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681.
We use a new method of studying the Hurst exponent with time and scale dependency. This new approach allows us to recover the major events affecting worldwide markets (such as the September 11th terrorist attack) and analyze the way those effects propagate through the different scales. The time-scale dependence of the referred measures demonstrates the relevance of entropy measures in distinguishing the several characteristics of market indices: “effects” include early awareness, patterns of evolution as well as comparative behaviour distinctions in emergent/established markets. 相似文献
682.
Nelson DA Donaldson GS Kreft H 《The Journal of the Acoustical Society of America》2008,123(3):1522-1543
Forward-masked psychophysical spatial tuning curves (fmSTCs) were measured in twelve cochlear-implant subjects, six using bipolar stimulation (Nucleus devices) and six using monopolar stimulation (Clarion devices). fmSTCs were measured at several probe levels on a middle electrode using a fixed-level probe stimulus and variable-level maskers. The average fmSTC slopes obtained in subjects using bipolar stimulation (3.7 dBmm) were approximately three times steeper than average slopes obtained in subjects using monopolar stimulation (1.2 dBmm). Average spatial bandwidths were about half as wide for subjects with bipolar stimulation (2.6 mm) than for subjects with monopolar stimulation (4.6 mm). None of the tuning curve characteristics changed significantly with probe level. fmSTCs replotted in terms of acoustic frequency, using Greenwood's [J. Acoust. Soc. Am. 33, 1344-1356 (1961)] frequency-to-place equation, were compared with forward-masked psychophysical tuning curves obtained previously from normal-hearing and hearing-impaired acoustic listeners. The average tuning characteristics of fmSTCs in electric hearing were similar to the broad tuning observed in normal-hearing and hearing-impaired acoustic listeners at high stimulus levels. This suggests that spatial tuning is not the primary factor limiting speech perception in many cochlear implant users. 相似文献
683.
The acoustical cues for sound location are generated by spatial- and frequency-dependent filtering of propagating sound waves by the head and external ears. Although rats have been a common model system for anatomy, physiology, and psychophysics of localization, there have been few studies of the acoustical cues available to rats. Here, directional transfer functions (DTFs), the directional components of the head-related transfer functions, were measured in six adult rats. The cues to location were computed from the DTFs. In the frontal hemisphere, spectral notches were present for frequencies from approximately 16 to 30 kHz; in general, the frequency corresponding to the notch increased with increases in source elevation and in azimuth toward the ipsilateral ear. The maximum high-frequency envelope-based interaural time differences (ITDs) were 130 mus, whereas low-frequency (<3.5 kHz) fine-structure ITDs were 160 mus; both types of ITDs were larger than predicted from spherical head models. Interaural level differences (ILDs) strongly depended on location and frequency. Maximum ILDs were <10 dB for frequencies <8 kHz and were as large as 20-40 dB for frequencies >20 kHz. Removal of the pinna eliminated the spectral notches, reduced the acoustic gain and ILDs, altered the acoustical axis, and reduced the ITDs. 相似文献
684.
To elucidate the reasons underlying the poor penetration of non-viral vectors in tissues, relating transport properties to
physico-chemical parameters of vectors may be crucial. These properties can be influenced by the presence of multiples labels
that are used. Therefore utilizing a vector with minimum of labels preferably not more than one is important to studying penetration
in tissues. The cell impermeant bisintercalating dye YOYO-1 was found suitable to both monitor the formation of complexes
between DNA and an amphipathic peptide LK15 and, to track their penetration in HCT116 spheroids by confocal microscopy. The
results revealed a limited decrease of fluorescence ascribed to the high affinity of YOYO-1 to bind DNA. The residual fluorescence
of complexes can be exploited to monitor penetration into spheroids, after correction for YOYO-1 attenuation, and to revealing
hyaluronidase-induced reduced binding. Hence high affinity dyes such as YOYO-1 with inefficiently quenched fluorescence may
be important to establish a relation between novel medicines characteristics and penetration in tissues. 相似文献
685.
Automation of solid-phase microextraction-gas chromatography-mass spectrometry extraction of eucalyptus volatiles 总被引:1,自引:0,他引:1
Zini CA Lord H Christensen E de AT Caramão EB Pawliszyn J 《Journal of chromatographic science》2002,40(3):140-146
Solid-phase microextraction (SPME) coupled with gas chromatography (GC)-ion-trap mass spectrometry (ITMS) is employed to analyze fragrance compounds from different species of eucalyptus trees: Eucalyptus dunnii, Eucalyptus saligna, Eucalyptus grandis, and hybrids of other species. The analyses are performed using an automated system for preincubation, extraction, injection, and analysis of samples. The autosampler used is a CombiPAL and has much flexibility for the development of SPME methods and accommodates a variety of vial sizes. For automated fragrance analysis the 10- and 20-mL vials are the most appropriate. The chromatographic separation and identification of the analytes are performed with a Varian Saturn 4D GC-ITMS using an HP-5MS capillary column. Several compounds of eucalyptus volatiles are identified, with good reproducibility for both the peak areas and retention times. Equilibrium extraction provides maximal sensitivity but requires additional consideration for the effect of carryover. Preequilibrium extraction allows good sensitivity with minimal carryover. 相似文献
686.
Dong SD Oberthür M Losey HC Anderson JW Eggert US Peczuh MW Walsh CT Kahne D 《Journal of the American Chemical Society》2002,124(31):9064-9065
Because teicoplanin and vancomycin are the last line of defense for many bacterial infections, the emergence of resistance to glycopeptide antibiotics in enterococci and streptococci has aroused concern. Despite their similarity in terms of structure and mechanism of action, vancomycin induces the expression of genes that leads to bacterial resistance, and teicoplanin does not. We have used a combination of chemical and enzymatic methods to produce sets of vancomycin and teicoplanin analogues that allow us to consider whether the aglycon, the carbohydrate, or other parts of these molecules stimulate VanB resistance. We show that the teicoplanin and vancomycin aglycons are the structural elements that lead to induction of resistance. We think that lipid-containing analogues of vancomycin, like teicoplanin itself, circumvent resistance because the lipid chain changes the periplasmic distribution of the glycopeptide and, therefore, changes the biosynthetic step that it blocks. 相似文献
687.
Laser desorption-atmospheric pressure chemical ionization-mass spectrometry (LD-APCI-MS) is presented for the atmospheric pressure (AP) sampling of tryptic peptides directly from a polyacrylamide gel. In contrast to other gel sampling mass spectrometric approaches, this technique does not require the addition of any exogenous matrices to the gel to assist with ionization. In this arrangement, a CO(2) laser at 10.6 micro m is used to desorb intact neutral peptide molecules from the gel, followed by ionization in the gas-phase with APCI. The ions are then sampled via a heated capillary inlet and transferred to a quadrupole ion trap mass spectrometer for mass analysis. Preliminary results suggest the polyacrylamide gel electrophoresis-LD-APCI-MS technique provides several advantages that could translate into a more convenient, robust methodology for the rapid identification and characterization of proteins. Finally, strategies regarding the further development of the method are presented. 相似文献
688.
Ring opening of 1-alkyl-2-methyleneaziridines 1 or 2 is accomplished with organocopper reagents (R(2)CuLi or RMgX/CuI) in the presence of boron trifluoride diethyl etherate giving 1-substituted propan-2-ones 3-9 in 42-88% yield. Ring opening with RMgCl/CuI in the absence of the Lewis acid allows further alkylation of the metalloenamine (metalated imine) intermediate in a regiocontrolled manner. The sequential formation of two new intermolecular carbon-carbon bonds in this reaction provides a rapid entry into a variety of 1,3-disubstituted propan-2-ones, including 11 and 14-23. The scope and mechanism of this multicomponent reaction (MCR) has been assessed. It is established that this MCR tolerates alkyl, aryl, and benzylic Grignard reagents and a wide range of electrophiles, including alkyl iodides, bromides, and tosylates, as well as epoxides and aldehydes. In addition, gem-dimethyl substitution on the exocyclic double bond of the 2-methyleneaziridine is tolerated. This MCR has been applied to the one-pot synthesis of (Z)-6-heneicosen-11-one, 25, an important sex attractant of the Tussock moth. Using 3-deuterio-1-(1-phenylethyl)-2-methyleneaziridine, 26, we determined that this MCR occurs predominantly by direct ring opening at the sp(3)-hybridized aziridine carbon atom (C-3). 相似文献
689.
We report the existence of a novel C48N12 molecular structure to the recently reported thin-film formation of nano-onions of carbon and nitrogen with similar composition [Phys. Rev. Lett. 2001, 87, 225503]. An extended local aromaticity of eight all-carbon hexagonal rings is the driving force toward the maximum stability of this molecule, which is found to be 13.1 kcal/mol energetically more stable at the B3LYP/6-31G* level of theory than the recently reported structure [Chem. Phys. Lett. 2001, 340, 227]. The extended region of electron delocalization enhances the stability of this molecule via resonance energy contribution. On the basis of HUMO-LUMO gap of 2.74 eV, the new material is predicted to be an insulator. 相似文献
690.
Burford N Spinney HA Ferguson MJ McDonald R 《Chemical communications (Cambridge, England)》2004,(23):2696-2697
[Mes*NP(DMAP)2][OTf] represents the first ligand-rich coordination complex of a phosphorus(III) Lewis acceptor, and a three coordinate hypervalent electron-rich (lone pair bearing) center; further diversification is demonstrated by, [(Mes*NP)2(4,4'-BIPY)][OTf]2, representing an acceptor-rich dication. 相似文献