Biomedical subjects

Eric W Kaler

Publications and source records attributed to Eric W Kaler.

18 recordsLinked to original sources

PubMed2006

Effects of additives on surfactant phase behavior relevant to bacteriorhodopsin crystallization.

The interactions leading to crystallization of the integral membrane protein bacteriorhodopsin solubilized in n-octyl-beta-D-glucoside were investigated. Osmotic second virial coefficients (B(22)) were measured by self-interaction chromatography using a wide range of additives and precipitants, including polyethylene glycol (PEG) and heptane-1,2,3-triol (HT). In all cases, attractive protein-detergent complex (PDC) interactions were observed near the surfactant cloud point temperature, and there is a correlation between the surfactant cloud point temperatures and PDC B(22) values. Light scattering, isothermal titration calorimetry, and tensiometry reveal that although the underlying reasons for the patterns of interaction may be different for various combinations of precipitants and additives, surfactant phase behavior plays an important role in promoting crystallization. In most cases, solution conditions that led to crystallization fell within a similar range of slightly negative B(22) values, suggesting that weakly attractive interactions are important as they are for soluble proteins. However, the sensitivity of the cloud point temperatures and resultant coexistence curves varied significantly as a function of precipitant type, which suggests that different types of forces are involved in driving phase separation depending on the precipitant used.

Alkanes↗

PubMed2006

Stabilization of catanionic vesicles via polymerization.

Polymerizable cationic surfactant methacryloyloxyoctyl trimethylammonium bromide (MOTB) and anionic surfactant sodium 4-(omega-methacryloyloxyoctyl)oxy benzene sulfonate (MOBS) were synthesized. Stable catanionic vesicles can spontaneously form upon mixing the two oppositely charged surfactants in aqueous solution, which was further permanently fixed by polymerization. Surface tensiometry, nuclear magnetic resonance (NMR), static and dynamic laser light scattering (LLS), and cryogenic transmission electron microscopy (cryo-TEM) were used in combination to characterize the catanionic vesicles before and after polymerization. The kinetics of formation and breakdown of unpolymerized catanionic vesicles were studied in detail employing stopped-flow light scattering. In contrast to unpolymerized vesicles, the polymerized ones exhibit permanent stability under external perturbations such as dilution or adding excess MOTB. A tentative explanation is proposed about why free radical polymerization can successfully fix the catanionic vesicles, the structure of which is well-known to be in dynamic equilibrium exchange with unimers.

Journal Article↗

PubMed2006

Surfactant and polyelectrolyte gel particles that swell reversibly.

Mixing of oppositely charged surfactants and polyelectrolytes in aqueous solutions can lead to associative phase separation, where the concentrated phase is a viscous liquid, gel, or precipitate. In recent years, this phenomenon has been exploited to form gel-like particles, ranging from approximately 100 to 4000 microm in diameter, whose stability depends on equilibrium phase behavior. As the sample composition is varied, these particles either remain stable (in a two-phase mixture) or dissolve over time. Here, we present the formation of reversibly swelling gel particles from mixtures of N,N,N-trimethylammonium-derivatized hydroxyethyl cellulose (JR-400) and sodium dodecyl sulfate (SDS), whose swelling is controlled by the ambient solution conditions. The effects of cross-linking density and surfactant concentration are investigated by gravimetry and confocal microscopy. The resulting particles have a core/shell morphology and undergo reversible swelling/collapse transitions which, depending on the cross-link density, can be either gradual or abrupt with changing SDS concentration.

Journal Article↗

PubMed2006

Polymerization of anionic wormlike micelles.

Polymerizable anionic wormlike micelles are obtained upon mixing the hydrotropic salt p-toluidine hydrochloride (PTHC) with the reactive anionic surfactant sodium 4-(8-methacryloyloxyoctyl)oxybenzene sulfonate (MOBS). Polymerization captures the cross-sectional radius of the micelles (approximately 2 nm), induces micellar growth, and leads to the formation of a stable single-phase dispersion of wormlike micellar polymers. The unpolymerized and polymerized micelles were characterized using static and dynamic laser light scattering, small-angle neutron scattering, 1H NMR, and stopped-flow light scattering. Stopped-flow light scattering was also used to measure the average lifetime of the unpolymerized wormlike micelles. A comparison of the average lifetime of unpolymerized wormlike micelles with the surfactant monomer propagation rate was used to elucidate the mechanism of polymerization. There is a significant correlation between the ratio of the average lifetime to the monomer propagation rate and the average aggregation number of the polymerized wormlike micelles.

Journal Article↗

PubMed2005

Mixtures of n-octyl-beta-D-glucoside and triethylene glycol mono-n-octyl ether: phase behavior and micellar structure near the liquid-liquid phase boundary.

The phase behavior and microstructure of aqueous mixtures of n-octyl-beta-D-glucoside (C8betaG1) and triethylene glycol mono-n-octyl ether (C8E3) is presented. C8betaG1 forms a one-phase micellar solution in water at surfactant concentrations up to 60 wt %, whereas mixtures with C8E3 show a liquid-liquid phase transition at low surfactant concentration. The position of this phase boundary for mixtures can be rationally shifted in the temperature-composition window by altering the ratio of the two surfactants. Small-angle neutron scattering is used to determine the size and shape of the mixed micelles and to characterize the nature of the fluctuations near the cloud point of the micellar solutions. The C8betaG1/C8E3 solutions are characterized by concentration fluctuations that become progressively stronger upon approach to the liquid-liquid phase boundary, whereas micellar growth is negligible. Such observations confirm previous views of the role of the surfactant phase boundary in tuning attractive micellar interactions, which can be used effectively to change the nature and strength of interparticle interactions in colloidal dispersions. Colloidal silica particles were then added to these surfactant mixtures and were found to aggregate at conditions near the cloud point. This finding is relevant to current strategies for protein crystallization.

Colloids↗

PubMed2005

Fibrous assemblies and water gelation in mixtures of lysine with sodium alkyl sulfates.

Thermoresponsive, pH-sensitive fibrous structures and gels are formed in aqueous mixtures of the amino acid lysine with oppositely charged sodium alkyl sulfate surfactants. The formation of these assemblies depends on the chain length of the surfactant, which is varied between 8 and 16, the chirality and degree of protonation of the amino acid, and the molar ratio of these species. Self-assembly of the fibers occurs when specific lysine enantiomers are in solution and for pH conditions in which the majority of the amine groups are protonated (i.e., at near-equimolar amounts of HCl and lysine). Racemic mixtures of lysine do not form fibers with sodium dodecyl sulfate. Micelles are the fiber precursors, and the fibers, which can be hundreds of micrometers long, entangle to form gels. With increasing temperature, the gels melt, the fibers dissolve, and a single micellar phase forms. The micelles elongate with decreasing pH when the acid concentration is greater than equimolar with respect to lysine, and they shrink with increasing temperature.

Gels↗

PubMed2005

Small-angle neutron scattering from mixtures of sodium dodecyl sulfate and a cationic, bolaform surfactant containing azobenzene.

This paper reports on the microstructures formed in aqueous solutions containing mixtures of sodium dodecyl sulfate (SDS) and a photosensitive, bolaform surfactant, bis(trimethylammoniumhexyloxy)azobenzene dibromide (BTHA). By using quasi-elastic light scattering and small-angle neutron scattering, we determined that aqueous solutions containing SDS and the trans isomer of BTHA (0.1 wt % total surfactant, 15 mol % BTHA, 85 mol % SDS) form vesicles with average hydrodynamic diameters of 1350 +/- 50 angstroms and bilayer thicknesses of 35 +/- 2 angstroms. The measured bilayer thickness is consistent with a model of the vesicle bilayer in which the trans isomer of BTHA spans the bilayer. Upon illumination with UV light, the BTHA underwent photoisomerization to produce a cis-rich photostationary state (80% cis isomer). We measured this photoisomerization to drive the reorganization of vesicles into cylindrical aggregates with cross-sectional radii of 19 +/- 3 angstroms and average hydrodynamic diameters of 240 +/- 50 angstroms. Equilibration of the cis-rich solution in the dark at 25 degrees C for 12 h or illumination of the solution with visible light leads to the recovery of the trans-rich photostationary state of the solution and the reformation of vesicles, thus demonstrating the potential utility of this system as the basis of a tunable fluid.

Azo Compounds↗

PubMed2005

Influence of pH on the micelle-to-vesicle transition in aqueous mixtures of sodium dodecyl benzenesulfonate with histidine.

Small unilamellar vesicles (approximately 100 nm in diameter) form spontaneously in aqueous mixtures of histidine and sodium dodecyl benzenesulfonate. By manipulating pH, a gradual transition from micelles to vesicles to bilayers to precipitate is observed. The self-assembly of vesicles occurs over a wide range of compositions when the solution pH is lower than 6.0, the pKa of the imidazole moiety on the histidine molecule. This phenomenon is likely the result of attractive interactions between the negatively charged benzenesulfonate headgroups and the positively charged imidazole group in the amino acid. Similar results are obtained when imidazole salt itself is used.

Benzenesulfonates↗

PubMed2005

The role of protein and surfactant interactions in membrane-protein crystallization.

The interactions leading to crystallization of the integral membrane protein bacteriorhodopsin solubilized in n-octyl-beta-D-glucoside were investigated. Osmotic second virial coefficients were measured by self-interaction chromatography in the presence of sodium malonate, sodium formate and ammonium sulfate. Attractive protein-detergent complex (PDC) interactions were observed as the surfactant cloud-point temperature was approached for each salt, suggesting that surfactant interactions may play an important role in promoting PDC crystallization. Dynamic light scattering and tensiometry measurements show that the interaction trends are strongly influenced by micelle structure and surfactant phase behavior, both of which are sensitive to salt and surfactant concentration. Overall, detailed investigations using a combination of experimental techniques can provide insight into the complex nature of PDC interactions, which is essential to developing rational approaches to membrane-protein crystallization.

Bacteriorhodopsins↗

PubMed2005

Quantitative imaging by confocal scanning fluorescence microscopy of protein crystallization via liquid-liquid phase separation.

Metastable states such as liquid-liquid phase separation, aggregation and gelation can affect protein crystallization but their positive or negative effects are only partially understood. In this work, mixtures of PEG (MW 10 kDa) and a large model protein, glucose isomerase (MW 173 kDa), have been studied to characterize the effect of a metastable liquid-liquid phase separation on protein crystallization. Fluorescence labeling allowed confocal fluorescence microscopy observations and quantification of the partitioning of the protein and PEG between the liquid phases and showed two steps in the crystallization process. Two crystallization mechanisms within the liquid domain were revealed, yielding two different polymorphs. With one polymorph, few crystals nucleated and grew droplet-by-droplet in the dispersed concentrated liquid phase, while for the other homogeneous crystal nucleation and growth occurred independently and simultaneously in numerous droplets of the concentrated phase. The results demonstrate the substantial possible complexity of crystallization behavior, as well as its sensitivity to the location of the conditions on the phase diagram and to the physicochemical properties of the system.

Aldose-Ketose Isomerases↗

PubMed2005

Relating surfactant properties to activity and solubilization of the human adenosine a3 receptor.

The effects of various surfactants on the activity and stability of the human adenosine A3 receptor (A3) were investigated. The receptor was expressed using stably transfected HEK293 cells at a concentration of 44 pmol functional receptor per milligram membrane protein and purified using over 50 different nonionic surfactants. A strong correlation was observed between a surfactant's ability to remove A3 from the membrane and the ability of the surfactant to remove A3 selectively relative to other membrane proteins. The activity of A3 once purified also correlates well with the selectivity of the surfactant used. The effects of varying the surfactant were much stronger than those achieved by including A3 ligands in the purification scheme. Notably, all surfactants that gave high efficiency, selectivity and activity fall within a narrow range of hydrophile-lipophile balance values. This effect may reflect the ability of the surfactant to pack effectively at the hydrophobic transmembrane interface. These findings emphasize the importance of identifying appropriate surfactants for a particular membrane protein, and offer promise for the development of rapid, efficient, and systematic methods to facilitate membrane protein purification.

Binding, Competitive↗

PubMed2005

Structural characteristics and growth of pentagonal silver nanorods prepared by a surfactant method.

The crystal structure and growth mechanism of silver nanorods prepared by a seed-mediated surfactant method using the cationic surfactant cetyltrimethylammonium tosylate (CTAT) and its wormlike micelles are characterized by conventional and high-resolution transmission electron microscopy. Depending on the nanorod orientations, two types of electron diffraction patterns are obtained from a truncated decahedral structure consisting of five crystal units packing along [111] twining planes with five [111] planes on each end and five circumferential [001] side surfaces parallel to a <110> longitudinal direction. High-resolution images of the nanorods and the corresponding Fourier transform patterns confirm the results from the morphological and diffraction analyses. The silver nanorods grow only from multiply twinned decahedral seeds, and the high selectivity of surfactant attachment results in a barrier to the transfer of silver atoms from the solution to the circumferential [100] planes. Blockage of circumferential growth causes the aspect ratio of the rod to grow.

Journal Article↗

PubMed2004

Spontaneous vesicle formation and phase behavior in mixtures of an anionic surfactant with imidazoline compounds.

Unexpected colloidal assemblies form in aqueous mixtures of sodium dodecylbenzenesulfonate (SDBS) with the following imidazoline compounds: 2,2'-azobis[2-(2-imidazolin-2-yl)propane] dihydrochloride (V-44, which is a commonly used free-radical initiator), 2,2'-tetramethylenedi-2-imidazoline (TMI), and the main recombination product (RP) from the decomposition of V-44. All of these imidazoline compounds act as hydrotropes. As the molar ratio of imidazoline to SDBS increases, a gradual transition from micelles to vesicles to bilayers to precipitate is observed. V-44 decomposes slowly at 25 degrees C, and the phase diagrams of V-44/SDBS and RP/SDBS are similar. The vesicular region observed in mixtures of TMI/SDBS is larger in composition than that of V-44/SDBS and RP/SDBS mixtures. At equimolar compositions of SDBS and RP, a novel colloidal structure with multiple closely packed bilayers is observed. In these mixtures, small unilamellar vesicles (<80 nm in diameter) form spontaneously, although with time they coexist with a small amount of precipitate and their size increases steadily. The self-assembly of vesicles occurs over a wide range of compositions when the solution pH is lower than the pK(a) of the imidazoline moiety. Quasi-elastic light scattering, cryogenic transmission electron microscopy, nuclear magnetic resonance, and small-angle neutron scattering were used to determine the characteristic length scales and properties of the assemblies.

Journal Article↗

PubMed2004

Shear-induced phase separation in solutions of wormlike micelles.

Polymer solutions in the vicinity of the theta-point are known to undergo shear-induced turbidity or phase separation. The present study shows that a similar phenomenon also occurs for certain wormlike micellar solutions. Wormlike micelles are the self-assembled counterparts of polymers and are characterized by their ability to reversibly break and recombine. In the system of interest, the micelles are formed by the cationic surfactant erucyl bis(hydroxyethyl)methylammonium chloride (EHAC), in conjunction with a salt such as sodium chloride (NaCl) or sodium salicylate (NaSal). Micellar samples that become turbid under shear show evidence of critical concentration fluctuations and may contain predominantly branched micelles. The shear-induced turbidity in these samples correlates with the appearance of flow-dichroism in rheooptic experiments and with an increase in low-q scattering in small-angle light scattering under flow (flow-SALS) experiments. The characteristic "butterfly" pattern, with enhanced scattering in the flow direction and a dark streak perpendicular to the flow direction, is typically observed in flow-SALS. The results suggest that the turbidity is due to a shear-induced growth of concentration fluctuations, which in turn manifests as large anisotropic domains, typically oriented along the vorticity axis.

Journal Article↗

PubMed2004

Two-dimensional crystallization of microspheres by a coplanar AC electric field.

The particle-field and particle-particle interactions induced by alternating electric fields can be conveniently used for on-chip assembly of colloidal crystals. Two coplanar electrodes with a millimeter-sized gap between them are used here to assemble two-dimensional crystals from suspensions of either latex or silica microspheres. When an AC voltage is applied, the particles accumulate and crystallize on the surface between the electrodes. Light diffraction and microscopic observations demonstrate that the hexagonal crystal is always oriented with one axis along the direction of the field. The particles disassemble when the field is turned off, and the process can be repeated many times. The diffraction patterns from all consecutively formed crystals are identical. This assembly is driven by forces that depend on the electric field gradient, and a model is proposed involving a combination of dielectrophoresis and induced dipole chaining. The organization of large two-dimensional crystals allows characterization of the electrostatic interactions in the particle ensembles. The process can be controlled via the field strength, the frequency, and the viscosity of the liquid media. It could be used to make rudimentary optical switches or to separate mixtures of particles of different sizes.

Journal Article↗

PubMed2003

Effect of multiple scattering on SANS spectra from bicontinuous microemulsions.

Small-angle neutron scattering is a powerful tool for investigating the microstructure of self-assembled systems. The domain length, d, and the correlation length, xi, are two measures of bicontinuous microemulsions that are often determined from coherent SANS spectra. Some microemulsions scatter strongly, however, so measured spectra can contain multiple coherent scattering, and neglect of that multiple scattering can lead to incorrect values of d and xi. In addition, multiple scattering can give rise to artifacts in the spectra, most notably an apparent scattering peak at twice the value of the scattering vector of the main peak. Here, changes in the SANS spectra from strongly scattering microemulsions and the parameters derived from them are reported as a function of relative scattering probability by varying both sample thickness and scattering contrast. A linear extrapolation of the results to zero scattering probability yields good estimates for the microstructural parameters, and the numerical procedure of Schelten and Schmatz is used to calculate the specific effects of multiple scattering on typical bicontinuous microemulsion scattering spectra.

Journal Article↗

PubMed2003

Small angle neutron scattering study of sodium dodecyl sulfate micellar growth driven by addition of a hydrotropic salt.

The structures of aggregates formed in aqueous solutions of an anionic surfactant, sodium dodecyl sulfate (SDS), with the addition of a cationic hydrotropic salt, p-toluidine hydrochloride (PTHC), have been investigated by small angle neutron scattering (SANS). The SANS spectra exhibit a pronounced peak at low salt concentration, indicating the presence of repulsive intermicellar interactions. Model-independent real space information about the structure is obtained from a generalized indirect Fourier transformation (GIFT) technique in combination with a suitable model for the interparticle structure factor. The interparticle interaction is captured using the rescaled mean spherical approximation (RMSA) closure relation and a Yukawa form of the interaction potential. Further quantification of the geometrical parameters of the micelles was achieved by a complete fit of the SANS data using a prolate ellipsoidal form factor and the RMSA structure factor. The present study shows that PTHC induces a decrease in the fractional charge of the micelles due to adsorption at the micellar surface and consequent growth of the SDS micelles from nearly globular to rodlike as the concentration of PTHC increases.

Journal Article↗

PubMed2002

Effect of pH and Ca2+-induced associations of soybean proteins.

An experimental procedure was developed to characterize the solubility of the soybean protein fractions close to the isoelectric point. The results show that the 7S fraction is precipitated in a much narrower range of pH values than the 11S fraction. Surprisingly, the addition of salt to the solutions leads to increased solubility of proteins, unlike the common "salting out" effect generally expected for proteins in solution in this range of salt concentrations. The precipitation equilibria of both soybean fractions in the presence of calcium ions and electrolyte were characterized. The amount of calcium ions required to precipitate a mole of the 7S fraction is much larger than that required for the 11S fractions. The precipitation pattern can be correlated to the charge density per surface area of the proteins.

Calcium↗