Biomedical subjects

Stuart Allison

Publications and source records attributed to Stuart Allison.

9 recordsLinked to original sources

PubMed2006

Diffusion-controlled reactions: hydrodynamic interaction between charged, uniformly reactive spherical reactants.

In this work, different models of hydrodynamic interaction (HI) are examined in the diffusion-controlled reaction between uniformly reactive charged spherical particles. In addition to Oseen "stick" and "slip" models of HI, one is considered that accounts for the disturbance of fluid flow by the ions around one reactive partner as they interact with a neighboring reactive species. This interaction is closely related to the "electrophoretic effect" in electrokinetics and can be described by a fairly simple electrophoretic, or E-tensor. These models are applied to the electron-transfer quenching reaction of Ru(bpy)3(2+) and methyl viologen (MV2+) over a wide range of NaCl concentrations (Chiorboli, C. et al., J. Phys. Chem. 1988, 92, 156). The back reaction is also considered. From a comparison of the salt dependence of the model and experimental rates, it is concluded that the "E-tensor" model works best and ignoring HI altogether works worst. The Oseen "stick" and "slip" models fall between these.

Journal Article↗

PubMed2006

Modeling the electrophoresis of peptides and proteins: improvements in the "bead method" to include ion relaxation and "finite size effects".

A bead model methodology developed in our lab (Xin et al. J. Phys. Chem. B 2006, 110, 1038) and applicable to modeling the free solution electrophoretic mobility of peptides and proteins is generalized in two significant ways. First, an approximate account is taken of the relaxation effect, which makes the methodology applicable to more highly charged peptides and proteins than was previously possible. Second, a more accurate account is taken of the finite size of the beads making up the model structure. This improvement makes the method applicable at higher salt concentrations and/or to models consisting of larger sized subunits. The relaxation effect is accounted for by correcting "unrelaxed" mobilities on the basis of model size and average electrostatic surface, or zeta potential. Correction factors are estimated using those of spheres with the same hydrodynamic radius and zeta potential as the model structure. The correction factors of spheres are readily determined. The more general methodology is first applied to two sets of peptides (74 different peptides total) varying in size from 2 to 42 amino acids. The sets also cover a wide range of net charges. It is shown that accounting for finite bead size results in a small change in model mobilities under the conditions of the experiments (35 mM monovalent salt). The correction for ion relaxation, however, can be significant for highly charged peptides and improves agreement between model and experimental mobilities. Our correction procedure is also tested by examining the electrophoretic mobility of a particular protein "charge ladder" (Carbeck et al. J. Am. Chem. Soc. 1999, 121, 10,671), where the protein charge is varied over a wide range yet the conformation remains essentially constant. In summary, the effects of ion relaxation can be significant if the absolute electrophoretic mobility of a peptide exceeds approximately 0.20 cm2/(kV s).

Amino Acid Sequence↗

PubMed2006

Electrokinetic transport of a spherical gel-layer model particle: inclusion of charge regulation and application to polystyrene sulfonate.

An electrokinetic gel-layer model of a spherical, highly charged colloid particle developed previously [S. Allison, J. Colloid Interface Sci. 277 (2004) 248], is extended in several ways. The charge of the particle is assumed to arise from deprotonation of acidic groups that are present, in uniform concentration, over a portion (or all) of the gel layer. Free energy considerations coupled with Poisson-Boltzmann theory are used to estimate how the local electrostatic environment of a charged gel layer alters the local pK(a) of the acidic groups. This modulation of the charge of the colloidal particle, or "charge regulation," can be significant even for colloidal particles with strongly acidic groups at moderate pH if the ambient salt concentration is low. The methodology is applied to the viscosity and electrophoretic mobility data of a particular polystyrene sulfonate latex over a broad range of monovalent salt (NaCl) concentration [M.J. Garcia-Salinas, F.J. de las Nieves, Langmuir 16 (2000) 715]. The experimental data can be accounted for by a gel layer model that decreases in thickness, but does not vanish, as the salt concentration is increased. Viscosity data provides valuable information about the degree of solvation of the colloidal particle and the thickness of the gel layer. The mobility data is best explained by a model in which only the outermost portion of the gel layer is charged. Charge regulation is significant at a monovalent salt concentration of 3 x 10(-3) mol/l and increases as the salt concentration decreases.

Journal Article↗

PubMed2005

Electrophoretic mobility and primary electroviscous effect of dilute "hard" prolate ellipsoids.

Electrophoretic mobilities and primary electroviscous coefficients are determined for "hard" prolate ellipsoids of axial ratio less than or equal to 4 in KCl, NaCl, and Tris-glycine salt solutions. Account is taken of the steady state distortion of the ion atmosphere around the ellipsoid by numerical solution of the coupled Poisson, low-Reynolds-number Navier-Stokes, and ion transport equations. This is accomplished by a boundary element procedure. Results are presented as ratios of mobilities and primary electroviscous coefficients of ellipsoids to those of corresponding spheres. It is shown that the electrophoretic mobility of an ellipsoid is very similar to that of a sphere under similar conditions of size, ionic strength, salt type, and zeta potential. Other factors being equal, shape has little effect on electrophoretic mobility. For the primary electroviscous coefficient, on the other hand, there is a substantial shape effect. It is argued that the complementary techniques of electrophoresis and viscosity together provide an effective means of studying the size, charge, and shape of macroions and colloidal particles.

Journal Article↗

PubMed2004

Analysis of the electrophoretic mobility and viscosity of dilute Ludox solutions in terms of a spherical gel layer model.

A spherical gel layer model of colloidal particles is used to analyze the electrophoretic mobility and viscosity of a dilute suspension of the silica sol, Ludox, reported previously by Laven and Stein (J. Laven, H.N. Stein, J. Colloid Interface Sci. 238 (2001) 8-15). The colloid is modeled as a sphere with a solid inner core surrounded by a diffuse gel layer of uniform thickness and comprising a specific fraction, f, of the colloidal particle's mass. The gel layer is accessible to solvent and ions, but the gel layer retards the flow of solvent, which is assumed to obey the Brinkman equation. The colloidal charge is assumed to be spherically symmetric, but its disposition on the surface of the core particle and gel layer (alpha = fraction of charge in the gel layer) is left as an adjustable parameter. Experiments at pH 5.7 and 8.7 over a KCl concentration range of 0.3 to 80 mM are examined. At high salt and/or low pH, the thickness of the gel layer is estimated to be 1.0 to 1.6 nm depending on the assumed fraction of silica present in the gel layer. At low salt (0.3 mM) and high pH, where the net absolute charge of Ludox is large, the thickness of the gel layer is estimated to be 3.7 to 4.1 nm. Thus, the thickness of the gel layer appears to increase with decreasing salt at high pH. The net charge required to simultaneously match experimental and model mobilities and viscosities is sensitive to the choice of f and alpha. Nonetheless, for reasonable choices of these parameters (f = 0.13 and alpha approximately equal to 1.0), the estimated net absolute charges of Ludox from present modeling are in good agreement with the titration charges of Bolt (G.H. Bolt, J. Phys. Chem. 61 (1957) 1166-1169), and Milonjic (S.K. Milonjic, Colloids Surf. 23 (1987) 301-311) over the entire salt concentration range at pH 8.7. At pH 5.7, however, the estimated net absolute charge from current modeling exceeds the Bolt values by about 50%.

Journal Article↗

PubMed2003

Study of lipid and apolipoprotein binding interactions using vesicle affinity capillary electrophoresis.

Vesicle affinity capillary electrophoresis (VCE), a newly developed technique, was designed to assess the effect of physicochemical properties of apolipoprotein (apo) on the binding to lipoproteins, under physiological conditions (phosphate-saline buffer system at pH 7.4 and 37 degrees C), using vesicle as a model. The technique results in similar lipid binding properties of apo CIII (CIII) and its peptides compared to other techniques. It also offers a fast and more sensitive tool in determining the lipid affinity of apos in a unique system simulating the dynamic binding properties of apo in vivo. A noncompetitive binding model is used to determine the multiple binding properties of CIII and its peptides to vesicle. The VCE binding constants are dependent on temperature, physicochemical properties of the protein (hydrophobicity and charge), and nature of the vesicle. The vesicles used in the VCE experiments described here have been fully characterized and found to be stable under different temperatures (4 and 37 degrees C) and voltage conditions. Migration behavior of CIII and related peptides is reported in terms of relative mobility in order to correct for variability in viscosity at different vesicle concentrations. The VCE method provides very precise data on the migration time from 0.1 to 3.3% RSD at the highest concentration of vesicle. The model and current data have been used to determine VCE binding constants and protein-to-lipid binding ratios. The model predicts that higher lipid affinity (K(B)), protein-lipid binding ratio (n), and lower protein concentration result in a shift of the binding isotherm toward a lower concentration range of vesicle. A higher vesicle mobility, reflecting the size and charge of the vesicle, results in a larger separation window between the migration time of the free protein and the complex. The value of VCE for structure-function studies and drug design for peptides and proteins that are strongly bound to lipids has been illustrated.

Apolipoproteins↗

PubMed2003

A general gel layer model for the transport of colloids and macroions in dilute solution.

A general boundary element methodology for studying the dilute solution transport of rigid macroions that contain gel layers on their outer surfaces is developed and applied to several model systems. The methodology can be applied to particles of arbitrary size, shape, charge distribution, and gel layer geometry. Account is also taken of the steady state distortion of the ion atmosphere from equilibrium, which makes it applicable to the transport of highly charged structures. The coupled field equations (Poisson, ion-transport, low-Reynolds-number Navier-Stokes, and Brinkman) are solved numerically and from this, transport properties (diffusion constants, electrophoretic mobilities, excess viscosities) can be computed. In the present work, the methodology is first applied to a gel sphere model over a wide range of particle charge and the resulting transport properties are found to be in excellent agreement with independent theory under those conditions where independent theory is available. It is then applied to several prolate spheroidal models of a particular silica sol sample in an attempt to identify possible solution structures. A single model, that is able to account simultaneously for all of the transport behavior, which does not undergo significant conformational change with salt concentration, could not be found. A model with a thin (</=1-nm) gel layer at high salt content that expands on going to low salt content is able to explain the salt dependence of the intrinsic viscosity, but not the electrophoretic mobility. However, a model with a fairly thick (2-nm) gel layer at high salt content, which expands slightly (2.5-nm) at low salt content, is in fairly good agreement with experiment. In addition, the influence of particle charge and the presence of a gel layer on the Scheraga-Mandelkern parameter are examined. This parameter is proportional to the product of the translational diffusion constant and the cube root of the intrinsic viscosity. It is found to be very robust with regard to net particle charge as well as properties of the gel layer.

Journal Article↗

PubMed2003

The primary electroviscous effect of prolate silica sols.

The intrinsic viscosity and the dynamic mobility of four silica sols have been measured as a function of the ionic strength. It was found that intrinsic viscosity decreased with increasing ionic strength, which we attribute to the primary electroviscous effect. The geometry and the charge of the particles were fitted using experimental viscosity, light scattering, and dynamic mobility data, where the intrinsic viscosity measured at the highest ionic strength for a given sol was used as input data in our analysis. Further, the boundary element (BE) method was used to calculate the primary electroviscous effect and electrophoretic mobility of charged prolate ellipsoids. These calculations were then compared with experimental data, and the primary electroviscous effect was subtracted from the intrinsic viscosity at a given ionic strength, which led to a slightly altered geometry of the particles. This revised geometry was used as input data using the BE method, and the procedure was repeated iteratively until agreement was obtained at high ionic strength. In general, good agreement between theory and experiment was found.

Journal Article↗

PubMed2003

The primary electroviscous effect, free solution electrophoretic mobility, and diffusion of dilute prolate ellipsoid particles (minor axis = 3 nm) in monovalent salt solution.

The principal objective of the present work is the modeling of the primary electroviscous effect of charged prolate ellipsoid models of low axial ratio. Other transport properties examined include (free solution) electrophoretic mobilities and translational diffusion constants. A numerical boundary element method is employed to solve the coupled Poisson, low Reynolds number Navier-Stokes, and ion transport equations. The methodology is first applied to the primary electroviscous effect of spheres with a centrosymmetric charge distribution and excellent agreement with independent theory is obtained. Specific model studies are also carried out for prolate ellipsoid models with axial ratios less than 4 and a minor axis equal to 3 nm. Most studies are carried out in aqueous NaCl solution (2 to 50 mM) at 20 degrees C for a range of different particle charges, although limited results are also presented in LiCl and KCl solution. The primary electroviscous effect for weakly charged prolate ellipsoids is smaller than that of a sphere under similar conditions. These studies are also carried out at high absolute particle charge. A comparison is made between the primary electroviscous effect and electrophoretic mobilities of prolate ellipsoids and corresponding spherical models.

Journal Article↗