全文获取类型
收费全文 | 2427篇 |
免费 | 203篇 |
国内免费 | 52篇 |
出版年
2024年 | 5篇 |
2023年 | 27篇 |
2022年 | 35篇 |
2021年 | 41篇 |
2020年 | 54篇 |
2019年 | 54篇 |
2018年 | 66篇 |
2017年 | 58篇 |
2016年 | 51篇 |
2015年 | 62篇 |
2014年 | 81篇 |
2013年 | 119篇 |
2012年 | 54篇 |
2011年 | 138篇 |
2010年 | 121篇 |
2009年 | 153篇 |
2008年 | 113篇 |
2007年 | 146篇 |
2006年 | 143篇 |
2005年 | 139篇 |
2004年 | 153篇 |
2003年 | 75篇 |
2002年 | 134篇 |
2001年 | 59篇 |
2000年 | 42篇 |
1999年 | 44篇 |
1998年 | 77篇 |
1997年 | 31篇 |
1996年 | 51篇 |
1995年 | 72篇 |
1994年 | 28篇 |
1993年 | 16篇 |
1992年 | 24篇 |
1991年 | 19篇 |
1990年 | 21篇 |
1989年 | 12篇 |
1988年 | 17篇 |
1987年 | 13篇 |
1986年 | 13篇 |
1985年 | 22篇 |
1984年 | 14篇 |
1983年 | 6篇 |
1982年 | 17篇 |
1981年 | 9篇 |
1980年 | 10篇 |
1979年 | 6篇 |
1976年 | 6篇 |
1973年 | 8篇 |
1972年 | 7篇 |
1971年 | 5篇 |
排序方式: 共有2682条查询结果,搜索用时 269 毫秒
31.
The reactions of cadmium halides with the 15-membered macrocyclic crown ethers, 15-crown-5 and benzo-15-crown-5, have been carried out and six new complexes have been isolated and structurally characterized. Metal to ligand stoichiometries of 1:1, 2:1, 3:1 and 3:2 have been observed with a variety of different formulations. Examples of charge separated ion pairs ([(NH4)(benzo-15-crown-5)2]2[Cd2I6]), halogen bridged monomers, dimers or polymers ([Cd(15-crown-5)(OHMe)(μ-Br)CdBr3], [Cd(15-crown-5)(μ-Br)2CdBr(μ-Br)]2(isolated from the same reaction mixture) and [(CdCl2)2CdCl2(15-crown-5)]n), and hydrogen bonded finite chains or polymers ([(Cd(OH2)2(15-crown-5)][CdI3(OH2)]2·2(15-crown-5)·2CH3CN and [CdI2(OH2)2(THF)]·benzo-15-crown-5) have been isolated. Three different types of 15-crown-5 coordination modes have been observed in these complexes. In-cavity coordination resulting in pentagonal bipyramidal geometries about Cd2+ was observed in [(CdCl2)2CdCl2(15-crown-5)]n, [Cd(15-crown-5)(OHMe)(μ-Br)CdBr3], and [Cd(OH2)2(15-crown-5)][CdI3(OH2)]2·2(15-crown-5)·2CH3CN, [Cd(15-crown-5)(μ-Br)2CdBr(μ-Br)]2 displays out-of-cavity coordination with one etheric donor distorted into an axial position of a distorted pentagonal bipyramid. The third coordination mode is secondary sphere coordination via hydrogen bonding which is observed for [Cd(OH2)2(15-crown-5)][CdI3(OH2)]2·2(15-crown-5)·2CH3CN. The good fit of Cd2+ within the cavity of 15-crown-5 results in shorter bonding contacts and a more narrow distribution in Cd---O values (2.273(7)-2.344(6) Å) than observed for cadmium halide complexes of 18-crown-6 (Cd---O = 2.69(1)–2.81(1) Å). 相似文献
32.
Harry Adams Neil A. Bailey Philippe Bertrand Simon R. Collinson David E. Fenton Simon J. Kitchen 《Inorganica chimica acta》1996,250(1-2):139-148
The cyclocondensation of 2,5-diformylthiophene and the amines N,N-bis-(2-aminoethyl)-2-phenylethylamine, N,N-bis-(2aminoethyl)-t-butyl-amine and N,N-bis-(2-aminoethyl)-t-butyl-amine in the presence of silver(I) salts yields homodinuclear bibracchial tetraimine Schiff base macrocyclic complexes. The structures of two such complexes are also reported. The complex Ag2L4(NO3)(PF6) (2) crystallises in the triclinic space group
, No. 2) and has unit-cell dimensions a = 12.834(6), B = 13.183(6), C = 14.588(7) Å, = 64.86(4), β = 79.77(4), γ = 69.44(3)° with Z = 2; there is a monodentate and singly bridging nitrate anion present and the Ag---Ag separation is 4.161 Å. The complex Ag2L4(CH3CN)2(BF4)2·CH3CN (9) crystallises in the triclinic space group
, No. 2) and has unit-cell dimensions a = 9.297(4), B = 12.985(3), C = 21.770(5) Å, = 91.570(10), β = 92.33(3), γ = 97.92(3) ° with Z = 2; there is a strongly bonded acetonitrile molecule coordinated to each silver atom and the Ag---Ag separation is 4.920 Å. 相似文献
33.
N. Kositzyna M. Antipin K. Lyssenko P. S. Pregosin G. Trabesinger 《Inorganica chimica acta》1996,250(1-2):365-373
A series of cationic allyl complexes of Pd(II) derived from the diterpene carvone
have been synthesised and characterised using the chelating ligands, bipyridine, 1,8-phenanthroline, neocuproin, S,S (-)-Chiraphos and R (+)-Binap as co-ligands. These complexes can be readily converted to new organic products in which one C---H bond of the allyl methyl has been substituted. The structure of a bipyridyl complex as its tetraphenyl borate salt has been determined via X-ray diffraction. 2-D NOESY studies on the Chiraphos and Binap complexes are reported. The Binap, but not the Chiraphos, complex reveals η3-η3η3 isomerisation at ambient temperature. 相似文献
34.
New mixed metal complexes SrCu2(O2CR)3(bdmap)3 (R = CF3 (1a), CH3 (1b)) and a new dinuclear bismuth complex Bi2(O2CCH3)4(bdmap)2(H2O) (2) have been synthesized. Their crystal structures have been determined by single-crystal X-ray diffraction analyses. Thermal decomposition behaviors of these complexes have been examined by TGA and X-ray powder diffraction analyses. While compound 1a decomposes to SrF2 and CuO at about 380°C, compound 1b decomposes to the corresponding oxides above 800°C. Compound 2 decomposes cleanly to Bi2O3 at 330°C. The magnetism of 1a was examined by the measurement of susceptibility from 5–300 K. Theoretical fitting for the susceptibility data revealed that 1a is an antiferromagnetically coupled system with g = 2.012(7), −2J = 34.0(8) cm−1. Crystal data for 1a: C27H51N6O9F9Cu2Sr/THF, monoclinic space group P21/m, A = 10.708(6), B = 15.20(1), C = 15.404(7) Å, β = 107.94(4)°, V = 2386(2) Å3, Z = 2; for 1b: C27H60N6O9Cu2Sr/THF, orthorhombic space group Pbcn, A = 19.164(9), B = 26.829(8), C = 17.240(9) Å, V = 8864(5) Å3, Z = 8; for 2: C22H48O11N4Bi2, monoclinic space group P21/c, A = 17.614(9), B = 10.741(3), C = 18.910(7) Å, β = 109.99(3)°, V = 3362(2) Å3, Z = 4. 相似文献
35.
The presence of galanin was examined in the cat gut and related autonomic nervous structures using radioimmunoassay (RIA) and high performance liquid chromatography (HPLC). In the gut wall, the concentration of galanin-like immunoreactivity (GAL-LI) was assayed separately in the muscular layers with the nervous plexuses and in the mucosa and ranged from 0.35 to 0.55 pmol/g wet tissue. In the autonomic nervous structures, GAL-LI concentrations ranged from 0.22 (thoracic spinal ganglia) to 0.81 (inferior mesenteric ganglion) pmol/g wet tissue. The presence of galanin was checked by HPLC in the antrum, intestine, and colon. HPLC of extractable material revealed a major peak coeluting with the synthetic porcine peptide and minor earlier peaks representing likely different molecular forms of galanin. Our study strengthens the notion that galanin acts in nervous control of the cat gut functions. 相似文献
36.
James M. Chen Spero Manolatos Paul W. Brandt-Rauf Randall B. Murphy Regina Monaco Matthew R. Pincus 《Journal of Protein Chemistry》1996,15(6):511-518
The three-dimensional structures of theras-p21 protein and its protein inhibitor, rap-1A, have been computed bound to theras-binding domain, RBD (residues 55–131), of theraf-p74 protein, a critical target protein ofras-p21 in theras-induced mitogenic signal transduction pathway. The coordinates of RBD have been reconstructed from the stereoview of an X-ray crystal structure of this domain bound to rap-1A and have been subjected to energy minimization. The energy-minimized structures of bothras- p21 and rap-1A, obtained in previous studies, have been docked against RBD, using the stereo figure of the RBD-rap-1A complex, based on a six-step procedure. The final energy-minimized structure of rap-1A-RBD is identical to the X-ray crystal structure. Comparison of theras-p21- and rap-1A-RBD complexes reveals differences in the structures of effector domains ofras-p21 and rap-1a, including residues 32–47, a domain that directly interacts with RBD, 60–66, 96–110, involved in the interaction ofras-p21 withjun kinase (JNK) andjun protein, and 115–126, involved in the interaction of p21 with JNK. The structure of the RBD remained the same in both complexes with the exception of small deviations in its-2 binding loop (residues 63–71) and residues 89–91, also involved in binding to rap-1A. The results suggest that the binding of these two proteins to RBD may allow them to interact with other cellular target proteins such as JNK andjun. 相似文献
37.
Manfred Tacker Peter F. Stadler Erich G. Bornberg-Bauer Ivo L. Hofacker P. Schuster 《European biophysics journal : EBJ》1996,25(2):115-130
Algorithms predicting RNA secondary structures based on different folding criteria – minimum free energies (mfe), kinetic
folding (kin), maximum matching (mm) – and different parameter sets are studied systematically. Two base pairing alphabets
were used: the binary GC and the natural four-letter AUGC alphabet. Computed structures and free energies depend strongly on both the algorithm and the parameter set. Statistical
properties, such as mean number of base pairs, mean numbers of stacks, mean loop sizes, etc., are much less sensitive to the
choice of parameter set and even of algorithm. Some features of RNA secondary structures, such as structure correlation functions,
shape space covering and neutral networks, seem to depend only on the base pairing logic (GC or AUGC alphabet).
Received: 16 May 1996 / Accepted: 10 July 1996 相似文献
38.
Treatment of the A-ring aromatic steroids estrone 3-methyl ether and β-estradiol 3, 17-dimethyl ether with Mn(CO)5+BF4− in CH2Cl2 yields the corresponding [(steroid)Mn(CO)3]BF4 salts 1 and 2 as mixtures of and β isomers. The X-ray structure of [(estrone 3-methyl ether)Mn(CO)3]BF4 · CH2Cl2 (1) having the Mn(CO)3 moiety on the side of the steroid is reported: space group P21 with a=10.3958(9), b=10.9020(6), c=12.6848(9) Å, β=111.857(6)°, Z=2, V=1334.3(2) Å3, calc=.481 cm−3, R=0.0508, and wR=0.0635. The molecule has the traditional ‘piano stool’ structure with a planar arene ring and linear Mn---C---O linkages. The nucleophiles NaBH4 and LiCH2C(O)CMe3 add to [(β-estradiol 3,17-dimethyl ether)Mn(CO)3]BF4 (2) in high yield to give the corresponding - and β-cyclohexadienyl manganese tricarbonyl complexes (3). The nucleophiles add meta to the arene -OMe substituent and exo to the metal. The and β isomers of 3 were separated by fractional crystallization and the X-ray structure of the β isomer with an exo-CH2C(O)CMe3 substituent is reported (complex 4): space group P212121 with a=7.5154(8), b=15.160(2), c=25.230(3) Å, Z=4, V=2874.4(5) Å3, calc=1.244 g cm−3, R=0.0529 and wR2=0.1176. The molecule 4 has a planar set of dienyl carbon atoms with the saturated C(1) carbon being 0.592 Å out of the plane away from the metal. The results suggest that the manganese-mediated functionalization of aromatic steroids is a viable synthetic procedure with a range of nucleophiles of varying strengths. 相似文献
39.
Comparison of the intermediate complexes of human growth hormone bound to the human growth hormone and prolactin receptors. 总被引:4,自引:2,他引:2 下载免费PDF全文
A. A. Kossiakoff W. Somers M. Ultsch K. Andow Y. A. Muller A. M. De Vos 《Protein science : a publication of the Protein Society》1994,3(10):1697-1705
The crystal structures of complexes of human growth hormone (hGH) with the growth hormone and prolactin receptors (hGHR and hPRLR, respectively), together with the mutational data available for these systems, suggest that an extraordinary combination of conformational adaptability, together with finely tuned specificity, governs the molecular recognition processes operative in these systems. On the one hand, in the active 1:2 ligand-receptor complexes, 2 copies of the same receptor use the identical set of binding determinants to recognize topographically different surfaces on the hormone. On the other hand, comparing the 1:1 hGH-hGHR and hGH-hPRLR complexes, 2 distinct receptors use this same set of binding determinants to interact with the identical binding site on the ligand, even though few residues among the binding determinants are conserved. The structural evidence demonstrates that this versatility is accomplished by local conformational flexibility of the binding loops, allowing adaptation to different binding environments, together with rigid-body movements of the receptor domains, necessary for the creation of specific interactions with the same binding site. 相似文献
40.
Summary In the twelve Drosophila obscura group species studied, belonging to the affinis, obscura, and pseudoobscura subgroups, the mitochondrial DNA length ranges from 15.8 to 17.2 kb. This length polymorphism is mainly due to insertions/deletions in the variable region of the A+T-rich region. In addition, one species (D. tristis) possess a tandem duplication of a 470-bp fragment that contains the replication origin.The same duplication has occurred at least twice in the Drosophila evolutionary history due to the fact that the repetition is analogous to repetitions found in the four species of the D. melanogaster complex.By comparing the nucleotide sequence of the conserved region in D. ambigua, D. obscura, D. yakuba, D. teissieri, and D. virilis, we show the presence of a secondary structure, likely implied in the replication origin, which could favor the generation of this kind of duplications.Finally, we propose that the high A and T content in the variable region of the A + T-rich region favors the formation of less-stable secondary structures, which could explain the generation of minor insertion/deletions found in this region.Offprint requests to: A. Latorre 相似文献