全文获取类型
收费全文 | 6227篇 |
免费 | 586篇 |
国内免费 | 97篇 |
专业分类
6910篇 |
出版年
2024年 | 17篇 |
2023年 | 80篇 |
2022年 | 94篇 |
2021年 | 128篇 |
2020年 | 173篇 |
2019年 | 213篇 |
2018年 | 253篇 |
2017年 | 170篇 |
2016年 | 173篇 |
2015年 | 189篇 |
2014年 | 296篇 |
2013年 | 386篇 |
2012年 | 137篇 |
2011年 | 316篇 |
2010年 | 340篇 |
2009年 | 365篇 |
2008年 | 466篇 |
2007年 | 423篇 |
2006年 | 377篇 |
2005年 | 348篇 |
2004年 | 276篇 |
2003年 | 266篇 |
2002年 | 200篇 |
2001年 | 101篇 |
2000年 | 91篇 |
1999年 | 101篇 |
1998年 | 118篇 |
1997年 | 90篇 |
1996年 | 67篇 |
1995年 | 85篇 |
1994年 | 63篇 |
1993年 | 62篇 |
1992年 | 45篇 |
1991年 | 40篇 |
1990年 | 32篇 |
1989年 | 31篇 |
1988年 | 29篇 |
1987年 | 24篇 |
1986年 | 16篇 |
1985年 | 24篇 |
1984年 | 57篇 |
1983年 | 34篇 |
1982年 | 39篇 |
1981年 | 24篇 |
1980年 | 24篇 |
1979年 | 12篇 |
1978年 | 4篇 |
1977年 | 4篇 |
1975年 | 3篇 |
1958年 | 1篇 |
排序方式: 共有6910条查询结果,搜索用时 0 毫秒
51.
Two blue-light responses of Phaeophyta that are expressed within a few seconds of a blue-light stimulus were characterized with respect to their photoreception properties. The first response is the activation of red-light-saturated photosynthesis which can be stimulated to values up to 5 times the rates in red light, depending on the species. The second response is a blue-light-induced acidification measurable at the plant surface. Both responses have similar kinetic characteristics and thus led us initially to hypothesise that they were causally connected in the same transduction mechanism. The two responses have action spectra [measured for Ectocarpus siliculosus (Dillwyn) Lyngb. and Laminaria saccharina (L.) Lamouroux] that are indistinguishable within the relatively large limits of error. However, in all species tested, the threshold sensitivity for blue light of the photosynthetic response is lower than that of the pH-shift by a factor of 2 to 150. Furthermore, stimulation of photosynthesis is sensitive to the flavin inhibitors, KI and phenylacetic acid, but the pH response is not affected by these inhibitors. Thus, the blue-light-induced pH-shift does not cause the stimulation of photosynthesis. In contrast, the different fluence-response relationships of the two responses and particularly the differential effect of the inhibitors are clear evidence for the action of two independent transduction pathways and photoreceptor systems for blue light. At least photoreception for stimulation of photosynthesis involves a flavin-or and a pterin.Abbreviations DCMU
3-(3,4-dichlorophenyl)-1,1-dimethylurea
- PAA
phenylacetic acid
We thank Dr. C. A. Maggs for collecting P. pavonica. This research was supported by National Environment Research Council grant No. GR3/8102. 相似文献
52.
Takayoshi Yoshida Hiroshi Watari 《European journal of applied physiology and occupational physiology》1993,66(6):494-499
This study evaluated the time courses of intracellular pH and the metabolism of phosphocreatine (PCr) and inorganic phosphate (P) at the onset of four exercise intensities and recoveries. Non-invasive evaluation of continuous changes in phosphorus metabolites has become possible using31P-nuclear magnetic resonance spectroscopy (31P-MRS). After measurements at rest, six healthy male subjects performed 4 min of femoral flexion exercise at intensities of 0 (loadless), 10, 20 and 30 kg · m · min–1 in a 2.1 T superconducting magnet with a 67-cm bore. Measurements were continuously made during 5 min of recovery. During a series of rest-exercise-recovery procedures,31P-MRS were accumulated using 32 scans · spectrum–1 requiring 12.8 s each. At the onset of exercise, PCr decreased exponentially with a time constant of 27–32 s regardless of the exercise intensity. The time constant PCr resynthesis during recovery was about 27–40 s. The PCr kinetics were independent of exercise intensity. There were similar Pi kinetics at the onset of all types of exercise, while those of Pi recovery became significantly longer at the higher exercise intensities (P < 0.05). Furthermore, the intracellular pH indicated temporary alkalosis just at the onset of exercise, probably due to absorption of hydrogen ions by PCr hydrolysis, and then decrease at a point about 40%–50% of the preexercise PCr. The pH recovery time was longer than that for the Pi or PCr kinetics. By using a more efficient resolution system it was possible to obtain the phosphorus kinetics during exercise and to follow PCr resynthesis within the first few minutes of recovery. From our results it was concluded that in general the time course of PCr and Pi metabolism were unaffected by the exercise intensity, both at the onset of exercise and during recovery, with the exception of Pi recovery. 相似文献
53.
Takayoshi Yoshida Hiroshi Watari 《European journal of applied physiology and occupational physiology》1993,67(3):274-278
The rates of change in intracellular pH during repeated exercise sessions with rest periods was determined by 31 phosphorus-nuclear magnetic resonance spectroscopy (31P-MRS). Five long-distance runners and six healthy male subjects as controls performed a 2-min femoral flexion at 20 kg · m · min–1 in a 2.1 T superconducting magnet with a 67-cm bore and repeated this exercise four times with 2-min rest periods intervening. In all cases during exercise the inorganic phosphate (Pi) peak split into two, the earlier increased rapidly (high-pH Pi) and the later (low-pH Pi) increased more slowly. The Pi peaks were separated by a fitting procedure using the least square mean method. The high-pH Pi area during exercise decreased as the number of repeated exercise periods increased, while the low-pH Pi area gradually increased. Although the total Pi area decreased exponentially during the recovery period, the high-pH Pi area decreased first and then the low-pH Pi area reduced gradually. The pH values were estimated from the chemical shift between the phosphocreatine peak and each split peak in the Pi. The high-pH in pooled data ranged from 6.6 to 7.0 during exercise and recovery, while the low pH decreased to 6.2 during exercise. As the number of exercise periods increased, each pH value gradually became less acidic, although there was a tendency to more acidity in the control subjects than in the long-distance runners. In conclusion, it was possible to obtain by non-invasive, continuous31P-MRS, a split pattern of Pi peaks during exercise and there were at least tow different intracellular pH values during exercise, suggesting that each Pi peak might be attributed to the types of muscle fibre recruited. 相似文献
54.
55.
Elizabeth J. Fear Frida H. Torkelsen Elisa Zamboni Kuan-Ju Chen Martin Scott Glenn Jeffery Heidi Baseler Aneurin J. Kennerley 《Aging cell》2023,22(11):e14005
Mitochondrial function declines with age, and many pathological processes in neurodegenerative diseases stem from this dysfunction when mitochondria fail to produce the necessary energy required. Photobiomodulation (PBM), long-wavelength light therapy, has been shown to rescue mitochondrial function in animal models and improve human health, but clinical uptake is limited due to uncertainty around efficacy and the mechanisms responsible. Using 31P magnetisation transfer magnetic resonance spectroscopy (MT-MRS) we quantify, for the first time, the effects of 670 nm PBM treatment on healthy ageing human brains. We find a significant increase in the rate of ATP synthase flux in the brain after PBM in a cohort of older adults. Our study provides initial evidence of PBM therapeutic efficacy for improving mitochondrial function and restoring ATP flux with age, but recognises that wider studies are now required to confirm any resultant cognitive benefits. 相似文献
56.
Representing an ensemble of NMR-derived protein structures by a single structure. 总被引:1,自引:0,他引:1 下载免费PDF全文
M. J. Sutcliffe 《Protein science : a publication of the Protein Society》1993,2(6):936-944
The usefulness of representing an ensemble of NMR-derived protein structures by a single structure has been investigated. Two stereochemical properties have been used to assess how a single structure relates to the ensemble from which it was derived, namely the distribution of phi psi torsion angles and the distribution of chi 1 torsion angles. The results show that the minimized average structure derived from the ensemble (a total of 11 ensembles from the Brookhaven Protein Data Bank were analyzed) does not always correspond well with this ensemble, particularly for those ensembles generated with a smaller number of experimentally derived restraints per residue. An alternative method that selects the member of the ensemble which is closest to the "average" of the ensemble has been investigated (a total of 23 ensembles from the Brookhaven Protein Data Bank were analyzed). Although this method selected a structure that on the whole corresponded more closely to the ensemble than did the minimized average structure, this is still not a totally reliable means of selecting a single structure to represent the ensemble. This suggests that it is advisable to study the ensemble as a whole. A study has also been made of the practice of selecting the "best" rather than the most representative member of the ensemble. This too suggests that the ensemble should be studied as a whole. A study of the conformational space occupied by the ensemble also suggests the need to consider the ensemble as a whole, particularly for those ensembles generated with a smaller number of experimentally derived restraints per residue. 相似文献
57.
To investigate the role of proline in defining β turn conformations within cyclic hexa- and pentapeptides we synthesized and determined the conformations of a series of L - and D -proline-containing peptides by means of 2D NMR spectroscopy and restrained molecular dynamics simulations. Due to cis/trans isomerism the L -proline peptides adopt at least two different conformations that are analyzed and compared to the structures of the corresponding D -proline peptides. The cis conformations of the compounds cyclo(-Pro-Ala-Ala-Pro-Ala-Ala-), cyclo(-Arg-Gly-Asp-Phe-Pro-Gly-), cyclo(-Arg-Gly-Asp-Phe-Pro-Ala-), cyclo(-Pro-Ala-Ala-Ala-Ala--), and cyclo(-Pro-Ala-Pro-Ala-Ala-) form uncommon βVI turns that mimic the turn geometries found in crystallographically refined protein structures at such a detailed level that even preferred side chain orientations are reproduced. The ratios of the cis/trans isomers are analyzed in terms of the steric demand of the proline-following residue. The conformational details derived from this study illustrate the importance of the examination of small model compounds derived from protein loop regions, especially if bioactive recognition sequences, such as RGD (Arg-Gly-Asp), are incorporated. © 1993 Wiley-Liss, Inc. 相似文献
58.
Olga V. Beluzhenko Galina V. Bogdanova Vitalii A. Tumenev Yurii A. Zhdanov 《Chirality》1993,5(6):455-463
To control stereoselectivity in aldol-like reactions with chiral carbohydrate templates, we studied the interaction between completely protected dialdo compounds and magnesium enediolates of arylacetic acids. Diastereomeric mixtures of the highly functionalized acids obtained were esterified to isolate individual methyl uronates. It was found that all the diastereomeric esters exhibit Cotton effects of the same positive sign in the 220–230 nm region and so possess the same S configuration of the aryl chiral center C(6). Chiral center C(5) configurational assignments were performed using IR and ORD spectroscopy. We separated and specified four pairs of diastereomeric methyl uronates. It follows that the precursory acids have the same 5R*, 6S (major isomers) and 5S*, 6S (minor isomers) configurations. A tentative mechanism for complexation and possible models of Mg2+ -protected dialdose intermediate complexes has been proposed. We have concluded that a kind of orbital steering is realized, accompanied by some “tuning” of molecular assembly conditioned by two-point coordination between Mg2+ and potential cation-binding sites in the substrate molecules. Thus it has been demonstrated that reasonable diastereo-selectivity can be achieved even through the use of small matrix molecules using rather small functional groups, which do not impose any stringent steric requirements. © 1993 Wiley-Liss, Inc. 相似文献
59.
Jens Chr. Madsen Ole Winneche Sørensen Poul Sørensen Flemming M. Poulsen 《Journal of biomolecular NMR》1993,3(2):239-244
Summary NMR pulse sequences for measuring coupling constants in 13C, 15N-labeled proteins are presented. These pulse sequences represent improvements over earlier experiments with respect to resolution and number of radiofrequency pulses. The experiments are useful for measuring JNH
, JNCO, JNC
, JH
N
CO and JH
N
H
. Applications to chymotrypsin inhibitor 2 (CI-2) are shown. 相似文献
60.
L. Vannini R. Lanciotti F. Gardini M. E. Guerzoni 《World journal of microbiology & biotechnology》1996,12(1):85-90
Chemical changes in the medium, induced by the fermentative species Lactobacillus plantarum and Lactobacillus brevis and by the enzymatic action of a proteolytic, spoilage species, Yarrowia lipolytica, were analysed using Fourier-transform i.r. spectroscopy (FTIR). Changes in the absorbance data over time could be modelled using one of the more current predictive, mathematical models of microbial growth, such as the Gompertz equation. Moreover, a linear correlation between FTIR data (expressed as absorbance of some selected peaks) and viability data (expressed as log10 c.f.u./g or ml) was observed during the fermentation process, both for L. plantarum and L. brevis. 相似文献