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131.
The first complexes that contain the 2,6-bis(dicyclohexylphosphinomethyl)pyridine ligand (PNP) have been isolated and characterized. The reactions of K4Mo2Cl8, (n-Bu4N)2Re2Cl8 and PdBr2(1,5-COD) afford Mo2Cl4(PNP)(HPCy2) (1), ReCl3(PNP) (2) and PdBr2(PNP) (4), respectively, while from the reaction of PNP with cis-Re2(μ-O2CCH3)2Cl4(H2O)2 the heteromacrocylic dication [Cy2P{CH2pyCH2}2PCy2]2+ has been isolated as its mixed [Cl]−/[ReO4]− salt (3). The reaction of cis-Re2(μ-O2CCH3)2Cl4(H2O)2 with bis(diphenylphosphinomethyl)sulfide (PSP) gives the mononuclear Re(V) complex ReO(OEt)Cl2(PSP) (5) in which the S atom is not coordinated. The structures of 1-5 have been established by X-ray crystallography, that of 5 being the first for a complex of this ligand. 相似文献
132.
Heather J. GilfoyKatherine N. Robertson T.Stanley CameronManuel A.S. Aquino 《Inorganica chimica acta》2002,331(1):330-335
The first sulfur-donor adduct of a diruthenium tetracarboxylate, [Ru2(μ-O2CCH3)4(THT)2]PF6 (1), where THT=tetrahydrothiophene, has been synthesized and characterized by X-ray diffraction, IR and UV-Vis spectroscopy, magnetic susceptibility and electrochemistry. Complex 1 displays a typical RuRu bond length of 2.285(4) Å but a very long RuS axial bond of 2.627(13) Å. The redox potential for the [Ru2(μ-O2CCH3)4(THT)2]+/0 couple is −322 mV (vs. Fc/Fc+) which is comparable to other moderately strong Lewis base adducts. Axial ligand binding is driven by σ-donation with little π-backdonation from the metal centers. 相似文献
133.
We have collected all aromatic pairs (3152) involving an N-phenyl partner in a dataset of 593 proteins of the PDB: 728 of these pairs involve a partner residue less than 6 apart in the sequence. These near-sequence Phe-X pairs correspond to specific conformations that stabilize secondary structures, mainly alpha-helices when the residues are 1, 3, and 4 apart, and beta-strands when they are 2 apart in the sequence. These conformations are not spatially random and have been examined in detail. The remaining phenylalanine pairs (2424) are between partners more than 5 apart in the sequence. Of these far-sequence pairs, 34% of occurrences are in sheets. Next in frequencies are pairs that bridge a beta-strand to a helix (24%), followed by pairs that bridge a beta-strand to a random coiled structure (15%). Helix to helix pairs only constitute 12% of these far-sequence pairs. Analysis of the pairing frequency supports the hypothesis that aromatic interactions are late events of protein folding. 相似文献
134.
In a data set of 593 nonhomologous proteins from the PDB, we have analyzed the pairing of phenylalanine, tyrosine, tryptophan, and histidine residues with their closest aromatic partner. The frequency distribution of the shortest interatomic distance of partners is bimodal with a sharp peak at approximately 3.8 A and a wider one at a longer distance. Only the 3.8 A peak corresponds to direct ring-ring interactions thus aromatic pairs. The aromatic pairs were separated into two classes, near-sequence pairs and far-sequence pairs. Near sequence pairs stabilize local structure, and far-sequence pairs stabilize tertiary structure. Far-sequence pairs (74% of all pairs) mainly bridge two beta-strands, followed by pairs that bridge a beta-strand and a helix, and pairs that bridge a beta-strand and a random coil structure. Pairs that bridge helices are rare. The secondary structure of the near-sequence pairs depends on the partner distance in the sequence. When the partners are 1, 3, or 4 residues apart in the sequence, pairs are mostly found in helical structures. When the partners are two apart, pairs are mostly found in the same beta-strand. Analysis of the frequency of near sequence pairs supports the hypothesis that aromatic pairing occurs after, rather than before, the formation of secondary structures. 相似文献
135.
Understanding protein aggregation may hold important clues to understanding what goes wrong with protein folding in neurodegenerative disorders and in bioreactors in which proteins are overexpressed. Unfortunately, aggregates tend to be intractable to most standard methods of biochemical investigation. Thus, relatively little is even now known about the micro- and macro-structural features of aggregates. To gain insights into the thermal aggregation of a model globular protein [bovine carbonic anhydrase (BCA)], we have used spectrofluorimetry to examine the binding of a hydrophobic dye, 8-anilinonaphthalene sulfonate (ANS), to hydrophobic clusters on the protein's surface both before and after heat-induced aggregation and upon cooling. Whereas native BCA shows no surface hydrophobicity, thermally aggregated BCA displays significant hydrophobicity both in the heated state and upon cooling. The timing of the addition of ANS in the course of aggregation makes no net difference to the ANS bound; we argue that this suggests that aggregates are essentially porous. Cooling of aggregates results in a dramatic, fully reversible increase in ANS binding that cannot be explained by the temperature dependence of fluorescence quantum yield alone; we argue that the enhancement of fluorescence upon cooling indicates possible structural consolidation of unfolded regions within aggregates (akin to refolding), with the required structural reorganization being facilitated by porosity. Finally, implications of porosity in aggregates are discussed, in particular, for the possible immobilization of enzymes through fusion with aggregation-prone protein domains. 相似文献
136.
The preparation and structural characterization of {Ru3(CO)11}2(1,4-bis(diphenylphosphino)benzene), a modified synthesis of 1,4-bis(diphenylphosphino)benzene, and the structural characterization of {Ru3(CO)11}2(bis(diphenylphosphino)ethane) are reported. In both compounds two metal cluster units are connected through ditertiary-phosphine ligands. Both molecules consist of centrosymmetric units in which the diphosphine ligands are largely covered by the triangular ruthenium clusters. No direct interaction between the two cluster units occurs within individual molecules. Molecular packing in the solid state is dominated by interactions between sets of carbon monoxide ligands in motifs that were previously identified in the solid state structure of the parent cluster, Ru3(CO)12. 相似文献
137.
Ilia Levitin Marina Tsikalova Natalia Sazikova Vladimir Bakhmutov Zoya Starikova Aleksandr Yanovsky Yuri Struchkov 《Inorganica chimica acta》1998,270(1-2):169-176
Organocobalt(III) complexes with Schiff bases derived from a β-diketone bearing both an alkyl and an aryl group have been prepared. The template syntheses using benzoylacetone and ethylenediamine as complexing agents provide a route to alkylcobalt chelates with the corresponding tri- and tetradentate Schiff bases. However, if a β-diketone with two aryl groups, e.g. dibenzoylmethane, was employed as the starting ketoenol component, no organometallic products were detected; a new mixed-ligand ‘inorganic’ chelate of cobalt(II), [Co{O=C(Ph)CH=C(Ph)O}2(en)], was isolated instead. Its structure as well as that of one of the alkylcobalt complexes with a tridentate Schiff base composed of benzoylacetone and ethylenediamine have been established by X-ray techniques. The current scope of the template synthesis of alkylcobalt complexes with Schiff bases is summarized. 相似文献
138.
Antonio Zanotti-Gerosa Euro Solari Luca Giannini Carlo Floriani Nazzareno Re Angiola Chiesi-Villa Corrado Rizzoli 《Inorganica chimica acta》1998,270(1-2):298-311
Lithiation of [p-But-calix[4]-(OMe)2(OH)2] (1), followed by reaction with TiCl3(thf)3 or TiCl4(thf)2, led to the corresponding titanium-calix[4]arene complexes [p-But-calix[4]-(OMe)2(O)2]TiCl] (2) and [p-But-calix[4]-(OMe)2(O)2]TiCl2] (3), respectively. Reaction of 1 with TiCl4(thf)2 results in demethylation of the calix[4]arene and the obtention of [p-But-calix[4]-(OMe)2(O)3]TiCl] (4), whose hydrolysis led to [p-But-calix[4]-(OMe)(OH)3] (6). The preparation of 6 can be carried out as a one-pot synthesis. Both 2 and 4 undergo alkylation reactions using conventional procedures, thus forming surprisingly stable organometallic species, namely [p-But-calix[4]-(OMe)2(O)2Ti(R)] (R = Me (7); CH2Ph (8), p-MeC6H4 (9) and [p-But-calix[4]-(OMe)(O)3Ti(R)] (R = Me (10); CH2Ph (11); p-MeC6H4 (12)). Complexes 7 and 9 undergo a thermal oxidative conversion into 10 and 12, occurring with the demethylation of one of the methoxy groups. A solid state structural property of 9 and 12 has been revealed by X-ray analysis showing a self-assembly of the monomeric units into a columnar polymer, where the p-tolyl substituent at the metal functions as a guest group for an adjacent titanium-calixarene. Reductive alkylation of 3 with Mg(CH2Ph)2 gave 8 instead of forming the corresponding dialkyl derivative. Two synthetic routes have been devised for the synthesis of the Ti(III)-Ti(III) dimer [p-But-calix[4]-(OMe)(O)3Ti]2] (13): the reduction of 4 and the reaction of TiCl3(thf)3 with the lithiated form of 6. A very strong antiferromagnetic coupling is responsible for the peculiar magnetic behavior of 13. The proposed structures have been supported by the X-ray analyses of 4, 9, 12 and 13. 相似文献
139.
Structural differences between the repertoires of mouse and human germline genes and their evolutionary implications 总被引:1,自引:0,他引:1
J. C. Almagro Ismael Hernández Maria Carmen Ramírez Enrique Vargas-Madrazo 《Immunogenetics》1998,47(5):355-363
Although human and mouse antibodies are similar when one considers their diversification strategies, they differ in the extent
to which kappa and lambda light chains are present in their respective variable light chain repertoires. While the Igk-V germline genes are preponderant in mice (95% or more), they comprise only 60% in humans. This may account for differences
in the structural repertoire encoded in the Igk-V germline genes of these species. However, this subject has not been properly investigated, partially because a systematic
structural characterization of the mouse Igk-V germline genes has not been undertaken. In the present study we compiled all available information on mouse Igk-V germline genes to characterize their structural repertoire. As expected, comparison with the structural repertoire of human
Igk-V germline genes indicates differences. The most interesting is that the mouse Igk-V germline gene repertoire is more diverse in structural terms than its human counterpart: the mouse encodes seven canonical
structure classes (combination of canonical structures in L1 and L3). In contrast, the human encodes only four. Analysis of
the evolutionary relationships of human and mouse Igk-V germline genes led us to propose that the difference reflects a strategy of mice to compensate for the small lambda chain
contribution to the repertoire of their variable light chains.
Received: 1 June 1997 / Revised: 6 October 1997 相似文献
140.
The role of disturbed photoperiod on the developing surface layers of ripening Scots pine (Pinus sylvestris L.) seeds was studied from August 14 to October 30. The embryo sizes of both the light-treated and the control seeds were
examined by the X-ray method prior to germination tests. The anatomical details of resin-embedded seeds were examined by fluorescence
and light microscopy. The timing of the ripening process of the surface structures was described and documented. The greatest
anatomical changes in the ripening seeds occurred in the sarcotesta and in the nucellar layers. Maturation of the surface
structures was essentially slower than could have been interpreted by the size of the embryo and responded clearly to the
disturbance of photoperiod. Accumulation of phenolic substances and degeneration of cells, particularly at the chalazal region,
advanced faster in the light-treated than in the control seeds up to mid-September. The ripening effect of the altered photoperiod
diminished, however, after mid-September. This was also confirmed by the brown colour of the seed coat getting darker only
in the control seeds at the end of the test period. Consequently, fully ripe structures were first found about a fortnight
earlier in the control than in the light-treated seeds. The coincidental advancement of the anatomical potential examined
by the X-ray method supported the role of the surface structures on the anatomical maturity of pine seeds and the timing of
cone collecting.
Received: 26 January 1998 / Accepted: 30 March 1998 相似文献