scispace - formally typeset
Search or ask a question

Showing papers on "Relaxation (NMR) published in 1995"


Journal ArticleDOI
TL;DR: Simulations demonstrate that the method is accurate for a wide range of protein motions and correlation times, and experimental data establish the validity of the methodology.
Abstract: A method is presented for the determination of values of the spectral density function, J(ω), describing the dynamics of amide bond vectors from 15N relaxation parameters alone. Assuming that the spectral density is given by the sum of Lorentzian functions, the approach allows values of J(ω) to be obtained at ω=0, ωN and 0.870ωH, where ωN and ωH are Larmor frequencies of nitrogen and proton nuclei, respectively, from measurements of 15N T1, T2 and 1H−15N steady-state NOE values at a single spectrometer frequency. Alternatively, when measurements are performed at two different spectrometer frequencies of i and j MHz, J(ω) can be mapped at ω=0, ωiN, ωjN, 0.870ωiH and 0.870ωjH, where ωiN, for example, is the 15N Larmor frequency for a spectrometer operating at i MHz. Additionally, measurements made at two different spectrometer frequencies enable contributions to trasverse relaxation from motions on millisecond-microsecond time scales to be evaluated and permit assessment of whether a description of the internal dynamics is consistent with a correlation function consisting of a sum of exponentials. No assumptions about the specific form of the spectral density function describing the dynamics of the 15N−NH bond vector are necessary, provided that dJ(ω)/dω is relatively constant between ω=ωH+ωN to ω=ωH−ωN. Simulations demonstrate that the method is accurate for a wide range of protein motions and correlation times, and experimental data establish the validity of the methodology. Results are presented for a folded and an unfolded form of the N-terminal SH3 domain of the protein drk.

476 citations


Journal ArticleDOI
TL;DR: A two-cluster drifting state with zero magnetization forms spontaneously at very small temperatures; at larger temperatures an initial density modulation produces this state, which relaxes very slowly, which suggests the possibility of exciting magnetized states in a mean-field antiferromagnetic system.
Abstract: We study the dynamics of a fully coupled network of N classical rotators, which can also be viewed as a mean-field XY Heisenberg (HMF) model, in the attractive (ferromagnetic) and repulsive (antiferromagnetic) cases. The exact free energy and the spectral properties of a Vlasov-Poisson equation give hints on the values of dynamical observables and on time relaxation properties. At high energy (high temperature T) the system relaxes to Maxwellian equilibrium with vanishing magnetization, but the relaxation time to the equilibrium momentum distribution diverges with N as ${\mathit{NT}}^{2}$ in the ferromagnetic case and as ${\mathit{NT}}^{3/2}$ in the antiferromagnetic case. The N dependence of the relaxation time is suggested by an analogy of the HMF model with gravitational and charged sheets dynamics in one dimension and is verified in numerical simulations. Below the critical temperature the ferromagnetic HMF mode shows a collective phenomenon where the rotators form a drifting cluster; we argue that the drifting speed vanishes as ${\mathit{N}}^{\mathrm{\ensuremath{-}}1/2}$ but increases as one approaches the critical point (a manifestation of critical slowing down). For the antiferromagnetic HMF model a two-cluster drifting state with zero magnetization forms spontaneously at very small temperatures; at larger temperatures an initial density modulation produces this state, which relaxes very slowly. This suggests the possibility of exciting magnetized states in a mean-field antiferromagnetic system.

475 citations


Journal ArticleDOI
12 May 1995-Science
TL;DR: The data indicate that the motions of the individual zinc-finger domains are highly correlated on time scales shorter than 10 nanoseconds and that the average conformation of the three-finger polypeptide is elongated.
Abstract: Structural characterization of biomolecules in solution by nuclear magnetic resonance (NMR) spectroscopy is based primarily on the use of interproton distances derived from homonuclear cross-relaxation experiments. Information about short time-scale dynamics, on the other hand, is obtained from relaxation rates of heteronuclear spin pairs such as 15N-1H. By combining the two types of data and utilizing the dependence of heteronuclear NMR relaxation rates on anisotropic diffusional rotational tumbling, it is possible to obtain structural information about long-range motional correlations between protein domains. This approach was applied to characterize the relative orientations and mobilities of the first three zinc-finger domains of the Xenopus transcription factor TFIIIA in aqueous solution. The data indicate that the motions of the individual zinc-finger domains are highly correlated on time scales shorter than 10 nanoseconds and that the average conformation of the three-finger polypeptide is elongated.

315 citations


Journal ArticleDOI
TL;DR: The authors have reformulated outer sphere relaxation theory to incorporate progressive magnetic saturation of solute nanoparticles and indicate how to use empirical magnetization data for realistic particles when their magnetic properties are not ideal.
Abstract: Organically coated iron oxide crystallites with diameters of 5-50 nm ("nanoparticles") are potential magnetic resonance imaging contrast agents. 1/T1 and 1/T2 of solvent water protons are increased dramatically by magnetic interactions in the "outer sphere" environment of the nanoparticles; subsequent diffusive mixing distributes this relaxation throughout the solvent. Published theory, valid for the solute magnetic energy small compared with thermal energy, is applicable to small magnetic solutes (e.g., gadolinium and manganese diethylenetriaminopentaacetic acid, and nitroxide free radicals) at generally accessible fields (< or = 50 T). It fails for nanoparticles at fields above approximately 0.05 T, i.e., at most imaging fields. The authors have reformulated outer sphere relaxation theory to incorporate progressive magnetic saturation of solute nanoparticles and, in addition, indicate how to use empirical magnetization data for realistic particles when their magnetic properties are not ideal. It is important to handle the effects of rapid thermally induced reorientation of the magnetization of the nanoparticles (their "superparamagnetism") effectively, including their sensitivity to particle size. The theoretical results are presented as the magnetic field dependence (NMRD profiles) of 1/T1 and 1/T2, normalized to Fe content, for three sizes of particles, and then compared with the limited data extant for well-characterized material.

304 citations


Journal ArticleDOI
TL;DR: In this paper, a photobleaching technique has been used to observe rotational dynamics of dilute probe molecules in supercooled o−terphenyl (OTP) and the nonexponential rotational relaxation of the probe molecules is due, at least in part, to the presence of spatial heterogeneity in the host dynamics.
Abstract: A photobleaching technique has been used to observe rotational dynamics of dilute probe molecules in supercooled o‐terphenyl (OTP) The nonexponential rotational relaxation of the probe molecules is shown to be due, at least in part, to the presence of spatial heterogeneity in the host dynamics Under appropriate photobleaching conditions, a nonequilibrium distribution of probe molecule mobilities can be created by preferentially bleaching the more mobile probe molecules in a sample Near Tg this nonequilibrium distribution is observed to return to an equilibrium distribution of relaxation times over times on the order of 103 τc, where τc is the average rotational correlation time of a probe molecule The time required to return to equilibrium is interpreted as a structural relaxation time for dynamic heterogeneities in OTP

292 citations


Journal ArticleDOI
TL;DR: The backbone motions of calcium-free Xenopus calmodulin have been characterized by measurements of the 15N longitudinal relaxation times and the difference in the transverse relaxation rates observed between the amides in helices C and D suggests that the change in interhelical angle upon calcium binding is less than predicted by Herzberg et al.
Abstract: The backbone motions of calcium-free Xenopus calmodulin have been characterized by measurements of the 15N longitudinal relaxation times (T1) at 51 and 61 MHz, and by conducting transverse relaxation (T2), spin-locked transverse relaxation (T1 rho), and 15N-[1H] heteronuclear NOE measurements at 61 MHz 15N frequency. Although backbone amide hydrogen exchange experiments indicate that the N-terminal domain is more stable than calmodulin's C-terminal half, slowly exchanging backbone amide protons are found in all eight alpha-helices and in three of the four short beta-strands. This confirms that the calcium-free form consists of stable secondary structure and does not adopt a 'molten globule' type of structure. However, the C-terminal domain of calmodulin is subject to conformational exchange on a time scale of about 350 microseconds, which affects many of the C-terminal domain residues. This results in significant shortening of the 15N T2 values relative to T1 rho, whereas the T1 rho and T2 values are of similar magnitude in the N-terminal half of the protein. A model in which the motion of the protein is assumed to be isotropic suggests a rotational correlation time for the protein of about 8 ns but quantitatively does not agree with the magnetic field dependence of the T1 values and does not explain the different T2 values found for different alpha-helices in the N-terminal domain. These latter parameters are compatible with a flexible dumb-bell model in which each of calmodulin's two domains freely diffuse in a cone with a semi-angle of about 30 degrees and a time constant of about 3 ns, whereas the overall rotation of the protein occurs on a much slower time scale of about 12 ns. The difference in the transverse relaxation rates observed between the amides in helices C and D suggests that the change in interhelical angle upon calcium binding is less than predicted by Herzberg et al. Strynadka and James [Strynadka, N. C. J. & James, M. N. G. (1988) Proteins Struct. Funct. Genet. 3, 1-17].

268 citations


Journal ArticleDOI
T. Jonsson1, J. Mattsson1, C. Djurberg1, F.A. Khan1, Per Nordblad1, Peter Svedlindh1 
TL;DR: In this paper, the influence of dipolar interaction in a frozen ferrofluid has been experimentally studied and the magnetic relaxation of the most concentrated particle system shows typical spin glass dynamics at low temperature, e.g., the relaxation depends on the time spent at constant temperature before applying the magnetic field.
Abstract: The influence of dipolar interaction in a frozen ferrofluid has been experimentally studied. The ferrofluid consisted of particles of $\ensuremath{\gamma}$- ${\mathrm{Fe}}_{2}$${\mathrm{O}}_{3}$ with mean diameter 70 \AA{}. Four samples with volume concentration of magnetic particles ranging from 0.03% to 17% have been investigated. The magnetic relaxation of the most concentrated particle system shows typical spin glass dynamics at low temperature, e.g., the relaxation depends on the time spent at constant temperature before applying the magnetic field---the system ages. The most diluted sample shows isolated particle dynamics and no aging.

260 citations


Journal ArticleDOI
TL;DR: In this paper, the authors extended the theory of pulsed-gradient spin-echo NMR in the case of molecules trapped within pores to include wall-relaxation effects, and showed that deviations in the pore radius, obtained from the position of the echo minimum, are weak in practice, provided that the time between the gradient pulses exceeds a 2 /D, where D is the molecular self-diffusion coefficient.

221 citations



Journal ArticleDOI
TL;DR: In this paper, the authors investigated the relaxation problem for the hydrodynamic isentropic Euler-Poisson system when the momentum relaxation time tends to zero and obtained very sharp estimates on the solutions, independent of the relaxation time.
Abstract: We investigate the relaxation problem for the hydrodynamic isentropic Euler-Poisson system when the momentum relaxation time tends to zero. Very sharp estimates on the solutions, independent of the relaxation time, are obtained and used to establish compactness.

209 citations


Book
13 Jul 1995
TL;DR: In this article, the authors introduce chemical shifts and chemical exchange for spin spin coupling and spin relaxation, as well as spin-spin coupling with chemical exchange and spinner spin relaxation.
Abstract: 1. Introduction 2. Chemical shifts 3. Spin-spin coupling 4. Chemical exchange 5. Spin relaxation 6. Experimental methods Bibliography Index

Journal ArticleDOI
TL;DR: It is shown experimentally that a simple modification to the WATERGATE water suppression scheme results in a 20% gain in intensity for all signals when using a relaxation delay of 1.5 s, and that avoiding a semisaturated state for the water magnetization allows the amide protons as well as other proton resonances to relax to equilibrium with their proper relaxation time.
Abstract: A simple modification to the WATERGATE water suppression scheme [Piotto, M., Saudek, V. and Sklenař, V. (1992) J. Biomol. NMR, 2, 661–665] is proposed. Radiation damping is used as an active element during the mixing time of a NOESY experiment, in order to obtain a reproducable state of the water magnetization at the end of the mixing time. Through the use of a water flip-back pulse and a gradient-tailored excitation scheme, we obtain both an excellent water suppression and a water magnetization close to equilibrium at the beginning of the acquisition time. We show experimentally that this modification results in a 20% gain in intensity for all signals when using a relaxation delay of 1.5 s, and also that avoiding a semisaturated state for the water magnetization allows the amide protons as well as other proton resonances to relax to equilibrium with their proper relaxation time.

Journal ArticleDOI
TL;DR: In this paper, the electrochemical behavior of conducting polymers was modelled using a conformational relaxation treatment, defined as a rearrangement of the chain conformations along the polarization time.

Journal ArticleDOI
TL;DR: To investigate the relationship between multicomponent T2 relaxation and the MT process, multiecho T2 measurements have been combined with MT measurements for freshly excised samples of cardiac muscle, striated muscle, and white matter, and quantitative MT measurements were identical for the two major T2 components.
Abstract: T2 relaxation makes an important contribution to tissue contrast in magnetic resonance (MR) imaging. Many tissues are known to exhibit multicomponent T2 relaxation that suggests some compartmental segregation of mobile protons on a T2 timescale. Magnetization transfer (MT) is another relaxation mechanism that can be used to produce tissue contrast in MR imaging. The MT process depends strongly on water-macromolecular interactions. To investigate the relationship between multicomponent T2 relaxation and the MT process, multiecho T2 measurements have been combined with MT measurements for freshly excised samples of cardiac muscle, striated muscle, and white matter. For muscle, short T2 components show greater MT than long T2 components, consistent with the belief that they represent distinct water environments. For white matter, quantitative MT measurements were identical for the two major T2 components, apparently because of exchange between the T2 compartments on a time-scale characteristic of the MT experiment. Implications for accurate modeling of MT in tissue and the use of MT for MR image contrast are discussed.

BookDOI
01 Jan 1995
TL;DR: Theoretical and experimental results on the Electronic Structure of the Water Molecule (1925-1970) have been reported in this paper, showing that water can be interpreted as a mixture of spin-spin couplings.
Abstract: 1 Introduction-Water, the Unique Chemical.- 1. Introduction.- 2. The Occurrence and Distribution of Water on the Earth.- 3. Water and Life.- 4. The Scientific Study of Water-A Short History.- 5. The Place of Water among Liquids.- 2 The Water Molecule.- 1. Introduction.- 2. Principles of Structure and Spectra: The Born-Oppenheimer Separation.- 3. The Electronic Motion.- 3.1. The Ground Electronic State of Water.- 3.2. The Excited Electronic States of Water.- 4. The Nuclear Motion.- 5. External-Field Effects.- 5.1. Perturbed Hartree-Fock Method.- 5.2. Perturbed Configuration Interaction Method.- 6. Conclusion.- Appendix A. Bibliography of Theoretical Calculations on the Electronic Structure of the Water Molecule (1925-1970).- Appendix B. Definition of the Symbols Appearing in TN.- Appendix C. Definition of the Symbols Appearing in Evib and Hrotv.- Appendix D. Definition of the Symbols Appearing in the Rotational Matrix Elements.- 3 Theory of Hydrogen Bonding in Water.- 1. Introduction.- 2. Early Theoretical Studies of the Hydrogen Bond.- 3. Potential Function for the Hydrogen Bond.- 4. Recent Theoretical Methods.- 5. Water Dimer.- 6. Water Polymers.- 7. Spectroscopic Properties, Proton Potential Functions, Charge Distribution, and Related Aspects.- 8. Conclusions.- 4 The Properties of Ice.- 1. Introduction.- 2. Phase Behavior of Ice.- 3. Structures of Crystalline Ice Phases.- 4. Thermodynamic Properties of Ice Polymorphs.- 5. Mechanical Properties.- 6. Lattice Dynamics.- 7. Molecular and Ionic Transport in Ice.- 7.1. Dielectric Properties.- 7.2. Self-Diffusion.- 7.3. Electrical Conductivity.- 7.4. Orientational and Ionic Defects in Ice Theories of Transport Mechanisms.- 8. Summary.- 5 Raman and Infrared Spectral Investigations of Water Structure.- 1. Introduction.- 2. New Experimental Techniques.- 2.1. Stimulated Raman Scattering.- 2.2. Hyper-Raman or Inelastic Harmonic Light Scattering.- 2.3. Fixed-Beam Laser-Raman Method.- 3. Intermodular Vibrations of H2O and D2O.- 3.1. The Restricted Translational Region.- 3.2. The Librational Region.- 3.3. The Five-Molecule, Fully Hydrogen-Bonded C2V Structural Model.- 4. Intramolecular Vibrational Spectra from H2O in H2O and D2O.- 4.1. Raman Spectra.- 4.2. Infrared Absorbance Spectra.- 4.3. Stimulated Raman Spectra.- 4.4. Near-Infrared Spectra.- 5. Intramolecular Vibrational Spectra from H2O and D2O.- 5.1. Spontaneous and Stimulated Raman Spectra.- 5.2. Infrared Spectra.- 6. Relation of Component Properties to Water Structure.- 7. Thermodynamic Tests of the Consecutive Hydrogen-Bond Disruption Model.- 8. Current and Future Work.- 6 Nuclear Magnetic Resonance Studies on Water and Ice.- 1. Theoretical and Experimental Foundations of Magnetic Resonance.- 1.1. Properties of Nuclei in Electric and Magnetic Fields.- 1.2. Spectroscopic Parameters Describing Nuclear Magnetic Resonance.- 1.3. Experimental Arrangements.- 2. Magnetic Shift and Spin Coupling Phenomena in Water Vapor, Water, and Ice.- 2.1. Theory of Chemical Shift.- 2.2. Measurement of Deuteron and 17O Chemical Shifts in Water.- 2.3. Structural Interpretations of Chemical Shift Measurements.- 2.4. Measurements of Spin-Spin Coupling Constants.- 3. Measurement and Interpretation of Magnetic Relaxation Times in Water and Ice.- 3.1. Dipole-Induced Relaxation.- 3.2. Measurement and Interpretation of Proton Relaxation in Water.- 3.3. Measurement and Interpretation of Proton Relaxation in Ice.- 3.4. Quadrupole-Induced Relaxation.- 3.5. Measurement and Interpretation of Deuteron and 17O Relaxation in Water.- 3.6. Summary of Relaxation Experiments on Liquid Water.- 3.7. Measurement of Self-Diffusion Coefficient.- 3.8. Measurement of Kinetic Rate Constants in Water.- 4. The Contributions of NMR Methods to the Structure Problem in Liquid Water.- 4.1. Results from Chemical Shift Measurements.- 4.2. Results from Relaxation Measurements.- 7 Liquid Water: Dielectric Properties.- 1. Introduction.- 2. The Electrical Properties of the Free Water Molecule.- 3. The Measured Static Dielectric Constant.- 4. Theory of the Static Dielectric Constant.- 5. The Static Dielectric Constant Calculated on Various Models.- 6. Interpretation of Static Dielectric Constant with Induced-Dipole Contribution.- 7. Microwave and Submillimeter Dielectric Constants.- 8. Kinks?.- 9. Submillimeter Measurements.- 10. Theory of the Dielectric Constant in a Time-Varying Field.- 11. Interpretation of the Relaxation Times and Absorption Bands.- 12. Interpretation of Data in the Submillimeter Band. Conclusions.- 8 Liquid Water: Scattering of X-Rays.- 1. Introduction.- 2. Experimental Methods.- 2.1. Large-Angle Scattering.- 2.2. Small-Angle Scattering.- 2.3. Data Reduction.- 3. Analysis of Diffraction Data.- 3.1. Scattering by Heteroatomic Liquids.- 3.2. Scattering by Model Liquids.- 4. Diffraction Pattern of Liquid Water.- 4.1. Survey of Experimental Data.- 4.2. Interpretation of the Diffraction Pattern.- 5. Conclusions.- 9 The Scattering of Neutrons by Liquid Water.- 1. Introduction.- 2. The Neutron Scattering Method.- 2.1. Nuclear Scattering of Neutrons.- 2.2. Coherent and Incoherent Scattering.- 2.3. Isotopic Substitution.- 2.4. The Scattering Law and Correlation Functions.- 2.5. Neutron Spectra from Water.- 3. Structural Measurements.- 3.1. Structure Factor for Water.- 3.2. Molecular Structure Factors.- 4. Quasielastic Scattering of Neutrons by Water.- 4.1. Debye-Waller Factor.- 4.2. Diffusive Motions.- 5. Inelastic Scattering of Neutrons by Water.- 5.1. Spectral Density Function.- 5.2. Experimental Results.- 6. Discussion.- Appendix. Calculation of Orientational Correlation Factors for Molecules.- 10 Thermodynamic and Transport Properties of Fluid Water.- 1. Introduction.- 2. Thermodynamic Properties.- 2.1. The Macroscopic Viewpoint.- 2.2. Volume Properties.- 2.3. Thermal Properties.- 2.4. Liquid-Vapor Equilibria.- 2.5. Critical Point.- 3. Transport Properties.- 3.1. The Hydrodynamic Viewpoint.- 3.2. Vapor.- 3.3. Liquid.- 11 Application of Statistical Mechanics in the Study of Liquid Water.- 1. Introduction.- 2. Characteristic Features of an Effective Pair Potential for Liquid Water.- 3. Application of the Percus-Yevick Equation.- 4. Application of the Monte Carlo Technique.- 5. Some Specific Applications of the Pair Correlation Function.- 5.1. Density Fluctuations in Water.- 5.2. Local Structure Index for Water.- 5.3. Hole and Particle Distributions in Water.- 5.4. A Possible Exact Definition of the Structure of Water.- 12 Liquid Water-Acoustic Properties: Absorption and Relaxation.- 1. Introduction.- 2. Ultrasonic Absorption.- 3. Ultrasonic Techniques.- 3.1. Acoustic System.- 3.2. Pressure- and Temperature-Control Systems.- 3.3. Electronics.- 4. Experimental Results.- 4.1. Volume Viscosity.- 5. Relaxational Compressibility.- 6. Structural Relaxation Time.- 7. Conclusions.- 13 Water at High Temperatures and Pressures.- 1. Introduction.- 2. Thermodynamic Properties.- 2.1. PVT Data and Equation of State.- 2.2. Thermodynamic Functions.- 3. Transport Properties.- 3.1. Viscosity.- 3.2. Self-Diffusion.- 3.3. Thermal Conductivity.- 4. Electrical Properties.- 4.1. Static Dielectric Constant.- 4.2. Specific Conductance and Ionic Conductances.- 5. Ionic Product.- 6. Spectroscopic Studies.- 6.1. Infrared Spectral Studies.- 6.2. Raman Spectral Studies.- 6.3. Interpretation of Spectroscopic Results with Respect to Water Structure.- 7. Concluding Remarks.- 14 Structural Models.- 1. The Nature of Models.- 2. Requirements a Water Model Must Attempt to Satisfy.- 3. Some Properties of Water of Special Current Interest.- 3.1. Thermodynamic and Mechanical Properties Structure and Fluctuations.- 3.2. X-Ray Scattering Structure and Fluctuations.- 3.3. Hydrogen Bonds and Structure.- 3.4. Spectroscopic Properties: Broken Hydrogen Bonds.- 3.5. Vapor-Pressure Isotope Effect.- 3.6. Molecular Motions.- 4. Brief Survey of Models.- 4.1. Mixture Models.- 4.2. Uniformist Models.- 4.3. Interstitial Models.- 5. The Present "Best Guess".- 5.1. Bonding.- 5.2. Structure Pattern.- 5.3. Equilibrium Relations.- 5.4. Molecular Motions.- 5.5. Heat Capacity.- References.

Journal ArticleDOI
TL;DR: It is shown that electron scattering by confined acoustic phonons interacting through the deformation potential is substantially suppressed up to the electron energies corresponding to the energy of the first dilatational mode.
Abstract: The confined acoustic phonons in free-standing quantum wells are considered in detail. The Hamiltonian describing interactions of the confined acoustic phonons with electrons in the approximation of the deformation potential and the corresponding electron transition probability density are derived. They are used to analyze the electron scattering times (inverse scattering rate, momentum relaxation time, and the energy relaxation time) in the test-particle approximation as well as in the kinetic approximation. It is shown that the first dilatational mode makes the main contribution to electron scattering in the lowest electron subband. The contribution of the zeroth mode and the second mode are also essential while the modes of higher order are insignificant. Our analysis is performed for both nondegenerate and degenerate electron gases. It is shown that electron scattering by confined acoustic phonons interacting through the deformation potential is substantially suppressed up to the electron energies corresponding to the energy of the first dilatational mode.

Journal ArticleDOI
Robert Tycko1, S. E. Barrett1, Gary Dabbagh1, L. N. Pfeiffer1, K. W. West1 
09 Jun 1995-Science
TL;DR: Radio-frequency measurements reveal effects of electron-electron interactions on the energy levels and spin states of the two-dimensional electron system confined in the GaAs wells and rapid, temperature-independent relaxation at intermediate v values indicates a manifold of low-lying electronic states with mixed spin polarizations.
Abstract: An optical pumping technique was used to enhance and localize nuclear magnetic resonance (NMR) signals from an n-doped GaAs/Al0.1Ga0.9As multiple quantum well structure, permitting direct radio-frequency measurements of gallium-71 NMR spectra and nuclear spin-lattice relaxation rates (1/T1) as functions of temperature (1.6 K < T < 4.2 K) and the Landau level filling factor (0.66 < v < 1.76). The measurements reveal effects of electron-electron interactions on the energy levels and spin states of the two-dimensional electron system confined in the GaAs wells. Minima in 1/T1 at v approximately 1 and v approximately 2/3 indicate energy gaps for electronic excitations in both integer and fractional quantum Hall states. Rapid, temperature-independent relaxation at intermediate v values indicates a manifold of low-lying electronic states with mixed spin polarizations.

Journal ArticleDOI
TL;DR: The present results resolve the long-standing controversy regarding the mechanism behind the spin relaxation dispersion of water nuclei in protein solutions, thus establishing oxygen-17 relaxation as a powerful tool for studies of structurally and functionally important water molecules in proteins and other biomolecules.

Journal ArticleDOI
TL;DR: In this article, the standard theory of NMR relaxation in liquids (with molecular motion described as a classical Brownian motion, and including intermolecular spin-spin couplings) is re-examined, taking great care not to drop significant contributions from the dipolar coupling between distant molecules.
Abstract: The standard theory of NMR relaxation in liquids (with molecular motion described as a classical Brownian motion, and including intermolecular spin–spin couplings) is re‐examined, taking great care not to drop significant contributions from the dipolar coupling between distant molecules. This results in ‘‘modified Bloch–Redfield equations’’ for the spins in a single molecule, valid at all spin temperatures, which contain both the usual relaxation terms and a coupling of each spin with a classical average dipolar field. Delicate issues raised in this derivation, like the neglect of quantum correlations between spins on different molecules at (repeated) initial times, are discussed with the help of exact calculations (for all spin temperatures) performed on a simplified model which includes equal couplings between all N spins of a system. The same model is used to compare the merits of different forms of ‘‘high temperature’’ approximation. We also propose an iterative scheme for solving the ‘‘modified Bloch...

Journal ArticleDOI
TL;DR: In this article, the dynamics of α-relaxation in homogeneous mixtures of polystyrene (PS) and poly(cyclohexyl acrylate-stat-butyl methacrylate) (PCHA-stat BMA)) are analyzed within the framework of a concentration fluctuation model.
Abstract: The dynamics of the α-relaxation in homogeneous mixtures of polystyrene (PS) and poly(cyclohexyl acrylate-stat-butyl methacrylate) (P(CHA-stat-BMA)) is analyzed within the framework of a concentration fluctuation model. The shape and widths of the dielectric relaxation spectra as well as the range of calorimetric glass transition in the mixtures are respectively associated with distributions of relaxation time and T arising from the presence of concentration fluctuations. The magnitude of these fluctuations is obtained by fitting the dielectric loss curves to a model function obtained as a convolution of a relaxation time distribution for the mixture with a function describing the dielectric loss of the dielectrically active component. The relaxation time distribution is calculated from William-Landel-Ferry (WLF) or Vogel-Fulcher-Tammann (VFT) free volume scaling with the assumption of a Gaussian concentration distribution in the samples. Fits of the measured α-relaxation spectra which are dominated by P(CHA-stat-BMA) offer as a reset the mean square concentration fluctuations in the mixtures. The fluctuations are interpreted in terms of the random phase approximation, and it is shown that is correlated to the length scale of cooperativity governing the relaxation process near the glass transition. The temperature and composition dependence of both and the size of the domains of cooperativity in PS/P(CHA-stat-BMA) are determined

Journal ArticleDOI
TL;DR: In this article, molecular mobility in propylene glycol and its three oligomers confined to the ∼100 A pores of a controlled porous glass has been investigated using dielectric and neutron spectroscopy.
Abstract: In this paper we present and discuss experimental results on molecular mobility in propylene glycol and its three oligomers confined to the ∼100 A pores of a controlled porous glass. The objective is to elucidate the finite size effects on the dynamics of hydrogen‐bonded liquids of different molecular weights but identical chemical composition. The methods of dielectric and neutron spectroscopy have been employed to investigate both the low‐ and high‐frequency features as a function of temperature. We find that all fluids in pores separate into two distinct liquid phases. (i) molecules physisorbed at the surface which exhibit a dramatic frustration of their mobility related to a substantial positive shift of the glass transition temperature Tg by up to ΔTg≊+47 K; and (ii) relatively ‘‘free’’ molecules in the inner pore space subject to only moderate retardation of the α and normal mode relaxation and substantial broadening of the distribution of relaxation times. The shift in Tg for the α process with ΔTg...

Journal ArticleDOI
15 Sep 1995
TL;DR: In this paper, a method is described for using measurements of nuclear magnetic resonance (NMR) relaxation times in hydrogels to obtain pore radius distribution profiles in a rapid and noninvasive fashion.
Abstract: A method is described for using measurements of nuclear magnetic resonance (NMR) relaxation times in hydrogels to obtain pore radius distribution profiles in a rapid and noninvasive fashion. Data for spin-lattice relaxation times of the water in the gel are interpreted by using the "magnetization-diffusion" equation that has been used previously for similar measurements in porous glass, sandstones and wood pulp. To account for the fibrous microstructure of hydrogels, a fiber-cell model is introduced and used to demonstrate the existence of a "fast-diffusion" limit, in which NMR relaxation is exponential with a single time constant. It is demonstrated that agar, agarose, and polyacrylamide gels easily satisfy the criterion for fast diffusion, and the model is used to calculate pore-size distributions for those materials. This technique will prove very valuable for real-time, noninvasive monitoring of variations in microstructure occurring over length scales of approximately 100 μm in both synthetic gels and many foods.

Journal ArticleDOI
TL;DR: In this article, the collective normal mode dynamics and their effects on correlations between torsion angle fluctuations and heteronuclear NMR relaxation parameters were investigated using the protein crambin as a model system.
Abstract: Theoretical methods are developed and applied to the protein crambin as a model system to characterize collective normal mode dynamics and their effects on correlations between torsion angle fluctuations and heteronuclear NMR relaxation parameters. Backbone N–H NMR S2 order parameters are found to be predominantly determined by local φ and ψ torsion angle fluctuations induced by collective protein modes. The ratio between Cβ–Hβ and Cα–Hα order parameters directly yields fluctuation amplitudes of the sidechain χ1 torsion angles. The results allow a more direct interpretation of motional effects monitored by nuclear spin relaxation.

Journal ArticleDOI
TL;DR: In this paper, femtosecond measurements of the dynamics of photoinduced electrons at the liquid-metal interface produced by exciting the surface plasmon band of aqueous silver colloidal particles are reported.
Abstract: We report direct femtosecond measurements of the dynamics of photoinduced electrons at the liquid–metal interface produced by exciting the surface plasmon band of aqueous silver colloidal particles. The electron plasma resonance created initially dephases into individually excited electrons in less than 150 fs. This is followed by a large component, fast exponential decay with a time constant of 2 ps, which is attributed to electronic energy relaxation through electron–phonon coupling. A slower 40 ps decay is also observed and attributed to subsequent cooling of the excited phonons due to phonon–solvent interaction. The decay dynamics, especially the 2 ps decay, are relatively insensitive to the solvent environment, indicating that the early time decay is dominated by the properties of the silver particles. The solvent molecules play an important role in the phonon cooling process following the electronic energy relaxation. The results show that the majority of the electrons created through photoexcitatio...

Journal ArticleDOI
TL;DR: A good correlation between 6Li isotropic chemical shifts and oxygen coordination number is demonstrated, and this result is used to describe the range of coordination environments for Li in silicate glasses and the second-order quadrupolar shift for 7Li is derived from 7Li and 6Li MAS spectra acquired at a single magnetic field.

Journal ArticleDOI
TL;DR: In this paper, the effect of changing Nd2O3 concentration on 29Si magic-angle spinning nuclear magnetic resonance (MAS-NMR) spin-lattice relaxation rates was investigated.
Abstract: High field strength cations such as rare earth ions are known to show strong clustering when incorporated in minor quantities (hundreds to up to a few thousands of ppm) in highly polymerized silicate liquids and glasses. In this study, formation, distribution and growth of such clusters in Nd2O3-doped SiO2 glasses have been detected by monitoring the effect of changing Nd2O3 concentration on 29Si magic-angle spinning nuclear magnetic resonance (MAS-NMR) spin-lattice relaxation rates. The results indicate the presence of clusters of Nd ions even at the lowest doping level of 300 ppm of Nd2O3 by weight. Once the Nd2O3 concentration reaches 1400 ppm, no new clusters are found to form on further addition of Nd2O3 and only the growth of existing clusters is observed. These observations have significance for both dipolar resonant energy transfer among Nd ions and for fluorescence quenching. Electron paramagnetic resonance spectroscopy and 29Si MAS-NMR spin-lattice relaxation studies indicate that, on doping SiO2 glass with Al2O3, paramagnetic defect sites are created. 29Si relaxation results also show that, on co-doping the Nd2O3-doped glasses with Al2O3, a completely homogeneous distribution of the Nd ions is obtained at an Al:Nd atomic ratio of 10:1.

Journal ArticleDOI
TL;DR: Groot et al. as discussed by the authors applied an off-lattice simulation model to obtain the mechanical spectrum as a function of frequency and polymer concentration, using a Green-Kubo relation for the time-dependent modulus.
Abstract: An off-lattice simulation model for associative polymer gels as introduced recently (Groot, R. D. ; Agterof, W. G. M. J. Chem. Phys. 1994, 100, 1649) has been applied to obtain the mechanical spectrum as a function of frequency and polymer concentration, using a Green-Kubo relation for the time-dependent modulus. Two stages of relaxation are observable in our simulations. The early-time decay is consistent with a -2/3 power law, whose form is insensitive to large variations in polymer concentration, association lifetime, and degree of association. The late stage, which relaxes like the end-to-end vector in the Rouse model, has a characteristic stress that scales as the cube of the concentration and a relaxation time that is proportional to the monomer-monomer dissociation rate. The simulation results have been compared with experiments found in the literature for several physical gels. The quantitative agreement calls into question other postulated mechanisms involving hydrodynamic interaction or reptation, since the simulation contains neither of these features. As an alternative explanation for the observed early-time decay, an explicit relation between the power law exponent and the polymer fractal dimension is given.

Journal ArticleDOI
TL;DR: In this article, the authors used rheological data together with an emulsion model to determine the volume weighted sphere-size distribution up to a scaling depending on the interfacial tension.
Abstract: Often, blends of two immisible polymers have a morphology with one component building a matrix in which spherical inclusions of the other component are embedded. The rheological response of such blends contains an elastic contribution which can be attributed to the form relaxation of the inclusions. This process has a characteristic relaxation time which is proportional to the radius of the inclusions divided by the interfacial tension between the blends’ components. Thus a distribution of radii leads to a distribution of relaxation times. It is shown that rheological data together with an emulsion model can be used to determine the volume weighted sphere‐size distribution up to a scaling depending on the interfacial tension. The procedure is applied to data of four PMMA/PS blends and the results are compared with the corresponding distributions obtained from transmission electron microscopy (TEM). If the concentration of the spherical inclusions is small, both results are in excellent agreement. For larg...

Journal ArticleDOI
TL;DR: In this paper, the effect of spatial variations in both the diffusivity and relaxation properties within the porous medium, occurring over lengthscales of order > 100 μm, was considered, and the results suggest that macroscopically heterogeneous materials have a greater tortuosity than more homogeneous materials.

Journal ArticleDOI
TL;DR: In this article, the authors have borrowed terminology from chemical gelation and applied it to an example of physical gelation: the isothermal crystallization of isotactic polypropylene.
Abstract: Polymeric materials at the liquid–solid transition exhibit unusual simplicity and regularity in their relaxation pattern. This expresses itself in a self-similar relaxation modulus G(t)=St–n at long times λ0 < t < ∞, where λ0 is the characteristic time for the crossover to a different relaxation regime (e.g. crossover to glass transition or entanglement region). Rheological features of liquid–solid transitions are very similar for chemical and physical gelation: (1) broadening of the relaxation time spectrum, (2) divergence of the longest relaxation time (with an upper cut-off for physical gels) and (3) self-similar relaxation patterns. We have borrowed terminology from chemical gelation and applied it to an example of physical gelation: the isothermal crystallization of isotactic polypropylene. The transition through the gel point has been investigated by dynamic mechanical experiments. The influence of temperature and crystallization rate have been studied. The degree of crystallinity (estimated by the Avrami equation) at the gel point was very low (6–15% depending on the crystallization temperature).