Slow Diffusive Motion Research Articles

Slow filament dynamics and viscoelasticity in entangled and active actin networks.

This paper deals with correlations between the viscoelastic impedance of entangled actin networks and the slow conformational dynamics and diffusive motions of single filaments. The single filament dynamics is visualized and analysed by analysing the Brownian motion of attached colloidal beads, which enables independent measurements of characteristic viscoelastic response times such as the entanglement and reptation times. We further studied the frequency-dependent viscoelastic impedance of active actin-heavy-meromyosin II networks by magnetic-tweezers microrheometry to gain insight into the effect of such highly dynamic and force-generating crosslinkers (exhibiting bond lifetimes of less than 1 s) on the rheological properties. We show that at high frequencies (higher than 1 Hz) the viscoelastic loss modulus is slightly increased relative to the entangled network (associated with an increase in the energy dissipated during mechanical excitations), while at low frequencies the plateau of the impedance spectrum becomes more pronounced as a consequence of the cross-linking of the network and the suppression of the terminal regime. Our data provide evidence that the myosin motor protein may play a role as softener of the actin cortex, enabling the adaptive reduction of the yield stress of cells and thus facilitating cellular deformations.

Static and Dynamic Solution Properties of Corn Amylose in N,N-Dimethylacetamide with 3 LiCl

Static and dynamic light scattering techniques were used to study the solution properties of corn amylose in N,N-dimethylacetamide with 3% (w/v) LiCl. Static properties show three distinguishable regions, which are designated as the dilute, the semidilute, and the concentrated regimes. The concentrations at which these transitions occur are designated as the overlap concentration c* and the semidilute−concentrated transition concentration c**. Two master curves are used successfully to fit the static data in the dilute and semidilute regimes. Dynamic data show that at c < c*, a single diffusion motion representing the translational diffusion of the amylose macromolecule is present in the solution. At c > c*, two dynamic motions are observed: a fast diffusion representing the cooperative diffusion and a slow one representing the cluster diffusion. The slow diffusion motion is found to follow the “stretched exponential” diffusion law. The fast diffusion motion is found to have two distinguishable concentration dependence patterns: its concentration dependence at c > c** is stronger than that at c < c**. The relative amounts of these two modes are also analyzed in terms of their concentration dependence.

Influence of Long-Range Interactions on Diffusion Behavior in Semidilute Solution: Dynamics of Cellulose Diacetate in Quiescent State

The dynamics of cellulose diacetate (CDA) in semidilute solution (1.4c* < c < 5c*) was investigated in a good and high-polarity solvent, dimethylacetamide (DMAc), by dynamic light scattering in the quiescent state at 30 °C. Two fast diffusion motions and a very slow diffusion motion were detected, all the behavior of which is not expected from the framework of dynamics for usual flexible polymers in good solvents. The fast (fast mode) and medium-fast (medium mode) motions gave the diffusion coefficients DF and DM as DF ∝ c0.40 and DM ∝ c0.55. Both were the cooperative diffusion motions, or the relaxations of concentration fluctuations in two types of coarse-dense structures, which were realized as a nest of structures in a dynamic sense in semidilute solution. However, the relaxation rates were suppressed by the long-ranged hydrogen-bond interaction between hydroxyl (OH) groups on CDA molecules, and the interactions brought the systems into the medium, not good, solvent state. The slow diffusion (slow mod...

A STOCHASTIC CAGE MODEL FOR THE ORIENTATIONAL DYNAMICS OF SINGLE MOLECULES IN NEMATIC PHASES

A stochastic cage model for the orientational dynamics of a molecule in isotropic and nematic phases of a liquid crystal has been developed, following the methodology introduced in Refs. 1, 2. The model has been parameterized on the basis of statistical data obtained from the analysis of Molecular Dynamics (MD) simulations of a Gay–Berne mesogen and is based on the general assumption of a timescale separation between the fast inertial librational motion inside the instantaneous cage potential and the slow diffusive motion of the cage itself. The model is able to reproduce single molecule time correlation functions both for the angular momentum and the reorientation of the long molecular axis of the molecule. A complete description of the dynamics of a Gay–Berne particle is given with a single set of physical parameters, from a very fast (hundreds of femtoseconds) timescale up to a timescale of nanoseconds.

Slow motions in bilayers containing anionic phospholipid

Lung surfactant monolayer formation and maintenance requires substantial reorganization of phospholipid assemblies containing up to 10% anionic lipid. Nuclear magnetic resonance was used to study chain order and slow reorientation in bilayer mixtures of dipalmitoylphosphatidylcholine (DPPC) and an anionic phospholipid, dipalmitoylphosphatidylglycerol (DPPG), with one or both lipids chain-perdeuterated and in the presence and absence of calcium in the aqueous medium. Anionic lipid broadens the chain-melting transition in the presence of calcium but has little effect on phase behaviour or chain order in its absence. Chain spectra suggest that any calcium-induced DPPG aggregation in the liquid crystalline phase is short-lived. While bilayers containing DPPG can exist in a metastable hydration state, their stable hydration state is found to be one in which the contribution of slow collective or diffusive bilayer motions to liquid crystal phase quadrupole echo decay is substantially greater than observed for DPPC bilayers.

Correlated motion of two particles in a dense fluid of hard rods

An exact equation describing the stochastic motion of a droplet of neighbouring equal mass hard points with velocities ± c, immerzed in a hard point fluid, is derived. The derivation generalizes Resibois' idea originally applied to the study of self-diffusion. The explicit solution for a two-particle droplet is obtained, showing a rapid relaxation of the interparticle distance to the Poisson distribution, superposed on a slow diffusive motion of the centre of mass of the two particles. An approximate description of the dynamics of N-particle droplets ( N > 2) is proposed.

Polyexponential kinetics of chemical reactions in condensed media within the quasiclassical approximation

Polyexponential kinetical behavior typical for condensed phase reactions in highly viscous media is studied on a simple example of one-dimensional diffusion equation with a sink modeling a chemical conversion of reactants. The corresponding polyexponential regime is demonstrated to have a thorough analogy with the quasiclassical approximation of one-dimensional quantum mechanics and a relevant approximation for the Green’s function is developed. The asymptotic short- and long-time kinetics are examined at the analytical level. Contrary to the frozen medium approximation according to which the slow diffusion motion of the medium is entirely ignored, the present quasiclassical model is fit for a qualitative description of the total time interval covering the reaction events from the initial moment up to the ultimate steady-state monoexponential evolution. The range of validity of the quasiclassical approach is discussed. Numerical tests expose some peculiarities of the present treatment for equilibrium and nonequilibrium initial distributions. The work presents a qualitative development of the theory of nonexponential kinetics pioneered by papers of Agmon and Hopfield, Sumi and Marcus, and Nadler and Marcus.

Direct pump/probe spectroscopy of the near-IR band of the solvated electron in alcohols

We report femtosecond direct pump/probe data on the s-state to p-state transition of the solvated electron in alcohols. The data reveal a p-state lifetime of ≈ 0.5 ps and a large spectral component due to displacement along the ground-state solvent coordinate involving slow diffusive solvent motions and subsequent relaxation. The large displacement is surprising considering the short excited-state lifetime and the long relaxation times of these modes. This effect is apparently the result of a previously unknown p-state to s-state nonadiabatic rearrangement of the solvent coordinate of solvated electrons in alcohols.

Quasielastic neutron scattering measurements of fast local translational diffusion of lipid molecules in phospholipid bilayers

Quasielastic incoherent neutron scattering has been used to investigate the rate of local translational diffusion of lipid molecules in phospholipid bilayers of dipalmitoyl-phosphatidylcholine. The measured translational diffusion constants (4 · 10 −10 m 2 s −1 at 63°C and 1.4 · 10 −11 m 2 s −1 at 30°C) are considerably faster than those deduced using other less direct methods, but are in agreement with those measured in soap-water lyotropic liquid crystals, and with calculated values. This disagreement is attributed to differences in the time and distance scales characterising the various measurements. Quasielastic neutron scattering experiments observe fast motions over molecular distances, whereas other methods tend to measure a rate of diffusion which is averaged over macroscopic distances, and may thus contain contributions from long distance slow diffusive motion such as diffusion between the bilayers.

General behavior of completely anisotropic ESR hyperfine centers under slow orientational diffusion motion.

Based on the solution of an isotropic orientational diffusion equation, a restricted ensemble-averaging process has been formulated for completely anisotropic electron-spin-resonance (ESR) centers undergoing slow orientational diffusion motion in a restricted angular interval during their spin-spin relaxation time. By employing this averaging process, we have calculated the slow-tumbling motional linewidth and line shift of completely asymmetric ESR hyperfine centers (${g}_{x}$\ensuremath{\ne}${g}_{y}$\ensuremath{\ne}${g}_{z}$, ${A}_{x}$\ensuremath{\ne}${A}_{y}$\ensuremath{\ne}${A}_{z}$) for arbitrary orientations of their symmetric axes with respect to the ESR external magnetic field B\ensuremath{\rightarrow}. For B\ensuremath{\rightarrow} parallel to the ESR (x,y,z) principal axes, both the motional linewidth and line shift depend on the associated motional correlation time \ensuremath{\tau} as ${\ensuremath{\tau}}^{\mathrm{\ensuremath{-}}1/2}$. When B\ensuremath{\rightarrow} is oriented in the vicinity of 45\ifmmode^\circ\else\textdegree\fi{} with the principal z axis, the motional linewidth is proportional to ${\ensuremath{\tau}}^{\mathrm{\ensuremath{-}}1/3}$. There are ``magic'' orientations of B\ensuremath{\rightarrow} for which the motional line shift vanishes identically. It will be discussed that the analytical expressions obtained for the motional linewidth and line shift for B\ensuremath{\rightarrow} parallel to the principal axes enable one to determine reliable experimental motional correlation times from slow-tumbling ESR hyperfine spectra in polycrystalline-amorphous substances.

Experimental application of the new theory of slowly tumbling axially symmetric ESR hyperfine centers in amorphous samples

The results of new theoretical treatments of axially symmetric ESR hyperfine centers undergoing small angle, slow orientational diffusion motion in amorphous samples are applied to a polycarbonate polymer containing nitroxide spin probes over the temperature range 77–295 K. In this experimental application, the conventional first-derivative as well as second-derivative nitroxide hyperfine spectra at X-band microwave frequencies are obtained and the motional linewidths and shifts in spectral line positions of the low-field (M=+1) and the high-field (M=−1) nitroxide hyperfine parallel-edge lines are determined from the experimental spectra. By correlating the experimental data with the theory, it is possible to determine the temperature dependences of the nitroxide tumbling motional correlation times τ, the motional narrowing of the inhomogeneous linewidths, and the nitroxide hyperfine coupling constant. The present method provides a simple and direct way of determining experimental τ values in the range ∼1×10−5 to ∼8×10−8 s for nitroxide spin probes/labels tumbling slowly around the external magnetic field direction in amorphous samples.

Generalized theoretical treatment of axially symmetric ESR hyperfine centers under slow orientational diffusion motion

The ensemble averaging based on the solution of an isotropic diffusion equation is employed to calculate analytical expressions for angular-dependent motional linewidths and shifts in spectral line position of axially symmetric ESR hyperfine centers, which undergo isotropic, small-angle, slow-orientational diffusion motion. The resultant theoretical expressions are consistent with those calculated by other investigators using different theoretical approaches. As a result of the present calculations, the entire ESR hyperfine spectra associated with slow-tumbling motions can be calculated in polycrystalline/amorphous substances. A detailed investigation of these theoretical spectra supports the validity of the previously proposed simple method of determining experimental rotational correlation times from the behavior of the parallel-edge lines of nitroxide ESR powder spectra.

On the direct energy transfer via exchange to moving acceptors

In a recent work [K. Allinger and A. Blumen, J. Chem. Phys. 72, 4608 (1980)] we derived expressions for the energy decay of an excited donor due to its interactions with moving acceptors. As we show here, this approach is related to path-integral methods which occur in different fields. We apply the formalism to interactions mediated by exchange. Analytic expressions are found for the decay due to acceptors moving slowly or rapidly on the time scale of the energy transfer. If the motion is frozen we retrieve the decay law for acceptors imbedded randomly in a solid matrix [A. Blumen, J. Chem. Phys. 72, 2632 (1980)]. For slow diffusive motion, as in the three-dimensional dipolar case [M. Yokota and O. Tanimoto, J. Phys. Soc. Jpn. 22, 779 (1967)], the decay may be expressed by means of a power series in the diffusion coefficients. Here we obtain the coefficients of the series from a recurrence formula and present the first ten terms. An approximate, compact formula for the decay law is also given. In the rapid motion case the decay law depends on the distance of nearest approach between donor and acceptors, but not on the details of the motion.

An investigation of the segmental flexibility of spin labelled synthetic macromolecules in solutions

Within the framework of a model, which takes into account fast anisotropic rotation of the spin label and slow diffusion motion of the polymer chain segment, the ESR spectra of 1% solutions of spin labelled poly(4-vinylpyridine) have been analysed. It has been shown that the model describes the observed spectra satisfactorily. The rotational correlation times for the segment together with its effective dimensions (∼ 12 monomer units) have been calculated. The dependence of segment correlation time on temperature is described by the Arrhenius equation with E = 4 kcal/mol. τ o = 2·10 −13 sec.

Kinetic theory of chemical reactions in liquids. II. Spatial nonequilibrium effects for a reversible reaction

The effects of particle diffusion on the rate coefficients of a bimolecular isomerization reaction occurring in a dense fluid medium are investigated by kinetic theory. Emphasis is placed on the spatial nonequilibrium effects which occur as a result of the slow diffusive motion in a liquid, and the structure of the chemical relaxation time and rate coefficients when both forward and reverse rate processes are treated consistently. Since the kinetic theory calculations are microscopic and do not rely on a diffusion equation approach, a more accurate representation of the short time evolution in such reacting systems is obtained. Competition effects which occur at higher densities of the reacting species are also studied.

Nuclear Relaxation and Slow Diffusive Motions in CaF2 and NaF

Fluorine relaxation times T1Z , T1D and T2 are measured as functions of temperature in single crystals of CaF2 and NaF. Typical quantities like the activation energy for vacancy diffusion and the formation energy of defect pairs are deduced and reported.