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991.
We extend to ternary solutions our previous study of conformational, thermodynamic, and rheological properties of semidilute polymer solutions in good solvent. Osmotic pressure and viscosity measurements were performed in several mixtures of two compatible polymers in a common solvent. Renormalization group results were used to analyze the data, using de Gennes's blobs model to connect dynamic and conformational properties. © 2002 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 40: 1071–1079, 2002  相似文献   
992.
This communication reports, for the first time, the dependence of the SERS intensities under resonant CT conditions (SERS-CT) on the electrode potential. SERS-CT intensities have been estimated from the properties of S(0)-CT(i) transitions ranging between 200-1200 nm of selected [Ag(n)-pyridine](q) and [Ag(n)-pyrazine](q) complexes.  相似文献   
993.
Tonsil Actisil FF, a commercial bentonitic clay, promotes the formation of a series of electrophilic-aromatic-substitution products from para-methoxybenzyl acetate in carbon disulfide. The molecules obtained correspond to linear isomeric dimers, trimers, tetramers and a pentamer, according to their spectroscopic data. A clear indication of the title mechanistic pathway for the oligomerization growth was obtained from the analysis of a set of computational-chemistry calculations using the density-functional-theory level B3LYP/6-311++G(d,p). The corresponding conclusions were based on the computed dipole moments, the HOMO/LUMO distributions, and a natural-populations analysis of the studied molecules.  相似文献   
994.
The luminescent patterns of several members of the aminopyrimidine family are very different, showing not fluorescence at all, only a fluorescence band, normal or anomalous, or dual fluorescence, depending on the substituents and on the environment (gas phase vs. polar solvents). In this work, we study the lowest excited states of several members of this family that exhibit different fluorescence patterns to try to explain their photochemistry and to understand the effect of the substituents and the environment. We have found that several excited states (local excited (LE), charge transfer (CT) and n N?C??* states) have minima on the lowest excited potential energy surface (S1), being their relative energy the determinant factor of the luminescent behavior. If the more stable S1 minima are of n N?C??* character, a non-radiative deexcitation channel is the most efficient and the system shows no fluorescence. If the CT and/or LE states are the most stable, the non-radiative deactivation channel is not accessible and the system fluoresces. The relative energies of the CT and LE minima (affected by substituents and by the presence of a polar solvent) and the different magnitude of the oscillator strength for the radiative transition to the ground state determine which emission is more efficient, giving place to normal, anomalous or dual fluorescence. The study has been carried out by CASSCF/CASPT2 computations, including the solvent effect by means of the PCM model.  相似文献   
995.
Protein stability is a major bottleneck in the biotechnological application of ionic liquid-containing solvents, either in the frame of biocatalysis or protein storage. Herein, differential scanning fluorimetry was successfully implemented as a high throughput method to fast scan the impact of a number of cholinium-based ionic liquids on the stability of proteins.  相似文献   
996.
The chemistry in low pressure (0.8-8 Pa) plasmas of H(2) + 10% N(2) mixtures has been experimentally investigated in a hollow cathode dc reactor using electrical probes for the estimation of electron temperatures and densities, and mass spectrometry to determine the concentration of ions and stable neutral species. The analysis of the measurements by means of a kinetic model has allowed the identification of the main physicochemical mechanisms responsible for the observed distributions of neutrals and ions and for their evolution with discharge pressure. The chemistry of neutral species is dominated by the formation of appreciable amounts of NH(3) at the metallic walls of the reactor through the successive hydrogenation of atomic nitrogen and nitrogen containing radicals. Both Eley-Rideal and Langmuir-Hinshelwood mechanisms are needed in the chain of hydrogenation steps in order to account satisfactorily for the observed ammonia concentrations, which, in the steady state, are found to reach values ~30-70% of those of N(2). The ionic composition of the plasma, which is entirely due to gas-phase processes, is the result of a competition between direct electron impact dissociation, more relevant for high electron temperatures (lower pressures), and ion-molecule chemistry that prevails for the lower electron temperatures (higher pressures). At the lowest pressure, products from the protonation of the precursor molecules (H(3)(+), N(2)H(+) and NH(4)(+)) and others from direct ionization (H(2)(+) and NH(3)(+)) are found in comparable amounts. At the higher pressures, the ionic distribution is largely dominated by ammonium. It is found that collisions of H(3)(+), NH(3)(+) and N(2)H(+) with the minor neutral component NH(3) are to a great extent responsible for the final prevalence of NH(4)(+).  相似文献   
997.
In order to explore parity violating effects in chiral molecules, of interest in some models of evolution towards homochirality, quantum stochastic resonance (QSR) is studied for the population difference between the two enantiomers of a chiral molecule (hence for the optical activity of the sample), under low viscous friction and in the deep quantum regime. The molecule is described by a two-state model in an asymmetric double well potential where the asymmetry is given by the known predicted parity violating energy difference (PVED) between enantiomers. In the linear response to an external driving field that lowers and rises alternatively each one of the minima of the well, a signal of QSR is predicted only in the case that the PVED is different from zero, the resonance condition being independent on tunneling between the two enantiomers. It is shown that, at resonance, the fluctuations of the first order contribution to the internal energy are zero. Due to the small value of the PVED, the resonance would occur in the ultracold regime. Some proposals concerning the external driving field are suggested.  相似文献   
998.
Ng WY  Ramos A  Lam YC  Wijaya IP  Rodriguez I 《Lab on a chip》2011,11(24):4241-4247
This paper studies the principles of fluid flow manipulation based on DC-biased AC-electrokinetics. This method makes use of planar parallel electrodes in a microfluidic channel in contact with an electrolyte solution, with a DC biased AC electrical signal applied to the electrode pair. Due to the application of DC bias, incipient Faradaic electrolytic reactions take place resulting in an increase of the ionic content of the bulk solution. The ionic content was found to be dissimilar at the cathodic and anodic sides of the channel and a conductivity difference of approximately 10% was measured for 2 V(DC). Fluid flow is generated by the action of the DC biased AC electric signal acting on the transverse conductivity gradient generated across the microchannel. The induced flow in the form of vortex was characterized experimentally and the results substantiated theoretically. The velocity of the induced flow vortex under the employed experimental conditions was ~600 to 700 μm s(-1) which is faster than those obtained in conventional AC-electroosmosis and AC-electrothermal types of flows.  相似文献   
999.
Cancer chemotherapy has been used since the early 1950s and still remains one the major therapeutic options for many malignant tumours. A major obstacle to successful cancer chemotherapy is drug resistance. Frequently resistance is intrinsic to the cancer, but as therapy becomes more effective, acquired resistance has also become more frequent. One form of resistance, named multidrug resistance (MDR), is responsible for the failure of tumours to respond to a wide spectrum of chemotherapeutic agents. The in vivo monitoring of MDR could assist in the selection of patients for therapy and can avoid ineffective and potentially toxic treatments. Therefore, methods for functionally interrogating MDR transport activity have been sought, namely single photon emission computed tomography (SPECT) and positron emission tomography (PET). Cationic radiotracers originally developed as SPECT myocardial imaging agents, such as [(99m)Tc(MIBI)(6)](+) and [(99m)Tc(tetrofosmin)(2)O(2)](+), are used for both early cancer detection and non-invasive monitoring of the tumour MDR transport function. With the ultimate goal of obtaining better performing radioprobes for MDR imaging, other metal-based complexes and/or small molecules have also been synthesized and biologically evaluated. In this perspective we will report on the chemical efforts made to find metalloprobes for in vivo monitoring of MDR by nuclear imaging techniques. The current knowledge on the biological mechanisms and proteins involved in tumour MDR will be also briefly presented, as its understanding is invaluable for the rational design and biological evaluation of new radioprobes.  相似文献   
1000.
Hydrogen abstraction from the C-7 position of cholesterol (Ch) by triplet excited benzophenone (BZP) exhibits remarkable solvent-dependence in product studies. Kinetic measurements on the intramolecular version of the process in dyads containing covalently linked Ch and BZP units reveal important solvent effects and significant stereodifferentiation.  相似文献   
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