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排序方式: 共有151条查询结果,搜索用时 15 毫秒
1.
The carbobenzyloxy (cbz) protecting group is evaluated for it's potential to enhance the resolution of chiral amine enantiomers using high-performance liquid chromatography (HPLC) and supercritical fluid chromatography (SFC). A series of cbz derivatives of commercially available racemates was prepared and analyzed by enantioselective chromatography using a variety of mobile phases and polysaccharide and Pirkle-type chiral stationary phases (CSPs). The cbz-derivatized product consistently demonstrated enhanced chiral resolution under HPLC and SFC conditions. Improved selectivity and resolution combined with an automated preparative HPLC or SFC system can lead to the rapid generation of highly purified enantiomers of desirable starting materials, intermediates or final products. 相似文献
2.
Jayaram B McConnell K Dixit SB Das A Beveridge DL 《Journal of computational chemistry》2002,23(1):1-14
Noncovalent association of proteins to specific target sites on DNA--a process central to gene expression and regulation--has thus far proven to be idiosyncratic and elusive to generalizations on the nature of the driving forces. The spate of structural information on protein--DNA complexes sets the stage for theoretical investigations on the molecular thermodynamics of binding aimed at identifying forces responsible for specific macromolecular recognition. Computation of absolute binding free energies for systems of this complexity transiting from structural information is a stupendous task. Adopting some recent progresses in treating atomic level interactions in proteins and nucleic acids including solvent and salt effects, we have put together an energy component methodology cast in a phenomenological mode and amenable to systematic improvements and developed a computational first atlas of the free energy contributors to binding in approximately 40 protein-DNA complexes representing a variety of structural motifs and functions. Illustrating vividly the compensatory nature of the free energy components contributing to the energetics of recognition for attaining optimal binding, our results highlight unambiguously the roles played by packing, electrostatics including hydrogen bonds, ion and water release (cavitation) in protein-DNA binding. Cavitation and van der Waals contributions without exception favor complexation. The electrostatics is marginally unfavorable in a consensus view. Basic residues on the protein contribute favorably to binding despite the desolvation expense. The electrostatics arising from the acidic and neutral residues proves unfavorable to binding. An enveloping mode of binding to short stretches of DNA makes for a strong unfavorable net electrostatics but a highly favorable van der Waals and cavitation contribution. Thus, noncovalent protein-DNA association is a system-specific fine balancing act of these diverse competing forces. With the advances in computational methods as applied to macromolecular recognition, the challenge now seems to be to correlate the differential (initial vs. final) energetics to substituent effects in drug design and to move from affinity to specificity. 相似文献
3.
H. W. Coover Richard L. McConnell F. B. Joyner D. F. Slonaker R. L. Combs 《Journal of polymer science. Part A, Polymer chemistry》1966,4(10):2563-2582
Copolymerization of propylene and 1-butene with highly stereospecific three-component coordination catalysts produced multiblock crystalline copolymers having stereo-regular sequences of both propylene and 1-butene. Copolymers containing from 3 to about 80% 1-butene had two DTA melting points which were attributable to polypropylene and poly-1-butene crystallinity. Those containing from 18 to about 70% 1-butene had x-ray diffraction patterns showing peaks characteristic of polypropylene and form I poly-1-butene, but form II poly-1-butene crystallinity was never observed. The multiblock copolymer structure observed is also supported by the fact that the product of the reactivity ratios is greater than unity. The composition distributions of low-conversion and continuously prepared copolymers were similar and relatively broad. For example, copolymers containing an average of 12% 1-butene had species containing from 5–30% 1-butene. High-conversion copolymers had an even broader composition distribution due to the gradual increase of the 1-butene concentration in the comonomer mixture as the copolymerization proceeded. The absence of homopolymers was demonstrated by fractionation. The ability to detect homopolymers was proved by the fact that a mixture of stereoregular polypropylene and poly-1-butene were readily separated. Increasing the amount of 1-butene tended to decrease those properties dependent upon crystallinity such as hardness, tensile strength, stiffness, density, and melting point, but tended to improve significantly the impact strength, low temperature properties, and clarity of molded objects. These duocrystalline copolymers retained a much higher level of properties than that observed for random copolymers prepared with less stereospecific coordination catalysts. 相似文献
4.
Terry R. McConnell 《Probability Theory and Related Fields》1984,67(2):213-233
Summary We study the exit time T of a sum of independent, identically distributed random vectors, X
1, X
2, ..., from a subset R of N dimensional Euclidean space when N2. We assume that R is invariant under positive dilations and that the boundary of R satisfies certain regularity conditions. The random vector X
1 is to have mean zero and a nonsingular covariance matrix. We show that there is a critical exponent, e, independent of X
1, X
2, ..., such that 0<pET
p/2<. In addition, if X
1 is bounded and slightly more restrictive assumptions are imposed on R, then p>e implies ET
p/2=.This paper constitutes a portion of the author's Ph.D. dissertation written at the University of Illinois 相似文献
5.
Petrovic AG Polavarapu PL Drabowicz J Zhang Y McConnell OJ Duddeck H 《Chemistry (Weinheim an der Bergstrasse, Germany)》2005,11(14):4257-4262
The enantiomers of 3,3,3',3'-tetramethyl-1,1'-spirobi[3 H,2,1]benzoxaselenole have been separated on a chiral preparative chromatographic column. The experimental vibrational circular dichroism (VCD) spectra have been obtained for both enantiomers in CH(2)Cl(2). The theoretical VCD spectra have been obtained by means of density functional theoretical calculations with the B3 LYP density functional. From a comparison of experimental and theoretical VCD spectra, the absolute configuration of an enantiomer with positive specific rotation in CH(2)Cl(2) at 589 nm is determined to be R. This conclusion has been verified by comparing results of experimental optical rotatory dispersion (ORD) and electronic circular dichroism (ECD) to predictions of the same properties using the B3 LYP functional for the title compound. 相似文献
6.
Cai Y Kingery D McConnell O Bach AC 《Rapid communications in mass spectrometry : RCM》2005,19(12):1717-1724
The performance of the atmospheric pressure photoionization (APPI) technique was evaluated against five sets of standards and drug-like compounds and compared to atmospheric pressure chemical ionization (APCI) and electrospray ionization (ESI). The APPI technique was first used to analyze a set of 86 drug standards with diverse structures and polarities with a 100% detection rate. More detailed studies were then performed for another three sets of both drug standards and proprietary drug candidates. All 60 test compounds in these three sets were detected by APPI with an overall higher ionization efficiency than either APCI or ESI. Most of the non-polar compounds in these three sets were not ionized by APCI or ESI. Analysis of a final set of 201 Wyeth proprietary drug candidates by APPI, APCI and ESI provided an additional comparison of the ionization techniques. The detection rates in positive ion mode were 94% for APPI, 84% for APCI, and 84% for ESI. Combining positive and negative ion mode detection, APPI detected 98% of the compounds, while APCI and ESI detected 91%, respectively. This analysis shows that APPI is a valuable tool for day-to-day usage in a pharmaceutical company setting because it is able to successfully ionize more compounds, with greater structural diversity, than the other two ionization techniques. Consequently, APPI could be considered a more universal ionization method, and therefore has great potential in high-throughput drug discovery especially for open access liquid chromatography/mass spectrometry (LC/MS) applications. 相似文献
7.
We give an overview of some remarkable connections between symmetric informationally complete measurements (SIC-POVMs, or SICs) and algebraic number theory, in particular, a connection with Hilbert’s 12th problem. The paper is meant to be intelligible to a physicist who has no prior knowledge of either Galois theory or algebraic number theory. 相似文献
8.
9.
Karny M Grzywacz RK Batchelder JC Bingham CR Gross CJ Hagino K Hamilton JH Janas Z Kulp WD McConnell JW Momayezi M Piechaczek A Rykaczewski KP Semmes PA Tantawy MN Winger JA Yu CH Zganjar EF 《Physical review letters》2003,90(1):012502
Fine structure in proton emission from the 3.1(3) mus activity of 145Tm was discovered by using a novel technique of digital processing of overlapping recoil implantation and decay signals. Proton transitions to the ground state of 144Er and to its first excited 2(+) state at 0.33(1) MeV with a branching ratio I(p)(2(+))=9.6+/-1.5% were observed. The structure of the 145Tm wave function and the emission process were analyzed by using particle-core vibration coupling models. 相似文献
10.
We report a robust and reliable platform source for visible-wavelength multiphoton microscopy that is based on nonlinear optical methods. We demonstrate a synchronously pumped, singly resonant optical parametric oscillator with simultaneous intracavity third-order quasi-phase matching in a single crystal that generates continuously tunable, visible, and femtosecond-pulsed radiation. The application of the system is demonstrated by two-photon laser-scanning fluorescence microscopy of rabbit cardiac myocytes loaded with the fluorescent Ca2+ indicator fura-2. 相似文献