adducts exhibit distinctive spectrum which can be used to identify the radicals being generated and can provide a wealth of information about the nature and kinetics of radical production9.The cyclic nitrones, 5,5-dimethyl-pyrroline-N-oxide, DMPO10, the phosphoryl-substituted DEPMPO11, and the ester-substituted, EMPO12 and BMPO13, have been widely employed as spin traps--the latter spin traps exhibiting longer half-lives for O2•- adduct. Iron (II)-N-methyl-D-glucamine dithiocarbamate, Fe(MGD)2 is commonly used to trap NO due to high rate of adduct formation and the high stability of the spin adduct14. 相似文献
17.
A deletion mutant that lacks the Psb30 protein, one of the small subunits of Photosystem II, was constructed in a Thermosynechococcus elongatus strain in which the D1 protein is expressed from the psbA3 gene (WT*). The ΔPsb30 mutant appears more susceptible to photodamage, has a cytochrome b559 that is converted into the low potential form, and probably also lacks the PsbY subunit. In the presence of an inhibitor of protein synthesis, the ?Psb30 lost more rapidly the water oxidation function than the WT* under the high light conditions. These results suggest that Psb30 contributes to structurally and functionally stabilise the Photosystem II complex in preventing the conversion of cytochrome b559 into the low potential form. Structural reasons for such effects are discussed. 相似文献
18.
Diazotization of primary aromatic amines with isoamyl nitrite in benzene at room temperature was studied employing EPR and spin trapping techniques. Nitrosodurene (ND). 2-methyl-2-nitrosopropane (MNP). and 5,5-dimethyl-pyrroline N-oxide (DMPO) were used as spin trapping agents. Aryl radicals were detected employing ND and MNP. Using DMPO as a spin trap most of the amines produced EPR spectra ascribed to adducts with aniline-type radicals (N-centred radicals). The assignments were verified using 15JN-labeled anilines. Similar spectra of DMPO adducts were recorded from amines treated with benzoyl peroxide or benzophenone plus UV. Possible mechanisms of formation of these adducts (radical trapping versus nucleophilic addition to DMPO followed by oxidation) during treatment of the amines with isoamyl nitrite are discussed. 相似文献
19.
Aryna V. Lado Alexander V. Piskunov Georgy K. Fukin Vladimir N. Ikorskii Vladimir K. Cherkasov 《Inorganica chimica acta》2005,358(15):4443-4450
Addition of 3,6-di-tert-butyl-o-benzoquinone (3,6-DBBQ) to SnCl2 in THF leads to the oxidation of Sn(II) to Sn(IV) with formation of catecholate complex (3,6-DBCat)SnCl2 · 2THF (1), where 3,6-DBCat is 3,6-di-tert-butyl-catecholate dianion. The reaction of 4,6-di-tert-butyl-N-(2,6-di-iso-propylphenyl)-o-iminobenzoquinone (IBQ-Pri) also proceeds on the oxidative-addition mechanism yielding bis-iminosemiquinonato species (ISQ-Pri)2SnCl2(2), where ISQ-Pri is anion-radical 4,6-di-tert-butyl-N-(2,6-di-iso-propylphenyl)-o-iminobenzosemiquinolate. The complexes have been characterized by IR, X-band EPR, 1H NMR (for 1) spectroscopy and magnetochemistry (for 2). X-ray analysis data show the distorted octahedral environment of tin(IV) for both complexes. Complex 1 is diamagnetic (ground state S = 0), while 2 has triplet ground state (S = 1, biradical). Catecholate complex 1 is able to be a spin trap for different organic radicals. 相似文献
20.
Membranes made from binary mixtures of egg sphingomyelin (ESM) and cholesterol were investigated using conventional and saturation-recovery
EPR observations of the 5-doxylstearic acid spin label (5-SASL). The effects of cholesterol on membrane order and the oxygen
transport parameter (bimolecular collision rate of molecular oxygen with the nitroxide spin label) were monitored at the depth
of the fifth carbon in fluid- and gel-phase ESM membranes. The saturation-recovery EPR discrimination by oxygen transport
(DOT) method allowed the discrimination of the liquid-ordered (l
o), liquid-disordered (l
d), and solid-ordered (s
o) phases because the bimolecular collision rates of the molecular oxygen with the nitroxide spin label differ in these phases.
Additionally, oxygen collision rates (the oxygen transport parameter) were obtained in coexisting phases without the need
for their separation, which provides information about the internal dynamics of each phase. The addition of cholesterol causes
a dramatic decrease in the oxygen transport parameter around the nitroxide moiety of 5-SASL in the l
o phase, which at 50 mol% cholesterol becomes ∼5 times smaller than in the pure ESM membrane in the l
d phase, and ∼2 times smaller than in the pure ESM membrane in the s
o phase. The overall change in the oxygen transport parameter is as large as ∼20-fold. Conventional EPR spectra show that 5-SASL
is maximally immobilized at the phase boundary between regions with coexisting l
d and l
o phases or s
o and l
o phases and the region with a single l
o phase. The obtained results all
owed for the construction of a phase diagram for the ESM-cholesterol membrane. 相似文献