Showing papers on "Relaxation (NMR) published in 1997"
TL;DR: The TROSY principle should benefit a variety of multidimensional solution NMR experiments, especially with future use of yet somewhat higher polarizing magnetic fields than are presently available, and thus largely eliminate one of the key factors that limit work with larger molecules.
Abstract: Fast transverse relaxation of 1H, 15N, and 13C by dipole-dipole coupling (DD) and chemical shift anisotropy (CSA) modulated by rotational molecular motions has a dominant impact on the size limit for biomacromolecular structures that can be studied by NMR spectroscopy in solution. Transverse relaxation-optimized spectroscopy (TROSY) is an approach for suppression of transverse relaxation in multidimensional NMR experiments, which is based on constructive use of interference between DD coupling and CSA. For example, a TROSY-type two-dimensional 1H,15N-correlation experiment with a uniformly 15N-labeled protein in a DNA complex of molecular mass 17 kDa at a 1H frequency of 750 MHz showed that 15N relaxation during 15N chemical shift evolution and 1HN relaxation during signal acquisition both are significantly reduced by mutual compensation of the DD and CSA interactions. The reduction of the linewidths when compared with a conventional two-dimensional 1H,15N-correlation experiment was 60% and 40%, respectively, and the residual linewidths were 5 Hz for 15N and 15 Hz for 1HN at 4°C. Because the ratio of the DD and CSA relaxation rates is nearly independent of the molecular size, a similar percentagewise reduction of the overall transverse relaxation rates is expected for larger proteins. For a 15N-labeled protein of 150 kDa at 750 MHz and 20°C one predicts residual linewidths of 10 Hz for 15N and 45 Hz for 1HN, and for the corresponding uniformly 15N,2H-labeled protein the residual linewidths are predicted to be smaller than 5 Hz and 15 Hz, respectively. The TROSY principle should benefit a variety of multidimensional solution NMR experiments, especially with future use of yet somewhat higher polarizing magnetic fields than are presently available, and thus largely eliminate one of the key factors that limit work with larger molecules.
2,262 citations
TL;DR: In this article, a wide variety of rheological observations of materials in the vicinity of an LST are discussed with respect to their universality, and a variety of applications for polymers near the liquid-solid transition are presented that either already exist or can be envisioned.
Abstract: Polymeric materials near the liquid-solid transition (LST) exhibit a very distinct relaxation pattern. The reference point for analyzing these patterns is the instant of LST at which relaxation becomes self-similar over wide ranges of the relaxation time. The universality of this transition and its consequences have been explored extensively during the past decade. This study will present an overview of rheological implications inherent in liquid-solid transitions of polymers. The LST can be most reliably detected in a dynamic mechanical experiment in which the frequency independence of the loss tangent marks the LST. A wide variety of rheological observations of materials in the vicinity of an LST are discussed with respect to their universality. It is shown that polymer chemistry, molecular weight, stoichiometry, temperature, inhomogeneities, etc. greatly influence the material behavior near the LST. However, the characteristic self-similar relaxation is shown by all investigated materials, independent of the nature of the LST (e.g., both, physically and chemically crosslinking polymers). Several theories predict chemical and rheological properties in the vicinity of an LST. They are briefly discussed and compared with experimental results. A variety of applications for polymers near LST are presented that either already exist or can be envisioned. The self-similar relaxation behavior which results in a power law relaxation spectrum and modulus is not restricted to materials near LST. Different classes of polymers are described that also show power law relaxation behavior. What makes the self-similar relaxation specific for materials at LST is its occurrence at long times with the longest relaxation time diverging to infinity.
669 citations
TL;DR: In this paper, the real and imaginary part of the dielectric constant of liquid water in the far-infrared region from 0.1 to 2.0 THz in a temperature range from 271.1 K to 366.7 K were obtained with the use of THz time domain reflection spectroscopy.
Abstract: We report measurements of the real and imaginary part of the dielectric constant of liquid water in the far-infrared region from 0.1 to 2.0 THz in a temperature range from 271.1 to 366.7 K. The data have been obtained with the use of THz time domain reflection spectroscopy, utilizing ultrashort electromagnetic pulses generated from a photoconductive antenna driven by femtosecond laser pulses. A Debye model with an additional relaxation time is used to fit the frequency dependence of the complex dielectric constants. We obtain a fast (fs) and a Debye (ps) relaxation time for the macroscopic polarization. The corresponding time correlation functions have been calculated with molecular dynamics simulations and are compared with experimental relaxation times. The temperature dependence of the Debye relaxation time is analyzed using three models: Transition state theory, a Debye–Stoke–Einstein relation between the viscosity and the Debye time, and a model stating that its temperature dependence can be extrapolated from a singularity of liquid water at 228 K. We find an excellent agreement between experiment and the two latter models. The simulations, however, present results with too large statistical error for establishing a relation for the temperature dependence.
564 citations
TL;DR: In this article, the magnetic relaxation of clusters of eight iron ions characterized by a spin ground state of ten and an Ising anisotropy was investigated, and it was shown that tunneling of the magnetic moment across its anisotropic energy barrier occurs.
Abstract: We have investigated the magnetic relaxation of clusters of eight iron ions characterized by a spin ground state of ten and an Ising anisotropy. Below 400 mK the relaxation rate is temperature independent suggesting that tunneling of the magnetic moment across its anisotropy energy barrier occurs. Using the anisotropy constants derived from EPR data, we can calculate both the crossover temperature ${T}_{c}$ and the expected tunneling frequency $1/\ensuremath{\tau}$. The field dependence of the relaxation shows evidence of resonant tunneling.
562 citations
TL;DR: A femtosecond mid-infrared pump-probe study of the vibrational and orientational dynamics of the OH-stretching mode of HDO dissolved in D2O is presented in this article.
Abstract: A femtosecond mid-infrared pump-probe study of the vibrational and orientational dynamics of the OH-stretching mode of HDO dissolved in D2O is presented. The orientational relaxation of the HDO molecules was observed to occur on either a very slow or a very fast time scale, with associated time constants of τ R = 13 picoseconds and τ R = 0.7 picosecond. It was observed that strongly hydrogen-bonded water molecules only relax through the slow orientational relaxation process, whereas the fast process dominates for weakly hydrogen-bonded molecules. This suggests that, with respect to orientional dynamics, two distinct molecular species exist in liquid water.
527 citations
TL;DR: In this paper, the authors demonstrate inelastic phonon scattering to be the dominant intradot carrier-relaxation mechanism in self-organized InAs/GaAs quantum dots.
Abstract: Carrier relaxation and recombination in self-organized InAs/GaAs quantum dots (QD's) is investigated by photoluminescence (PL), PL excitation (PLE), and time-resolved PL spectroscopy We demonstrate inelastic phonon scattering to be the dominant intradot carrier-relaxation mechanism Multiphonon processes involving up to four LO phonons from either the InAs QD's, the InAs wetting layer, or the GaAs barrier are resolved The observation of multiphonon resonances in the PLE spectra of the QD's is discussed in analogy to hot exciton relaxation in higher-dimensional semiconductor systems and proposed to be intricately bound to the inhomogeneity of the QD ensemble in conjunction with a competing nonradiative recombination channel observed for the excited hole states Carrier capture is found to be a cascade process with the initial capture into excited states taking less than a few picoseconds and the multiphonon (involving three LO phonons) relaxation time of the first excited hole state being 40 ps The |001〉 hole state presents a relaxation bottleneck that determines the ground-state population time after nonresonant excitation For the small self-organized InAs/GaAs QD's the intradot carrier relaxation is shown to be faster than radiative (g1 ns) and nonradiative (\ensuremath{\approx}100 ps) recombination explaining the absence of a ``phonon bottleneck'' effect in the PL spectra
408 citations
TL;DR: In this paper, it is shown that for the classical model where spin-bath interactions are described by stochastic Langevin fields and spin-spin interactions are treated within the mean-field approximation (MFA), such a LLB equation can be derived exactly from the Fokker-Planck equation, if the external conditions change slowly enough.
Abstract: A macroscopic equation of motion for the magnetization of a ferromagnet at elevated temperatures should contain both transverse and longitudinal relaxation terms and interpolate between the Landau-Lifshitz equation at low temperatures and the Bloch equation at high temperatures. It is shown that for the classical model where spin-bath interactions are described by stochastic Langevin fields and spin-spin interactions are treated within the mean-field approximation (MFA), such a ``Landau-Lifshitz-Bloch'' (LLB) equation can be derived exactly from the Fokker-Planck equation, if the external conditions change slowly enough. For weakly anisotropic ferromagnets within the MFA the LLB equation can be written in a macroscopic form based on the free-energy functional interpolating between the Landau free energy near ${\mathrm{T}}_{\mathrm{C}}$ and the ``micromagnetic'' free energy, which neglects changes of the magnetization magnitude |M|, at low temperatures.
354 citations
TL;DR: A new model, the kinetic version of the equilibrium 'zipper' model for helix<==>coil transitions, which predicts a value of the cooperativity parameter sigma which is approximately 3-fold larger than previously reported values obtained from fitting equilibrium data only.
Abstract: The kinetics of the helix coil transition of an alanine-based peptide following a laser-induced temperature jump were monitored by the fluorescence of an N-terminal probe, 4-(methylamino)benzoic acid (MABA). This probe forms a peptide hydrogen bond to the helix backbone, which changes its fluorescence quantum yield. The MABA fluorescence intensity decreases in a single exponential relaxation, with relaxation times that are weakly temperature dependent, exhibiting a maximum value of approximately 20 ns near the midpoint of the melting transition. We have developed a new model, the kinetic version of the equilibrium 'zipper' model for helix coil transitions to explain these results. In this 'kinetic zipper' model, an enormous reduction in the number of possible species results from the assumption that each molecule contains either no helical residues or a single contiguous region of helix (the single-sequence approximation). The decay of the fraction of N-terminal residues that are helical, calculated from numerical solutions of the kinetic equations which describe the model, can be approximately described by two exponential relaxations having comparable amplitudes. The shorter relaxation time results from rapid unzipping (and zipping) of the helix ends in response to the temperature jump, while the longer relaxation time results from equilibration of helix-containing and non-helix-containing structures by passage over the nucleation free energy barrier. The decay of the average helix content is dominated by the slower process. The model therefore explains the experimental observation that relaxation for the N-terminal fluorescent probe is approximately 8-fold faster than that for the infrared probe of Williams et al. [(1996) Biochemistry 35, 691-697], which measures the average helix content, but does not account for the absence of observable amplitude for the slow relaxation in the fluorescence experiments ( helix rate is purely entropic, the model can also explain the maximum in the temperature dependence of the relaxation time for the fluorescent probe. Parameters that best reproduce the melting curves and the ratio of relaxation times predict a value of the cooperativity parameter sigma which is approximately 3-fold larger than previously reported values obtained from fitting equilibrium data only. The helix growth rate of approximately 10(8) s-1 that reproduces the experimental relaxation times is approximately 100-fold slower than those observed in molecular dynamics simulations. These parameters can be used to simulate the kinetically cooperative formation of a helix from the all-coil state.
344 citations
TL;DR: In this article, self-diffusion coefficients, D, have been measured in the glass forming liquids salol, glycerol, phenolphthaleine dimethyl ether (PDE), CDE, and ααβ-trinaphthylbenzene (TNB) in the supercooled regime.
Abstract: Self-diffusion coefficients, D, have been measured in the glass forming liquids salol, glycerol, phenolphthaleine dimethyl ether (PDE), cresolphthaleine dimethyl ether (CDE), and ααβ-trinaphthylbenzene (TNB) in the supercooled regime. The NMR static magnetic field gradient technique was applied where D >10-14 m2 s-1 can be attained. The results are similar to previous diffusion experiments where an enhancement of translational diffusion was found in comparison with rotational diffusion and shear viscosity. Various models of spatial heterogeneity are related to a phenomenological environmental fluctuation model in view of recent diffusion and relaxation data close to the glass transition.
335 citations
TL;DR: Structural analysis of delta131delta, a fragment model of the denatured state of staphylococcal nuclease, has been extended by obtaining long-range distance restraints between protein chain segments based on paramagnetic relaxation enhancement methods.
318 citations
TL;DR: In this paper, several samples of antiferromagnetic NiO nanoparticles with average sizes ranging from 50 to >800 A were investigated and the reversible magnetization could not fit with a Langevin function that was consistent with the physically reasonable moment representing the uncompensated spins.
Abstract: As first noted by Neel, antiferromagnetic nanoparticles could exhibit superparamagnetic relaxation of their spin lattices as well as permanent moments arising from uncompensated surface spins. Several samples of antiferromagnetic NiO nanoparticles with average sizes ranging from 50 to >800 A were investigated in the present study. In addition to the inverse dependence on average particle size of the susceptibility predicted by Neel, and previously reported, some unusual behavior was observed. Above the blocking temperatures (TB) of the particles, the reversible magnetization could not be fit with a Langevin function that was consistent with the physically reasonable moment representing the uncompensated spins. For the 53 A diameter particles, both zero-field-cooled (ZFC) and field-cooled (FC) loops below TB exhibit large coercive forces (several kOe) and the loops showed irreversibility up to 50 kOe. In addition, in the FC state below TB the hysteresis loops were strongly shifted. The latter behavior may ...
TL;DR: The measured rates can be directly related to the angular geometry without need for calibration of a Karplus-type curve, and depend only on the rotational correlation time of the molecule as an empirical parameter, which makes the determination of torsional angles independent from the measurement of coupling constants.
Abstract: Angles between two interatomic vectors are measured for structure elucidation in solution nuclear magnetic resonance (NMR). The angles can be determined directly by using the effects of dipole-dipole cross-correlated relaxation of double-quantum and zero-quantum coherences. The measured rates can be directly related to the angular geometry without need for calibration of a Karplus-type curve, as is the case for scalar coupling measurements, and depend only on the rotational correlation time of the molecule as an empirical parameter. This makes the determination of torsional angles independent from the measurement of coupling constants. The two interatomic vectors can in principle be arbitrarily far apart. The method was demonstrated on the measurement of the peptide backbone angle ψ in the protein rhodniin, which is difficult to determine in solution by NMR spectroscopy.
TL;DR: There is significant agreement between predicted and experimental NMR parameters suggesting that local conformations of the denatured states are largely determined by short-range interactions within the polypeptide chain.
Abstract: Oxidized and reduced hen lysozyme denatured in 8 M urea at low pH have been studied in detail by NMR methods 15N correlated NOESY and TOCSY experiments have provided near complete sequential assignment for both 1H and 15N resonances Over 900 NOEs, including 130 (i, i + 2) and 23 (i, i + 3) NOEs, could be identified by analysis of the NOESY spectra of the denatured states, and 3J(HN, Hα) coupling constants and 15N relaxation rates have been measured The coupling constant and NOE data were analyzed by comparisons with theoretical predictions from a random coil polypeptide model based on amino acid specific φ,ψ distributions extracted from the protein data bank There is significant agreement between predicted and experimental NMR parameters suggesting that local conformations of the denatured states are largely determined by short-range interactions within the polypeptide chain This result is supported by the observation that the chemical shift, coupling constant, and NOE data are little affected by whe
TL;DR: In this article, the T junctions of single-walled carbon nanotubes forming one of the smallest prototypes of microscopic metal-semiconductor-metal contacts are proposed, which are found to be local minima of the total energy on relaxation with a generalized tight-binding molecular dynamics scheme.
Abstract: Stable ``T junctions'' of single-walled carbon nanotubes forming one of the smallest prototypes of microscopic metal-semiconductor-metal contacts are proposed. The structures have been found to be local minima of the total energy on relaxation with a generalized tight-binding molecular dynamics scheme. These quasi-2D junctions could be the building blocks of nanoscale tunnel junctions in a 2D network of nanoelectronic devices.
TL;DR: How water diffusion within biological compartments and water exchange between these compartments affect MR signal enhancement and therefore the ability to extract physiologic information is discussed.
Abstract: The contrast-enhanced magnetic resonance imaging (MRI) signal is rarely a direct measure of contrast concentration; rather it depends on the effect that the contrast agent has on the tissue water magnetization. To correctly interpret such studies, an understanding of the effects of water movement on the magnetic resonance (MR) signal is critical. In this review, we discuss how water diffusion within biological compartments and water exchange between these compartments affect MR signal enhancement and therefore our ability to extract physiologic information. The two primary ways by which contrast agents affect water magnetization are discussed: (1) direct relaxivity and (2) indirect susceptibility effects. For relaxivity agents, for which T1 effects usually dominate, the theory of relaxation enhancement is presented, along with a review of the relevant physiologic time constants for water movement affecting this relaxation enhancement. Experimental issues that impact accurate measurement of the relaxation enhancement are discussed. Finally, the impact of these effects on extracting physiologic information is presented. Susceptibility effects depend on the size and shape of the contrast agent, the size and shape of the compartment in which it resides, as well as the characteristics of the water movement through the resulting magnetic field inhomogeneity. Therefore, modeling of this effect is complex and is the subject of active study. However, since susceptibility effects can be much stronger than relaxivity effects in certain situations, they may be useful even without full quantitation.
TL;DR: Direct monitoring of the cooling dynamics of the heme cofactor within the globin matrix allows the characterization of the vibrational energy flow through the protein moiety and to the water bath.
Abstract: The formation of vibrationally excited heme upon photodissociation of carbonmonoxy myoglobin and its subsequent vibrational energy relaxation was monitored by picosecond anti-Stokes resonance Raman spectroscopy. The anti-Stokes intensity of the nu4 band showed immediate generation of vibrationally excited hemes and biphasic decay of the excited populations. The best fit to double exponentials gave time constants of 1.9 +/- 0.6 and 16 +/- 9 picoseconds for vibrational population decay and 3.0 +/- 1.0 and 25 +/- 14 picoseconds for temperature relaxation of the photolyzed heme when a Boltzmann distribution was assumed. The decay of the nu4 anti-Stokes intensity was accompanied by narrowing and frequency upshift of the Stokes counterpart. This direct monitoring of the cooling dynamics of the heme cofactor within the globin matrix allows the characterization of the vibrational energy flow through the protein moiety and to the water bath.
TL;DR: In this article, the presence of the fourth order term in the total spin justifies the irregularities in the spacing of the jumps, recently observed in the hysteresis loop of Mn12ac and attributed to acceleration of the relaxation of the magnetization due to Quantum Tunneling between degenerate M states of the ground S=10 multiplet of the cluster.
Abstract: EPR spectra have been recorded in very high field, up to 25T, and at high frequency, up to 525 GHz, on a polycristalline sample of Mn12ac (see paper for detailed formula), the first example of molecular cluster behaving like a nanomagnet. The simulation of the spectra has provided an accurate determination of the parameters of the spin hamiltonian (see paper for formula and values of the various parameters). The presence of the fourth order term in the total spin justifies the irregularities in the spacing of the jumps, recently observed in the hysteresis loop of Mn12ac and attributed to acceleration of the relaxation of the magnetization due to Quantum Tunneling between degenerate M states of the ground S=10 multiplet of the cluster. The term in (S_+^4 + S_-^4) is responsible of the transverse magnetic anisotropy and plays a crucial role in the mechanism of Quantum Tunneling. The HF-EPR spectra have for the first time evidenced its presence and quantified it.
TL;DR: In this paper, the lifetime of majority-spin electrons at 1 eV above the Fermi energy was shown to be twice as long as minority-spin electron at the same energy level.
Abstract: The spin dependence of the lifetime of electrons excited in ferromagnetic cobalt is measured directly in a femtosecond real-time experiment. Using time- and spin-resolved two photon photoemission, we show that the lifetime of majority-spin electrons at 1 eV above the Fermi energy is twice as long as that of minority-spin electrons. The results demonstrate the feasibility of studying spin-dependent electron relaxation in ferromagnetic solids directly in the time domain and provide a basis for understanding the dynamics of electron transport in ferromagnetic solids and thin films.
TL;DR: In this article, a tutorial on dielectric (relaxation) spectrometry of liquids is given, and some methods of measuring complex (electric) permittivity spectra are briefly described.
Abstract: A tutorial on dielectric (relaxation) spectrometry of liquids is given in this article. Some methods of measuring complex (electric) permittivity spectra are briefly described. Results for water are presented and related to characteristic properties of the liquid structure and to models of the molecular dynamics, particularly as resulting from computer simulation studies. Dielectric spectra for aqueous solutions of low weight electrolytes, polyelectrolytes, small molecules, and polymers are discussed to illustrate effects of kinetic depolarization, structure saturation, as well as positive, negative, and hydrophobic hydration. Reference is also made to fluctuations in the hydrogen bond network of mixtures of water with liquids that are completely miscible with this unique solvent.
TL;DR: A simple mathematical formalism is presented which allows closed form expressions for the echo attenuation, E(q), in spin echo diffusion experiments, for practically all gradient waveforms and for the case of restricted diffusion in enclosing pores, with or without wall relaxation.
TL;DR: Structure and dynamics of the partially folded A state of ubiquitin in a 60%/40% methanol/water mixture at pH 2 was studied by two- and three-dimensional nuclear magnetic resonance spectroscopy (NMR) using fully 13C,15N, and 1HN-labeled ubiquitins.
Abstract: Structure and dynamics of the partially folded A state of ubiquitin in a 60%/40% methanol/water mixture at pH 2 was studied by two- and three-dimensional nuclear magnetic resonance spectroscopy (NMR) using fully 13C,15N-labeled ubiquitin. Complete backbone 13CO, 13Cα, 15N, and 1HN assignment was achieved. 13CO and 13Cα chemical shifts and 1H−1H nuclear Overhauser enhancement (NOE) connectivities indicate different behavior for the N-terminal and the C-terminal halves of the protein. In the N-terminal half of the A state, comprising the antiparallel β-sheet and the central α-helix, the native secondary structural elements are largely conserved. The C-terminal half, which is in the native form rich in β-strand character, undergoes a methanol-induced transition to a dynamic state with a uniformly high propensity for helical structure. This behavior is also reflected in backbone 15N relaxation data, indicating the presence of three loosely coupled secondary structural segments with enhanced internal mobility ...
TL;DR: In this paper, femtosecond measurements of transient absorption, bleach, and stimulated emission were used to study the excited-state dynamics of phthalocyanine tetrasulfonate (PcS4) and zinc PHTHC-S4 in solution.
Abstract: Femtosecond measurements of transient absorption, bleach, and stimulated emission are used to study the excited-state dynamics of phthalocyanine tetrasulfonate (PcS4) and zinc phthalocyanine tetrasulfonate (ZnPcS4) in solution. In water the excited-state decay process is fast and dominated by energy relaxation due to intermolecular aggregation. In dimethyl sulfoxide (DMSO) both PcS4 and ZnPcS4 exist predominantly in the monomeric form and exhibit very different dynamics from that of the aggregates. The decays are much slower and the observed processes are strongly dependent on the probe wavelength. For PcS4 in DMSO, when probed at 790 nm, the dynamics are dominated by stimulated emission which is observed for the first time in solution. At other wavelengths either transient absorption or bleach dominates the signal. All the observed dynamics can be well fit using a double-exponential function with a fast and slow component. The fast decay has a time constant of 10 ± 4 ps for both phthalocyanines while the...
TL;DR: Using the Broadwell model of the nonlinear Boltzmann equation, a second-order scheme is developed that works effectively, with a fixed spatial and temporal discretization, for all ranges of the mean free path.
Abstract: We develop high-resolution shock-capturing numerical schemes for hyperbolic systems with relaxation. In such systems the relaxation time may vary from order-1 to much less than unity. When the relaxation time is small, the relaxation term becomes very strong and highly stiff, and underresolved numerical schemes may produce spurious results. Usually one cannot decouple the problem into separate regimes and handle different regimes with different methods. Thus it is important to have a scheme that works uniformly with respect to the relaxation time. Using the Broadwell model of the nonlinear Boltzmann equation we develop a second-order scheme that works effectively, with a fixed spatial and temporal discretization, for all ranges of the mean free path. Formal uniform consistency proof for a first-order scheme and numerical convergence proof for the second-order scheme are also presented. We also make numerical comparisons of the new scheme with some other schemes. This study is motivated by the reentry problem in hypersonic computations.
TL;DR: A review of the muon spin rotation and relaxation studies on magnetic materials published from July 1993 is presented in this paper, which covers the investigation of magnetic phase diagrams, that of spin dynamics and the analysis of the magnetic properties of superconductors.
Abstract: A review of the muon spin rotation and relaxation studies on magnetic materials published from July 1993 is presented. It covers the investigation of magnetic phase diagrams, that of spin dynamics and the analysis of the magnetic properties of superconductors. We have chosen to focus on selected experimental works in these different topics. In addition, a list of published works is provided.
TL;DR: This paper shows how the dependence of heteronuclear longitudinal and transverse relaxation times on the rotational diffusion anisotropy of non-spherical molecules can be readily used to directly provide restraints for simulated annealing structure refinement that characterize long range order a priori.
Abstract: Structure determination by NMR presently relies on short range restraints between atoms in close spatial proximity, principally in the form of short (< 5 A) interproton distances. In the case of modular or multidomain proteins and linear nucleic acids, the density of short interproton distance contacts between structural elements far apart in the sequence may be insufficient to define their relative orientations. In this paper we show how the dependence of heteronuclear longitudinal and transverse relaxation times on the rotational diffusion anisotropy of non-spherical molecules can be readily used to directly provide restraints for simulated annealing structure refinement that characterize long range order a priori. The method is demonstrated using the N-terminal domain of Enzyme I,a protein of 259 residues comprising two distinct domains with a diffusion anisotropy(Dparallel/Dperpendicular)of approximately 2.
TL;DR: In this paper, a planar relaxation of polystyrene was found in the first 1 and 10 nm from the free surface, whereas the dominant relaxation was normal to the surface.
Abstract: Near-edge X-ray absorption fine structure, NEXAFS, spectroscopy was used to investigate the relaxations of polystyrene, a typical amorphous polymer, near a free surface after the imposition of a small deformation. Using synchrotron radiation, the NEXAFS dichroic ratio was determined for both the Auger and total electron yield processes as a function of temperature to determine the orientation of the polymer in the first 1 and 10 nm from the free surface, respectively. Complete relaxation of the polymer was not seen for temperatures less than the bulk glass transition temperature. No evidence of enhanced mobility at the free surface was found. A planar relaxation of the polymer was found in the first nanometer from the free surface, whereas in the first 10 nm, the dominant relaxation was normal to the surface.
TL;DR: Anisotropy of the magnetic susceptibility tensor, x , of relatively efficient H– H spin flips of the methylene protons a molecule results in an orientation-dependent interaction complicate the measurement, and there is a residual N– H dipolar coupling.
TL;DR: In this article, the authors presented an extensive study of these important aspects, metastability and dynamics, of the \ensuremath{\alpha}-\ensuresuremath{varepsilon} transition in iron.
Abstract: The shock-induced \ensuremath{\alpha}(bcc)\ensuremath{\rightarrow}\ensuremath{\varepsilon}(hcp) transition in iron begins at 13 GPa on the Hugoniot. In the two-phase region above 13 GPa, the Hugoniot lies well above the equilibrium surface defined by ${\mathrm{G}}_{\mathrm{\ensuremath{\alpha}}}$ =${\mathrm{G}}_{\mathrm{\ensuremath{\varepsilon}}}$ , with G the Gibbs free energy. Also, the phase transition relaxation time \ensuremath{\tau} is uncertain, with estimates ranging from 50 ns to \ensuremath{\approx}180 ns. Here we present an extensive study of these important aspects, metastability and dynamics, of the \ensuremath{\alpha}-\ensuremath{\varepsilon} transition in iron. Our primary theoretical tools are (a) accurate theoretically based free energies for \ensuremath{\alpha} and \ensuremath{\varepsilon} phases of iron and (b) accurate calculations of the wave evolution following planar impacts. We define metastable surfaces for forward and reverse transitions by the condition that the thermodynamic driving force ${\mathrm{G}}_{\mathrm{\ensuremath{\alpha}}}$ -${\mathrm{G}}_{\mathrm{\ensuremath{\varepsilon}}}$ is just balanced by an opposing force resulting from elastic stresses, and we calibrate the forward surface from the Hugoniot and the reverse surface from the phase interface reflection feature of shock profiles. These metastable surfaces, corresponding to \ensuremath{\alpha}\ensuremath{\leftrightarrows}\ensuremath{\varepsilon} transitions proceeding at a rate of tens of nanoseconds, are in remarkable agreement with quasistatic diamond cell measurements. When the relaxation time \ensuremath{\tau} is calibrated from the rise time of the P2 wave, our calculated wave profiles are in good agreement with VISAR data. The overall comparison of theory and experiment indicates that (a) \ensuremath{\tau} depends on shock strength and is approximately 60\ensuremath{\rightarrow}12 ns for shocks of 17\ensuremath{\rightarrow}30 GPa, and (b) while \ensuremath{\tau} expresses linear irreversible-thermodynamic relaxation, some nonlinear relaxation must also be present in the shock process in iron.
TL;DR: In this article, a forward reconvolution fit procedure based on a microscopic incoherent transport model, including diagonal disorder, dipolar intersite coupling, and a density-of-states (DOS) of molecular site excitations, is presented.
Abstract: Exciton relaxation in poly(phenylenevinylene), PPV, has been probed by femtosecond luminescence-up-conversion. We report on excitonic luminescence profiles that depend on the spectral position of the detection window (e=2.7,2.6,2.5, and 2.4 eV, respectively). In an attempt to reveal the transient steps implied in fs relaxation, we present a quantitative forward reconvolution fit procedure that is based on a microscopic incoherent transport model, including diagonal disorder, dipolar intersite coupling, and a density-of-states (DOS) of molecular site excitations. Special emphasis has been placed (i) on the analysis of luminescence lifetime distributions 〈φ(τ;e,τ0)〉 which directly map out the spectra of hopping modes of energy-cascading neutral excitations, and (ii) on the rigorous evaluation of (radiationless) transfer population from high-energy subensembles to low-energy tail states of the DOS. We quantitatively show that the absence of significant rise terms in the S0ν=0←S1ν=0 luminescence transition ...
TL;DR: In this article, the free radical formation of the amino acid l-α-alanine has been studied using X-band and K-band EPR, ENDOR, and EIE (ENDOR-induced EPR) spectroscopy.
Abstract: Single crystals of the amino acid l-α-alanine have been X-irradiated at room temperature, and the free radical formation has been studied using X-band and K-band EPR, ENDOR, and EIE (ENDOR-induced EPR) spectroscopy in the temperature interval 220−295 K. Aided by the results from EIE, as well as ENDOR from selected magnetic field positions, nine hyperfine coupling tensors were obtained and assigned to three different radicals. Room-temperature relaxation behaviors characterized by efficient W1x and W1e and by slow W1n relaxation rates allowed for determination of the signs of the various hyperfine couplings from the ENDOR spectra obtained at room temperature. The temperature dependence of the W1x relaxation is qualitatively discussed. The EPR spectra from alanine are dominated by the well-known resonance of the “stable alanine radical”, SAR, formed by a net deamination of the protonated alanine anion. Precise hyperfine coupling tensors due to the α-proton coupling, the methyl group coupling, and a dipolar ...