全文获取类型
收费全文 | 8088篇 |
免费 | 713篇 |
国内免费 | 253篇 |
出版年
2024年 | 12篇 |
2023年 | 90篇 |
2022年 | 118篇 |
2021年 | 156篇 |
2020年 | 203篇 |
2019年 | 266篇 |
2018年 | 299篇 |
2017年 | 204篇 |
2016年 | 231篇 |
2015年 | 243篇 |
2014年 | 351篇 |
2013年 | 553篇 |
2012年 | 199篇 |
2011年 | 374篇 |
2010年 | 401篇 |
2009年 | 423篇 |
2008年 | 542篇 |
2007年 | 518篇 |
2006年 | 477篇 |
2005年 | 428篇 |
2004年 | 342篇 |
2003年 | 333篇 |
2002年 | 262篇 |
2001年 | 152篇 |
2000年 | 140篇 |
1999年 | 160篇 |
1998年 | 167篇 |
1997年 | 137篇 |
1996年 | 122篇 |
1995年 | 122篇 |
1994年 | 117篇 |
1993年 | 119篇 |
1992年 | 87篇 |
1991年 | 69篇 |
1990年 | 53篇 |
1989年 | 52篇 |
1988年 | 54篇 |
1987年 | 48篇 |
1986年 | 31篇 |
1985年 | 42篇 |
1984年 | 91篇 |
1983年 | 55篇 |
1982年 | 59篇 |
1981年 | 40篇 |
1980年 | 42篇 |
1979年 | 29篇 |
1978年 | 8篇 |
1977年 | 11篇 |
1974年 | 4篇 |
1973年 | 7篇 |
排序方式: 共有9054条查询结果,搜索用时 859 毫秒
991.
Stephan BeilAnna Schamberger Wolfgang NaumannSusanne Machill Karl-Heinz van Pée 《Carbohydrate polymers》2012,87(1):117-122
A new method for the determination of the degree of N-acetylation (DA) of chitin and chitosan is described using first derivative diamond ATR FTIR spectroscopy. Applying the derivative values of the amide III band at 1327 cm−1 and the CH deformation band of the N-acetyl group at 1383 cm−1 as measure of the N-acetyl content of the sample in relation to the derivative value of the bridge oxygen vibration at 1163 cm−1 as internal standard, a linear correlation to the results of first derivative UV spectroscopy was obtained and confirmed by elemental analysis and Raman spectroscopy. The described method allows the determination of the degree of N-acetylation of chitosan and chitin in the presence of water thus making drying procedures unnecessary. 相似文献
992.
The chemical structure and antioxidant of natural and ultrasonic degraded polysaccharides from Porphyra yezoensis Udea was investigated. The degraded polysaccharide (PYPSUD) was purified, and F2 (a homogeneous fraction) was obtained. FT-IR, 1H and 13C NMR spectral analysis revealed that F2 have typical porphyran structure. It has a backbone of alternating (1 → 4)-3,6-anhydro-α-l-galactopyranose) units and (1 → 3)-linked β-d-galactose or (1 → 4)-linked α-l-galactose 6-sulfate units. The result ascertained ultrasound degradation did not change the main structure of polysaccharides in the test conditions. Antioxidant proved that the activity of scavenging superoxide and hydroxyl radical is F2 > VC > PSPYUD > PSPY. It was possible that ultrasonic treatment is an effective way for enhancing PSPY's antioxidant activity ascribing to decreasing molecular weight of polysaccharides. 相似文献
993.
994.
995.
996.
Merten JA Schultz KM Klug CS 《Protein science : a publication of the Protein Society》2012,21(2):211-218
Gram-negative bacteria such as Escherichia coli have an inner membrane and an asymmetric outer membrane (OM) that together protect the cytoplasm and act as a highly selective permeability barrier. Lipopolysaccharide (LPS) is the major component of the outer leaflet of the OM and is essential for the survival of nearly all Gram-negative bacteria. Recent advances in understanding the proteins involved in the transport of LPS across the periplasm and into the outer leaflet of the OM include the identification of seven proteins suggested to comprise the LPS transport (Lpt) system. Crystal structures of the periplasmic Lpt protein LptA have recently been reported and show that LptA forms oligomers in either an end-to-end arrangement or a side-by-side dimer. It is not known if LptA oligomers bridge the periplasm to form a large, connected protein complex or if monomeric LptA acts as a periplasmic shuttle to transport LPS across the periplasm. Therefore, the studies presented here focus specifically on the LptA protein and its oligomeric arrangement and concentration dependence in solution using experimental data from several biophysical approaches, including laser light scattering, crosslinking, and double electron electron resonance spectroscopy. The results of these complementary techniques clearly show that LptA readily associates into stable, end-to-end, rod-shaped oligomers even at relatively low local protein concentrations and that LptA forms a continuous array of higher order oligomeric end-to-end structures as a function of increasing protein concentration. 相似文献
997.
998.
Felicia L. V. Gray Marcelo J. Murai Jolanta Grembecka Tomasz Cierpicki 《Protein science : a publication of the Protein Society》2012,21(12):1954-1960
Characterization of disordered regions in globular proteins constitutes a significant challenge. Here, we report an approach based on 13C‐detected nuclear magnetic resonance experiments for the identification and assignment of disordered regions in large proteins. Using this method, we demonstrate that disordered fragments can be accurately identified in two homologs of menin, a globular protein with a molecular weight over 50 kDa. Our work provides an efficient way to characterize disordered fragments in globular proteins for structural biology applications. 相似文献
999.
Yin‐Quan Chen Pei‐lun Chou Chen‐Yu Cheng Chia‐Chun Chiang Ming‐Tzo Wei Chin‐Ting Chuang Yi‐Lin Sophia Chen Arthur Chiou 《Journal of biophotonics》2012,5(10):777-784
The viscoelastic properties of synovial fluid (SF) are critical to its functions of lubrication and shock‐absorption of joints in human body; a change in the viscoelastic properties, even of only a few percents, is often concomitant with arthritis. In this work, the elastic modulus G ′(f) and the viscous modulus G ′′(f) of SF from patients suffering from three kinds of joint diseases, namely, osteoarthritis (OA), rheumatoid arthritis (RA), and gouty arthritis (GA), were determined as a function of frequency “f ” (in the low frequency range from f ~ 0.1 to 10 Hz) by Diffusing Wave Spectroscopy (DWS) and correlated with the white blood cell (WBC) count. A strong correlation was observed, showing a higher WBC count corresponding to lower elastic and viscous moduli, G ′ and G ′′; further details depend on inflammatory vs. non‐inflammatory, and on the severity of inflammation. Different types of arthritis lead to different degrees of decreasing viscoelasticity. Identical measurements were carried out with a commercial visco‐supplementation (or artificial SF) to serve as reference. In general, the reduction in both G ′ and G ′′ was most severe in the case of GA and least severe in the case of OA. Besides, in all cases, the reduction in G ′ was more prominent than the reduction in G ′′, indicating that in general, the deterioration in the elasticity of SF by inflammation is more severe than that in the viscosity. This simple method for quantitative physical characterization of synovial fluid may serve as a useful complementary metric to the conventional biochemical analysis in clinical diagnosis of arthritis. (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim) 相似文献
1000.
Marieke Pudlas Heike Walles Ulrich A. Stock Katja Schenke‐Layland 《Journal of biophotonics》2012,5(1):47-56
The non‐destructive and label‐free monitoring of extracellular matrix (ECM) remodeling and degradation processes is a great challenge. Raman spectroscopy is a non‐contact method that offers the possibility to analyze ECM in situ without the need for tissue processing. Here, we employed Raman spectroscopy for the detection of heart valve ECM, focusing on collagen fibers. We screened the leaflets of porcine aortic valves either directly after dissection or after treatment with collagenase. By comparing the fingerprint region of the Raman spectra of control and treated tissues (400–1800 cm–1), we detected no significant differences based on Raman shifts; however, we found that increasing collagen degradation translated into decreasing Raman signal intensities. After these proof‐of‐principal experiments, we compared Raman spectra of native and cryopreserved valve tissues and revealed that the signal intensities of the frozen samples were significantly lower compared to those of native tissues, similar to the data seen in the enzymatically‐degraded tissues. In conclusion, our data demonstrate that Raman microscopy is a promising, non‐destructive and non‐contact tool to probe ECM state in situ. (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim) 相似文献