Small-Angle Scattering Measurements Research Articles

Methods for Small-Angle Scattering Measurements on Peptiplexes of DNA with Cell-Penetrating Peptides.

Peptiplexes are soft biomaterials formed through the noncovalent association between cell-penetrating peptides and nucleic acids. Although internalization often involves electrostatic anchoring followed by endocytosis, the mode of action of these transporters remains elusive in many cases, and proper understanding of mechanisms behind their penetrating capabilities necessarily entails structural data at the nanoscopic scale. In this chapter, we examine the structural landscape of peptiplexes, emphasizing the complex behavior of these polyelectrolyte self-assemblies and how supramolecular order impacts their translocation efficiency. We discuss experimental tools commonly used to investigate the structure of peptiplexes and pay special attention to small-angle X-ray scattering (SAXS) as a suitable method for unveiling their nanoscale organization. A roadmap for standard SAXS measurements in CPP/DNA samples is presented alongside a selection of observations from our own experience dealing with SAXS applied to the investigation of CPPs.

Structural Analysis of DDS Nanocarriers by Neutron and X-ray Small-angle Scattering Measurements

Structural Analysis of DDS Nanocarriers by Neutron and X-ray Small-angle Scattering Measurements

Casein micelles in milk as sticky spheres.

Milk is a ubiquitous foodstuff and food ingredient, and milk caseins are key to the structural properties of milk during processing and storage. Caseins self-assemble into nanometer-sized colloids, referred to as "micelles", and particles of this size are ideally suited to study by small-angle scattering (SAS). Previous SAS measurements have almost exclusively focussed on the internal structure of the micelles. While important for milk's properties, this attention to the interior of the micelles provides limited information about the structure-forming properties of milk and milk ingredients. The ultra-small-angle X-ray scattering (USAXS) measurements and analysis in this study extend to the micrometer scale, which makes it possible to characterize the interaction between the micelles. Until now, SAS studies have generally excluded a consideration of the interparticle interactions between casein micelles. This is inconsistent with these new data, and it is not possible to model the data without some interparticle attraction. If the micelles are treated as sticky spheres, excellent agreement between experimental data and model fits can be obtained over the length scales studied, from micrometers to ångströms. The stickiness of casein micelles will impact ultra-small-angle scattering and small-angle scattering measurements of casein micelles, but it particularly limits the application of simple approximations, which generally assume that particles are dilute and noninteracting. In summary, this analysis provides an approach to modelling scattering data over many orders of magnitude, which will provide better understanding of interactions between caseins and during food processing.

Characterization of components of nano-energetics by small-angle scattering techniques

Small-angle scattering (SAS) and ultra small-angle scattering techniques, employing x-rays and neutrons, were used to characterize six different aluminum nanopowders and nanopowders composed of molybdenum trioxide and tungsten trioxide nanoparticles. Each material has different primary particle morphology and aggregate and agglomerate geometry, and each is important to the development of nano-energetic materials. The combination of small-angle and ultra small-angle techniques allowed a wide range of length scales to be probed, providing a more complete characterization of the materials. For the aluminum-based materials, differences in the scattering of x-rays and neutrons from aluminum and aluminum oxide provided sensitivity to the metal core and metal oxide shell structure of the primary nanoparticles. Small-angle scattering was able to discriminate between particle size and shape and agglomerate and aggregate geometry, allowing analysis of both aspects of the structure. Using the results of these analyses and guided by scanning electron microscopy (SEM) images, physical models were developed, allowing for a quantitative determination of particle morphology, mean nanoparticle size, nanoparticle size distribution, surface layer thickness, and aggregate and agglomerate fractal dimension. Particle size distributions calculated using a maximum entropy algorithm or by assuming a log-normal particle size distribution function were comparable. Surface area and density determinations from the small-angle scattering measurements were comparable to those obtained from other, more commonly used analytical techniques: gas sorption using Brunauer–Emmett–Teller analysis, thermogravimetric analysis, and helium pycnometry. Particle size distribution functions derived from the SAS measurements agreed well with those obtained from SEM.

Neutron Diffraction with an Excess-Water Cell

As part of a study of the molecular basis of membrane fusion by enveloped viruses, we have used neutron diffraction to study the lamellar (L(α)) to inverse hexagonal (H(II)) phase transition in the phospholipid N-methylated dioleoylphosphatidylethanolamine. This lipid was chosen because its phase transitions are particularly sensitive to the presence of agents that have been demonstrated to promote or inhibit membrane fusion. Two different geometries of neutron diffraction were used: small angle scattering (SANS) and a membrane diffractometer. The SANS measurements were carried out on the SWAN instrument at KEK, Japan, using dispersions of multilamellar vesicles (MLVs). The diffractometer measurements used the V1 instrument at BeNSC-HMI, Germany, with a specially-constructed cell that holds a stack of lipid bilayers in an excess-water state. The two approaches are compared and discussed. Although the diffractometer takes considerably longer to collect the data, it records much higher resolution than the SANS instrument. The samples recorded in the excess-water cell were shown to be well aligned, despite the lipids being fully hydrated, allowing for the production of high-resolution data. Trial measurements performed have demonstrated that sample alignment is preserved throughout the L(α) to H(II) phase transition, thereby opening up possibilities for obtaining high-resolution data from non-lamellar phases.

A Model for the Three-Dimensional Structure of Human Plasma Vitronectin from Small-Angle Scattering Measurements

Small-angle X-ray scattering (SAXS) measurements were used to characterize vitronectin, a circulatory protein found in human plasma that functions in regulating cell adhesion and migration, as well as proteolytic cascades that affect blood coagulation, fibrinolysis, and pericellular proteolysis. SAXS measurements were taken over a 3-fold range of protein concentrations, yielding data that characterize a monodisperse system of particles with an average radius of gyration of 30.3 +/- 0.6 A and a maximum linear dimension of 110 A. Shape restoration was applied to the data to produce two models of the solution structure of the ligand-free protein. A low-resolution model of the protein was generated that indicates the protein to be roughly peanut-shaped. A better understanding of the domain structure of vitronectin resulted from low-resolution models developed from available high-resolution structures of the domains. These domains include the N-terminal domain that was determined experimentally by NMR [Mayasundari, A., Whittemore, N. A., Serpersu, E. H., and Peterson, C. B. (2004) J. Biol. Chem. 279, 29359-29366] and the docked structure of the central and C-terminal domains that were determined by computational threading [Xu, D., Baburaj, K., Peterson, C. B., and Xu, Y. (2001) Proteins: Struct., Funct., Genet. 44, 312-320]. This model provides an indication of the disposition of the central domain and C-terminal heparin-binding domains of vitronectin with respect to the N-terminal somatomedin B (SMB) domain. This model constructed from the available domain structures, which agrees with the low-resolution model produced from the SAXS data, shows the SMB domain well separated from the central and heparin-binding domains by a disordered linker (residues 54-130). Also, binding sites within the SMB domain are predicted to be well exposed to the surrounding solvent for ease of access to its various ligands.

X-ray Pole Figure and Small Angle Scattering Measurements on Tubular Blown Low-Density Poly(ethylene) Films

Pole figures obtained by wide-angle X-ray diffraction have been used to quantify and characterize the molecular orientations induced in low-density poly(ethylene) films prepared by the tubular film process. Small-angle X-ray scattering was employed to determine the type and orientation of the morphology within the films. All films were found to possess a combination of two morphologies: a surface transcrystalline layer and a row-nucleated morphology caused by crystallization in low-stress conditions. For the first series of blown films, in which the draw-down ratio was increased from 1.9 to 7.9 for a constant blow-up ratio of 1.68, the a axis was observed to increase its orientation along the extrusion direction, signifying that the row-nucleated structures intensify their orientation along this same direction. The amount of transcrystalline material decreases upon increasing the draw-down ratio. Unexpectedly, the amorphous regions tend to become biaxially oriented in the higher drawn samples. In the second series of films, the blow-up ratio was increased from 1.60 to 2.74 for a constant draw-down ratio of 4.2. In this case, the row-nucleated morphology became progressively oriented in the film plane upon bubble enlargement, thus creating an uniplanar orientation of the molecular chains in the film plane. The amorphous regions, weakly oriented on the onset along the extrusion direction, gradually become biaxially oriented in the film plane upon increasing the blow-up ratio. Finally, it was shown that molecular orientation measurements can be used to verify the consistency of the processing conditions in blown film production.

Improved Double-Crystal Spectrometer for Small Angle Scattering Measurements

Improved Double-Crystal Spectrometer for Small Angle Scattering Measurements