Showing papers on "Relaxation (NMR) published in 1978"

Dynamics of rod-like macromolecules in concentrated solution. Part 1

Abstract: A concentrated solution of rod-like macromolecules is thermodynamically ideal if the rods are thin enough, but its dynamical behaviour is completely different from that of a dilute solution because of the “entanglement” constraint that the rods cannot pass through each other. This characteristic feature is discussed. First the basic kinetic equation which describes the Brownian motion is set up. This equation is then applied to two particular experiments, the Kerr effect and dynamic light scattering. The principal results are : (1) The characteristic relaxation time τr of the Kerr effect is very long, and depends strongly on the concentration ρ and the molecular weight M such that τr∝ρ2M7, and the relaxation is non-exponential. (2) The dynamical structure factor S(k, t) measured by light scattering has a characteristic shape which is independent of concentration; the initial slope of S(k, t) being about half the value predicted for dilute solution; S(k, t) has a very long tail as S(k, t)∝ 1/√t.

Theory of NMR relaxation in macromolecules: Restricted diffusion and jump models for multiple internal rotations in amino acid side chains

Abstract: The theory required to extract detailed motional information from NMR relaxation times of nuclei in an amino acid side chain containing multiple internal rotation axes attached to a large macromolecule of at least cylindrical symmetry is developed. Emphasis is placed on the analysis of 13C‐NMR of protonated carbons where dipolar relaxation is predominant. Extension to other relaxation processes is straightforward. The spectral density from which the relaxation times can be calculated is obtained for various models for the motion of the side chain. The existing theory which assumes that internal rotations are both independent and free is generalized to incorporate excluded volume effects in a heuristic way by restricting the amplitude of the internal rotations. It is found that small amplitude motions are ineffective in causing relaxation. Thus jump models involving a relatively small number of configurations are appropriate to describe the motion of the side chain. The advantage of jump models is twofold:...

Magnetic cross-relaxation among protons in protein solutions.

Abstract: The magnetic spin-lattice relaxation rates of solvent water nuclei are known to increase upon addition of diamagnetic solute protein. This enhancement of the relaxation rate is a function of magnetic field, and the orientational relaxation time of the protein molecules can be deduced from analysis of the field-dependent relaxation rates. Although the nature of the interactions that convey information about the dynamics of protein motion to the solvent molecules is not established, it is known that there is a contribution to the relaxation rates of solvent protons that plays no role in the relaxation of solvent deuterons and 17O nuclei. We show here that the additional interaction arises from a cross-relaxation process between solvent and solute protons. We introduce a heuristic three-parameter model in which protein protons and solvent protons are considered as two separate thermodynamic systems that interact across the protein-solvent interface. The three parameters are the intrinsic relaxation rates of each system and a cross-relaxation term. The sign of the latter term must always be positive, for all values of magnetic field, in order for magnetization energy to flow from the hotter to the cooler system. We find that the magnetic field-dependence of the cross-relaxation contribution is much like that of the remaining solvent proton relaxation, i.e., about the same as the deuteron relaxation field dependence. This finding is not compatible with the predictions of expressions for the cross-relaxation that have been used by other authors, but not applied to data over a wide range of magnetic field strength. The model predicts that the relaxation behavior of both the protein protons and the solvent protons is the sum of two exponentials, the relative contributions of which would vary with protein concentration and solvent isotopic composition in a fashion suggestive of the presence of two classes of protein protons, when there is in reality only one. This finding has immediate implications for the interpretation of published proton relaxation rates in complex systems such as tissues; these data should be reexamined with cross-relaxation taken into account.

Rotation of Molecules and Nuclear Spin Relaxation

Abstract: Nuclear spin relaxation has been developed as a standard method for studying molecular motions in liquids, solids, polymers, and—to a lesser extent—gases, staring with the pioneering work of Bloembergen, Purcell, and Pound [1]. Of the great variety of molecular motions possible (e.g., translations, rotations, vibrations) rotations are particularly important for nuclear spin relaxation. Conversely, nuclear spin relaxation can be especially successful if information about rotational motions is desired. In this case nuclear spin relaxation can yield quantitative information over an extraordinary wide range of characteristic frequencies, from about 1 Hz to 1014Hz. It shoud be noted that, typically, the nuclear spin relaxation times actually observed are much longer than the characteristic times of rotation of molecules. Therefore, a theory is needed which links the time constants of nuclear spin relaxation to the correlation functions describing the rotational motion, and so a considerable part of this volume is devoted to this goal. For rapid motions in liquids of low viscosity nuclear spin relaxation thus in principle, is inferior to scattering experiments (e.g., light and neutron scattering) which allow the determination of correlation functions.

Measurement of protein rotational diffusion in membranes by flash photolysis.

Abstract: Publisher Summary This chapter describes the measurement of protein rotational diffusion in membranes by flash photolysis. Spectroscopic methods of measuring rotation depend on photoselection, whereby an oriented population of excited molecules is optically selected from an initial random distribution. This is achieved by excitation with plane-polarized light, so that those molecules whose transition moment for absorption is parallel or at a small angle to the electric vector of the incident light are preferentially excited. Signals arising from the excited molecules in general reflect their anisotropic distribution so that emission signals are polarized and absorption signals are dichroic. When excitation is by a brief pulse of light, the initial emission or absorption anisotropy decays as the molecules again become randomized by Brownian rotation. From the rate of decay, rotational relaxation times may be determined. To make measurements, one must have a molecule with suitable spectroscopic properties. While a few membrane proteins have a suitable intrinsic chromophore, it is generally necessary to attach an artificial chromophore or probe.

Chemical Shifts of 119Sn Nuclei in Organotin Compounds

Abstract: Publisher Summary This chapter describes the measurement of, and factors influencing, 1I9 Sn chemical shifts of organotin compounds. Elemental tin contains 10 naturally occurring isotopes, and only three of these have nonzero nuclear magnetic moments: 115 Sn, 117 Sn, and 119 Sn. Each of the three magnetic tin isotopes has a nuclear spin of 4 and therefore zero quadrupole moment. All three isotopes have approximately equal nuclear magnetic moments. However, in nuclear magnetic resonance (NMR) investigations of tin compounds, the chemical shifts are usually measured for 119 Sn nuclei because of the greater intensity of their NMR signals and because of their higher natural abundance. Fourier transform (FT) NMR spectroscopy, although used extensively to study 13 C and 15 N nuclei, has only very recently been applied to 119 Sn. Pulse FT NMR has been used to study the spin-lattice relaxation times, T 1 , of l19 Sn in a number of organic and inorganic tin compounds. The most important relaxation mechanism for this nucleus in a series of tetraorganotins appears to be spinrotation (SR), although for larger molecules, such as hexabutylditin, dipole–dipole (DD) relaxation is important, even at room temperature.

The depolarized Rayleigh scattering from fluids of spherical molecules

Abstract: A calculation of the depolarized Rayleigh scattering from a liquid of isotropically polarizable molecules is described. The spectrum reflects cooperative, many-particle, structural relaxation which is represented by a generalized Langevin equation for the bilinear number density fluctuations. The theory is applied to the particular case of liquid argon at the triple point and is shown to give a very close quantitative agreement with the experimental lineshape, using data from neutron-scattering experiments on liquid argon and no adjustable parameters. The various features of the observed spectra are related to the behaviour of the correlation functions of different Fourier components of the bilinear number density fluctuations which are interpreted in different limits of the molecular motion, diffusion, gas-like, etc. The sensitivity of the lineshape to modifications of the induced polarizability functional from DID form is investigated. Within the range of physically reasonable modifications little chang...

Surface Relaxation Times of Conduction-Electron Spins in Superconductors and Normal Metals

Abstract: Electron spin relaxation times in surface collisions were deduced from measurements on superconductors of the NMR Knight shift, critical magnetic fields, and tunneling conductance as well as from measurements on normal metals by conduction-electron spin resonance. The relaxation times ${\ensuremath{\tau}}_{\mathrm{so}}$ as determined by the various methods are consistent and depend on the atomic number of the metal in a manner similar to that proposed by Abrikosov and Gor'kov.

Pulsed FT-NMR double resonance studies of yeast tRNAPhe: specific nuclear Overhauser effects and reinterpretation of low temperature relaxation data

Abstract: Cross-relaxation effects are demonstrated between the imino protons and other protons in yeast tRNAPhe and H2O. A detailed examination has been made of the observed relaxation rate of the proton resonance at 11.8 ppm from DSS as a function of the D2O content in the solvent. This result, as well as the size and number of observed nuclear Overhauser effects, suggests that dipolar magnetization transfer between solvent H2O, amino, imino, and other tRNA protons may dominate the relaxation processes of the imino protons at low temperature. At higher temperatures the observed relaxation rate is dominated by chemical exchange. The selective nuclear Overhauser effects are shown to be an important aid in resonance assignments. By these means we were able to identify tow protons from the wobble base pair GU4 at 11.8 ppm and 10.4 ppm.

Broadening mechanism in semiconductor (GaAs) lasers: Limitations to single mode power emission

Abstract: A nonlinear steady-state theory of the emission spectrum of semiconductor (GaAs) lasers above threshold is developed, and limitation to power in a single longitudinal mode is studied. The nonlinear steady-state rate equations describing the power and the gain are solved iteratively. The model is based on the well known idea that the gain always saturates somewhere below the loss, and power sharing among the modes is dependent on the relative gain of the modes with respect to the loss level. The limitation to single mode power is essentially due to the uneven rate of saturation of the gain of the different modes as they approach the loss level asymptotically, with the dominant mode having the fastest saturation. The rate of saturation of the gain of different modes depends on the power emission spectrum and the intraband relaxation rate of the carriers. In this work, the relaxation is accounted for by using the generalized spectral weight function to describe the carriers. The dependence of maximum single mode power on intraband relaxation time is obtained. It ranges from a few milliwatts for relaxation time of the order of 10-12s to hundreds of milliwatts for relaxation time of 10-13s. The predictions of the model on gain saturation spectra, and carrier lifetime spectra agree well with experimental observations. The gain is seen to saturate near the lasing energy but continues to increase at a reduced rate at higher energy levels. The carrier radiative lifetime is found to decrease sharply in the vicinity of the lasing mode energies.

NMR Relaxation Studies of Solute-Solvent Interactions

Abstract: This review focuses on the application of nuclear magnetic resonance relaxation measurements to the study of solvent-solute interactions. It primarily deals with solutions of low molecular weight solutes and with aqueous protein solutions in an attempt to compare the approaches to these rather different systems. Magnetic resonance has been so widely used to characterize solution chemistry of all sorts that it is difficult to distinguish clearly between those experiments that deal directly with solvent-solute interactions and those that do not. This review therefore omits major areas of activity and makes no attempt to be comprehensive. Discussion of solvent or solute relaxation measurements in systems containing liquid crystals, lipids, polynucleotides, polysaccharides, inorganic solids such as clay, and most of the data on whole tissues is largely omitted for lack of space, not interest. Fundamental aspects of NMR have previously been presented (1-7). Reviews of magnetic resonance appear regularly in several series (8-12). Discussions of NMR relaxation and related topics appear in related review series (13-15). Grandly stated, the goal is to understand solutions. Unfortunately one is limited by the observations that are possible and the preconceived notions that are brought to the problem. For example, the existence of a solvent in the first coordination sphere of the transition metal ions is clearly documented. The first coordination sphere water molecules remain bonded to chromium(III) ions in water for times exceeding a day at room temperature, while other metal ions exchange solvent more rapidly (16). In either case the first coordination sphere is included in formu­ lating the chemistry of these ions; we write Cr(H20)�+, rather than Cr3+. The situation is less clear for other electrolyte ions such as the halide or alkali metal ions. One may expect to discover the coordination number for an ion by using a new experimental approach; however, if the solvent lifetime in the first coordination sphere region of the ion at a temperature of interest is not significantly longer than the time required for a solvent molecule to translate one jump length in the pure

Quadrupolar spin-lattice relaxation in solids

Abstract: Calculations have been carried out for quadrupolar spin-lattice relaxation in NMR experiments, involving nuclei in non-cubic environments, which extend and, in certain cases, correct previous treatments for I=3/2, 5/2, 7/2 and 9/2. In particular, pulsed NMR experiments are treated and the results are presented in a way which reveals non-exponential effects. Since it is not in general meaningful to introduce a spin-lattice relaxation time some emphasis is placed on possible methods for measuring the quantities W2 and W1 which are related to the transition rates for Delta m=+or-2 and Delta m=+or-1 transitions involved in two-phonon relaxation processes. Results for 59Co in K3Co(CN)6 are briefly discussed.

Theory of nuclear spin relaxation by translational self-diffusion in liquid crystals: Nematic phase

Abstract: The theory of intermolecular nuclear spin relaxation by translational self-diffusion in liquid crystals is developed. Torrey's treatment of simple liquids is extended and modified for the liquid-crystalline phases by taking into account the anisotropy of the molecular motion, the elongated molecular shape, and the spin distribution on a molecule. Results, obtained for the frequency and angular dependence of ${T}_{1}$ in the nematic phase, are presented graphically for a variety of parameters and are compared with Torrey's results for classical liquids. A brief comparison with the available experimental data is presented.

Magnetic field dependence of relaxation times in nonequilibrium superconductors

Abstract: We have investigated the effects of a magnetic field H on nonequilibrium quasiparticle relaxation processes in superconductors by analyzing the I–V characteristics of long tin microbridges in a parallel field near T c (H). Following Skocpol, Beasley, and Tinkham, we take the differential resistance of a given phase-slip center (PSC) to be approximately equal to the normal resistance R n of a nonequilibrium region of the bridge with length 2(DΤR)1/2, where D is the quasiparticle diffusion constant andΤ R is the transverse mode (branch imbalance) relaxation time. The magnitude and the temperature and field dependence ofΤ R as inferred from our data agree well with those of the transverse mode relaxation time in the presence of pair-breaking, derived by Schmid and Schon. The longitudinal mode disequilibrium is interpreted in terms of local heating proportional to IV, and the resulting distortions in the I–V characteristics are corrected. Most of our samples contain a deliberate weak spot with a depressed critical current I c, which isolates a single PSC and lowers the heat dissipation(~I c 2Rn).

The state of water in normal and senile cataractous lenses studied by nuclear magnetic resonance

Abstract: Normal and senile cataractous lenses were studied by nuclear magnetic resonance in order to get information about the state of water during the progression of cataract. Water proton spin-spin (T2) and spin-lattice relaxation times—in the laboratory and the rotating frame—() were measured at 34 and 90 MHz. At temperatures below −9°C the relaxation times of protons in “non-freezable” component of lens water were measured as a function of the temperature, and T1 and T2 relaxation times were found to be the same for both the normal and the cataractous lenses. At temperatures above −9°C a marked difference in relaxation times T1 and T2 was found between normal and completely opaque lenses. It was concluded that the most characteristic difference between the normal and completely opaque lenses should be accounted for by the change in the “bound”-to-“free” ratio of lens water.

A study of 'absorption currents' in low-density polyethylene

Abstract: A study of variations of charging and discharging transients in low-density (0.918 gm cm-3) polyethylene has been made over a wide range of fields and temperature and a limited range of sample thickness and electrode materials. In the temperature range 113-273K two broad isochronal (i.e. current at constant time) peaks were observed at 138K ( gamma -relaxation) and 203K respectively. The observed behaviour of both peaks may be explained by dipolar processes accompanying molecular relaxations in the bulk. Above 273K the 'absorption current' may be due to a charge carrier hopping mechanism.

Nuclear magnetic resonance relaxation times and plasmalemma water exchange in ivy bark.

Abstract: Measurement of nuclear magnetic resonance (NMR) relaxation times (transverse [T 2 ] and longitudinal [T 1 ]) for Hedera helix L. cv. Thorndale (ivy) bark water indicates the presence of at least two populations of water with different relaxation characteristics. One population of water with short T 2 and T 1 was found to be composed of both hydration water and extracellular free water. The second population of water with long T 2 and T 1 was identified as intracellular bulk water. NMR relaxation of extracellular water protons is controlled by cell wall surface effects, possibly due to binding of paramagnetic cations by the cell walls. NMR relaxation of intracellular water protons is controlled by both water exchange to the extracellular environment and chemical exchange with a population of protons that is chemically shifted from that of the bulk water. The relaxation time of intracellular water is not measurably affected, either by intracellular paramagnetic ions or by increased viscosity of intracellular water. Manganese flux into the cells occurs at 1.7 × 10 −15 moles cm −2 seconds −1 and is independent of extracellular Mn 2+ concentration in the range 5 to 20 mm. The intracellular-extracellular water exchange time of ivy bark was found to be predominantly limited by membrane water permeability. A diffusional water permeability coefficient (P d ) of approximately 3 × 10 −2 cm seconds −1 was calculated for ivy cell membranes at 20 C.

Brillouin Scattering and Segmental Motion of a Polymeric Liquid, I.

Abstract: The effect of segmental motion on the density–density correlation function of a viscous polymer liquid has been analyzed using a generalized relaxation equation developed by Zwanzig and Mori. It is shown that for polymer liquids of high viscosity, Brillouin scattering is closely associated with the structural relaxation associated with the motion chain segments. A single relaxation time theory is shown to yield good agreement with the experimental results on polypropylene glycol. The torsional motion involving a small number of monomer units is shown to be responsible for the dispersion and attenuation of the hypersonic wave. The fact that the Brillouin scattering spectrum of a polymer liquid is insensitive to the change of molecular weight is discussed. We have shown that temporal modulation of the spatial second moment of the intermolecular or intersegmental interaction energy is responsible for the relaxation process involved in Brillouin scattering.

Fluorotyrosine M13 coat protein: fluorine-19 nuclear magnetic resonance study of the motional properties of an integral membrane protein in phospholipid vesicles.

Abstract: We have prepared in vivo a fluorotyrosyl derivative of M13 coat protein and have incorporated it at high levels in small phospholipid vesicles, using a urea-cholate dialysis procedure. 19F nuclear magnetic resonance experiments at 254 MHz with this system indicate a T1 of 0.32 s and line width of 300 Hz. The observed line width increases dramatically below the gel to liquid-crystalline transition temperature for the lipid, indicating that the probe is sensitive to the phase state of the bilayer. Neclear Overhauser enhancement and field dependence of line width were used to establish the relative contributions of dipolar interactions and chemical-shift anisotropy to the observed T1 and line width. From this relaxation data, we have constructeda model for the motional properties of the protein in the lipid bilayer. This model is characterized by correlation times for rotation about the alphabeta and betagamma bonds of the two tyrosyl residues of 2 x 10(-8) and k x 10(-9) s, respectively. Rapid intermolecular dipole-dipole interactions are required to account for theestimated dipolar contribution to T1. A reasonable model for these interactions is that lipid methylene protons are involved in relaxation of the fluorine probes (which reside in the hydrophobic region of this integral membrane protein). We estimate a minimum translational diffusion coefficient for such lipids of D greater than 3 x 10(-9) cm2/s.