Linda J. Magid

Structural investigation of PAMAM dendrimers in aqueous solutions using small-angle neutron scattering: effect of generation.

We investigate a series of poly(amidoamine) starburst dendrimers (PAMAM) of different generations in acidic, aqueous solutions using small-angle neutron scattering (SANS). While the overall molecular size is found to be practically unaffected by a pD change, a strong generational dependence of counterion association is revealed. Upon increasing the dendrimer generation, the effective charge obtained from our SANS experiments only shows a small increase in contrast to the nearly exponential increase predicted by a recent atomic simulation. We also find that with the same degree of molecular protonation the specific counterion association, which is defined as the ratio of bound chloride anions to positively charged amines in solutions, is larger for higher-generation PAMAM dendrimer. The associated counterion density also increases upon increasing generation number.

Research report on proteins in reverse micelles. Structural aspects and enzymology

Abstract In this research report we present various aspects of our basic investigations on proteins solubilized in AOT reverse micelles. In a first section, we present data on the rate of formation of water pools of reverse micelles: the time course of water solubilization is biphasic and it is often accelerated by the presence of proteins. Upon the protein uptake, the various components of the micellar system redistribute themselves completely into a new thermodynamic equilibrium. A detailed insight of these changes has been obtained by the ‘double-dye ultracentrifugation technique’ by which filled (i.e. containing proteins) and unfilled micelles can be simultaneously monitored in a sedimentation run. The nature of the final equilibrium is also approached theoretically on the basis of a thermodynamic treatment which deals in particular with the nature of the driving forces which are responsible for the protein solubilization in reverse micelles. The stability of the solubilized proteins has been studied with differential scanning calorimetry and circular dichroism spectroscopy from − 16 up to + 80°C. The temperature induced unfolding of the proteins within the reverse micelles is a two-state process. In an other section, kinetic studies of trypsin in reverse micelles are presented with the aim of a comparison with α-chymotrypsin. In contrast to the latter protease, trypsin solubilized in AOT/isooctane did not show ‘superactivity’ at the pH optimum of the reaction. Finally, preliminary solubilization studies with lecithin as the surfactant in organic solvents are presented. In formic acid butyl ester for example, proteins up to a molecular weight of 2500 D could be solubilized in the presence of 100 inM soybean lecithin.

The elucidation of micellar and microemulsion architecture using small-angle neutron scattering

Abstract Small-angle neutron scattering measurements on micellar and microemulsion systems studied by the author are reviewed. For such systems, the particles a

Charge-remote fragmentation in a hybrid (BEqQ) mass spectrometer to determine isotopic purity in selectively polydeuterated surfactants

The combination of fast atom bombardment with charge-remote fragmentation using a hybrid (BEqQ) mass spectrometer was used successfully to assess and localize the extent of selective deuterium isotope labeling of tetradecyItrimethylammonium bromides. Spectral details reveal a new reaction that can give rise to ions isobaric with those formed by chargeremote fragmentation.

“Over the horizon” SANS: Measurements on near-surface Poiseuille shear-induced ordering of dilute solutions of threadlike micelles

Abstract Although the behavior of a fluid under shear near a surface can be expected to be critically important to its drag and lubrication properties, most shear measurements to date have been of the bulk. This paper outlines the use of a specially developed Poiseuille shear cell at grazing incidence to measure the small-angle neutron scattering (SANS) signal from the first few tens of microns in the interfacial region. We illustrate the technique with measurements made on the near-surface ordering in flow past a quartz surface of dilute surfactant solutions comprising highly extended self-assembling “thread-like” micelles.

A small angle neutron scattering (SANS) study of micellar structure and growth of a straight-chain benzene sulfonate: comparison with an isomeric branched chain surfactant

Abstract Sodium p-n-dodecylbenzenesulfonate solutions have been studied by means of small angle neutron scattering measurements at 65°C as a function of the surfactant concentration, as a function of the contrast of the solvent, and as a function of the concentration of added electrolyte. In addition micelles of the isomeric branched chain surfactant sodium p-1-(pentylheptyl)benzenesulfonate have been studied at the same temperature as a function of the contrast of the solvent and as a function of the concentration of added electrolyte to compare the structure and the micellar growth of the two surfactants. Good fits to the experimental results have been obtained by calculating the structure factor by means of the mean spherical approximation using a screened coulombic potential plus hard-core repulsions. Several different models have been used to calculate the particle form factor. For some of the models, the measured geometry and the charge of the aggregates are, within the experimental uncertainties, independent of the contrast between the micelles and the solvent. The thickness of the shell which surrounds the hydrocarbon core implies a rough micellar surface for both surfactants; thus we propose that the planar phenyl group functions as a sort of supplementary head group, leading to a thicker polar shell.

Small‐angle neutron scattering from H2O/D2O solutions of a sodium alkylbenzenesulfonate having a branched alkyl group

Small‐angle neutron scattering results from 0.055 M sodium p(1‐pentylheptyl)benzenesulfonate solutions in D2O–H2O solutions of 57% to 100% volume percent D2O are reported. Strong interaction peaks indicate interparticle ordering. Good fits to the results can be obtained taking account of hard‐sphere and electrostatic repulsion with the mean spherical approximation. The model used is a spherical micelle, comprising a hydrocarbon core of 11.9 A radius surrounded by a shell. The thickness of the shell derived from the results is 8.9 A, implying a rough micellar surface. The measured geometry and change are essentially independent of the contrast between the micelles and the solvent.

New High-flux Small-angle Neutron Scattering Instrumentation and the Center for Structural and Molecular Biology at Oak Ridge National Laboratory

A number of upgrades are currently being undertaken at the High Flux Isotope Reactor (HFIR), including the installation of a supercritical hydrogen moderator (T ∼ 20 K) that will be one of the brightest cold sources currently available. It will feed four cold neutron guides (CGI-4), each with new instrumentation. CG2 and CG3 are reserved for two new small-angle neutron scattering (SANS) instruments. A 40 m SANS instrument (SANSI), funded by the Department of Energy (DOE) Office of Basic Energy Sciences and the University of Tennessee, Knoxville is designed for CG2. Our 35 m small-angle neutron scattering facility (SANS2 on CG3) is optimized for the study of biological systems as part of a Center for Structural and Molecular Biology (CSMB), funded by the DOE Office of Biological and Environmental Research. In addition to the new SANS facilities, there will be suite of instruments including a reflectometer and a cold triple-axis spectrometer. Both SANS facilities will have variable wavelength and large area (1m2) high count-rate detectors that can translate 45 cm off axis to increase the dynamic Q-range ( 10 15 neutrons/s/cm 2 , the beam intensities (up to 10 7 /s/cm 2 ) will be comparable to the best facilities worldwide and this will enhance our ability to collect data from synthetic and biological macromolecules more quickly and study smaller sample quantities.

SANS study of ternary non-ionic microemulsions.

SANS measurements were performed for the ternary microemulsion D2O-octane-[n-dodecylpenta(oxyethylene glycol) monoether (C12E5)] at several compositions: (1) as a function of C12E5 wt fraction γ at constant oil/water wt ratio α (α = 0.4, so the oil-to-water volume ratio is one); and (2) as a function of α (at γ = 0.07 wt fraction) through the main isotropic channel. A formalism recently developed by Teubner & Strey [J. Chem. Phys. (1987), 87, 3195–3200] was used to describe the scattering data. Some problems with the fitting of the data are discussed. The values obtained for d, the domain size, from these fits agree rather well with the conduit sizes measured by electron microscopy.

Small-angle neutron scattering from an oil-in-water microemulsion as a function of temperature

Abstract A multicomponent oil-in-water microemulsion containing CTAB (C 16 H 33 NMe 3 Br), n -octane, 1-butanol, NaBr, and D 2 O has been studied by small-angle neutron scattering at several temperatures. The size of the microemulsion droplets was found to be independent of temperature. Correlation lengths for the critical concentration fluctuations agree with those determined earlier by light scattering.