Biomedical subjects

J R Abney

Publications and source records attributed to J R Abney.

9 recordsLinked to original sources

PubMed1992

Evanescent interference patterns for fluorescence microscopy.

The increasing experimental use of total internal reflection/fluorescence photobleaching recovery has motivated a theoretical study of the spatial intensity profiles generated by two interfering evanescent waves. The interference patterns generated by evanescent waves differ considerably from those generated by plane waves in a homogenous medium because evanescent waves are not transverse and because the evanescent propagation number depends on the incidence angle of the totally internally reflected light. The periodicity and contrast of the evanescent interference patterns under various conditions are calculated; these parameters depend on the intensities, polarizations, and incidence angles of the two incident beams, as well as the refractive indices of the two media that form the planar interface where total internal reflection occurs. The derived intensity profiles are used to develop expressions for the shapes of fluorescence photobleaching recovery curves when evanescent interference patterns are used for fluorescence excitation and bleaching. The calculations also suggest that colliding beam experiments may confirm theoretically predicted evanescent field polarizations.

Binding Sites↗

PubMed1991

Dynamics, structure, and function are coupled in the mitochondrial matrix.

The coupling between molecular diffusion and the structure and function of the rat liver mitochondrial matrix was explored using fluorescence anisotropy techniques and electron microscopy. The results confirm that matrix ultrastructure and the concentration of matrix protein are influenced by the respiratory state of mitochondria and the osmolarity of the external medium. At physiological osmolarity, a fluorescent metabolite-sized probe was found to diffuse slowly in the mitochondrial matrix but not to be completely immobile. In addition, significant differences in diffusion rates were found to exist between different mitochondrial respiratory states, with the slowest diffusion occurring in states with the highest matrix protein concentration. These data support the concept of a matrix structure in which diffusion is considerably hindered due to limited probe-accessible water and further suggest that volume-dependent regulation of matrix protein packing may modulate metabolite diffusion and, in turn, mitochondrial metabolism.

Animals↗

PubMed1990

On the measurement of particle number and mobility in nonideal solutions by fluorescence correlation spectroscopy.

Interparticle interactions are incorporated into the theoretical description of the initial amplitude, G(0), of the normalized fluorescence correlation spectroscopy autocorrelation function. Measurements of particle number, aggregate size, and interaction-dependent diffusion are then analyzed in the context of this generalized theory. It is shown that the neglect of interactions can introduce order-of-magnitude errors into estimates of particle number and aggregate size. It is also shown that measurement of G(0) provides an essentially unique method for testing the validity of theories of interaction-dependent membrane protein diffusion.

Mathematics↗

PubMed1989

Mutual diffusion of interacting membrane proteins.

The generalized Stokes-Einstein equation is used, together with the two-dimensional pressure equation, to analyze mutual diffusion in concentrated membrane systems. These equations can be used to investigate the role that both direct and hydrodynamic interactions play in determining diffusive behavior. Here only direct interactions are explicitly incorporated into the theory at high densities; however, both direct and hydrodynamic interactions are analyzed for some dilute solutions. We look at diffusion in the presence of weak attractions, soft repulsions, and hard-core repulsions. It is found that, at low densities, attractions retard mutual diffusion while repulsions enhance it. Mechanistically, attractions tend to tether particles together and oppose the dissipation of gradients or fluctuations in concentration, while repulsions provide a driving force that pushes particles apart. At higher concentrations, changes in the structure of the fluid enhance mutual diffusion even in the presence of attractions. It is shown that the theoretical description of postelectrophoresis relaxation and fluorescence correlation spectroscopy experiments must be modified if interacting systems are studied. The effects of interactions on mutual diffusion coefficients have probably already been seen in postelectrophoresis relaxation experiments.

Diffusion↗

PubMed1989

Self diffusion of interacting membrane proteins.

A two-dimensional version of the generalized Smoluchowski equation is used to analyze the time (or distance) dependent self diffusion of interacting membrane proteins in concentrated membrane systems. This equation provides a well established starting point for descriptions of the diffusion of particles that interact through both direct and hydrodynamic forces; in this initial work only the effects of direct interactions are explicitly considered. Data describing diffusion in the presence of hard-core repulsions, soft repulsions, and soft repulsions with weak attractions are presented. The effect that interactions have on the self-diffusion coefficient of a real protein molecule from mouse liver gap junctions is also calculated. The results indicate that self diffusion is always inhibited by direct interactions; this observation is interpreted in terms of the caging that will exist at finite protein concentration. It is also noted that, over small distance scales, the diffusion coefficient is determined entirely by the very strong Brownian forces; therefore, as a function of displacement the self-diffusion coefficient decays (rapidly) from its value at infinite dilution to its steady-state interaction-averaged value. The steady-state self-diffusion coefficient describes motion over distance scales that range from approximately 10 nm to cellular dimensions and is the quantity measured in fluorescence recovery after photobleaching experiments. The short-ranged behavior of the diffusion coefficient is important on the interparticle-distance scale and may therefore influence the rate at which nearest-neighbor collisional processes take place. The hard-disk theoretical results presented here are in excellent agreement with lattice Monte-Carlo results obtained by other workers. The concentration dependence of experimentally measured diffusion coefficients of antibody-hapten complexes bound to the membrane surface is consistent with that predicted by the theory. The variation in experimental diffusion coefficients of integral membrane proteins is greater than that predicted by the theory, and may also reflect protein-induced perturbations in membrane viscosity.

Animals↗

PubMed1988

Theoretical comparison of the self diffusion and mutual diffusion of interacting membrane proteins.

Self diffusion and mutual diffusion in two-dimensional membrane systems are analyzed. It is shown that interprotein interactions can produce markedly different density-dependent changes in the diffusion coefficients describing these two processes; the qualitative differences are illustrated by using a theoretical formalism valid for dilute solutions. Results are obtained for three analytical potentials: hard-core repulsions, soft repulsions, and soft repulsions with weak attractions. Self diffusion is inhibited by all three interactions. In contrast, mutual diffusion is inhibited by attractions but is enhanced by repulsions. It is shown that such interaction-dependent differences in self diffusion and mutual diffusion could underlie, among other things, the disparity in protein diffusion coefficients extracted from fluorescence recovery after photobleaching and postelectrophoresis relaxation data.

Algorithms↗

PubMed1987

Lateral interactions among membrane proteins. Valid estimates based on freeze-fracture electron microscopy.

We consider the lateral distribution of intrinsic membrane proteins from the viewpoint of the statistical-mechanical theory of liquids. We connect the information in freeze-fracture electron micrographs--positions of proteins but not lipids or aqueous species--to a well developed theory of liquid mixtures. An algorithm, based on the Born-Green-Yvon integral equation, is presented for deducing forces between proteins from correlations among protein positions that are observed in micrographs. The algorithm is tested on simulated micrographs, obtained by Monte-Carlo methods, where forces between proteins are known analytically. We conclude that valid estimates of such forces, both attractions and repulsions, can be obtained from the positions of a few thousand proteins.

Algorithms↗

PubMed1987

Lateral interactions among membrane proteins. Implications for the organization of gap junctions.

We have studied the relationship between interprotein forces and the lateral distribution of proteins in disordered mouse liver gap junctions. Data on protein positions are obtained from freeze-fracture electron micrographs. Short-ranged correlations in observed positions are characteristic of interacting particles in a fluid state. An analysis derived from statistical mechanics allows the determination of the magnitude and functional form of interprotein forces. We find that jap junction proteins are mutually repulsive, in a manner consistent with electrostatics and excluded volume. This dictates that long-ranged protein aggregation into jap junction plaques cannot arise solely from interparticle interactions. An alternative is the balance of lateral pressures between the junction and the surrounding glycocalyx. This idea is quantified into a model. Junctional pressure arises from protein-protein interactions and is computed from a pressure equation based on the force and a radial distribution function describing order. The pressure from the glycocalyx is assumed to arise from mixing, electrostatic, and elastic interactions of sugar residues, and is described with terms from Flory-Krigbaum and McMillan-Mayer theories. The results of this modeling are in reasonable agreement with available experimental data.

Animals↗

PubMedDate not supplied

How a gap junction maintains its structure.

In gap junctions, identical membrane proteins are linked up in pairs (dyads) that bridge the extracellular space between two apposed cell membranes. Typically, several thousand of these dyads are aggregated in the plane of the membranes and form a junctional plaque with a distinct boundary. The question thus arises as to what maintains the dyads in an aggregated state. From a statistical mechanical analysis of the positions of dyads in a freeze-fracture electron micrograph, we report here that the aggregates are not maintained by an attractive force between pairs of dyads, but probably by the minimization of the repulsive force between apposed membranes. On the basis of this analysis we present a model for the structure of mature gap junctions as well as certain aspects of the formation and disassembly of gap junctions.

Animals↗