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51.
In this paper, a mixture representation for the joint distribution function of progressively Type-II censored order statistics from heterogeneous distributions is established. Applications of this representation to stochastic orderings and inequalities are then illustrated. 相似文献
52.
Raphael F. Ribeiro Aleksandr V. Marenich Christopher J. Cramer Donald G. Truhlar 《Journal of computer-aided molecular design》2010,24(4):317-333
We applied the solvation models SM8, SM8AD, and SMD in combination with the Minnesota M06-2X density functional to predict
vacuum-water transfer free energies (Task 1) and tautomeric ratios in aqueous solution (Task 2) for the SAMPL2 test set. The
bulk-electrostatic contribution to the free energy of solvation is treated as follows: SM8 employs the generalized Born model
with the Coulomb field approximation, SM8AD employs the generalized Born approximation with asymmetric descreening, and SMD
solves the nonhomogeneous Poisson equation. The non-bulk-electrostatic contribution arising from short-range interactions
between the solute and solvent molecules in the first solvation shell is treated as a sum of terms that are products of geometry-dependent
atomic surface tensions and solvent-accessible surface areas of the individual atoms of the solute. On average, three models
tested in the present work perform similarly. In particular, we achieved mean unsigned errors of 1.3 (SM8), 2.0 (SM8AD), and
2.6 kcal/mol (SMD) for the aqueous free energies of 30 out of 31 compounds with known reference data involved in Task 1 and
mean unsigned errors of 2.7 (SM8), 1.8 (SM8AD), and 2.4 kcal/mol (SMD) in the free energy differences (tautomeric ratios)
for 21 tautomeric pairs in aqueous solution involved in Task 2. 相似文献
53.
Donoghue PJ Gupta AK Boyce DW Cramer CJ Tolman WB 《Journal of the American Chemical Society》2010,132(45):15869-15871
Insight into copper-oxygen species proposed as intermediates in oxidation catalysis is provided by the identification of a Cu(II)-superoxide complex supported by a sterically hindered, pyridinedicarboxamide ligand. A tetragonal, end-on superoxide structure is proposed based on DFT calculations and UV-vis, NMR, EPR, and resonance Raman spectroscopy. The complex yields a trans-1,2-peroxodicopper(II) species upon reaction with [(tmpa)Cu(CH(3)CN)]OTf and, unlike other known Cu(II)-superoxide complexes, acts as a base rather than an electrophilic (H-atom abstracting) reagent in reactions with phenols. 相似文献
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56.
Tobias Seiser Nicolai Cramer Dr. 《Chemistry (Weinheim an der Bergstrasse, Germany)》2010,16(11):3383-3391
The activation of carbon–carbon σ bonds is a complementary method to access uncommon and difficult‐to‐prepare organometallic species. Herein, we describe the activation of tert‐cyclobutanols through an enantioselective insertion of a chiral rhodium(I) complex into the C? C σ bond of the cyclobutane, forming a quaternary stereogenic center and an alkyl‐rhodium functionality that initiates ring‐closure reactions. This technology provides access to a variety of substituted cyclohexane derivatives with quaternary stereogenic centers. The formation of different product families can be controlled by the employed set of reaction conditions and additives. In general, high yields and excellent enantioselectivities of up to 99 % ee are obtained. 相似文献
57.
Urea is widely employed to facilitate protein separations in ion exchange chromatography at various scales. In this work, five model proteins were used to examine the chromatographic effects of protein conformational changes induced by urea in ion exchange chromatography. Linear gradient experiments were carried out at various urea concentrations and the protein secondary and tertiary structures were evaluated by far UV CD and fluorescence measurements, respectively. The results indicated that chromatographic retention times were well correlated with structural changes and that they were more sensitive to tertiary structural change. Steric Mass Action (SMA) isotherm parameters were also examined and the results indicated that urea induced protein conformational changes could affect both the characteristic charge and equilibrium constants in these systems. Dynamic light scattering analysis of changes in protein size due to urea-induced unfolding indicated that the size of the protein was not correlated with SMA parameter changes. These results indicate that while urea-induced structural changes can have a marked effect on protein chromatographic behavior in IEX, this behavior can be quite complicated and protein specific. These differences in protein behavior may provide insight into how these partially unfolded proteins are interacting with the resin material. 相似文献
58.
We analyze the scaling properties of the ac conductivity spectra of ion-conducting polyelectrolyte complexes of different compositions. Spectra were taken at ambient temperature but at different relative humidities. For the first time, we report on a scaling principle for conductivity spectra termed "time-humidity-superposition principle" in analogy with the well-known time-temperature-superposition principle. This model-free scaling holds for different materials over several decades in frequency. It implies that the hydration is activating ion motion over short and long distances in a similar general way, a concept so far only established for thermal energy. 相似文献
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