Kenneth H. Langley

Detection of a major conformational change in transfer ribonucleic acid by laser light scattering

The diffusion constants of Escherichia coli transfer ribonucleic acid and phenylalanine-specific tRNA from yeast have been determined under a variety of solution conditions using the technique of laser light scattering. The results reveal that a major change occurs in the conformation of tRNA when the ionic strength is reduced from 0·2 m to 0·1 m in the presence of 1·0 m m -magnesium at pH 7·2 and 20°C. The diffusion constant of the low salt conformer is greater by 11%, indicating that it is significantly more compact than the high salt form. A ‘block and hinge’ model is proposed to explain the conformational change. The results also reveal that a readily reversible association of bulk tRNA occurs in 10 m m -magnesium at high ionic strength at 20°C with an association constant of about 103 m −1. The association constant is similar to that reported by others for the formation of dimers between tRNAs with complementary anticodons under similar conditions. The surface charge of unfractionated E. coli tRNA has been estimated from the diffusive virial coefficients. In 1 m m -MgCl2 the average charge per tRNA is ten electrons, while at 10 m m -MgCl2 there are eight electrons per molecule.

Dynamic light scattering study of semidilute solutions of a stiff‐chain polymer

We report results from dynamic light scattering on the translational mobility of poly‐γ‐benzyl‐L‐glutamate in semidilute solutions with N, N‐dimethylformamide as solvent. Dimethylformamide is shown to be a good solvent for the virtually rodlike poly‐γ‐benzyl‐L‐glutamate molecules. The observed translational diffusion coefficient is seen to increase with concentration of polymer, in disagreement with the theory of Doi and Edwards, and also with the recent results of Zero and Pecora, who studied the poly‐γ‐benzyl‐L‐glutamate, 1,2‐dichlorethane system. The increase in diffusion coefficient with concentration is attributed to a repulsive thermodynamic contribution. The magnitude of this term was estimated from a linear virial expansion and subtracted from the measured diffusion coefficient. The difference represents diffusion in the absence of thermodynamic effects and does decrease with increasing concentration, perhaps even more strongly than expected. In qualitative agreement with the dynamic structure fac...

Zero-angle depolarized light scattering of a colloidal polymer

Abstract The potential of zero-angle depolarized scattering for measuring the size distributions of optically anisotropic particles is considered. To illustrate the method, an analysis of Fluon, a teflon latex suspension, is presented. Homodyne correlation functions having signal-to-noise characteristics every bit as good as conventional unpolarized finite-angle measurements enabled confident application of a smoothed exponential sampling algorithm to obtain the decay rate distribution, which was converted to a size distribution with certain approximations. The size distribution obtained from light scattering agrees well with that from electron microscopy.

Depolarized dynamic light scattering of a rigid macromolecule poly(p‐phenylene benzbisthiazole)

The rotational relaxation time of poly(p‐phenylene benzbisthiazole) (PBT) has been used to characterize its conformation in chlorosulfonic acid solutions. We present a method of extracting the persistence length from the forward depolarized photon correlation data. Using Hearst’s theory for the rotational diffusion coefficient of a wormlike chain, a persistence length of 640 A is determined. The results also suggest that under certain conditions depolarized photon correlation spectroscopy is a potentially useful method for estimating the polydispersity of a polymer sample.

Electrostatic and screening effects on the dynamic aspects of polyelectrolyte solutions

We present a quasielastic light scattering (QELS) study of a synthetic polyelectrolyte compound: poly(paraphenylene vinylene) derivative dissolved in methanol. The dynamical aspects of these solutions are found to be strongly dependent on whether or not we have added an ordinary electrolyte (salt for instance). For salt‐free solutions we measured two different, and a priori independent, diffusion coefficients, DC and DS, attributed, respectively, to cooperative and large scale motions. The small value of the hydrodynamical correlation length ξH, deduced from DC, reflects the large interpenetration of polyions in salt‐free solutions. The chains are, consequently, highly expanded even though not rod‐like in shape. This size, ξH, does not depend on the polymer concentration cp for the range scanned. The addition of even a small amount of sodium chloride progressively modifies the solution’s behavior. First, we measured only one diffusion coefficient when the salt concentration cs exceeded a certain threshold...

Time scale dependence of diffusion in porous material: Dynamic light scattering and computer simulation

Diffusion of polymer chains in liquid‐filled pores of controlled pore size silica glasses was studied experimentally by using dynamic light scattering (DLS), and by a computer simulation based on a hydrodynamic theory of hard spheres in cylindrical pores. Dynamic light scattering at fixed scattering wave vector revealed faster apparent diffusion at short times (corresponding to diffusion within a single pore) followed by a slower relaxation which we attribute to macroscopic diffusion over distances large enough to average over the microscopic nonuniformities of the glass‐pore matrix. The measured time at which the behavior crosses over from faster to slower diffusion was found to be independent of the light scattering wave vector, and is roughly equal to the time required for a polymer molecule to diffuse a distance comparable to the pore radius. At small ratio of polymer to pore radius, the crossover time was found to be proportional to the polymer radius, both in DLS measurements and in the computer sim...

Spectrum and intensity of light scattered from SF6 along the coexistence curve

Abstract Our two groups have measured independently the “Rayleigh linewidth” (Γ), the reduced compressibility (ϖϱ/ϖμ) T , and the pressure ( P ) of SF 6 along the liquid and vapor sides of the coexistence curve. We find that ( ϖϱ / ϖμ ) T = 1.67 × 10 -10 (1 - T / T c ) -1.22 ± 0.015 g 2 erg -1 cm -3 and (∂P/∂T)lχ = (7094 ± 0.1) × 10 5 dyne/cm 2 ° We analyze our linewidth measurements in terms of the Kadanoff-Swift-Kawasaki mode-mode coupling theory using estimates for the viscosity and correlation range.

Rayleigh linewidth measurements in liquid helium

A small-angle intensity fluctuation spectrometer has been constructed and used to measure the width Γ of the Rayleigh component of 514.5-nm light scattered from liquid helium. The techniques utilized to maximize the weak scattered signal are described. Measurements of Γ at small angles (65<θ < 104 mrad) as a function of temperature (2.6<T<4.3 K) and pressure (0.27<P<1.42 MPa) agree with predictions based on hydrodynamic theory.

7. Light Scattering and Other Optical Methods

Publisher Summary This chapter discusses the concept of light scattering and other optical methods. A number of studies employing various optical techniques have been carried out in porous materials in recent years. Generally they fall into two categories. First, if the matrix material is transparent and uniform in index of refraction and the pore spaces can be filled with a fluid having an index of refraction that closely matches that of the matrix material, the porous material is rendered transparent. So-called “index matching” removes the discontinuities at the pore-matrix boundaries to such an extent that the effects of multiple scattering and attenuation can be essentially ignored. Conventional imaging is employed and the detected light is considered as affected only by single scattering. Second, in complete contrast to the first category, in systems that scatter light strongly, the propagation of light can be described by a diffusion approximation. The key idea is that the light that reaches the detector has been scattered within the medium so many times that the phases of the scattered fields have been completely randomized and only the intensity of the light reaching the detector must be considered. An equivalent description is that the path of each scattered photon can be considered to be a random walk through the medium. The statistics of these random walks is well understood and can be used in the analysis of the scattered light. Dynamic light scattering (DLS), fluorescence recovery after photobleaching (FRAP), forced Rayleigh scattering (FRS), and interferometric methods belong to the first category. Diffusing-wave spectroscopy (DWS) belongs to the second category and is relatively new to this field. The distinguishing characteristics of each method are described in the chapter.

Conformational Transitions as Molecular Signalling Mechanisms in Synthetic Bilayer Membranes

Conformational transitions in membrane-bound macromolecules probably provide the most general and powerful mechanisms for chemical and physical signalling processes in biology. Changes in the conformations of membrane proteins and glycoproteins are exploited in cell biology to modulate enzymatic activity, to change membrane permeability, and to initiate the membrane reorganization events involved in endocytosis, in secretion and in the processing of ligands and receptors. In recent years, we have explored the consequences of conformational transitions in synthetic macromolecules bound to lipid bilayer membranes by adsorption or via hydrophobic anchoring groups.1-4 The present chapter examines the roles of polymer conformation and solvation in controlling membrane structure in such systems.