Rotational Diffusion Constants Research Articles

Nucleutide- and temperature- induced changes in myosin subfragment-1 structure

The effects of nucleotide binding and temperature on the internal structural dynamics of myosin subfragment 1 (S1) were monitored by intrinsic tryptophan phosphorescence lifetime and fluorescence anisotropy measurements. Changes in the global conformation of S1 were monitored by measuring its rate of rotational diffusion using transient electric birefringence techniques. At 5°C, the binding of MgADP, MgADP,P and MgADP,V (vanadate) progressively reduce the rotational freedom of S1 tryptophans, producing what appear to be increasingly more rigidified S1-nucleotide structures. The changes in the luminescence properties of the tryptophans suggest that at least one is located at the interface of two S1 subdomains. Increasing the temperature from 0 to 25°C increases the apparent internal mobility of S1 tryptophans in all cases and, in addition, a reversible temperature-dependent transition centered near 15°C was observed for S1,S1-MgADP and S1-MgADP,P, but not for S1-MgADP,V. The rotational diffusion constants of S1 and S1-MgADP were measured at temperatures between 0 and 25°C. After adjusting for the temperature and viscosity of the solvent, the data indicate that the thermally induced transition at 15°C comprises local conformational changes, but no global conformational change. Structural features of S1-MgADP,P, which may relate to its role in force generation while bound to actin, are presented.

Chain‐aggregate aerosols in smoke from the Kuwait oil fires

Electrooptical scattering was used to detect aggregated particle chains in the smoke from the Kuwait oil fires. Nonsphericity was detected by the change in light scattering brought about by induced alignment of particles when subjected to a pulsed, bipolar electric field. Measured parameters included the steady state enhancement of light scattering for complete orientation of the particles, and the rotational diffusion constant, calculated from the time required for the particles to relax to a random orientation after the electric field was removed. Chain aggregates of soot formed within seconds of combustion for those fires producing black smoke. These aggregates agglomerated to some extent in the smoke near the fires, but then remained relatively unchanged for several hours of travel downwind. Very little non sphericity was detected for particles in the plumes of white smoke, which consisted primarily of salt brine products emitted along with the oil.

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

Investigation of the reorientational dynamics of the bis (maleonitriledithiolato) nickel anion radical using electron spin resonance lineshapes

Electron spin resonance spectra of the bis(maleonitriledithiolato) nickel anion radical, Ni(mnt) 2 −, have been calculated and compared with the experimental results of Huang and Kivelson in n-butyl alcohol. The calculated spectra show that the well-defined principal line found experimentally (for all motional rates) is produced when the reorientational motion of Ni (mnt) 2 − is described by axially symmetric Brownian rotational diffusion. The long in-plane axis is the symmetry axis of the rotational diffusion tensor; the rotational diffusion constant for the reorientation about the symmetry axis is approximately three times larger than the rotational diffusion constant for the reorientation about the two axes perpendicular to the symmetry axis. This motional model also produces calculated widths of the principal line which are in agreement with the experimental widths when the reorientational modulation of the anisotropic Zeeman interaction makes the dominant contribution to the widths. There is a lack of agreement between the experimental and calculated widths in the fast motional region near the minimum experimental width. The lack of agreement is discussed in terms of the spin rotation and nonsecular Zeeman contributions as well as the temperature dependence of the g factors of Ni(mnt) 2 −. ESR spectra and principal linewidths calculated using computer programs with and without the nonsecular Zeeman (electron spin flip) terms have been compared for motional rates in the fast and intermediate motional regions.

C 60 Rotation in the Solid State: Dynamics of a Faceted Spherical Top

The rotational dynamics of C(60) in the solid state have been investigated with carbon-13 nuclear magnetic resonance ((13)C NMR). The relaxation rate due to chemical shift anisotropy (1/9T1(CSA)(1)) was precisely measured from the magnetic field dependence of T(1), allowing the molecular reorientational correlation time, tau, to be determined. At 283 kelvin, tau = 9.1 picoseconds; with the assumption of diffusional reorientation this implies a rotational diffusion constant D = 1.8 x 10(10) per second. This reorientation time is only three times as long as the calculated tau for free rotation and is shorter than the value measured for C(60) in solution (15.5 picoseconds). Below 260 kelvin a second phase with a much longer reorientation time was observed, consistent with recent reports of an orientational phase transition in solid C(60). In both phases tau showed Arrhenius behavior, with apparent activation energies of 1.4 and 4.2 kilocalories per mole for the high-temperature (rotator) and low-temperature (ratchet) phases, respectively. The results parallel those found for adamantane.

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.

Dynamics of ethane and propane in zeolite ZSM-5 studied by quasi-elastic neutron scattering

Quasi-elastic neutron scattering has been used to study the dynamics of ethane and propane in zeolite ZSM-5. The experiments were performed at different loadings for the two alkanes and at different temperatures for propane. The long-range translational motion of the molecules has been observed and has been interpreted with a jump diffusion model, the mean jump lengths being of the order of 10 Å, slightly decreasing with increasing loading. The self-diffusion coefficients of ethane are about 2 × 10 −9 m 2 s −1 at 300 K, those of propane at the same temperature being smaller by a factor of two. An activation energy of 5 kJ mol −1 is obtained for the self-diffusion of propane. These results are in good agreement with the pulsed-field gradient n.m.r. measurements. In addition to the translational motion, a rotational motion of the two molecules is also observed; it is described by an uniaxial rotational diffusion model and rotational diffusion constants are derived.

High Pressure NMR Studies of Confined Liquids

AbstractThe main goal of our novel NMR experiments on confined liquids was to determine the effects of pressure on the dynamics of liquids in the surface layer by using the twostate, fast exchange model and compare them to the pressure effects observed for bulk liquids. With this goal in mind, the deuteron NMR spin-lattice relaxation times, T1, in liquid pyridine-d5, nitrobenzene-d5, and methylcyclohexape-d confined to sol-gel porous silica glasses with pore radii in the range from 18Å to 49˚A were measured as a function of pressure up to 5 kbar at 300 K. In another set of high resolution natural abundance 13C NMR experiments, the 13C relaxation behavior of each carbon in 2- ethylhexyl benzoate model lubricant was measured as a function of pressure in porous silica glasses. In fact, the described experimental approach which allows investigation of the effects of pressure on the dynamic behavior of surface-layer liquids, may provide a new tool in studies of model liquid lubricants at extreme conditions of pressure and temperature. In addition selected results of a recent study of acetonitrile-d3 are reviewed. The 2H and 14N spin-lattice relaxation times of acetonitrile-d3 in porous silica glasses were measured in order to study the confinement effects on the anisotropic reorientation characterized by rotational diffusion constants, D⊥ and D‖, of this symmetric-top molecule.

Quadrupolar relaxation—an approach to rotational diffusion

The 7Li relaxational behavior of phenyl-lithium in isotropic solution has been investigated. The multiexponential quadrupolar relaxation dynamic facilitates the determination of the rotational diffusion constants together with the quadrupolar coupling constant, despite the zero average of the quadrupolar splitting on account of molecular tumbling. The rotational diffusion constants obtained are in very satisfactory agreement with those derived from our phenyl-lithium 13C relaxation studies.

Determination of FeCO geometry and heme rigidity in carbonmonoxyhemoglobin using femtosecond IR spectroscopy

The anisotropy of carbon monoxide hemoglobin at ambient temperatures following photolysis with 350 fs (fwhm) visible pulses was followed by monitoring the bound CO absorption bleach from 30 fs to 1 ns delays. The anisotropy was modeled assuming the porphyrin undergoes small damped harmonic oscillations about in-plane axes along and perpendicular to the projection of the CO bond onto the heme plane and expressed in terms of the protein rotational diffusion constant, the angle between the CO and the heme plane normal, the moment of inertia of the porphyrin ring, and the diffusion coefficient of the porphyrin within the protein. The anisotropy within the first 20 ps is constant at −0.170 ±0.003, while the average anisotropy from delays of 30 fs to 1 ns is nearly identical at −0.169 ±0.002 and hence it appears that the heme plane is rigidly held within the protein. The magnitude of the angle between the CO and the normal to the heme plane containing the absorption dipoles was calculated considering the fact that the heme is not a perfect circular absorber at the excitation wavelength, and was found to be 17.5° ±2.5°.

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)].

Rotational diffusion of liquid toluene in confined geometry

The effects of geometrical confinement on the molecular dynamics of liquid toluene in porous sol–gel silica glasses have been studied by the NMR method. The NMR deuteron spin–lattice relaxation times for toluene-d8, ortho-toluene-d1, and para-toluene-d1 were measured as a function of temperature for bulk and confined liquids. The rotational diffusion tensors for overall, internal, and anisotropic rotational diffusion have been determined. The effect of geometrical confinement results in a considerable decrease of the rotational diffusion constants and leads to a lowering of the symmetry of the anisotropic rotational diffusion tensor.

Experimental Characterization of Chain-Aggregate Aerosol by Electrooptic Scattering

A technique combining the electric field-induced alignment and the transient light scattering was applied to characterize Fe2O3 chain-aggregate aerosols. Particles were exposed to a pulsed square electric field with strengths up to 2.S kV/cm. Rotational diffusion constants of the particles were obtained from the decay rate of the relative change in light scattering intensity due to orientation of particles after the removal of electric field. Using a model for rodlike aerosols, average particle length was obtained from the electrooptic scattering data and was compared with the electron microscope analysis of the aggregates sampled with an aerosol centrifuge. The good agreement of the results obtained from the two independent methods demonstrates the applicability of the technique to the in-situ measurement of the fibrous aerosols.

Diffusion of single elements and steric limitations in kinetic theory of nucleation and crystallization

Translational and rotational diffusion equation of single elements in solution in the external orienting potential forces has been formulated. The equation should govern long-range diffusion effects in the kinetics of nucleation and crystal growth. Boundary conditions, adequate to the reversible reaction of cluster growth typical for kinetic model of nucleation and accounting for steric limitations, has been proposed. Uniaxial single elements in uniaxial orienting force field are considered. Depression of the concentration of single elements at the cluster boundary as controlled by kinetic factors, is predicted i. e., chemical rate constants, finite translational and rotational diffusion, supercolling, and steric limitations. Effective rate constants, controlled by long-range diffusion of single elements at steric limitations present, have been used. Two dimensionless kinetic factors (i. e., reduced addition-reaction rate constant and reduced rotational diffusion constant), supercooling, and steric tolerance anlge range, control process kinetics and distribution of single elements in the cluster's surroundings. Rate reduction factor responsible for the effects of long-range diffusion at steric limitations present is defined and applied for kinetic models of nucleation and crystal growth in unoriented and oriented systems. Computation examples are performed for a wide range of the model variables, and rate reduction effects of several orders of magnitude are predicted. The dominating role ranges of particular model variables, i. e., kinetic, thermodynamic, or steric variables, are discussed.

Dynamics of an ammonium ion in water: Molecular dynamics simulation

The translational and rotational dynamics of an aqueous ammonium ion are examined using an NVE molecular dynamics computer simulation and a rigid model of water. The linear momentum, angular momentum and reorientational autocorrelation functions of the ammonium ion have been measured. From these, the memory functions for the linear and angular momentum correlation functions are calculated. Reorientation of the ammonium ion about its center of mass shows unambiguous departure from Debye rotational behavior. The translational and rotational diffusion constants have been calculated.

Effect of interconversion between reactant configurational states on enzyme kinetics controlled by rototranslational diffusion motions

In this paper we extend a previous model [M. Baldo, A. Grassi, and A. Raudino, J. Chem. Phys. 91, 4658 (1989)] describing the orientational effects in diffusion-controlled enzyme (or membrane surface) reactions. The present generalization takes into account the reactants internal motions involving the interconversion between configurational states, one of them being much more reactive than the others. The problem leads to a system of rotational–translational diffusion equations (RT-DEs) coupled through the interconversion reactions between the conformers. For sake of simplicity, we have restricted the analysis to the case of only two conformational states. The steady-state RT-DE with the proper boundary conditions has been solved by an exact analytical procedure, leading to a set of linear algebraic equations which have been numerically solved. The model allows one to calculate the kinetic constants of the enzyme reactions as a function of available experimental parameters, such as the rotational and translational diffusion coefficients, the reactant’s orientational constraints and the rates of interconversion between its different conformations. The numerical results show a monotonous but very nonlinear increasing of the enzyme kinetic constant on raising either the rotational diffusion constant or the interconversion rate between the P+ and P− reactant’s conformations. Well-defined regions where the influence of the above parameters on the enzyme kinetics reaches a maximum have been identified.

Rotational diffusion of a steroid molecule in phosphatidylcholine-cholesterol membranes: fluid-phase microimmiscibility in unsaturated phosphatidylcholine-cholesterol membranes

Rotational diffusion of androstane spin-label (ASL), a sterol analogue, in various phosphatidylcholine (PC)-cholesterol membranes was systematically studied by computer simulation of steady-state ESR spectra as a function of the chain length and unsaturation of the alkyl chains, cholesterol mole fraction, and temperature for a better understanding of phospholipid-cholesterol and cholesterol-cholesterol interactions. Special attention was paid to the differences in the cholesterol effects on ASL motion between saturated and unsaturated PC membranes. ASL motion in the membrane was treated as Brownian rotational diffusion of a rigid rod within the confines of a cone imposed by the membrane environment. The wobbling rotational diffusion constant of the long axis, its activation energy, and the cone angle of the confines were obtained for various PC-cholesterol membranes in the liquid-crystalline phase. Cholesterol decreases both the cone angle and the wobbling rotational diffusion constant for ASL in all PC membranes studied in this work. The cholesterol effects are the largest in DMPC membranes. An increase of cholesterol mole fraction from 0 to 30% decreases the rotational diffusion constant by a factor of 9-15 (depending on temperature) and the cone angle by a factor of about 2. In dioleoyl-PC membranes, addition of 30 mol % cholesterol reduces both the rotational diffusion constant and the cone angle of ASL by factors of approximately 2.5 and approximately 1.3, respectively, while it was previously found to cause only modest effects on the motional freedom of phospholipid analogue spin probes [Kusumi, A., Subczynski, W. K., Pasenkiewicz-Gierula, M., Hyde, J. S., & Merkle, H. (1986) Biochim. Biophys. Acta 854, 307-317]. It is proposed that fluid-phase microimmiscibility takes place in dioleoyl-PC-cholesterol membranes at physiological temperatures, which induces cholesterol-rich domains in the membrane, partially due to the steric nonconformability between the rigid fused-ring structure of cholesterol and the 30 degrees bend at the C9-C10 cis double bond of the alkyl chains of dioleoyl-PC. The mechanism by which cholesterol influences the lipid dynamics in the membrane is different between saturated and unsaturated PC membranes.

Frequency dependence of electrically induced alignment of anisometric colloidal particles

The alignment of axisymmetric particles in an oscillating electric field depends on the alignment time τ al , associated with the induced dipole moment of each particle, on the angular frequency ω of the electric field, and on the rotary diffusion constant D T . Furthermore, for particles with a permanent dipole moment it depends on the alignment time τ p , associated with the permanent dipole. Thus the alignment depends on three dimensionless parameters, which we can choose as D r τ al , ω/ D r , and τ al / τ p . We have studied three colloidal systems of nonspherical particles, for which D r τ al is either much smaller or larger than one, or of order unity. The alignment is followed by measuring the intensity of light scattered by the particles after switching on the elctric field. It is found that the intensity approaches a final value approximately exponentially with an oscillation of frequency 2ω superimposed. The amplitude of this oscillation is maximum when D r τ al is of order unity. For the system for which D r τ al is of order unity, small deviations from theory were observed and were attributed to a small time dependence of τ al caused by a time dependence in the surface polarization. For the system with D r τ al ⩾1, all observed transients could be ascribed to anisotropic aggregates which break up under the influence of an elecric field, an effect largest at high frequencies. The system with D r τ al ⩽ 1 followed the expected trend for particles with a permanent dipole moment, except for a decrease in the change in light scattering intensity at high frequencies.

Time dependent behavior of electric birefringence caused by an ac field superimposed on a dc field

Transients of electric birefringence are calculated for the electric field E ( t ) = E 0 cos ωt + E c where E 0 and E c are the strengths of ac and dc fields, respectively, ω is the angular frequency and t is time. A systematic study of the time-dependent behavior of the electric birefringence is presented in the form of numerous graphs for different values of the reduced frequency ω '= ω / D , D being the rotary diffusion constant, and the parameter x = E 0 / E c . Each graph is referred to by a particular value of P . which measures the influence of induced dipolar moments with respect to permanent dipolar moments. For high values of x , the well-known result obtained by Morita and Watanabe in the case of a pure sinusoidal field is found again. The interest of that theoretical study is evidenced by the determination of the molecular parameters P and D from the transient and quasi-steady-state. For that latter regime, an original method of phase angle measurements is proposed.

Infrared spectra of HCl, HBr and HI dissolved in liquid xenon

Abstract A theoretical calculation of the fundamental vibration-rotation spectra of HCl, HBr and HI in liquid Xe is presented. The main characteristics of these spectra (essentially the existence of the P, Q, and R branches) are explained from the decomposition of the rotational motion of the diatomic as a quasi-free rotational motion and a rotational diffusion motion. The used spectral theory involves only a reduced set of statistical parameters concerning the diatomic and the diatomic-liquid medium interaction: the mean square of the interaction, its correlation time, the vibrational dephasing time, the rotational diffusion constant and the fraction of molecules executing rotational diffusion motion. By comparison between theoretical and experimental spectra, the behavior of the statistical parameters with the rotational constant of the diatomic molecules is also discussed.