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排序方式: 共有298条查询结果,搜索用时 11 毫秒
41.
Liu G Liang Y Bar-Shir A Chan KW Galpoththawela CS Bernard SM Tse T Yadav NN Walczak P McMahon MT Bulte JW van Zijl PC Gilad AA 《Journal of the American Chemical Society》2011,133(41):16326-16329
Chemical exchange saturation transfer (CEST) is a new approach for generating magnetic resonance imaging (MRI) contrast that allows monitoring of protein properties in vivo. In this method, a radiofrequency pulse is used to saturate the magnetization of specific protons on a target molecule, which is then transferred to water protons via chemical exchange and detected using MRI. One advantage of CEST imaging is that the magnetizations of different protons can be specifically saturated at different resonance frequencies. This enables the detection of multiple targets simultaneously in living tissue. We present here a CEST MRI approach for detecting the activity of cytosine deaminase (CDase), an enzyme that catalyzes the deamination of cytosine to uracil. Our findings suggest that metabolism of two substrates of the enzyme, cytosine and 5-fluorocytosine (5FC), can be detected using saturation pulses targeted specifically to protons at +2 ppm and +2.4 ppm (with respect to water), respectively. Indeed, after deamination by recombinant CDase, the CEST contrast disappears. In addition, expression of the enzyme in three different cell lines exhibiting different expression levels of CDase shows good agreement with the CDase activity measured with CEST MRI. Consequently, CDase activity was imaged with high-resolution CEST MRI. These data demonstrate the ability to detect enzyme activity based on proton exchange. Consequently, CEST MRI has the potential to follow the kinetics of multiple enzymes in real time in living tissue. 相似文献
42.
Wu R Marta RA Martens JK Eldridge KR McMahon TB 《Journal of the American Society for Mass Spectrometry》2011,22(9):1651-1659
The structure of the proton-bound lysine dimer has been investigated by infrared multiple photon dissociation (IRMPD) spectroscopy
and electronic structure calculations. The structures of different possible isomers of the proton-bound lysine dimer have
been optimized at the B3LYP/6-31 + G(d) level of theory and IR spectra calculated using the same computational method. Based
on relative Gibbs free energies (298 K) calculated at the MP2/aug-cc-pVTZ//B3LYP/6-31 + G(d) level of theory, LL-CS01, and
followed closely (1.1 kJ mol–1) by LL-CS02 are the most stable non-zwitterionic isomers. At the MP2/aug-cc-pVTZ//6-31 + G(d) and MP2/aug-cc-pVTZ//6-31 + (d,p)
levels of theory, isomer LL-CS02 is favored by 3.0 and 2.3 kJ mol–1, respectively. The relative Gibbs free energies calculated by the aforementioned levels of theory for LL-CS01 and LL-CS02
are very close and strongly suggest that diagnostic vibrational signatures found in the IRMPD spectrum of the proton-bound
dimer of lysine can be attributed to the existence of both isomers. LL-ZW01 is the most stable zwitterionic isomer, in which
the zwitterionic structure of the neutral lysine is well stabilized by the protonated lysine moiety via a very strong intermolecular
hydrogen bond. At the MP2/aug-cc-pVTZ//B3LYP/6-31 + G(d), MP2/aug-cc-pVTZ//6-31 + G(d) and MP2/aug-cc-pVTZ//6-31 + G(d,p)
levels of theory, the most stable zwitterionic isomer (LL-ZW01) is less favored than LL-CS01 by 7.3, 4.1 and 2.3 kJ mol–1, respectively. The experimental IRMPD spectrum also confirms that the proton-bound dimer of lysine largely exists as charge-solvated
isomers. Investigation of zwitterionic and charge-solvated species of amino acids in the gas phase will aid in a further understanding
of structure, property, and function of biological molecules. 相似文献
43.
44.
Caitlin M. McMahon Prof. Erik J. Alexanian 《Angewandte Chemie (International ed. in English)》2014,53(23):5974-5977
A palladium‐catalyzed, intermolecular Heck‐type coupling of alkyl iodides and alkenes is described. This process is successful with a variety of primary and secondary unactivated alkyl iodides as reaction partners, including those with hydrogen atoms in the β position. The mild catalytic conditions enable intermolecular C? C bond formations with a diverse set of alkyl iodides and alkenes, including substrates containing base‐ or nucleophile‐sensitive functionality. 相似文献
45.
Oku N Krishnamoorthy R Benson AG Ferguson RL Lipton MA Phillips LR Gustafson KR McMahon JB 《The Journal of organic chemistry》2005,70(17):6842-6847
The absolute stereochemistry of the three unresolved structural components in neamphamide A (1) was determined to be (R)-beta-methoxy-L-tyrosine, (2R,3R,4S)-4-amino-7-guanidino-2,3-dihydroxyheptanoic acid, and (2R,3R,4R)-3-hydroxy-2,4,6-trimethylheptanoic acid. Stereochemical assignments were made by chemical degradation of 1, derivatization of the resulting products, and then spectroscopic and chromatographic comparison of the derivatives with synthetically prepared standards. Using the same analytical protocol developed for 1, the beta-methoxytyrosine residue in papuamide B (2) was found to be (R)-beta-methoxy-D-tyrosine. This represents a rare example of divergent stereochemistry in an unusual amino acid residue that is present in two closely related classes of peptides. 相似文献
46.
V. L. Kasyutich A. McMahon T. Barnhart P. A. Martin 《Applied physics. B, Lasers and optics》2008,93(2-3):701-711
We demonstrate what is, to our knowledge, the first use of mid-infrared laser absorption spectroscopy for trace-gas measurements of cyclotron target outputs used for the generation of radioactive carbon-11 in positron emission tomography (PET). The spectrometer was based upon a liquid-nitrogen-cooled lead salt diode laser generating single-mode radiation in the wavenumber range of 2230–2240 cm?1. The sample flowed to a multiple-pass optical cell with a total path length of 15.23 m and the laser radiation was detected by two liquid-nitrogen-cooled InSb photodetectors. We present the results of CO, N2O and CO2 measurements on PET trace cyclotron output and discuss future work on 11CO and 11CO2 detection. 相似文献
47.
Extracts of the sponge genus Candidaspongia showed selective cytotoxicity toward melanoma cells in the NCI 60-cell-line screen. Continued investigation of the Candidaspongia sp. extracts led to the isolation of three new tedanolide analogues, precandidaspongiolides A (1) and B (2) and candidaspongiolide B (4), as well as candidaspongiolide A (3) and tedanolide (5). Semisynthetic derivatives were also generated to develop SAR. Candidaspongiolides A/B were the most potent and showed low nanomolar activity against several melanoma cell lines. 相似文献
48.
Muse Oke RosalindTanYan Ching LesterG. Carter KennethA. Johnson Huanting Liu StephenA. McMahon MalcolmF. White Carlos Bloch CatherineH. Botting MartinA. Walsh AishahA. Latiff MalcolmW. Kennedy Alan Cooper JamesH. Naismith 《Angewandte Chemie (Weinheim an der Bergstrasse, Germany)》2008,120(41):7971-7974
49.
M.J. Corden J.D. Dowell D. Eastwood J. Garvey R.J. Homer M. Jobes I.R. Kenyon T. McMahon R.J. Vallance P.M. Watkins J.A. Wilson J. Gago M. Jung P. Sonderegger D. Treille P.L. Woodworth V. Eckardt J. Fent K.C.T.O. Sumorok 《Physics letters. [Part B]》1977,68(1):96-100
Measurements have been made of relative production cross sections of the by π±, K±, p and p? at 39.5 GeV/c incident on copper. production rates from π?, K? and p? are similar. The relative particle/anti-particle production cross sections for x>0 are , σ(K+)/σ(K?)=(0.85±0.5) and . The small p/p? cross section ratio disagrees with models of J/ψ production by gluon amalgamation. 相似文献
50.
Diaz L Liauw CM Edge M Allen NS McMahon A Rhodes N 《Journal of colloid and interface science》2005,287(2):379-387
Flow microcalorimetry and infrared spectroscopy were used to study the surface structure and adsorptive properties of a series of calcined and uncalcined porous silicas. The adsorbates DL-menthol, (R)-(+)-limonene, (+/-)-citronellal and carvone were selected for their functionality, that included carbonyl, vinylic and hydroxyl groups. The amounts of probe retained by the silicas together with the energy exchange involved in the adsorption/desorption process were determined by flow microcalorimetry. The functional groups involved in these interactions were studied by means of infrared spectroscopy. It was observed that the strongest interactions with the silica surface took place through hydrogen bonding onto the surface silanol. The most retentive probes were found to be those with a carbonyl group in their structure. Adsorption onto calcined silicas was found to be less energetic than onto the equivalent calcined samples. The adsorption densities were compared with theoretical predictions based on molecular models. In all cases apart from citronellal monolayer coverage were not observed due to steric effects. 相似文献