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Processes Occurring (process + occurring)
Selected AbstractsCombined use of nuclear magnetic resonance and infrared spectroscopy for studying recognition processes between amphenicolic antibiotics and albuminMAGNETIC RESONANCE IN CHEMISTRY, Issue 7 2003Silvia Martini Abstract Biological reactions are mostly concerned with selective interactions between small ligands and macromolecular receptors. The same ligands may activate responses of different intensities and/or effects in the presence of different receptors. Many approaches based on spectroscopic and non-spectroscopic methods have been used to study interactions between small ligands and macromolecular receptors, including methods based on NMR and IR spectroscopic analysis of the solution behaviour of the ligand in the presence of receptors. In this work, we investigated the interaction between ovine serum albumin with two amphenicolic antibiotics [chloramphenicol (CAP) and thiamphenicol (TAP)], using a combined approach based on NMR and IR methodologies, furnishing complementary information about the recognition process occurring within the two systems. The two ligands, despite their similar structures, showed different affinities towards albumin. NMR methodology is based on the comparison of selective () and non-selective () spin,lattice relaxation rates of the ligands in the presence and absence of macromolecular receptors and and temperature dependence analysis. From these studies, the ligand,receptor binding strength was evaluated on the basis of the ,affinity index.' The derivation of the affinity index from chemical equilibrium kinetics for both the CAP,albumin and TAP,albumin systems allowed a comparison of the abilities of the two amphenicolic antibiotics to interact with the protein. IR methodology is based on the comparison of the ligand,protein ,complex' spectra with those of the non-interacting systems. On the basis of the differences revealed, a more thorough IR analysis was performed in order to understand the structural changes which occurred on both ligand and protein molecules within the interacting system. Copyright © 2003 John Wiley & Sons, Ltd. [source] Influence of the relative humidity on film formation by vapor induced phase separationPOLYMER ENGINEERING & SCIENCE, Issue 4 2003H. Caquineau The formation of polymer films produced by the phase separation process occurring when a cast poly(etherimide)/N-Methyl-2-Pyrrolidone solution was exposed to humid air was studied. It was found that above a relative humidity value of 27%, the films presented a cell-like structure. The size of the cells was shown to decrease when the relative humidity increased. This effect was more pronounced at the film/substrate interface than near the surface. A cell-size gradient from one face of the film to the other was also clearly observed. A phenomenological model has been proposed to explain the morphology obtained by a phase separation induced by the water vapor in the studied system, takin into account thermodynamics and kinetics considerations. In this model, the cell-like structure setting up is shown to result from a nucleation and growth process accompanied by a coalescence coarsening. It was illustrated by a composition path on the ternary phase diagram. It was shown how the relative humidity influenced the film composition leading to the preferential nucleation compared to the growth and coalescence of the cells. Finally, it was found that the cell-size anisotropy resulted in the solvent and non-solvent mass transfers in the film, bringing to the fore the determining role of kinetics. [source] Morphology development during blending of immiscible polymers in screw extrudersPOLYMER ENGINEERING & SCIENCE, Issue 6 2002Sandeep Tyagi The present work reports evolution of morphology from initial (presence of striation) to final (droplet formation) stages in a single-screw extruder. Morphology development during the blending process controls the final size of the dispersed phase, which in turn significantly affects the properties of the blends. The experiments were carried out using a 70/30 wt% polypropylene/ethylene vinyl acetate (PP/EVA) blend; samples were collected along the length of the screw, by screw pullout experiment, to analyze the size and size distribution of the dispersed phase present both as striated layers and subsequently as droplets. Average size of the dispersed phase and standard deviation were taken into account to monitor the morphology evolution along the length of the screw. Pre-breakup morphology development was studied by analyzing the sample collected from the feed zone of the extruder in terms of upper and lower layers along the cross section of screw channel. Examination of micrographs revealed the existence of pattern of ordered striations along the length of the melting zone containing striations from average size of 1000 ,m down to 50 ,m decreasing rapidly along the length of the screw. The breakup process was captured at the start of compression zone where step-up in the shear as well as elongational flow was applied due to decrease in the channel depth along the compression zone. The observed droplet size formed by the breakup of filaments is found to be in accordance with theory. The final droplet size is found to be governed by the emulsification process occurring as a result of stretching, breakup and coalescence in the metering section of the screw and is in the order of 2 ,m. [source] Sonovoltammetric Elucidation of Electron Transfer Rates: The Oxidation of Dimethyl- p -phenylenediamine in Aqueous SolutionELECTROANALYSIS, Issue 4 2003Craig Abstract The electrochemical oxidation of dimethyl- p -phenylenediamine (DMPD) in aqueous solution (pH 7 phosphate buffer) has been studied under conventional hydrodynamic and microelectrode voltammetric conditions and found to undergo a two-electron electrochemically reversible oxidation. Upon the application of ultrasound to the system an observed shoulder emerges in the oxidation wave. This effect has been attributed to the resolution of the two-electron transfer processes occurring: the first a relatively fast electron transfer (0.1,cm s,1) followed by a second slower (10,3 cm s,1) electron transfer: under the very high mass transport rates induced by insonation an overpotential develops for the second electron transfer so leading to the observed voltammetric resolution. The range of mass transport conditions accessible via sonication allows the estimation of the two rate constants reported. [source] Modelling the hydraulic preferences of benthic macroinvertebrates in small European streamsFRESHWATER BIOLOGY, Issue 1 2007SYLVAIN DOLÉDEC Summary 1. Relating processes occurring at a local scale to the natural variability of ecosystems at a larger scale requires the design of predictive models both to orientate stream management and to predict the effects of larger scale disturbances such as climate changes. Our study contributes to this effort by providing detailed models of the hydraulic preferences of 151 invertebrate taxa, mostly identified at the species level. We used an extensive data set comprising 580 invertebrate samples collected using a Surber net from nine sites of second and third order streams during one, two or three surveys at each site. We used nested non-linear mixed models to relate taxon local densities to bed shear stresses estimated from FliesswasserStammTisch hemisphere numbers. 2. An average model by taxon, i.e. independent from surveys, globally explained 25% of the density variations of taxa within surveys. A quadratic relationship existed between the average preferences and the niche breadth of taxa, indicating that taxa preferring extreme hemisphere numbers had a reduced hydraulic niche breadth. A more complete model, where taxa preferences vary across surveys, globally explained 38% of the variation of taxa densities within surveys. Variations in preferences across surveys were weak for taxa preferring extreme hemisphere numbers. 3. There was a significant taxonomic effect on preferences computed from the complete model. By contrast, season, site, average hemisphere number within a survey and average density of taxa within a survey used as covariates did not consistently explain shifts in taxon hydraulic preferences across surveys. 4. The average hydraulic preferences of taxa obtained from the extensive data set were well correlated to those obtained from two additional independent data sets collected in other regions. The consistency of taxon preferences across regions supports the use of regional preference curves for estimating the impact of river management on invertebrate communities. By contrast, the hydraulic niche breadths of taxa computed from the different data sets were not related. [source] Direct-temperature mass spectrometric detection of volatile terpenoids and natural terpenoid polymersin fresh and artificially aged resinsJOURNAL OF MASS SPECTROMETRY (INCORP BIOLOGICAL MASS SPECTROMETRY), Issue 6 2003Dominique Scalarone Abstract Electron impact (EI) ionization and ammonia chemical ionization (NH3/CI) direct-temperature mass spectrometry (DTMS) was used to characterize five natural terpenoid resins: dammar, mastic, colophony, Manila copal and sandarac. Compositional differences were highlighted by the identification of low molecular mass compounds, ranging from di- to triterpenoids, and polymeric components, based on polycadinene and polycommunic acid. Photo-ageing processes occurring under accelerated indoor and outdoor exposure conditions were also investigated. NH3/CI and tetramethylammonium hydroxide EI were applied to increase the sensitivity towards highly oxidized molecules. Oxidation and cross-linking reactions were found to affect mostly triterpenoid resins and diterpenoid abietane and pimarane molecules. Oxidation proceeds through a radical mechanism, generally starting from conjugated double bonds. Oxygen atoms are incorporated in the terpenoid structures in the form of alcohols, ketones and carboxylic acids. Oxidized cadinene oligomers released by pyrolytic degradation of the polycadinene fraction of dammar were detected even in unaged samples. Evidence is given indicating the occurrence of cleavages in the cross-linked polycommunic structure of aged sandarac and Manila copal. Bond scissions produce oligomeric fragments based on the communic acid structure and sufficiently volatile to be desorbed at low temperature in DTMS measurements. Copyright © 2003 John Wiley & Sons, Ltd. [source] What can we learn on the thermal history of the Universe from future cosmic microwave background spectrum measurements at long wavelengths?MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 2 2003C. Burigana ABSTRACT We analyse the implications of future observations of the cosmic microwave background (CMB) absolute temperature at centimetre and decimetre wavelengths, where both ground, balloon and space experiments are currently under way to complement the accurate COBE/FIRAS data available at ,, 1 cm. Our analysis shows that forthcoming ground and balloon measurements will allow a better understanding of free,free distortions but will not be able to significantly improve the constraints already provided by the FIRAS data on the possible energy exchanges in the primeval plasma. The same holds even for observations with sensitivities up to ,10 times better than those of forthcoming experiments. Thus, we have studied the impact of very high-quality data, such as those, in principle, achievable with a space experiment such as the Diffuse Microwave Emission Survey (DIMES) planned to measure the CMB absolute temperature at 0.5 ,,, 15 cm with a sensitivity of ,0.1 mK, close to that of FIRAS. We have demonstrated that such high-quality data would improve by a factor of ,50 the FIRAS results on the fractional energy exchanges, ,,/,i, associated with dissipation processes possibly occurred in a wide range of cosmic epochs, at intermediate and high redshifts (yh, 1), and that the energy dissipation epoch could also be significantly constrained. By jointly considering two dissipation processes occurring at different epochs, we demonstrated that the sensitivity and frequency coverage of a DIMES -like experiment would allow one to accurately recover the epoch and the amount of energy possibly injected into the radiation field at early and intermediate epochs even in the presence of a possible late distortion, while the constraints on the energy possibly dissipated at late epochs can be improved by a factor of ,2. In addition, such measurements can provide an independent and very accurate cross-check of FIRAS calibration. Finally, a DIMES -like experiment will be able to provide indicative independent estimates of the baryon density: the product ,bH20 can be recovered within a factor of ,2,5 even in the case of (very small) early distortions with ,,/,i, (5,2) × 10,6. On the other hand, for ,b (H0/50)2, 0.2, an independent baryon density determination with an accuracy at , per cent level, comparable to that achievable with CMB anisotropy experiments, would require an accuracy of ,1 mK or better in the measurement of possible early distortions but up to a wavelength from , few × dm to ,7 dm, according to the baryon density value. [source] Microsecond Protein Dynamics Measured by 13C, Rotating-Frame Spin RelaxationCHEMBIOCHEM, Issue 9 2005Patrik Lundström Abstract NMR spin relaxation in the rotating frame (R1,) is a unique method for atomic-resolution characterization of conformational (chemical) exchange processes occurring on the microsecond timescale. We present a rotating-frame13C,relaxation dispersion experiment for measuring conformational dynamics in uniformly13C-labeled proteins. The experiment was validated by using the E140Q mutant of the C-terminal fragment of calmodulin, which exhibits significant conformational exchange between two major conformations, as gauged from previous15N and1H relaxation studies. Consistent with previous work, the present13C, R1,experiment detects conformational-exchange dynamics throughout the protein. The average correlation time of ,,ex,=25±8 ,s is in excellent agreement with those determined previously from1H and15N R1,relaxation data: ,,ex,=19±7 and 21±3 ,s, respectively. The extracted chemical-shift differences between the exchanging states reveal significant fluctuations in dihedral angles within single regions of Ramachandran ,,, space, that were not identified from the1H and15N relaxation data. The present results underscore the advantage of using several types of nuclei to probe exchange dynamics in biomolecules. [source] | ||||||||||