全文获取类型
收费全文 | 4106篇 |
免费 | 83篇 |
国内免费 | 25篇 |
专业分类
4214篇 |
出版年
2023年 | 24篇 |
2022年 | 31篇 |
2021年 | 34篇 |
2020年 | 49篇 |
2019年 | 46篇 |
2018年 | 52篇 |
2017年 | 30篇 |
2016年 | 48篇 |
2015年 | 41篇 |
2014年 | 55篇 |
2013年 | 193篇 |
2012年 | 53篇 |
2011年 | 341篇 |
2010年 | 268篇 |
2009年 | 367篇 |
2008年 | 279篇 |
2007年 | 269篇 |
2006年 | 273篇 |
2005年 | 265篇 |
2004年 | 317篇 |
2003年 | 89篇 |
2002年 | 168篇 |
2001年 | 57篇 |
2000年 | 51篇 |
1999年 | 66篇 |
1998年 | 133篇 |
1997年 | 49篇 |
1996年 | 74篇 |
1995年 | 146篇 |
1994年 | 65篇 |
1993年 | 17篇 |
1992年 | 34篇 |
1991年 | 15篇 |
1990年 | 22篇 |
1989年 | 15篇 |
1988年 | 14篇 |
1987年 | 17篇 |
1986年 | 9篇 |
1985年 | 10篇 |
1984年 | 17篇 |
1983年 | 16篇 |
1982年 | 22篇 |
1981年 | 9篇 |
1980年 | 14篇 |
1979年 | 15篇 |
1978年 | 10篇 |
1977年 | 6篇 |
1976年 | 6篇 |
1975年 | 5篇 |
1974年 | 3篇 |
排序方式: 共有4214条查询结果,搜索用时 0 毫秒
31.
32.
Time-resolved fluorescence studies of tryptophan mutants of Escherichia coli glutamine synthetase: conformational analysis of intermediates and transition-state complexes. 下载免费PDF全文
W. M. Atkins J. J. Villafranca 《Protein science : a publication of the Protein Society》1992,1(3):342-355
Single tryptophan-containing mutants of low adenylylation state Escherichia coli glutamine synthetase have been studied by frequency-domain fluorescence spectroscopy in the presence of various substrates and inhibitors. At pH 6.5, the Mn-bound wild-type enzyme (wild type has two tryptophans/subunit) and the mutant enzymes exhibit heterogeneous fluorescence decay kinetics; the individual tryptophans are adequately described by a triple exponential decay scheme. The recovered lifetime values are 5.9 ns, 2.6 ns, and 0.4 ns for Trp-57 and 5.8 ns, 2.3 ns, and 0.4 ns for Trp-158. These values are nearly identical to the previously reported results at pH 7.5 (Atkins, W.M., Stayton, P.S., & Villafranca, J.J., 1991, Biochemistry 30, 3406-3416). In addition, Trp-57 and Trp-158 both exhibit an ATP-induced increase in the relative fraction of the long lifetime component, whereas only Trp-57 is affected by this ligand at pH 7.5. The transition-state analogue L-methionine-(R,S)-sulfoximine (MSOX) causes a dramatic increase in the fractional intensity of the long lifetime component of Trp-158. This ligand has no effect on the W158S mutant protein and causes a small increase in the fractional intensity of the long lifetime component of the W158F mutant protein. Addition of glutamate to the ATP complex, which affords the gamma-glutamylphosphate-ADP complex, results in the presence of new lifetime components at 7, 3.2, and 0.5 ns for Trp-158, but has no effect on Trp-57. Similar results were obtained when ATP was added to the MSOX complex; Trp-57 exhibits heterogeneous fluorescence decay with lifetimes of 7, 3.5, and 0.8 ns. Decay kinetics of Trp-158 are best fit to a nearly homogeneous decay with a lifetime of 5.5 ns in the MSOX-ATP inactivated complex. These results provide a model for the sequence of structural and dynamic changes that take place at the Trp-57 loop and the central loop (Trp-158) during several intermediate stages of catalysis. 相似文献
33.
Mark B. Bushuev Viktor P. Krivopalov Dmitrii Yu. Naumov Katerina A. Vinogradova Lilia A. Sheludyakova Stanislav V. Larionov 《Inorganica chimica acta》2010,363(7):1547-13900
A new bidentate chelating pyrazolylpyrimidine ligand bearing a strong electron-donating substituent, i.e. 4-(3,5-diphenyl-1H-pyrazol-1-yl)-6-(piperidin-1-yl)pyrimidine (L) (Scheme 1), has been synthesized and used to obtain the copper(II) complexes by reaction with CuCl2. The molar ratio Cu:L = 1:2 leads to isolation of a complex having CuL2Cl2 empirical formula, while the molar ratio Cu:L = 1:1 gives a complex with CuLCl2 empirical formula. The crystal structure of L as well as the structures of both complexes were studied by single crystal X-ray diffraction. The crystal structure of CuL2Cl2 compound is formed by trans-[CuL2Cl2] mononuclear molecules. Surprisingly, in contrast to the previous compound having molecular structure, the crystal structure of CuLCl2 consists of mononuclear [CuL2Cl]+ complex cations and dinuclear [Cu2Cl6]2− anions. Thus, formula of CuLCl2 complex can be represented as [CuL2Cl]2[Cu2Cl6]. In both complexes molecules of L adopt bidentate chelating coordination mode through N2 atom of pyrazole and N3 atom of pyrimidine rings forming five-membered CuN3C metallocycles. Owing to C-H···N interactions and π-π-stacking L molecules form 2D network. In the structure of trans-[CuL2Cl2] there exist double lone pair(N(piperidine))-π(pyrimidine) interactions and C-H···Cl contacts resulting in the formation of 1D chains. Layered 2D structure of [CuL2Cl]2[Cu2Cl6] results from C-H···Cl, C-H···π and double lone pair(Cl([CuL2Cl]+ complex cation)-π(pyrimidine) interactions. 相似文献
34.
Nearest neighbor analysis of immunocytolocalization experiments indicates that the enzymes glyceraldehyde-3-P dehydrogenase, triose-P isomerase and aldolase are located close to one another in the pea leaf chloroplast stroma, and that aldolase is located close to sedoheptulose bisphosphatase. Direct transfer of the triose phosphates between glyceraldehyde-3-P dehydrogenase and triose-P isomerase, and from glyceraldehyde-3-P dehydrogenase and triose-P isomerase to aldolase, is then a possibility, as is direct transfer of sedoheptulose bisphosphate from aldolase to sedoheptulose bisphosphatase. Spatial organization of these enzymes may be important for efficient CO2 fixation in photosynthetic organisms. In contrast, there is no indication that fructose bisphosphatase is co-localized with aldolase, and direct transfer of fructose bisphosphate from aldolase to fructose bisphosphatase seems unlikely. 相似文献
35.
The 2-methallyl complex [(η5-C9H7)Ru(η3-2-MeC3H4)(PPh3)] (3), prepared from [(η5-C9H7)Ru(PPh3)2Cl] (2) and 2-MeC3H4MgCl, reacts with HX (X = Cl, CF3CO2) in the presence of ethene to give the chiral-at-metal compounds [(η5-C9H7)Ru(C2H4)(PPh3)X] (4, 5) in nearly quantitative yields. Treatment of 2 with AgPF6 and ethene affords [(η5-C9H7)Ru(C2H4)(PPh3)2]PF6 (6), which reacts with acetone to give the substitution product [(η5-C9H7)Ru(OCMe2)(PPh3)2]PF6 (7). The molecular structure of 7 has been determined crystallographically. Whereas treatment of 4 with CH(CO2Et)N2 yields the olefin complex [(η5-C9H7)Ru{η2-(Z)-C2H2(CO2Et)2}(PPh3)Cl] (8), the reactions of 4 and 5 with Ph2CN2, PhCHN2 and (Me3Si)CHN2 lead to the formation of the carbeneruthenium(II) derivatives [(η5-C9H7)Ru(CRR′)(PPh3)Cl] (9-11) and [(η5-C9H7)Ru(CRR′)(PPh3)(κ1-O2CCF3)] (12-14), respectively. Treatment of 9 (R = R′ = Ph), 10 (R = H, R′ = Ph) and 11 (R = H, R′ = SiMe3) with MeLi produces the hydrido(olefin) complexes [(η5-C9H7)RuH(η2-CH2CPh2)(PPh3)] (15), [(η5-C9H7)RuH(η2-CH2CHPh)(PPh3)] (18a,b) and [(η5-C9H7)RuH(η2-CH2CHSiMe3)(PPh3)] (19) via C-C coupling and β-hydride shift. The analogous reactions of 11 with PhLi gives the η3-benzyl compound [(η5-C9H7)Ru{η3-(Me3Si)CHC6H5}(PPh3)] (20). The η3-allyl complex [(η5-C9H7)Ru(η3-1-PhC3H4)(PPh3)] (17) was prepared from 10 and CH2CHMgBr by nucleophilic attack. 相似文献
36.
Sotiris Zartilas Nick Hadjiliadis Nikolaos Kourkoumelis Maciej Kubicki Ian S. Butler Jan Balzarini 《Inorganica chimica acta》2009,362(3):1003-121
Silver(I) halides react with tri(p-tolyl)phosphine (tptp, C21H21P) in MeOH/MeCN solutions in 1:1 or 1:3 molar ratios to give complexes of formulae {[AgCl(tptp)]4} (1) or [AgX(tptp)3] (X = Cl (2), Br (3), I (4)), respectively. The complexes were characterized by elemental analyses, and FT-IR far-IR, FT-Raman, TG and 1H, 13C, 31P NMR spectroscopic techniques. Crystal structures of complexes 2-4 were determined by X-ray diffraction at room temperature (rt). The crystal structure of 1 and 4 was also determined at 100(1) and 140(2) K (lt), respectively. In complex 1 four μ3-Cl ions are bonded with four Ag(I) ions forming a cubane while the coordination sphere of silver(I) ions is completed by one P atom from a terminal tri(p-tolyl)phosphine ligand. In complexes 2-3 one terminal halogen and three P atoms from phosphine ligands form a tetrahedral arrangement around the metal ion. Complexes 1-4 were tested for in vitro cytostatic activity against sarcoma cancer cells (mesenchymal tissue) from the Wistar rat, polycyclic aromatic hydrocarbons (PAH, benzo[a]pyrene) carcinogenesis and against murine leukemia (L1210) and human T-lymphocyte (Molt4/C8 and CEM) cells. The silver(I) complexes 1-4 show strong activity. 相似文献
37.
P. Pérez-Tejeda G. López-Pérez F. Sánchez M. López-López G. González 《Inorganica chimica acta》2006,359(1):149-158
A study of the metal-to-metal charge-transfer (MMCT) transition within the binuclear cyano-bridged complexes cis-[L13CoIII(μ-NC)FeII(CN)5]− (L13 = 12-methyl-1,4,7,10-tetraazacyclotridecan-12-amine), trans-[L14CoIII(μ-NC)FeII(CN)5]− (L14 = 6-methyl-1,4,8,11-tetraazacyclotetradecan-6-amine) and trans-[L15CoIII(μ-NC)FeII(CN)5]− (L15 = 10-methyl-1,4,8,12-tetraazacyclopentadecan-10-amine) has been carried out in electrolyte solutions at varying concentrations. Using these data, as well as the reaction free energies obtained from electrochemical measurements, the reorganisation and activation free energies for the forward and reverse thermal electron-transfer processes have been estimated. The changes of these parameters with the electrolyte concentration, as well as those of the energy of the maximum MMCT band and the reaction free energy, are mainly due to ion-pairing effects. 相似文献
38.
Experimental evidence that flavonoid metal complexes may act as mimics of superoxide dismutase 总被引:1,自引:0,他引:1
Kostyuk VA Potapovich AI Strigunova EN Kostyuk TV Afanas'ev IB 《Archives of biochemistry and biophysics》2004,428(2):204-208
Radical scavenging activities of flavonoids rutin, taxifolin, (-)-epicatechin, luteolin, and their complexes with transition metal (Fe2+, Fe3+, and Cu2+) towards superoxide were determined using illumination of riboflavin as source and NBT as detector of O*2-. The scavenger potencies of flavonoid metal complexes were significantly higher than those of the parent flavonoids. To elucidate the mechanism of this phenomenon, the rates of superoxide-dependent oxidation of flavonoids and their metal complexes in photochemical system with riboflavin were examined. It was found for the first time that flavonoids bound to metal ions were much less subjected to oxidation compared with those of free compounds. The findings directly demonstrate superoxide scavenging activity of metal ions in complexes with flavonoids and support earlier suggestions that flavonoid metal complexes may exhibit superoxide dismuting activity. 相似文献
39.
D. P. Singh Ramesh Kumar Jitender Singh 《Journal of enzyme inhibition and medicinal chemistry》2013,28(3):883-889
A new series of complexes is synthesized by template condensation of oxalyldihydrazide and glyoxal in methanolic medium in the presence of trivalent chromium, manganese and iron salts forming complexes of the type: [M(C8H8N8O4)X]X2 where M = Cr(III), Mn(III), Fe(III) and X = Cl? 1, , CH3COO? 1. The complexes have been characterized with the help of elemental analyses, conductance measurements, magnetic susceptibility measurements, electronic, NMR, infrared and far infrared spectral studies. On the basis of these studies, a five coordinate square pyramidal geometry for these complexes has been proposed. The biological activities of the metal complexes were tested in vitro against a number of pathogenic bacteria and some of the complexes exhibited remarkable antibacterial activities. 相似文献
40.
The complex [Ru(SB12H11)(NH3)5]·2H2O has been prepared by the reaction of Cs2B12H11SH with [RuCl(NH3)5]Cl2 in aqueous solution. The complex represents the first reported example of the borocaptate anion acting as a ligand. The structure of the complex has been determined by single crystal X-ray diffraction analysis. The crystal parameters are monoclinic, space group P21/c, A = 8.056(1), B = 14.240(2), C = 15.172(2) Å, β=98.48° and Z = 4. The ruthenium atom has a distorted octahedral coordination. The distortion is probably due to the high (3−) charge and the large bulk of the borocaptate ligand. These features can also be observed in the spectroscopic properties of the complex. 相似文献