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排序方式: 共有214条查询结果,搜索用时 15 毫秒
1.
E. V. McCloskey A. Dey R. M. Parr N. Aras A. Balogh J. Bostock A. Borell S. Krishnan G. Lobo L. L. Qin Y. Zhang S. Cvijetic V. Zaichick M. Lim-Abraham K. Bose S. Wynchank G. V. Iyengar 《Journal of Radioanalytical and Nuclear Chemistry》2004,259(2):341-345
In 1994, the International Atomic Energy Agency (IAEA) initiated a 5-year Co-ordinated Research Project (CRP) to determine
geographical and racial differences in peak bone mineral density (BMD) in men and women aged 15-49 years. The study demonstrates
that there are distinct global differences in BMD at the hip and spine in both men and women approximating to one population
standard deviation between populations with the highest and lowest BMD. These differences persist following adjustments for
age, sex and body size. Such information is valuable in understanding the reasons for global differences in fracture rate
and predicting future trends in fracture incidence.
This revised version was published online in August 2006 with corrections to the Cover Date. 相似文献
2.
The action of ultra violet rays of 260 nm on aqueous solutions of bases, nucleosides and nucleotides of DNA at 77°K is studied by electron spin resonance. It is shown that the free radicals observed are similar, with a few noteworthy exceptions to those induced by X-rays, under the same conditions of temperature, in the solid state. Contrary to what might be excepted on the basis of the results obtained by X-rays, the variation in the yield in paramagnetic centres in each sequence studied does not seem to be important. 相似文献
3.
ALBERT VAN DE VORST 《Photochemistry and photobiology》1971,13(4):321-336
Abstract— –The photosensitization of amino acids by proflavine is studied using the technique of electron spin resonance spectroscopy. The analysis of the line shape as a function of the incident microwave power (both in the presence and absence of oxygen) and the dependence of the numbers of free radicals on the intensity of the incident light allow one to suggest that two types of radicals are formed. One is formed by a biphotonic process, the nature of the radicals being the same as in the case of ionising radiation, while the other is probably the RO2 radical formed as a result of photodynamic action. 相似文献
4.
Masakazu Uramoto Kimie Kobinata Kiyoshi Isono Tsutomu Higashijima Tatsuo Miyazawa Ellen E. Jenkins James A. McCloskey 《Tetrahedron》1982,38(11):1599-1608
New chitin synthetase inhibitors, neopolyoxins A, B, and C were isolated from the culture filtrate of Streptomyces cacaoi subsp. asoensis. Their absolute structures have been established on the basis of chemical and spectroscopic evidence. They are structural analogs of the polyoxins. As a nucleobase, neopolyoxin C possesses uracil, while neopolyoxins A and B contain the imidazoline moiety. A ring contraction reaction of pyrimidine nucleoside into imidazoline nucleoside played a key role in the structure determination of the nucleoside moiety. A similar transformation was suggested for the biosynthesis of neopolyoxins A and B. 相似文献
5.
BARB FLORIN DAN; IONESCU VLAD; KONING WILLEM DE 《IMA Journal of Mathematical Control and Information》1994,11(4):277-309
In this paper, we deal with the digital output-measurement-feedbackH control problem for Pritchard-Salamon infinite-dimensionalsystems with unbounded input and output operators. A discretePopov-theory-based solution is given in terms of the solvabilityof Kalman-Szegö-Popov-Yakubovitch systems associated withthe equivalent discrete-time time-invariant system obtainedby lifting the T-periodic continuous-time system. 相似文献
6.
P. F. Crain James A. McCloskey Arthur F. Lewis Karl H. Schram Leroy B. Townsend 《Journal of heterocyclic chemistry》1973,10(5):843-847
The mass spectrum of pyrazomycin has been obtained and reveals a fragmentation pattern with several important peaks which are not normally found in the mass spectra of C-nucleosides. It has now been established, using model compounds, that these unusual fragment ions are a direct result of the juxtaposition of the exocyclic hydroxy and carboxamido groups of the aglycon. It appears that a facile elimination of ammonia and ethanol from o-hydroxycarboxamides and o-hydroxyethylesters, respectively, may be a general fragmentation reaction for aromatic heterocycles. A mass spectrum of the TMS derivative of pyrazomycin has also been obtained and factors which may result in exceptions to the empirical B+30 (M-103) rule for C-nucleosides are discussed. 相似文献
7.
Abstract— Ellipsometric angles and reflectivity of black lipid membranes containing either egg lecithin or chloroplast extracts were measured at a wavelength of 6328 Å. Evidence was found for positive uniaxial anisotropy in membranes of lecithin. If the thickness of those films is assumed to be 62 Å, the refractive index normal to the film surface is 1.471 ± 0.004, and the refractive index parallel to the film surface is 1.454 ± 0.003. The results for membranes of chloroplast extracts also indicate a positive anisotropy in the indices of absorption. 相似文献
8.
Joachim G. Liehr Gareth A. Brenton John H. Beynon James A. McCloskey Wolfgang Blum Wilhelm J. Richter 《Helvetica chimica acta》1981,64(3):835-843
In isobutane chemical ionization mass spectrometry benzyl alcohol exhibits ions at m/z 147 (‘M + 39’) that arise by a loss of H2O from [M + C4H9]+, i.e. ‘M + 57’ complex ions. Electrophilic aromatic substitution of a proton at an ortho-position of neutral C6H5CH2OH with [t-C4H9]+ and, alternatively, nucleophilic substitution of H2O at the benzylic carbon in \documentclass{article}\pagestyle{empty}\begin{document}$ {\rm C_6 H_5 CH_2}\mathop {\rm O}\limits^+ {\rm H}_2 $\end{document} with CH2?C (CH3)2 are discussed as possible pathways. Evidence in favor of the latter is derived from the analysis of C6D5CH2OH and C6H5CD2OH for the origin of the H-atoms lost in H2O. The inferred ion structure of m/z 147 is verified by mass-analyzed ion kinetic energy (MIKE.) measurements of its collision-activated (CA.) decomposition. MIKE./CA. spectra of mass-selected m/z 147 ions, once generated by (CI(i-C4H10) from benzyl alcohol and, once, from 2-methyl-4-phenyl-2-butanol match closely and, thus, reflect identical ion structures. With reference to the simple genesis of this ion from the latter precursor, the structure in question can be concluded to be \documentclass{article}\pagestyle{empty}\begin{document}$ {\rm C_6 H_5 CH_2 CH_2}\mathop {\rm C}\limits^+ ({\rm CH}_3)_2 $\end{document} . 相似文献
9.
ELISABETH TAFFIN DE GIVENCHY FREDERIC GUITTARD FREDERIC BRACON AIME CAMBON 《Liquid crystals》2013,40(9):1371-1377
In order to determine the influence of 'sulphur-containing' spacers on the formation of mesophases in low molecular mass compounds, we have examined the mesomorphic behaviour of molecules which consists of a 4-biphenyl unit linked to an unbranched fluorinated chain via a short spacer including at least a sulphur atom. The synthesis of these compounds has been carried out from 2-F-butylethyl iodide or from the 2-F-alkylethyl mercaptans in the case of the F-hexyl and F-octyl tails. The mesomorphic properties have been characterized by polarized light microscopy and by differential thermal analysis showing the peculiar contribution of each of the spacers. The influence of the fluorinated chain and the shape of the connector on the stability of the mesophases has been investigated. The compounds with a thioether or a hemithioacetal spacer showed no mesomorphic properties, whereas the structures with a thioester spacer showed a very interesting enantiotropic behaviour of the smectic A type over a wide temperature range. Furthermore from the series exhibiting liquid crystalline behaviour, increasing the number of fluoromethylene units simultaneously increases both the melting and the clearing temperature. These mesomorphic properties within the fluorinated series are compared with those of their monocatenar hydrocarbon homologues. 相似文献
10.
We attempt to resolve the discrepancy between the Buckingham—Longuet-Higgins (BLH) and Imrie—Raab (IR) theories of linear birefringence induced in a gas of dipolar molecules by an electric field gradient. To this end we present a new calculation of the effect, based on forward scattering of a light beam incident on a thin lamina of gas molecules. We work to electric quadrupole—magnetic dipole order. The beam undergoes a time delay which is proportional to the thickness of the lamina and the electric field gradient, and can therefore be interpreted in terms of a contribution to the refractive index of the gas due to the field gradient. The birefringence is obtained by considering appropriate polarizations of the incident beam. To avoid the occurrence of a divergent quantity, such as appears in the BLH theory, it is essential to take account of the finite beam width. Calculations are performed using both primitive (traced) and traceless molecular quadrupole moments; as required on basic grounds, these results are equivalent. They are also identical to the BLH result. By contrast, the IR result is physically unacceptable because it is not invariant with respect to the use of traced and traceless moments. The source of error in the IR theory remains unclear. 相似文献