Effects Of Restricted Diffusion Research Articles

Effect of induced field inhomogeneity on transverse proton NMR relaxation in tissue water and model systems.

The effect of induced field inhomogeneity (IFI) on transverse NMR relaxation of water protons in tissue has been investigated by examining the field dependence of the effective transverse relaxation rates (1/T2 eff) for in vitro canine brain tissue samples. At fields of 0.47, 2.35, 7.05 T (corresponding to 20, 100, and 300 MHz, respectively) the transverse relaxation rates for both white and gray matter samples follow a field dependence of the form 1/T2 eff = C0 + C1 B0, where B0 is the applied field. The linearly dependent term, C1 B0, which reflects the IFI contribution, does not contribute much (i.e., less than 20%) at fields less than 2.0 T. However, at greater field strengths the contribution is appreciable, e.g., greater than 60% at 7.0 T. Results from model systems of glass beads are also reported to illustrate IFI effects. For both the model systems and canine brain tissue samples, the effects of restricted diffusion are qualitatively evident in Hahn spin-echo experiments.

A mathematical model for restricted diffusion effects on macromolecule impregnation in porous supports

A mathematical model for restricted diffusion effects on macromolecule impregnation in porous supports

The effect of restricted rotational diffusion on the dynamics of rodlike molecules in concentrated solution

AbstractThe model of Doi for the rotational dynamics of rodlike molecules in solution is compared with that of Warchol and Vaughan and Wang and Pecora. The similarities in the results for dielectric relaxation are discussed and the application of the models to the behaviour of isotropic solutions, anisotropic (lyotropic‐nematic) solutions, and mixtures of small molecules with polymers is briefly discussed.

Combined effect of restricted rotational diffusion plus jumps on nuclear magnetic resonance and fluorescence probes of aromatic ring motions in proteins

A simple model is presented for the motion of phenylalanine and tyrosine rings in proteins. The model consists of restricted rotational diffusion of the rings about the side chain C alpha--C beta (chi 1) and C beta--C gamma (chi 2) axes combined with 180 degree ring flips. The model is used to evaluate order parameters for nuclear magnetic resonance relaxation and fluorescence depolarization probes of aromatic ring motions in proteins. The dependence of the order parameters on orientation in the ring plane is examined and it is demonstrated that in the presence of ring flips, additional ring librations can have a large effect on the probe order parameters.

Non-equilibrium effects in the branched-chain peak broadening phenomenon in gas chromatography on Porapak P

Non-equilibrium coefficients ( C s and C m ) have been evaluated for iso-octane and n-octane on Porapak P at different temperatures in the range 130–180°. The reproducibility of the chromatographic parameters upon column conditioning has been investigated. An increase in the eddy diffusion contribution upon column conditioning is apparent. The stationary phase non-equilibrium coefficient for n-octane is affected due to a slight microstructural change in the polymer packing upon column condition, but that for iso-octane is unaltered under its existing large background effect. The temperature-sensitive and unusually high C s coefficients for iso-octane reflect a high activation energy of 17.6 kcal/mole. The unusual branched-chain peak broadening phenomenon recently observed on certain organic porous polymer column packings has been rationalized as being due to the effect of restricted intraparticle diffusion and the relative affinities of the various adsorbates for the adsorbent.

The effects of diffusion through locally inhomogeneous magnetic fields on transverse nuclear spin relaxation in heterogeneous systems. Proton transverse relaxation in striated muscle tissue

The effect on transverse nuclear spin relaxation of diffusion of molecules through magnetic field-gradients arising from heterogeneity of the magnetic susceptibility of the system are discussed. Similarities of these effects with the effects of spatially restricted diffusion in the presence of applied field-gradients and the motional narrowing of NMR lines are pointed out. An expression is deduced for the contribution of diffusion through internal inhomogeneous fields to the attenuation of a spin echo. The result is obtained from Robertson's approximate theory for the effect of restricted diffusion between plane parallel barriers and involves Δω, the width of the inhomogeneously broadened NMR absorption line and ξ the time taken for a spin to diffuse a distance of the order of the lattice constant for the structure of the system. The application of the result to striated muscle shows that diffusion through internal field-gradients is a negligible contribution to the transverse relaxation of the water protons.

Mathematics of band centrifugation: Concentration‐independent sedimentation and diffusion in shallow density gradients

AbstractIntegral expressions for concentration as a function of time and distance are derived from the continuity equation for centrifugation in a sector‐shaped cell for a macro‐molecular solute initially contained in a finite upper layer and a solute of low molecular weight in the supporting liquid. Computer patterns based on the sedimentation and diffusion coefficients of sucrose and of spherical and randomly coiled model solutes illustrate: (1) the time course of redistribution of both banded and supporting solutes from initial uniform concentrations; (2) the influence of the initial concentration, width, and solute concentration of the upper band; and (3) the effect of restricted diffusion at the meniscus on subsequent band shape. A Gaussian, approximation to band shape is derived and graphically tested. Rapid methods, not requiring computers, are out lined for the estimation of sedimentation and diffusion coefficients, where their concentration dependence is negligible, by band centrifugtion. The theoretical resolution of mixtures attainable by this technique is compared with moving‐boundary centrifugation, with the use of both integral (interferotmetric or absorption) and derivative (schlieren) optics.