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1.
From the intensity behaviour of a 29 cm?1 Raman shift of α-Al2O3:Cr3+ as a function of the incident power, it is concluded that the shift is due to an electronic Raman transition between the components of the excited state 2E. Population of the excited states is obtained through a pumping mechanism induced by the laser radiation λ = 514.5 nm which at the same time serves as a Raman probe. 相似文献
2.
Di Tullio V Proietti N Capitani D Nicolini I Mecchi AM 《Analytical and bioanalytical chemistry》2011,400(9):3151-3164
Hydrophobic treatment is one of the most important interventions usually carried out in the conservation of stone artifacts
and monuments. The analytical study reported in this paper was aimed at answering general questions such as the penetration
depth of a hydrophobic treatment into a porous material, its capability to impair the water absorption, how the presence of
a treatment may change the open porosity available to the water, and how a treatment may affect the diffusion of water inside
a porous structure. Also, inhomogeneities in treated stones due to sharp variations of the amount of the absorbed product
in the porous material were evidenced and scaled. The results of this fully non-invasive analytical study were rationalized
in terms of new parameters obtained by a suitable process of nuclear magnetic resonance data. These analytical parameters
reported here for the first time, namely the hydrophobic efficiency, the penetration depth, and angles describing changes
in slope in depth profiles, gave important information in assessing the performance of a treatment. 相似文献
3.
A mixture of Li and Mg vapors at 900°C was optically pumped at 488 nm by an Ar+ ion laser. The resulting Li2 fluorescence and LiMg* excimer chemiluminescence intensities were analyzed to determine rate constants for the reactions Li2(B1π) + Mg → LiMg* + Li and LiMg* + Li → Li2(a3π) + Mg and the radiative decay rates of Li2(B1π) and LiMg*. 相似文献
4.
Tikhonova IG Best RB Engel S Gershengorn MC Hummer G Costanzi S 《Journal of the American Chemical Society》2008,130(31):10141-10149
Rhodopsin, the light sensitive receptor responsible for blue-green vision, serves as a prototypical G protein-coupled receptor (GPCR). Upon light absorption, it undergoes a series of conformational changes that lead to the active form, metarhodopsin II (META II), initiating a signaling cascade through binding to the G protein transducin (G(t)). Here, we first develop a structural model of META II by applying experimental distance restraints to the structure of lumi-rhodopsin (LUMI), an earlier intermediate. The restraints are imposed by using a combination of biased molecular dynamics simulations and perturbations to an elastic network model. We characterize the motions of the transmembrane helices in the LUMI-to-META II transition and the rearrangement of interhelical hydrogen bonds. We then simulate rhodopsin activation in a dynamic model to study the path leading from LUMI to our META II model for wild-type rhodopsin and a series of mutants. The simulations show a strong correlation between the transition dynamics and the pharmacological phenotypes of the mutants. These results help identify the molecular mechanisms of activation in both wild type and mutant rhodopsin. While static models can provide insights into the mechanisms of ligand recognition and predict ligand affinity, a dynamic model of activation could be applicable to study the pharmacology of other GPCRs and their ligands, offering a key to predictions of basal activity and ligand efficacy. 相似文献
5.
Robbins AJ Ng WT Jochym D Keal TW Clark SJ Tozer DJ Hodgkinson P 《Physical chemistry chemical physics : PCCP》2007,9(19):2389-2396
Advances in solid-state NMR methodology and computational chemistry are applied to the (19)F NMR of solid octafluoronaphthalene. It is demonstrated experimentally, and confirmed by density functional theory (DFT) calculations, that the spectral resolution in the magic-angle spinning spectrum is limited by the anisotropy of the bulk magnetic susceptibility (ABMS). This leads to the unusual observation that the resolution improves as the sample is diluted. DFT calculations provide assignments of each of the peaks in the (19)F spectrum, but the predictions are close to the limits of accuracy and correlation information from 2-D NMR is invaluable in confirming the assignments. The effects of non-Gaussian lineshapes on the use of 2-D NMR for mapping correlations of spectral frequencies (e.g. due to the ABMS) are also discussed. 相似文献
6.
Angulo J Langpap B Blume A Biet T Meyer B Krishna NR Peters H Palcic MM Peters T 《Journal of the American Chemical Society》2006,128(41):13529-13538
The biosynthesis of human blood group B antigens is accomplished by a highly specific galactosyltransferase (GTB). On the basis of NMR experiments, we propose a "molecular tweezers mechanism" that accounts for the exquisite stereoselectivity of donor substrate selection. Transferred NOE experiments for the first time reveal the bioactive conformation of the donor substrate UDP-galactose (UDP-Gal) and of its enzymatically inactive analogue, UDP-glucose (UDP-Glc). Both bind to GTB in a folded conformation that is sparsely populated in solution, whereas acceptor ligands bind in a conformation that predominates in solution. The bound conformations of UDP-Gal and UDP-Glc are identical within experimental error. Therefore, GTB must discriminate between the two activated sugars on the basis of a hitherto unknown transition state that can only be formed in the case of UDP-Gal. A full relaxation and exchange matrix analysis of STD NMR experiments reveals that acceptor substrates dissociate significantly faster (k(off) > 100 Hz) from the binding pocket than donor substrates (k(off) approximately 10 Hz). STD NMR experiments also directly show that proper recognition of the hexopyranose rings of the UDP sugars requires bivalent metal cations. At the same time, this analysis furnishes the complete three-dimensional structure of the enzyme with its bound donor substrate UDP-Gal on the basis of a prior crystal structure analysis. We propose that, upon acceptor binding, GTB uses the Asp 302 and Glu 303 side chains as "molecular tweezers" to promote bound UDP-Gal but not UDP-Glc into a transition state that leads to product formation. 相似文献
7.
An optically pumped,highly polarized cesium beam for the study of spin-dependent electron scattering
G. Baum B. Granitza S. Hesse B. Leuer W. Raith K. Rott M. Tondera B. Witthuhn 《Zeitschrift für Physik D Atoms, Molecules and Clusters》1991,22(1):431-436
We have set up an atomic beam of cesium for the study of spin-dependent electron-cesium scattering. The beam is produced by an effusive oven with continuous recirculation of the condensed metal. The beam is optically pumped by circularly polarized light from two laser diodes tuned to the 62 S 1/2(F=3)→62 P 3/2(F′=4) and 62 S 1/2(F=4)→62 P 3/2(F′=5) transitions, respectively. Nearly all atoms are transferred into theF=4,m F =+4 orm F =?4 Zeeman level of the ground state, depending on the sense of circular polarization of the pumping light. The population distribution in the optically pumped beam is analyzed in terms of them J =?1/2 andm J =+1/2 components with a Stern-Gerlach magnet. We find the atomic polarization to be very close to unity at a density of 8×108 atoms/cm3 in the scattering center. The polarization decreases slightly with increasing density of the cesium beam due to radiation trapping. A spin flipper serves as a means of polarization reversal, introducing no systematic errors in the spin asymmetry measurements. Lock-in technique is used to stabilize the atomic beam polarization by detecting fluorescence light signals. 相似文献
8.
Shoshan MS Shalev DE Adriaens W Merkx M Hackeng TM Tshuva EY 《Chemical communications (Cambridge, England)》2011,47(22):6407-6409
The first NMR structure of a Cu(I)-bound metallochaperone model with the conserved sequence MT/HCXXC revealed that at pH ~3.0 and ~6.8 Cu(I) binds through one Cys and the Met rather than the two Cys residues, differently than at pH ~8.5. This suggests a possible role of Met in metal transport. 相似文献
9.
We report on the use of optical Faraday rotation to monitor the nuclear-spin signal in a set of model (19)F- and (1)H-rich fluids. Our approach integrates optical detection with high-field, pulsed NMR so as to record the time-resolved evolution of nuclear-spins after rf excitation. Comparison of chemical-shift-resolved resonances allows us to set order-of-magnitude constrains on the relative amplitudes of hyperfine coupling constants for different bonding geometries. When evaluated against coil induction, the present detection modality suffers from poorer sensitivity, but improvement could be attained via multipass schemes. Because illumination is off-resonant i.e., the medium is optically transparent, this methodology could find extensions in a broad class of fluids and soft condensed matter systems. 相似文献
10.
11.
Ullrich CA 《The Journal of chemical physics》2006,125(23):234108
Most applications of time-dependent density-functional theory (TDDFT) use the adiabatic local-density approximation (ALDA) for the dynamical exchange-correlation potential V(xc)(r,t). An exact (i.e., nonadiabatic) extension of the ground-state LDA into the dynamical regime leads to a V(xc)(r,t) with a memory, which causes the electron dynamics to become dissipative. To illustrate and explain this nonadiabatic behavior, this paper studies the dynamics of two interacting electrons on a two-dimensional quantum strip of finite size, comparing TDDFT within and beyond the ALDA with numerical solutions of the two-electron time-dependent Schrodinger equation. It is shown explicitly how dissipation arises through multiple particle-hole excitations, and how the nonadiabatic extension of the ALDA fails for finite systems but becomes correct in the thermodynamic limit. 相似文献
12.
Hisaya Sato Yasuyuki Tanaka Koichi Hatada 《Journal of Polymer Science.Polymer Physics》1983,21(9):1667-1674
Methylene and phenyl C1 carbon signals of polystyrene are assigned on the basis of the signal assignments of styrene oligomers. Polystyrenes prepared with benzoyl peroxide, n-butyllithium, and trifluoroboron etherate catalysts have random distributions with probabilities of racemic dyads of 0.54, 0.56, and 0.45, respectively. 相似文献
13.
Guella G Zanchetta C Patton B Miotello A 《The journal of physical chemistry. B》2006,110(34):17024-17033
To gain insight on the mechanistic aspects of the palladium-catalyzed hydrolysis of NaBH(4) in alkaline media, the kinetics of the reaction has been investigated by (11)B NMR (nuclear magnetic resonance) measurements taken at different times during the reaction course. Working with BH(4)(-) concentration in the range 0.05-0.1 M and with a [substrate]/[catalyst] molar ratio of 0.03-0.11, hydrolysis has been found to follow a first-order kinetic dependence from concentration of both the substrate and the catalyst (Pd/C 10 wt %). We followed the reaction of NaBH(4) and its perdeuterated analogue NaBD(4) in H(2)O, in D(2)O and H(2)O/D(2)O mixtures. When the process was carried out in D(2)O, deuterium incorporation in BH(4)(-) afforded BH(4)(-)(n)D(n)(-) (n = 1, 2, 3, 4) species, and a competition between hydrolysis and hydrogen/deuterium exchange processes was observed. By fitting the kinetics NMR data by nonlinear least-squares regression techniques, the rate constants of the elementary steps involved in the palladium-catalyzed borohydride hydrolysis have been evaluated. Such a regression analysis was performed on a reaction scheme wherein the starting reactant BH(4)(-) is allowed both to reversibly exchange hydrogen with deuterium atoms of D(2)O and to irreversibly hydrolyze into borohydroxy species B(OD)(4)(-). In contrast to acid-catalyzed hydrolysis of sodium borohydride, our results indicate that in the palladium-catalyzed process the rate constants of the exchange processes are higher than those of the corresponding hydrolysis reactions. 相似文献
14.
Monson PA 《Langmuir : the ACS journal of surfaces and colloids》2008,24(21):12295-12302
We discuss the thermodynamics of adsorption of fluids in pores when the solid-fluid interactions lead to partial wetting of the pore walls, a situation encountered, for example, in water adsorption in porous carbons. Our discussion is based on calculations for a lattice gas model of a fluid in a slit pore treated via mean field density functional theory (MFDFT). We calculate contact angles for pore walls as a function of solid-fluid interaction parameter, alpha, in the model, using Young's equation and the interfacial tensions calculated in MFDFT. We consider adsorption and desorption in both infinite pores and in finite length pores in contact with the bulk. In the latter case, contact with the bulk can promote evaporation or condensation, thereby dramatically reducing the width of hysteresis loops. We show how the observed behavior changes with alpha. By using a value of alpha that yields a contact angle of about 85 degrees and maintaining the bulk fluid in a supersaturated vapor state on adsorption, we find an adsorption/desorption isotherm qualitatively similar to those for graphitized carbon black where pore condensation occurs at supersaturated bulk vapor states in the spaces between the primary particles of the adsorbent. 相似文献
15.
H.J. Nolte 《Chemical physics letters》1975,31(1):134-139
A twisted band aggregate, based upon a brickwork arrangement of monomers, seems to be a reasonable model for the optically active Scheibe-aggregate of pseudoisocyanine. 相似文献
16.
A data processing method is presented making it possible to determine thin film thickness from GDOS depth profiles. It consists of fitting a piece-wise linear function to the depth profile and in using positions (abscissae) of their knots as parameters which are proportional to film thickness. For thickness calibration, such a linear combination of these parameters is used, which gives minimum variation coefficient of the residuals between the real and the estimated thickness of calibration samples. Relative error lower than 3% was obtained in calibration using 4 samples with nickel film thickness ranging from 0.1 to 8.3 m, each analyzed four times. 相似文献
17.
The structures of sodium aluminophosphate glasses prepared by both sol-gel as well as melt-cooling routes have been extensively characterized by high-resolution solid-state 23Na, 27Al, and 31P single and double-resonance NMR techniques, including quantitative connectivity studies by 27Al <--> 31P and 23Na <--> 31P rotational echo double-resonance (REDOR) methods. Studies along four compositional lines, I: (AlPO4)x -(NaPO3)1-x, II: (Na2O)x -(AlPO4)1-x, III: (NaAlO2)x -(NaPO3)1-x, and IV: (Al2O3)x (NaPO3)1-x, reveal that the network structures of those glasses that are accessible by either preparation method are essentially identical. However, the significantly extended glass-forming ranges available by the sol-gel route facilitate exploration of the structure/composition relationships in more detail, revealing a number of interesting universal features throughout the whole glass system. Both short- and medium-range order appear to be controlled strongly by the O/P ratio of the glasses studied: Up to an O/P ratio of 3.5 (pyrophosphate composition), aluminum is predominantly six-coordinated and fully connected to phosphorus (Al(OP)6 sites). In the region 3.5 < or = O/P < or = 4.0, a dramatic structural transformation takes place, leading to the appearance of additional four- and five-coordinated aluminum species whose second coordination spheres are also entirely dominated by phosphorus. The structure of glasses with an O/P ratio of precisely 4.0 (orthophosphate) is dominated by Al(OP)4 units. As the O/P ratio increases beyond 4.0, the average extent of Al-O-P connectivity is decreased significantly. Here, new types of five- and six-coordinated aluminum units, which are only weakly connected to phosphorus, are formed, while the network modifier is attracted mainly by the phosphate units. 相似文献
18.
ZFS parameters and kinetic constants of the lowest triplet state of chlorin and tetraphenylchlorin free base in n-octane have been determined by fluorescence-detected ODMR at 4.2 K. These compounds can be considered as model compounds for pheophytin, a compound of biological interest. For both compounds the middle spin-level is the most active one in the populating and depopulating pathway. In the lowest triplet state the NHNH axis in both chlorins is probably fixed to one orientation not involving the reduced ring, and no evidence was found for the occurrence of two tautomeric forms as in the corresponding porphyrins. 相似文献
19.
Fast gating is a unique property of chloride channels, where a permeating Cl(-) ion acts as its own ligand in opening the channel. The glutamate residue implicated in fast gating normally carries a unit negative charge. Whether this charge needs to be protonated to enable permeation of a Cl(-) ion is an important question that will affect how models of chloride channels are constructed. We investigate the energetic consequences of the charge state of this glutamate residue from continuum electrostatics using a schematic cylindrical channel model. Both analytical solutions of the Poisson equation for an infinite cylinder and numerical ones for a finite cylinder are employed in the calculations. 相似文献
20.
Kazuaki Kudo Daigo Nonokawa Jun Li Shinsaku Shiraishi 《Journal of polymer science. Part A, Polymer chemistry》2002,40(22):4038-4044
Optically active alicyclic polyimides were prepared for the first time from (?)‐[1S*,5R*,6S*]‐3‐oxabicyclo[3.2.1]octane‐2,4‐dione‐6‐spiro‐3′‐(tetrahydrofuran‐ 2′,5′‐dione) [(?)‐DAn] via polycondensation with diamines and subsequent chemical or thermal imidization. The dianhydride (?)‐DAn was synthesized by an asymmetric Diels–Alder reaction of a chiral itaconic acid derivative as a key step. Colorless or slightly yellow flexible films were obtained for the (?)‐DAn‐derived polyimides {PI[(?)‐DAn]s}. PI[(?)‐DAn]s showed good solubility toward dipolar aprotic solvents and pyridine. For 1,4‐dioxane and chloroform, the optically active polyimides showed slightly better solubility than the corresponding polyimides prepared from rac‐DAn [PI(rac‐DAn)s]. PI[(?)‐DAn]s showed glass‐transition temperatures of 267–268 °C and 10% weight‐loss temperatures of 416–424 °C in nitrogen. These values were almost identical to those of PI(rac‐DAn)s. The circular dichroism spectra of PI[(?)‐DAn]s showed exciton coupling patterns indicating that to some extent these polyimides had a higher order structure in solution. © 2002 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 40: 4038–4044, 2002 相似文献