全文获取类型
收费全文 | 1219篇 |
免费 | 47篇 |
国内免费 | 4篇 |
专业分类
化学 | 818篇 |
晶体学 | 10篇 |
力学 | 39篇 |
数学 | 240篇 |
物理学 | 163篇 |
出版年
2023年 | 14篇 |
2022年 | 32篇 |
2021年 | 33篇 |
2020年 | 37篇 |
2019年 | 37篇 |
2018年 | 19篇 |
2017年 | 22篇 |
2016年 | 41篇 |
2015年 | 40篇 |
2014年 | 34篇 |
2013年 | 69篇 |
2012年 | 93篇 |
2011年 | 83篇 |
2010年 | 52篇 |
2009年 | 46篇 |
2008年 | 56篇 |
2007年 | 61篇 |
2006年 | 57篇 |
2005年 | 65篇 |
2004年 | 52篇 |
2003年 | 44篇 |
2002年 | 37篇 |
2001年 | 10篇 |
2000年 | 9篇 |
1999年 | 7篇 |
1998年 | 10篇 |
1997年 | 16篇 |
1996年 | 12篇 |
1995年 | 7篇 |
1994年 | 14篇 |
1993年 | 11篇 |
1992年 | 14篇 |
1991年 | 5篇 |
1990年 | 11篇 |
1989年 | 8篇 |
1988年 | 7篇 |
1987年 | 6篇 |
1986年 | 7篇 |
1985年 | 9篇 |
1984年 | 4篇 |
1983年 | 4篇 |
1982年 | 8篇 |
1981年 | 7篇 |
1980年 | 4篇 |
1979年 | 3篇 |
1978年 | 4篇 |
1975年 | 3篇 |
1974年 | 5篇 |
1973年 | 3篇 |
1918年 | 3篇 |
排序方式: 共有1270条查询结果,搜索用时 15 毫秒
21.
Graham R. Underwood Brajeswar Paul M. Alice Becker 《Journal of heterocyclic chemistry》1976,13(6):1229-1232
Pyridoxol, protected by acetylation of the hydroxyl groups, has been converted to its N-oxide which upon reaction with perfluoroacetie anhydride yields a 2-nor-2-hydroxymethylpyridoxol derivative as an intermediate. This compound undergoes acyl migration from the 3-position. Protection of the pyridoxol hydroxyls by benzylalion followed by the same treatment yields the unrearranged α2-hydroxy derivative. This compound has been converted to a series of α2-substituted pyridoxols (X = -Cl, -Br, -OCOCH3, -OCH3, -OC2H5). 相似文献
22.
L. Avaldi R. Camilloni C. Dal Cappello E. Fainelli A. Lahmam-Bennani G. Stefani 《Zeitschrift für Physik D Atoms, Molecules and Clusters》1990,16(2):107-112
Relative triple differential cross section for the coplanar asymmetric (e, 2e) reaction in argon have been measured at 1.5 KeV incident energy and 40 eV ejected electron energy in several kinematics. Depending on the scattering angle, ? a , the chosen kinematics select either ionising collisions belonging to the Bethe ridge (? a =9.2°) or processes in the intermediate region between the pure dipolar and binary regimes. The more relevant finding is the presence of a minimum in the recoil lobe, almost opposite to the direction of the momentum transfer. This feature is qualitatively explained by a first Born model, which describes the ejected electron by a Coulomb wave-function. This result suggests that in the investigated kinematics the interaction of the slow ejected electron with the residual ion is the dominant effect beyond the first order electron-electron interactions. 相似文献
23.
Corsaro MM Piaz FD Lanzetta R Naldi T Parrilli M 《Rapid communications in mass spectrometry : RCM》2004,18(8):853-858
The use of the electrospray ionization quadrupole time-of-flight tandem mass spectrometry (ESI-QTOFMS) technique for the structural determination of Lipid A from Pseudomonas corrugata is described. This technique appears to be more sensitive with respect to other commonly used tandem mass spectrometric approaches, and was very valuable in the structural determination of the highly heterogeneous Lipid A fractions. The Lipid A fraction consists mainly of a pentaacyl component in which 3-hydroxydecanoyl [10:0(3-OH)] and 3-hydroxydodecanoyl [12:0(3-OH)] are linked as primary acyl substituents to the classical bisphosphorylated beta-(1' --> 6)-linked D-glucosamine disaccharide. Secondary substitution of N-acyl fatty acids with dodecanoyl residues [12:0] and/or its 2-OH derivatives was also observed. 相似文献
24.
A capillary electrophoresis method with UV detection was developed for the determination of 8-hydroxy-2'-deoxyguanosine (8-OHdG) in untreated urine samples. The calibration graph for 8-OHdG in urine is linear in the concentration range 10-500 mg/l. and the detection limit is 5 mg/l (17 microM). 8-OHdG was determined in urine from oncological patients treated by radiation therapy. Its concentrations relative to creatinine were found to be in the range 10-47 microg 8-OHdG/l mg creatinine (4-19 micromol 8-OHdG/mmol creatinine). The overall time of the analysis of a urine sample was less than 15 min. 相似文献
25.
Kourkoumelis N Demertzis MA Kovala-Demertzi D Koutsodimou A Moukarika A 《Spectrochimica acta. Part A, Molecular and biomolecular spectroscopy》2004,60(10):2253-2259
The reactions of the potent and widely used anti-inflammatory drug diclofenac, HL, with diorganotin(IV) oxides were studied. The dimeric tetraorganodistannoxane complexes [Me(2)LSnOSnLMe(2)](2), [Bu(2)LSnOSnLBu(2)](2), [Ph(2)LSnOSnLPh(2)](2) and the dibutyltin complex [Bu(2)SnL(2)], have been prepared and structurally characterized in the solid state by means of vibrational and 119Sn M?ssbauer spectroscopy. Determination of lattice dynamics by temperature-dependent 119Sn M?ssbauer spectroscopy. From the variable-temperature M?ssbauer effect, the Debye temperature was determined. The complexes have been characterized in solution by NMR (1H and 13C) spectroscopy. Vibrational, M?ssbauer, and NMR data are discussed in terms of the proposed structures. 相似文献
26.
Gray DE Messer D Porter A Ferguson S Harris RK Clark AP Algaier JW Overstreet JD Smith CS 《Journal of AOAC International》2005,88(6):1613-1620
We report here a liquid chromatography (LC) method with inline ultraviolet/evaporative light scattering (UV/ELS) detection for the simultaneous quantification of the terpenelactones and flavonol aglycones in a single sample of hydrolyzed Ginkgo biloba extract (GBE). The sample is hydrolyzed by a rapid and convenient oven heating method for 1 h at 90 degrees C with 10% hydrochloric acid. The 1 h hydrolysis was found to be equivalent to the 2.25 h reflux treatment for dry powder extract, where total flavonol glycosides were 28.4 and 28.1%, respectively. Acceptable precision was achieved for total terpenelactones [relative standard deviation (RSD) = 4.8%] by ELS detection, and total flavonol aglycones (RSD = 2.3%) by UV detection. The analytical range was 1.5 to 7.3% (w/w) for the individual terpenelactones (ELS) and 2.5 to 15.0% (w/w) for the individual glycosides (UV) calculated from the aglycones quercetin, kaempferol, and isorhamnetin. This improved method allows for the first time high throughput sample preparation coupled with the quantification of the predominant compounds generally used for quality control of GBE in a single assay. 相似文献
27.
Yang W Wilkins AL Ye Y Liu ZR Li SY Urbauer JL Hellinga HW Kearney A van der Merwe PA Yang JJ 《Journal of the American Chemical Society》2005,127(7):2085-2093
Ca2+, "a signal of life and death", controls numerous cellular processes through interactions with proteins. An effective approach to understanding the role of Ca2+ is the design of a Ca2+-binding protein with predicted structural and functional properties. To design de novo Ca2+-binding sites in proteins is challenging due to the high coordination numbers and the incorporation of charged ligand residues, in addition to Ca2+-induced conformational change. Here, we demonstrate the successful design of a Ca2+-binding site in the non-Ca2+-binding cell adhesion protein CD2. This designed protein, Ca.CD2, exhibits selectivity for Ca2+ versus other di- and monovalent cations. In addition, La3+ (Kd 5.0 microM) and Tb3+ (Kd 6.6 microM) bind to the designed protein somewhat more tightly than does Ca2+ (Kd 1.4 mM). More interestingly, Ca.CD2 retains the native ability to associate with the natural target molecule. The solution structure reveals that Ca.CD2 binds Ca2+ at the intended site with the designed arrangement, which validates our general strategy for designing de novo Ca2+-binding proteins. The structural information also provides a close view of structural determinants that are necessary for a functional protein to accommodate the metal-binding site. This first success in designing Ca2+-binding proteins with desired structural and functional properties opens a new avenue in unveiling key determinants to Ca2+ binding, the mechanism of Ca2+ signaling, and Ca2+-dependent cell adhesion, while avoiding the complexities of the global conformational changes and cooperativity in natural Ca2+-binding proteins. It also represents a major achievement toward designing functional proteins controlled by Ca2+ binding. 相似文献
28.
Magali Cordaillat-Simmons Alice Rouanet Bruno Pot 《Experimental & molecular medicine》2020,52(9):1397
Probiotics have been defined as “Live microorganisms that when administered in adequate amounts confer a health benefit on the host”. This definition covers a wide range of applications, target populations and (combinations of) microorganisms. Improved knowledge on the importance of the microbiota in terms of health and disease has further diversified the potential scope of a probiotic intervention, whether intended to reach the market as a food, a food supplement or a drug, depending on the intended use. However, the increased interest in the clinical application of probiotics may require specific attention given their administration in a diseased population. In addition to safety, the impact of the type of product, in terms of quality, production method and, e.g., the acceptance of side effects, is now part of the current regulatory constraints for developers. In the European Union, foods are regulated by the European Food Safety Authority and drugs by the European Medicines Agency; in the United States, the Food and Drug Administration (FDA) deals with both categories. More recently, the FDA has defined a new “live biotherapeutic products” (LBP) category, clarifying pharmaceutical expectations. Since 2019, the quality requirements for this category of drug products have also been clarified by the European Pharmacopoeia (Ph. Eur.). Similar to all products intended to prevent or treat diseases, LBPs will have to be registered as medicinal products to reach the market in the US and in Europe. In this area, regulatory authorities and the pharmaceutical industry will routinely use guidelines of the “International Council for Harmonization of Technical Requirements for Pharmaceuticals for Human Use” (ICH). Although ICH guidelines are not legally binding, they provide very important recommendations, recognized by almost all drug authorities in the world. In this review, we discuss some aspects of this regulatory framework, especially focusing on products with an intended use in a diseased or vulnerable target population.Subject terms: Biological therapy, Biologics 相似文献
29.
Ali Mohammad Akbar Edwards Alice A. Tuah Jauyah Hossain Mir Ezharul Nazimuddin Mohamed 《Transition Metal Chemistry》1997,23(1):41-44
A new, potentially polydentate sulfur–nitrogen chelating agent, 2,6–bis(N-methyl-S-methyldithiocarbazato)pyridine (L) has
been synthesized and characterized. With nickel(II) salts, the ligand yields complexes of empirical formula NiLX2·nH2O (X=Cl−,
NCS− or NO3−; n=0 or 1) in which it behaves as a quadridentate NSSN chelating agent, coordinating to the nickel(II) ion via
the two amino nitrogen atoms and the two sulfur atoms. Magnetic and spectral evidence support a distorted octahedral structure
for these complexes. The ligand reacts with copper(II), platinum(II) and palladium(II) salts to yield homo-binuclear complexes
of general formula [M2LX4]·nSol (M=CuII, PtII or PdII; X=Cl− or Br−; n=0.5, 1 or 2; Sol=H2O, MeOH or MeCOMe), in which each
of the metal ions is in a square-planar environment. These complexes have been characterized by a variety of physicochemical
techniques.
This revised version was published online in June 2006 with corrections to the Cover Date. 相似文献
30.
Misbahul Ain Khan Alice Maria Coimbra Rolim Antonio Elydio Guaroni 《Journal of heterocyclic chemistry》1983,20(2):475-476
The synthesis of the parent ring system and some of the derivatives of 1H-pyrazolo[3,4-d]thieno[2,3-b]pyridine are described. 相似文献