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排序方式: 共有177条查询结果,搜索用时 46 毫秒
21.
22.
Pellett J Lukulay P Mao Y Bowen W Reed R Ma M Munger RC Dolan JW Wrisley L Medwid K Toltl NP Chan CC Skibic M Biswas K Wells KA Snyder LR 《Journal of chromatography. A》2006,1101(1-2):122-135
A general procedure is proposed for the rapid development of a reversed-phase liquid chromatographic (RP-LC) separation that is "orthogonal" to a pre-existing ("primary") method for the RP-LC separation of a given sample. The procedure involves a change of the mobile-phase organic solvent (B-solvent), the replacement of the primary column by one of very different selectivity, and (only if necessary) a change in mobile phase pH or the use of a third column. Following the selection of the "orthogonal" B-solvent, column and mobile phase pH, further optimization of peak spacing and resolution can be achieved by varying separation temperature and either isocratic %B or gradient time. The relative "orthogonality" of the primary and "orthogonal" RP-LC methods is then evaluated from plots of retention for one method versus the other. The present procedure was used to develop "orthogonal" methods for nine routine RP-LC methods from six pharmaceutical analysis laboratories. The relative success of this approach can be judged from the results reported here. 相似文献
23.
Nelson BN Schieber LJ Barich DH Lubach JW Offerdahl TJ Lewis DH Heinrich JP Munson EJ 《Solid state nuclear magnetic resonance》2006,29(1-3):204-213
Solid-state NMR spectroscopy (SSNMR) is an extremely powerful technique for the analysis of pharmaceutical dosage forms. A major limitation of SSNMR is the number of samples that can be analyzed in a given period of time. A solid-state magic-angle spinning (MAS) probe that can simultaneously acquire up to seven SSNMR spectra is being developed to increase throughput/signal-to-noise ratios. A prototype probe incorporating two MAS modules has been developed and spectra of ibuprofen and aspirin have been acquired simultaneously. This version is limited to being a two-module probe due to large amounts of space required for the tuning elements located next to the MAS modules. A new probe design incorporating coaxial transmission lines and smaller MAS modules has been constructed. This probe allows for close proximity of the MAS modules (within 3 cm), adequate proton decoupling power (>50 kHz), and the capability of remote tuning and sample changing. Spectra of hexamethylbenzene (HMB) have been acquired and show signal-to-noise ratios comparable to existing SSNMR probes. Adamantane line widths are also comparable to conventional probe technology. Decoupling powers of 70 kHz have been achieved using a MAS module suitable for 3 cm spacing between modules. Remote tuning has also been achieved with this new coaxial transmission line design. This probe design can be easily scaled to incorporate multiple MAS modules, which is a limitation of the previous design. The number of modules that can be incorporated is only limited by the number of transmission lines that will fit in a cross-sectional diameter of the bore and the axial field length of the magnet. 相似文献
24.
Chen D Heberling MS Nakata M Hough LE Maclennan JE Glaser MA Korblova E Walba DM Watanabe J Clark NA 《Chemphyschem》2012,13(1):155-159
The B4 liquid crystal phase of bent-core molecules, a smectic phase of helical nanofilaments, is one of the most complex hierarchical self-assemblies in soft materials. We describe the layer topology of the B4 phase of mesogens in the P-n-OPIMB homologous series near the liquid crystal/glass interface. Freeze-fracture transmission electron microscopy reveals that the twisted layer structure of the bulk is suppressed, the layers instead forming a structure with periodic layer undulations, with the topography depending on the distance from the glass. The surface layer structure is modeled as parabolic focal conic arrays generated by equidistant parabolas whose foci are defect lines along the glass surface. Nucleation and growth of toric focal conics near the glass substrate is also observed. Although the growth of twisted nanofilaments, the usual manifestation of structural chirality in the B4 phase, is suppressed near the surface, the smectic layers are intrinsically chiral, and the helical filaments that form on top of them grow with specific handedness. 相似文献
25.
Filippo Favretto Jeremy D. Baker Timo Strohker Loren B. Andreas Laura J. Blair Stefan Becker Markus Zweckstetter 《Angewandte Chemie (International ed. in English)》2020,59(14):5643-5646
Peptidylprolyl isomerases (PPIases) catalyze cis/trans isomerization of prolines. The PPIase CypA colocalizes with the Parkinson's disease (PD)‐associated protein α‐synuclein in cells and interacts with α‐synuclein oligomers. Herein, we describe atomic insights into the molecular details of the α‐synuclein/CypA interaction. NMR spectroscopy shows that CypA catalyzes isomerization of proline 128 in the C‐terminal domain of α‐synuclein. Strikingly, we reveal a second CypA‐binding site formed by the hydrophobic sequence 47GVVHGVATVA56, termed PreNAC. The 1.38 Å crystal structure of the CypA/PreNAC complex displays a contact between alanine 53 of α‐synuclein and glutamine 111 in the catalytic pocket of CypA. Mutation of alanine 53 to glutamate, as found in patients with early‐onset PD, weakens the interaction of α‐synuclein with CypA. Our study provides high‐resolution insights into the structure of the PD‐associated protein α‐synuclein in complex with the most abundant cellular cyclophilin. 相似文献
26.
27.
Calorimetric measurements of the enthalpy of reaction of WO3(c) with excess OH?(aq) have been made at 85°C. Similar measurements have been made with MoO3(c) at both 85 and 25°C, to permit estimation of ΔH°=?13.4 kcal mol?1 for the reaction WO3(c)+2OH?(aq)=WO2?4(aq)+H2O(liq) at 25°C. Combination of this ΔH° with ΔH°f for WO3(c) leads to ΔH°f=?256.5 kcal mol?1 for WO2?4(aq). We also obtain ΔH°f=?269.5 kcal mol?1 for H2WO4(c). Both of these values are discussed in relation to several earlier investigations. 相似文献
28.
Prem Paul Singh Keith G. McCurdy Earl M. Woolley Loren G. Hepler 《Journal of solution chemistry》1977,6(5):327-330
We have used a Picker flow calorimeter for measurements leading to apparent molal heat capacities of dilute aqueous solutions of HClO4 and NaClO4 at 298°K. Results have been used to derive
c
°
=–27.1J-°K
–1-mole
–1 for HClO4 (aq.),
c
°
=15.2J-°K
–1-mole
–1 for NaClO4 (aq.), and C
p=–213.8J-°K–1-mole–1 for ionization of water. 相似文献
29.
Vadim A. Medvedev Michael E. Efimov Paul J. Cerutti Robert M. McKay Lawrence H. Johnson Loren G. Hepler 《Thermochimica Acta》1978,23(1):87-92
We have made calorimetric measurements of the enthalpy of solution of NH4NO3(c, IV) in water at 298 K, where (c, IV) indicates the crystal form of amomonium nitrate that is stable from 256 to 305 K. Results of our measurements have been combined with enthalpy of dilution values from Parker to obtain the standard enthalpy of solution of NH4NO3 (c, IV) in water at 298.15 K to be ΔHo = 25.41 kJ mol?1. 相似文献
30.
Dioxetanone, a key component of the bioluminescence of firefly luciferin, is itself a chemiluminescent molecule due to two conical intersections on its decomposition reaction surface. While recent calculations of firefly luciferin have employed four electrons in four active orbitals [(4,4)] for the dioxetanone moiety, a study of dioxetanone [F. Liu et al., J. Am. Chem. Soc. 131, 6181 (2009)] indicates that a much larger active space is required. Using a variational calculation of the two-electron reduced-density-matrix (2-RDM) [D. A. Mazziotti, Acc. Chem. Res. 39, 207 (2006)], we present the ground-state potential energy surface as a function of active spaces from (4,4) to (20,17) to determine the number of molecular orbitals required for a correct treatment of the strong electron correlation near the conical intersections. Because the 2-RDM method replaces exponentially scaling diagonalizations with polynomially scaling semidefinite optimizations, we readily computed large (18,15) and (20,17) active spaces that are inaccessible to traditional wave function methods. Convergence of the electron correlation with active-space size was measured with complementary RDM-based metrics, the von Neumann entropy of the one-electron RDM as well as the Frobenius and infinity norms of the cumulant 2-RDM. Results show that the electron correlation is not correctly described until the (14,12) active space with small variations present through the (20,17) space. Specifically, for active spaces smaller than (14,12), we demonstrate that at the first conical intersection, the electron in the σ(?) orbital of the oxygen-oxygen bond is substantially undercorrelated with the electron of the σ orbital and overcorrelated with the electron of the carbonyl oxygen's p orbital. Based on these results, we estimate that in contrast to previous treatments, an accurate calculation of the strong electron correlation in firefly luciferin requires an active space of 28 electrons in 25 orbitals, beyond the capacity of traditional multireference wave function methods. 相似文献