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241.
Tetsuro Shinada Toshikazu Ishida Yasutaka Yoshida Yasufumi Ohfune 《Tetrahedron letters》2007,48(43):7614-7617
Novel Leu-enkephalin analogs 10a-c in which the Tyr1, Gly2, or Gly3 residue of Leu-enkephalin 9 was replaced with α-amino squaric acid analog Sq-Tyr 8b or Sq-Gly 8a were synthesized starting from 14 or 18. Conformational analysis of 10a-c together with its model compound 26 and their opioid receptor binding activity were evaluated. 相似文献
242.
The quantitative analysis of ketones using DNPH is usually conducted in the presence of an acid catalyst. However, this method may cause an analytical error because 2,4-dinitrophenylhydrazones have both E- and Z-stereoisomers. Purified ketone-2,4-dinitrophenylhydrazone comprised only the E-isomer. However, under the addition of acid, both E- and Z-isomers were seen. In the case of 2-butanone-, 2-pentanone- and 2-hexanone-2,4-dinitrophenylhydrazone, the equilibrium Z/E isomer ratios were 0.20, 0.21 and 0.22, respectively. In addition, when trace water was added to the hydrazone derivatives in acetonitrile solution, the concentration of ketone derivatives were seen to decrease and the concentration of free DNPH was seen to increase. The decomposition rate of 2-butanone-2,4-dinitrophenylhydrazone was dependent on the concentration of acid-catalysis and reached an equilibrium state - carbonyl, DNPH, hydrazone-derivative and H2O - within 10 h at 0.1 mol L−1 phosphoric acid solution. The equilibrium constants of ketone-2,4-dinitrophenylhydrazones, [carbonyl] [DNPH]/[hydrazone] [H2O], were relatively large and ranged from 0.74 × 10−4 to 5.9 × 10−4. Hydrazone derivatives formed from 2-ketones such as 2-pentanone, 2-hexanone and 4-methyl-2-pentanone showed lower equilibrium constants than corresponding 3-ketones. Consequently, only a minimum concentration of catalytic acid must be added. The best method for the determination of ketone-2,4-dinitrophenylhydrazones by HPLC or GC is to add phosphoric acid to both the standard reference solution and samples, forming a 0.001 mol L−1 acid solution, and analyze after 27 h. 相似文献
243.
Saito K Ishida T Sugiura M Kawakami K Umena Y Kamiya N Shen JR Ishikita H 《Journal of the American Chemical Society》2011,133(36):14379-14388
The reaction center chlorophylls a (Chla) of photosystem II (PSII) are composed of six Chla molecules including the special pair Chla P(D1)/P(D2) harbored by the D1/D2 heterodimer. They serve as the ultimate electron abstractors for water oxidation in the oxygen-evolving Mn(4)CaO(5) cluster. Using the PSII crystal structure analyzed at 1.9 ? resolution, the redox potentials of P(D1)/P(D2) for one-electron oxidation (E(m)) were calculated by considering all PSII subunits and the protonation pattern of all titratable residues. The E(m)(Chla) values were calculated to be 1015-1132 mV for P(D1) and 1141-1201 mV for P(D2), depending on the protonation state of the Mn(4)CaO(5) cluster. The results showed that E(m)(P(D1)) was lower than E(m)(P(D2)), favoring localization of the charge of the cationic state more on P(D1). The P(D1)(?+)/P(D2)(?+) charge ratio determined by the large-scale QM/MM calculations with the explicit PSII protein environment yielded a P(D1)(?+)/P(D2)(?+) ratio of ~80/~20, which was found to be due to the asymmetry in electrostatic characters of several conserved D1/D2 residue pairs that cause the E(m)(P(D1))/E(m)(P(D2)) difference, e.g., D1-Asn181/D2-Arg180, D1-Asn298/D2-Arg294, D1-Asp61/D2-His61, D1-Glu189/D2-Phe188, and D1-Asp170/D2-Phe169. The larger P(D1)(?+) population than P(D2)(?+) appears to be an inevitable fate of the intact PSII that possesses water oxidation activity. 相似文献
244.
Katsuya Sako Yasufumi MugishimaTetsuo Iwanaga Shinji ToyotaHiroyuki Takemura Motonori WatanabeTeruo Shinmyozu Michito ShiotsukaHitoshi Tatemitsu 《Tetrahedron letters》2011,52(44):5865-5868
We have synthesized π-conjugated acceptor-type molecules 3a-d containing a cyanomethylene unit as the electron acceptor site and a 4,5-diazafluorene ligand for metal complexation. In the crystal, the planar 3a molecules stack along the b axis in the head-to-tail fashion. Compound 3a shows distinctive electrochromism and its three differently colored redox states (dianion (32−), anion radical (3−), neutral (3)) exhibit remarkable stability. 相似文献
245.
Kawakami K Umena Y Kamiya N Shen JR 《Journal of photochemistry and photobiology. B, Biology》2011,104(1-2):9-18
The catalytic center for photosynthetic water-splitting consists of 4 Mn atoms and 1 Ca atom and is located near the lumenal surface of photosystem II. So far the structure of the Mn(4)Ca-cluster has been studied by a variety of techniques including X-ray spectroscopy and diffraction, and various structural models have been proposed. However, its exact structure is still unknown due to the limited resolution of crystal structures of PSII achieved so far, as well as possible radiation damages that might have occurred. Very recently, we have succeeded in solving the structure of photosystem II at 1.9 ?, which yielded a detailed picture of the Mn(4)CaO(5)-cluster for the first time. In the high resolution structure, the Mn(4)CaO(5)-cluster is arranged in a distorted chair form, with a cubane-like structure formed by 3 Mn and 1 Ca, 4 oxygen atoms as the distorted base of the chair, and 1 Mn and 1 oxygen atom outside of the cubane as the back of the chair. In addition, four water molecules were associated with the cluster, among which, two are associated with the terminal Mn atom and two are associated with the Ca atom. Some of these water molecules may therefore serve as the substrates for water-splitting. The high resolution structure of the catalytic center provided a solid basis for elucidation of the mechanism of photosynthetic water splitting. We review here the structural features of the Mn(4)CaO(5)-cluster analyzed at 1.9 ? resolution, and compare them with the structures reported previously. 相似文献
246.
A short and stereoselective synthesis of manzacidins A and C, and their enantiomers was achieved via stereoselective hydrogenation reactions of dehydroamino acid esters 5-8 using a chiral Rh catalyst. 相似文献
247.
Takamiya T Hosobuchi S Noguchi T Asai K Nakamura E Habu Y Paterson AH Iijima H Murakami Y Okuizumi H 《Electrophoresis》2008,29(19):4088-4095
We analyzed the inheritance of DNA methylation in the first filial generation(F1) hybrid of Oryza sativa L. ("Nipponbare"x"Kasalath") by restriction landmark genome scanning (RLGS). Most parental RLGS spots were found in the F1, but eight spots (4%) showed abnormal inheritance: seven of the eight spots were missing in the F1, and one was newly detected in the F1. Here we show demethylation at restriction enzyme sites in the F1. We also found a candidate site of stable heterozygous methylation in the genome. These results show the applicability of the RLGS method for analysis of the inheritance and alteration of methylation in F1 hybrid plants. 相似文献
248.
249.
Sakaguchi K Ayabe M Watanabe Y Okada T Kawamura K Shiada T Ohfune Y 《Organic letters》2008,10(23):5449-5452
The stereoselective total synthesis of the marine alkaloid (-)-amathaspiramide F (1) was achieved from the alpha-hydoxy-alpha-ethynylsilane 2. The crucial steps in this synthesis involved not only the enolate Claisen rearrangement of the alpha-acyloxy-alpha-alkenylsilane 6 for the construction of the nitrogen-containing quaternary carbon center, but also the chemoselective formation of the azaspirohemiaminal 12 using heptamethyldisilazane as the methylamine equivalent and the regioselective dibromination of the phenol moiety of 12 using n-Bu(4)NBrCl(2). 相似文献
250.
G Fukuhara T Nakamura Y Kawanami C Yang T Mori H Hiramatsu Y Dan-Oh K Tsujimoto Y Inoue 《Chemical communications (Cambridge, England)》2012,48(73):9156-9158
Photocyclodimerization of 2-anthracenecarboxylate tethered to a cyclic nigerosylnigerose scaffold gave a single chiral cyclodimer (out of two achiral and two chiral stereoisomers) in 99% optical and 96% chemical yields, achieving the ultimate stereocontrol of the supramolecular photochirogenesis in aqueous solution at 25 °C. 相似文献