Manifestation of one-patch attractive protein interactions in solution scattering and in solution structures

In this work, we show how static structure factors, obtained from small-angle X-ray scattering and static light scattering shed light as to how patchy, attractive interactions between globular proteins are manifested experimentally and can be distinguished from isotropic attractions. Furthermore, the data are well described by an integral equation theory due to Wertheim for associating liquids, which shows how the patch-patch attraction gives rise to a concentration-driven association into dimers, which to our knowledge has not been shown previously for a biologically relevant system. This fundamental study shows how proteins interactions under the right set of conditions become highly anisotropic and patchy and how this leads to a concentration-induced dimerization, which at yet higher concentrations lead to stripe-like structures being formed. Please click Additional Files below to see the full abstract

Analysis of small-angle X-ray scattering data in the presence of significant instrumental smearing.

A laboratory-scale small-angle X-ray scattering instrument with pinhole collimation has been used to assess smearing effects due to instrumental resolution. A new, numerically efficient method to smear ideal model intensities is developed and presented. It allows for directly using measured profiles of isotropic but otherwise arbitrary beams in smearing calculations. Samples of low-polydispersity polymer spheres have been used to show that scattering data can in this way be quantitatively modeled even when there is substantial distortion due to instrumental resolution

Monodisperse PEGylated spheres: an aqueous colloidal model system

[Image: see text] Fluorinated core–shell spheres have been synthesized using a novel semibatch emulsion polymerization protocol employing slow feeding of the initiator. The synthesis results in aqueous dispersions of highly monodisperse spheres bearing a well-defined poly(ethylene glycol) graft (PEGylation). Measurements are consistent with the synthesis achieving a high grafting density that moreover consists of a single PEG layer with the polymer significantly elongated beyond its radius of gyration in bulk. The fluorination of the core of the particles confers a low index of refraction such that the particles can be refractive index matched in water through addition of relatively small amounts of a cosolvent, which enables the use of optical and laser-based methods for studies of concentrated systems. The systems exhibit an extreme stability in NaCl solutions, but attractions among particles can be introduced by addition of other salts, in which case aggregation is shown to be reversible. The PEGylated sphere dispersions are expected to be ideally suited as model systems for studies of the effect of PEG-mediated interactions on, for instance, structure, dynamics, phase behavior, and rheology

Study of the micelle-to-vesicle transition and smallest possible vesicle size by temperature-jumps.

We have investigated the temperature induced micelle-to-vesicle transition in a binary non-ionic surfactant/water system, for which the spontaneous curvature decreases with increasing temperature. Temperature jumps with variable rate were performed in a microwave oven, from a micellar phase at 5 ◦C to a lamellar phase region at 35 ◦C, passing a liquid-liquid two phase region where dilute and concentrated micellar solutions coexist. It is shown that the obtained vesicle size decreases with increasing heating rate through this two phase region. Moreover, we demonstrate that there exists a minimum vesicle radius, Rv∗, as is also predicted by theory. In the present system we find Rv∗ ≈ 50 nm, in reasonable agreement with a theoretical estimate

Concentration-dependent effective attractions between PEGylated nanoparticles

Effective attractions between colloidal particles bearing a grafted poly(ethylene glycol) (PEG) layer in water have been studied and quantified by measurements of the collective diffusion coefficient and by quantitative analysis of small-angle neutron scattering (SANS) data. Results for the collective diffusion coefficient in the dilute limit indicate that effective attractions develop gradually as carbonate anions are added to the dispersions. Analysis of SANS data within a square-well interaction model at a constant salt concentration allows for quantitative analysis of scattering patterns of samples prior to crossing into an aggregation regime, where particles form large clusters, reached either through increasing the temperature or the particle concentration. Aggregation is observed visually and is also evident in the scattering as a lowering of the intensity at intermediate wavevectors while leaving enhanced scattering in the forward direction, suggesting a nearby fluid-fluid phase transition. In addition, at low and moderate particle concentrations the attraction strength is shown to depend mainly on temperature but at high particle concentrations a much stronger temperature dependence is observed, which shows that the attraction acquires a dependence on particle concentration at sufficiently high concentrations. The concentration dependence is attributed to a decreased solvation of PEG chains due to an increased ratio of ethylene oxide segments to water

In Situ Observation of the Genesis of Mesoporous Silica SBA-15: Dynamics on Length Scales from 1 nm to 1 mum.

We report on the mechanism of growth of mesoporous silica (SBA-15, plane group p6m). In situ studies of the formation using ultrasmall angle X-ray scattering (USAXS) and small-angle X-ray scattering (SAXS) covering length scales from 5 to 10 000 A, complemented with UV-vis and transmission electron microscopy (TEM), provide unique data on particle growth coupled with information regarding the progression of the mesostructure formation and the micellar evolution

Polymer-Graft-Mediated Interactions between Colloidal Spheres

Aqueous dispersions of fluorinated colloidal spheres bearing grafted poly­(ethylene glycol) (PEG) are studied as a function of salt and particle concentration with the aim of improving the understanding of interactions among polymer-grafted particles. These dispersions can sustain large concentrations of salt, but crystals nucleate in dilute dispersions when a sufficient Na₂CO₃ concentration is reached, which is attributed to the presence of attractions between particles. On further increasing the Na₂CO₃ concentration, the solvent is rapidly cleared of particles. Small-angle X-ray scattering and cryogenic transmission electron microscopy are employed in order to quantify the attractions. The former is used to extract a second virial coefficient, and the latter shows that the PEG-graft contracts as a function of increasing salt concentration. The contraction not only leads to a reduction in excluded volume but also is accompanied by attractions of moderate magnitude. In contrast, dispersion of the particles in ethanol, in which bulk PEG solutions crystallize, lead to fractal structures caused by strong attractions

Peptide Nanotube Nematic Phase.

The self-assembly of the trifluoroacetate salt of the short peptide (ala)(6)-lys (A(6)K) in water has been investigated by cryo-transmission electron microscopy and small-angle X-ray scattering. For concentrations below ca. 12%, the peptide does not self-assemble but forms a molecularly dispersed solution. Above this critical concentration, however, A(6)K self-assembles into several-micrometer-long hollow nanotubes with a monodisperse cross-sectional radius of 26 nm. Because the peptides carry a positive charge, the nanotubes are charge-stabilized. Because of the very large aspect ratio, the tubes form an ordered phase that presumably is nematic