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381.
Rafael Rodríguez Esteban Suárez-Picado Emilio Quiñoá Ricardo Riguera Félix Freire 《Angewandte Chemie (Weinheim an der Bergstrasse, Germany)》2020,132(22):8694-8700
Herein, macromolecular gears composed of helical poly(phenylacetylenes) (PPAs) bearing short oligopeptides as pendant groups are described, in which the two structural motifs (framework and substituents) are combined. These gears are obtained by polymerization of the acetylene groups introduced at the C-terminus of short oligopeptides formed by achiral (Aib)n units (n=1–3) derivatized at the N-terminus by a single enantiomer (R or S) of α-methoxy-α-trifluoromethylphenylacetic acid (MTPA, Mosher's reagent). The chiral information of the MTPA is transmitted to the achiral Aib fragments and, through either chiral tele-induction and/or chiral harvesting mechanisms, is further transferred to the polyene backbones, which adopt preferentially P or M helical senses. Moreover, these materials also show dynamic behavior and respond to the action of external stimuli by either inverting the P/M sense and/or modifying the elongation in fully reversible processes. 相似文献
382.
Keyller Bastos Borges Ellen Figueiredo Freire Maria Elisa Pereira Bastos de Siqueira 《Talanta》2009,78(1):233-4556
A method for simultaneous determination of seven benzodiazepines (BZPs) (flunitrazepam, clonazepam, oxazepam, lorazepam, chlordiazepoxide, nordiazepam and diazepam using N-desalkylflurazepam as internal standard) in human plasma using liquid-liquid and solid-phase extractions followed by high-performance liquid chromatography (HPLC) is described. The analytes were separated employing a LC-18 DB column (250 mm × 4.6 mm, 5 μm) at 35 °C under isocratic conditions using 5 mM KH2PO4 buffer solution pH 6.0:methanol:diethyl ether (55:40:5, v/v/v) as mobile phase at a flow rate of 0.8 mL min−1. UV detection was carried out at 245 nm. Employing LLE, the best conditions were achieved with double extraction of 0.5 mL plasma using ethyl acetate and Na2HPO4 pH 9.5 for pH adjusting. Employing SPE, the best conditions were achieved with 0.5 mL plasma plus 3 mL 0.1 M borate buffer pH 9.5, which were then passed through a C18 cartridge previously conditioned, washed for 3 times with these solvents: 3 mL 0.1 M borate buffer pH 9.5, 4 mL Milli-Q water and 1 mL acetonitrile 5%, finally the BZPs elution was carried with diethyl ether:n-hexane:methanol (50:30:20). In both methods the solvent was evaporated at 40 °C under nitrogen flow. The validation parameters obtained in LLE were linearity range of 50-1200 ng mL−1 plasma (r ≥ 0.9927), limits of quantification of 50 ng mL−1 plasma, within-day and between-day CV% and E% for precision and accuracy lower than 15%, and recovery above 65% for all BZPs. In SPE, the parameter obtained were linearity range of 30-1200 ng mL−1 plasma (r ≥ 0.9900), limits of quantification of 30 ng mL−1 plasma, within-day and between-day CV% and E% for precision and accuracy lower than 15% and recovery above 55% for all BZPs. These extracting procedures followed by HPLC analysis showed their suitable applicability in order to examine one or more BZPs in human plasma. Moreover, it could be suggested that these procedures might be employed in various analytical applications, in special for toxicological/forensic analysis. 相似文献
383.
J. A. Lima P. T. C. Freire F. E. A. Melo J. Mendes Filho G. P. De Sousa R. J. C. Lima P. F. Faanha Filho H. N. Bordallo 《Journal of Raman spectroscopy : JRS》2010,41(7):808-813
Raman spectra of DL ‐Alanine crystals were investigated in the 50–3200 cm−1 spectral region for temperatures ranging from 15 to 295 K. The crystalline structure of DL ‐Alanine represents a rare example of an amino acid racemate crystallizing in a non‐centrosymmetric space group. From this study, we have observed changes in the wavenumber of modes associated with both rocking of CO2− and skeletal vibrations. On the other hand, neither changes in the modes associated to CH or CH3 vibrations nor substantial modifications of the lattice modes of the crystal were observed. Such result indicates slight changes of the CO2 group orientation without observation of a solid–solid phase transition in the DL ‐Alanine crystal. Copyright © 2009 John Wiley & Sons, Ltd. 相似文献
384.
G. Barbero F. C. M. Freire M. Scalerandi 《The European physical journal. E, Soft matter》2009,30(3):245-256
The transient and equilibrium behaviors of an electrolytic cell containing two groups of ions submitted to an external voltage
is considered. The analysis is performed analytically and numerically, considering the electrolytic solution as a dispersion
of ions in an insulating liquid. According to our results, the dynamic of the system corresponds to a multirelaxation problem
and the relaxation time related to a new type of free diffusion is able to capture the essential characteristics of the evolution
of the variables of the system. We also show that, contrary to what expected, the relaxation time related to the ambipolar
diffusion is not a solution of the eigenvalues equation. 相似文献
385.
386.
387.
Predicting the Helical Sense of Poly(phenylacetylene)s from their Electron Circular Dichroism Spectra
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Prof. Berta Fernández Rafael Rodríguez Prof. Antonio Rizzo Prof. Emilio Quiñoá Prof. Ricardo Riguera Prof. Félix Freire 《Angewandte Chemie (International ed. in English)》2018,57(14):3666-3670
The calculated ECD spectrum (time‐dependent density functional theory TD‐DFT) for small oligomers of polyphenylacetylenes (PPAs) show a very good match with the experimental spectra of the PPA polymers, particularly with the first Cotton band associated to the helical sense of the internal polyenic backbone. This has been proven with a series of PPAs representative of cis‐cisoidal, cis‐transoidal, compressed and stretched polyene backbones, with identical or opposite internal/external rotational senses and allows the prediction of the helical sense of the internal helix of a PPA directly from its CD spectra. 相似文献
388.
G. P. De Sousa P. T. C. Freire J. Mendes Filho F. E. A. Melo C. L. Lima 《Brazilian Journal of Physics》2013,43(3):137-144
We present a Raman spectroscopy investigation of the vibrational properties of l-histidine crystals at low temperatures. The temperature dependence of the spectra show discontinuities at 165 K, which we identify with modifications in the bonds associated to both the NH3 + and CO2 ? motifs indicative of a conformational phase transition that changes the intermolecular bonds. Additional evidence of such a phase transition is provided by differential scanning calorimetry measurements, which identified an enthalpic anomaly at ~165 K. 相似文献
389.
Peter Arndt Rodrigo de Alvarenga Freire Odilon Otavio Luciano Hugo Luiz Mariano 《Logica Universalis》2007,1(1):3-39
We explore the possibility and some potential payoffs of using the theory of accessible categories in the study of categories
of logics. We illustrate this by two case studies focusing on the category of finitary structural logics and its subcategory
of algebraizable logics.
Mathematics Subject Classification (2000): Primary 03B22; Secondary 18C35. 相似文献
390.
In this work we study the presence of T-points, a kind of codimension-two heteroclinic loop, in a Z2-symmetric electronicoscillator. Our analysis proves that, in the parameter plane, whenthe equilibria involved are saddle-focus,three spiraling curves of global codimension-onebifurcations emerge from this T-point, corresponding tohomoclinic of the origin, homoclinic of the nontrivialequilibria and heteroclinic between the nontrivial equilibriaconnections. Some first-order features of these three curves are also shown.The analytical results, valid for all three-dimensionalZ2-symmetric systems, are successfully checked in themodified van der Pol–Duffing electronic oscillator considered. 相似文献