Translational Diffusion Constants Research Articles

Gelation and aggregation of mucin in relation to the stomach's protective barrier

AbstractDynamic light scattering was applied to study the effects of pH and concentration on pig gastric mucin. At low concentration (C = 2.0 mg/ml), the distribution of diffusion constants is independent of pH, with an average diffusion constant D = 4 × 10−8 cm2/s. At pH2, very large aggregates with 100‐time slower D (2 × 10−8cm2/s) are seen. Depolarized DLS can be observed at pH2 even at low concentration (C = 5.0 mg/ml). The rotational and translational diffusion constants were analyzed using the Kirkwood‐Riseman model for rods and show that at pH2 mucin is a rod‐like molecule of length 390 nm, diameter 55 nm, with an axial ratio (length/diameter) of 7. However, at pH7, no significant depolarized signals could be detected at any of the concentrations examined in the study. These results suggest that the mucin monomer undergoes a conformational change from a flexible to a rod‐like conformation upon lowering pH from 7 (neutral) to 2 (acidic). Such conformational change and aggregation of the gastric mucin molecule at acidic pH may provide a mechanism by which the gastric mucin layer prevents the stomach from digesting itself, since the back diffusion of acid from the lumen to the gastric epithelium is retarded by the gel layer.

Translational Diffusion of a Transient Charge‐Separated Species in Carbon Dioxide and Trifluoromethane Studied by the Transient Grating Method

AbstractBy means of the transient grating method, we have measured the translational diffusion constant of a transient charge‐separated species created by the photo‐isomerization of 1′,3′,3′‐trimethyl‐6‐nitrospiro[2H‐1‐benzopyran‐2,2′‐indoline] in carbon dioxide and trifluoromethane from ρr = 0.8 to 2.0, where ρr is the reduced density by the critical density of the solvent. It has been revealed that the effect of the charge‐separation on the diffusion constant is small in these fluids due to the large molecular size of the transient molecule. The viscosity dependence of the diffusion constant is well simulated by an empirical equation which also works well for benzene and naphthalene.

Molecular motion of a water-soluble nitroxyl radical in gelatin gels

Electron spin resonance spectroscopy (ESR) has been used to measure the rotational correlation times and translational diffusion constants of a radical spin probe in gelatin gels. The radical used is NaTMIOS, the sodium salt of sulphonated 1,1,3,3-tetramethylisoindolin-2-yloxyl. It was found that the mobility of the radical was influenced by the gelatin concentration and by temperature. It was concluded that the spin probe was constrained by the polymer chains and, when conditions were such that the distance between the chains was of the order of the size of the probe, rotational motion was severely impeded.

Measuring diffusion in supercooled liquids: The effect of kinetic inhomogeneities

There is growing evidence to suggest that cooperativity in glasses is characterized by a spatial distribution of relaxation times. In this paper we examine the consequences of such inhomogeneities in the measurement of translational and rotational diffusion constants. We find that the decay of a concentration profile (the basis of transient grating experiments) is accelerated by the development of additional gradients parallel to the relaxation time gradients. This may account for recently observed differences in the variation of rotational and translational diffusion constants with viscosity below the glass transition temperature. The additional gradients may also provide the first direct means of examining the length scale associated with these inhomogeneities.

Translational diffusion of chemically stable and reactive radicals in solution

Translational diffusion constants (D) of chemically stable radicals (TEMPO, its derivatives and DPPH) and their analogous non-radicals (closed-shell molecules) have been investigated by the Taylor dispersion method. It has been found that TEMPO and its derivatives diffuse more slowly than the closed-shell molecules with similar molecular sizes and shapes in alcoholic solvents. On the other hand, in aprotic and nonpolar solvents, the D values of TEMPO and the closed-shell molecules are similar. D for DPPH is close to that of the closed-shell molecule with the same size. A chemically reactive radical of a similar structure is created by the photoinduced hydrogen abstraction reaction of tetramethylcyclohexanone in ethanol, and D values for the radical and the parent molecule are measured by the transient grating method. D values for the chemically reactive and stable radicals are compared.

Translational diffusion of a carbene and radicals derived from carbenes

Translational diffusion constants (D) of a persistent carbene (hexachlorodiphenylcarbene) and, simultaneously, that of the precursor compound (2,2′,4,4′,6,6′-hexachlorodiphenyldiazomethane) are determined, after the laser photolysis of the diazo-compound in benzene, by using the transient grating (TG) method. D of the carbene is found to be anomalously (ca. five times) smaller than that of the parent molecule. At the same time, the TG signals of some short-lived carbenes (diphenylcarbene, fluorenylidene, α-(and β-)naphthylphenylcarbene) are investigated in various solvents (cyclohexane, methanol and acetonitrile). In methanol, monotonic decays of the population grating signals are observed and, from the rate constants, Ds of the precursors, diazo-compounds, are determined. In cyclohexane, rise and decay signals are observed as the TG signal and these components are interpreted in terms of the diffusion of the diazo compounds and radicals created from the carbenes by hydrogen abstraction from the solvents. Ds of the radicals are generally three–four times smaller than those of the parent molecules. The smaller Ds of the transient radicals indicate a larger friction of the radicals during the diffusion process and the origin of the friction with the matrix is attributed to an attractive intermolecular interaction between the radicals and the surrounding molecules. From these results, it is concluded that the intermolecular interaction between the carbene and the matrix is stronger than that between the radicals and the matrix. This could be further evidence for the important role of the unpaired electron in diffusion processes in solution.

Molecular Dynamics Simulations of Neat Alkanes: The Viscosity Dependence of Rotational Relaxation

Molecular dynamics simulations of neat octane, dodecane, hexadecane, and eicosane at 323 K are compared with experiment and analyzed with respect to terms that contribute to reorientational relaxation of CH vectors. Agreement with experiment for translational diffusion constants and rotational relaxational times is excellent. It is found that gauche−trans isomerization rates, while weakly dependent on chain position, are essentially independent of chain length and, hence, shear viscosity. In contrast, rotational correlation times of the symmetry (or “long”) axis of the instantaneous moment of inertia tensor increase monotonically with chain length, and, taking the shape dependence into account with a hydrodynamic cylinder model, are proportional to viscosity. Rotation about the long axis is strongly coupled with isomerization. Using an approximate separation of internal and overall motion, reorientation of octane is shown to be largely “rigid body”. For the central carbons in eicosane flexibility contributes at most 40% to the total relaxation time that would be observed in, for example, NMR T1 measurements.

Ternary Interactions in Dynamics of ?-State Polymers: A Conformation-Space Renormalization-Group Approach

We reconsider the t'Hooft-Veltman type renormalization treatment of polymer properties near the theta-point. We correct other recent studies, and now demonstrate that the equivalence between this renormalization scheme and a zero-component field theory or a direct renormalization method for polymer chains works perfectly. We also give an estimate for the magnitude of the renormalized third virial coefficient using a new result for the translational diffusion constant at the theta-point.

ESR measurement of translational diffusion constants

AbstractESR spectroscopy can be used to measure the translational diffusion constants of spin probes in a variety of media using a capillary method. Following preliminary work, it has been possible to improve upon the accuracy of determining diffusion constants in two ways. Both methods require an experimental measurement of the resonant cavity sampling profile. In the first procedure, this cavity profile is convolved with a trial Gaussian distribution and the result is compared with the experimental data obtained from the ESR measurements; the diffusion constant is then determined by graphical optimization. The second method is a graphical one based on a knowledge of the experimental diffusion profile and the cavity sampling profile. Both methods have been shown to give similar results but the second less rigorous corrected‐zero method is generally simpler to apply. For a medium such as water, it takes several days to acquire the necessary experimental data but the time taken to carry out the experiment can be shortened (or slower rates of diffusion can be measured) by using a shielded resonant cavity.

Modeling the electrophoresis of rigid polyions: application to lysozyme

An algorithm is developed to determine the electrophoretic mobility of a rigid polyion modeled as a low dielectric volume element of arbitrary shape containing an arbitrary charge distribution. The solvent is modeled as a high dielectric continuum with salt distributed according to the linearized Poisson Boltzmann equation. Account is also taken of a Stern layer that separates the molecular surface and the surface of hydrodynamic shear, or Stern surface. Relaxation of the ion atmosphere because of the presence of the external field is ignored. The electrostatic and hydrodynamic problems are both solved by boundary element methods. The procedure is first applied to spherical polyions containing monopolar, dipolar, and quadrupolar charge distributions, and calculated mobilities are found to be in excellent agreement with the theory of Yoon and Kim. It is then applied to lysozyme by using models that account for the detailed shape and charge distribution of the enzyme. For reasonable choices of the molecular and Stern surfaces, calculated and experimental mobilities are found to be in fair agreement with each other. However, if a pH independent Stern layer (or, equivalently, translational diffusion constant, Dt) is assumed, the calculated mobilities exhibit a stronger pH dependence than is observed experimentally. A small increase in Dt with increasing pH could correct this discrepancy.

Simple EPR method for measuring translational diffusion constants

The capillary method has been adapted to provide a simple EPR method for measuring translational diffusion constants of spin probes.

Translational and Rotational Diffusion of Proteins

We have investigated the translational and rotational diffusion of two proteins, bovine pancreatic trypsin inhibitor and hen egg white lysozyme, using molecular dynamics simulations in explicit solvent. The translational diffusion constants obtained form the simulations compare favourably with results from experiments. However, rotational diffusion constants indicate increased rotational diffusion compared to experiments. Further analysis, using simple hydration models, suggest that the observed rotational diffusion can be explained without invoking a fixed shell of hydration water associated with the protein. Within the assumptions of the hydration models, the results suggest that the strength of protein—water interactions in the current force field may be slightly underestimated.

Slow dynamics of water molecules in confined space

We present results of the analysis of quasi-elastic and inelastic incoherent neutron scattering of H 2O contained in pores of Vycor glass. At room temperature and 50% hydration, the single particle motions of water molecules can be described as a series of short time jump diffusions in a confined space of 5 Å with a residence time τ 0 of 8 ps. As the temperature is lowered the residence time increases rapidly to 19 ps at - 15°C. However, the translational diffusion constant is similar to the corresponding bulk value at each temperature. The peaks in the density of states for the translational motions as well as for the librational motions are slightly up-shifted compared to that of bulk water due to the confinement near the surface.

Dynamical properties of a ferroelectric nematic liquid crystal.

Molecular-dynamics simulations are used to investigate the dynamical properties of a ferroelectric nematic liquid crystal. The results are also relevant to electrorheological fluids and to ferrofluids. Translational and rotational diffusion constants parallel and perpendicular to the director are reported and discussed. Dipole-dipole reorientational correlation functions are calculated and compared with theoretical models

A dual‐mode interpretation of nuclear spin relaxation for 13CO2 sorbed in polystyrene

AbstractRecently we presented a dual‐mode interpretation of the nuclear spin relaxation for 13CO2 sorbed in polycarbonate [E. J. Cain, W.‐Y. Wen, A. A. Jones, P. T. Inglefield, B. J. Cauley, and J. T. Bendler, Journal of Polymer Science, Part B, Polymer Physics, Vol. 29, p. 1009 (1991)]. We have made similar investigation on spin relaxation for 13CO2 sorbed in polystyrene and are presenting the results in this report. 13C spin‐lattice relaxation, spin‐spin relaxation, and nuclear Overhauser enhancements (NOE) are reported for 13CO2 sorbed in glassy polystyrene. These spin relaxation parameters were determined as a function of temperature and Larmor frequency. A quantitative interpretation is based on a two‐site, rapid exchange model, which is consistent with the NMR result. Gas molecules in the dissolved site have rotational, collisional, and translational dynamics typical of low molecular weight liquids. Sorbed molecules in the Langmuir site have much slower rotational and translational motions. The activation energy for motions in the dissolved site is about 7.5 KJ/mol, while that for the Langmuir site is about 15.5 KJ/mol. The translational diffusion constants derived from the NMR data range from 10−7 to 10−9 cm2/s and are consistent with the apparent diffusion constant from the permeability measurements of 4.9 × 10−8cm2/s at 35°C. A relation between the permeability value and the NMR values is available from a theoretical description of the dual‐mode model in terms of a random walk on a lattice containing two sites. As a modification of our previous lattice model (E. J. Cain et al., Journal of Polymer Science, Part B, Polymer Physics, Vol. 29, p. 1009 (1991)), the temperature dependence of the fraction of Langmuir sites in the lattice is taken into account. Parameter values characterizing the motion of CO2 in polystyrene are found to be similar to the corresponding ones in polycarbonate. © 1993 John Wiley & Sons, Inc.

Intermolecular translational-rotational contribution to nuclear-spin relaxation in liquids.

The intermolecular contribution to spin relaxation by translational-rotational diffusion dynamics in molecular liquids is calculated for arbitrary relative time scales of the two stochastic processes. The resulting relaxation times are functions of variables that are proportional to ratios between rotational and translational diffusion constants, in substantial generalization of earlier theoretical approaches. In the first part the general treatment for symmetric-top molecules is given with a reduction to quadratures and a simpler version for the spherical case

Diffusive motion of hydrogen in mordenite studied by quasi-elastic neutron scattering

Neutron time-of-flight experiments have been done with hydrogen adsorbed in Na-mordenite at temperatures between 35 and 130 K and densities between n = 43 and n = 13 H 2 molecules in the unit cell of the zeolite lattice. The quasi-elastically scattered neutrons give rise to a peak that is not broadened at half-height with respect to the experimental energy resolution of 90 μeV (46 ns). A Lorentzian broadening at the bottom of the quasi-elastic peak has been interpreted in terms of a diffusion model yielding a rotational diffusion constant D r = 2 · 10 11rad s −1 and a translational diffusion constant D t = 2 · 10 −3cm 2 s −1 at T = 130K andn = 35 H 2 molecules/unit cell.

Molecular dynamics of discotics in the isotropic phase

Molecular dynamics (MD) simulation and multifield carbon-13 nuclear spin relaxation data are reported and used in an investigation of dynamic processes taking place in the isotropic phase, at 95 °C, of benzene-hexa-n-heptanoate (BHA7). The relaxation data are analyzed by using a dynamic model based on separation between a fast internal and a slow overall motion. The overall reorientations defined by the motion of the rigid aromatic core, occurs on nanosecond time scale, whereas the internal motion in the flexible, aliphatic side chains is much faster, with the correlation times in the picosecond range. Increased mobility and decreased orientational order along the chains—characterized by order parameters—is predicted by the experiments and also confirmed in the simulation. The translational diffusion constants are calculated in the MD simulation and measured using FT-PGSE technique. The molecular structure obtained from the MD simulation indicates that the aliphatic chains are situated alternatively up and down relative to the aromatic core. The radial distribution functions show that the molecules in the liquid are associated in pairs, with a parallel orientation of the planes defined by the aromatic core.

Comparison of Langevin and molecular dynamics simulations. Equilibrium and dynamics of ethylene glycol in water

The results of molecular dynamics (MD) simulations of one ethylene glycol molecule in 259 waters from trajectories totalling 5 ns are compared with those from Langevin dynamics simulations of a single ethylene glycol. It is found that while the trans–gauche equilibrium constant is relatively unperturbed by water, the effective barrier for isomerization is lowered by ca. 1kT. Once this effect is taken into account, a Langevin model without explicit waters with a collision frequency, γ, of ca. 50 ps–1 provides a reasonable description of the average isomerization rate. Overall rotation is well fitted with γ= 75 ps–1, although the fast librational motion present in MD is overdamped in LD. When hydrodynamic interaction is introduced with the Oseen tensor and the above collision frequencies are assumed, 1–2 bound waters are required to reproduce the translational diffusion constant of the glycol obtained in the MD simulations. These results indicate that Langevin dynamics simulations can reproduce some behaviour of polar molecules in aqueous solutions, but might be better carried out including a partial layer of bound water.

Quasielastic neutron scattering measurements of n-butane in its crystalline, plastic, and liquid phases

We report here measurements of quasielastic neutron scattering from n-butane at temperatures of 90, 115, 125 and 190 K and in a momentum transfer range of 0.8–2.4 Å−1. These measurements confirm that between 115 and 125 K butane forms a plastic crystal in which the centers of mass of the butane molecules form a crystalline structure, but the individual molecules are free to rotate. At these two intermediate temperatures, there exists both an elastic peak, characteristic of a solid structure, and quasielastic components arising from the rotational motions of the butane molecules. At 90 K, the butane scatters neutrons only elastically, while at 190 K, the butane scatters neutrons only quasielastically. In both the plastic and the liquid phases, the presence of at least two quasielastic processes must be assumed in order to explain the measurements. In the plastic crystal, we associate a broad Lorentzian component with intramolecular reorientations about the central carbon–carbon bond and a second, relatively narrow, Lorentzian component with whole molecule rotations. The latter process gives rise to a rotational quasielastic peak having a width of 400 μeV, which is constant to within the instrumental resolution of 70 μeV at both temperatures and at all measured momentum transfers. In a continuous diffusion model, this width corresponds to a rotational diffusion constant of 0.277 rad2/ps, a value which is about 3.5 times larger than one extracted from a molecular dynamics simulation of n-butane in the plastic phase recently published by Refson and Pawley [Mol. Phys. 61, 669 (1987); 61, 693 (1987)]. On the other hand, the first process, which corresponds to the carbon–carbon reorientation peak, is about 16 meV wide, indicating that this reorientation occurs on a time scale of about 0.1 ps. The absence of this broad peak in the solid butane at 90 K indicates that this fast carbon–carbon reorientation is coupled to an aspect of the structure or dynamics of the plastic phase. In the scattering from liquid butane at 190 K, there is a third, narrow quasielastic peak which also has a Lorentzian energy distribution. This peak width is about 200 μeV and corresponds to a translational diffusion constant of 0.23 Å2/ps, a result in rough agreement with a recent molecular dynamics simulation of liquid butane by Ullo and Yip. [J. Chem. Phys. 85, 4056 (1986)].