David Cookson

A low-background-intensity focusing small-angle X-ray scattering undulator beamline

The SAXS/WAXS beamline at the Australian Synchrotron is an advanced and flexible undulator X-ray scattering beamline used for small- and wide-angle X-ray scattering analysis on a wide variety of solids, fluids and surfaces across a diverse range of research and development fields. The beamline has numerous features that minimize the intensity of the instrument background, provide automated stable optics, and allow accurate analysis of very weakly scattering samples. The geometric and intensity requirements of a three-slit collimation system are described in detail for conventional metal and single-crystal germanium slits. Straightforward ray tracing and simple linear projections describe the observed direct beam as well as parasitic background scattering geometry of the beamline at its longest camera length, providing a methodology for the design and operation of similar beamlines. As an aid to instrument design, the limit of background intensity determined by the intensity incident on single-crystal germanium guard slit edges and its q dependence was quantified at 11 keV. Details of the beamlines implementation, underlying optical concept and measured performance are given.

The role of nano-roughness in antifouling

Nano-engineered superhydrophobic surfaces have been investigated for potential fouling resistance properties. Integrating hydrophobic materials with nanoscale roughness generates surfaces with superhydrophobicity that have water contact angles (θ) >150° and concomitant low hysteresis (<10°). Three superhydrophobic coatings (SHCs) differing in their chemical composition and architecture were tested against major fouling species (Amphora sp., Ulva rigida, Polysiphonia sphaerocarpa, Bugula neritina, Amphibalanus amphitrite) in settlement assays. The SHC which had nanoscale roughness alone (SHC 3) deterred the settlement of all the tested fouling organisms, compared to selective settlement on the SHCs with nano- and micro-scale architectures. The presence of air incursions or nanobubbles at the interface of the SHCs when immersed was characterized using small angle X-ray scattering, a technique sensitive to local changes in electron density contrast resulting from partial or complete wetting of a rough interface. The coating with broad spectrum antifouling properties (SHC 3) had a noticeably larger amount of unwetted interface when immersed, likely due to the comparatively high work of adhesion (60.77 mJ m−2 for SHC 3 compared to 5.78 mJ m−2 for the other two SHCs) required for creating solid/liquid interface from the solid/vapour interface. This is the first example of a non-toxic, fouling resistant surface against a broad spectrum of fouling organisms ranging from plant cells and non-motile spores, to complex invertebrate larvae with highly selective sensory mechanisms. The only physical property differentiating the immersed surfaces is the nano-architectured roughness which supports longer standing air incursions providing a novel non-toxic broad spectrum mechanism for the prevention of biofouling.

A Synthetic Resilin Is Largely Unstructured

Proresilin is the precursor protein for resilin, an extremely elastic, hydrated, cross-linked insoluble protein found in insects. We investigated the secondary-structure distribution in solution of a synthetic proresilin (AN16), based on 16 units of the consensus proresilin repeat from Anopheles gambiae. Raman spectroscopy was used to verify that the secondary-structure distributions in cross-linked AN16 resilin and in AN16 proresilin are similar, and hence that solution techniques (such as NMR and circular dichroism) may be used to gain information about the structure of the cross-linked solid. The synthetic proresilin AN16 is an intrinsically unstructured protein, displaying under native conditions many of the characteristics normally observed in denatured proteins. There are no apparent alpha-helical or beta-sheet features in the NMR spectra, and the majority of backbone protons and carbons exhibit chemical shifts characteristic of random-coil configurations. Relatively few peaks are observed in the nuclear Overhauser effect spectra, indicating that overall the protein is dynamic and unstructured. The radius of gyration of AN16 corresponds to the value expected for a denatured protein of similar chain length. This high degree of disorder is also consistent with observed circular dichroism and Raman spectra. The temperature dependences of the NH proton chemical shifts were also measured. Most values were indicative of protons exposed to water, although smaller dependences were observed for glycine and alanine within the Tyr-Gly-Ala-Pro sequence conserved in all resilins found to date, which is the site of dityrosine cross-link formation. This result implies that these residues are involved in hydrogen bonds, possibly to enable efficient self-association and subsequent cross-linking. The beta-spiral model for elastic proteins, where the protein is itself shaped like a spring, is not supported by the results for AN16. Both the random-network elastomer model and the sliding beta-turn model are consistent with the data. The results indicate a flat energy landscape for AN16, with very little energy required to switch between conformations. This ease of switching is likely to lead to the extremely low energy loss on deformation of resilin.

Cortactin Adopts a Globular Conformation and Bundles Actin into Sheets

Cortactin is a filamentous actin-binding protein that plays a pivotal role in translating environmental signals into coordinated rearrangement of the cytoskeleton. The dynamic reorganization of actin in the cytoskeleton drives processes including changes in cell morphology, cell migration, and phagocytosis. In general, structural proteins of the cytoskeleton bind in the N-terminal region of cortactin and regulatory proteins in the C-terminal region. Previous structural studies have reported an extended conformation for cortactin. It is therefore unclear how cortactin facilitates cross-talk between structural proteins and their regulators. In the study presented here, circular dichroism, chemical cross-linking, and small angle x-ray scattering are used to demonstrate that cortactin adopts a globular conformation, thereby bringing distant parts of the molecule into close proximity. In addition, the actin bundling activity of cortactin is characterized, showing that fully polymerized actin filaments are bundled into sheet-like structures. We present a low resolution structure that suggests how the various domains of cortactin interact to coordinate its array of binding partners at sites of actin branching.

Amorphization of embedded Cu nanocrystals by ion irradiation

While bulk crystalline elemental metals cannot be amorphized by ion irradiation in the absence of chemical impurities, the authors demonstrate that finite-size effects enable the amorphization of embedded Cu nanocrystals. The authors form and compare the atomic-scale structure of the polycrystalline, nanocrystalline, and amorphous phases, present an explanation for the extreme sensitivity to irradiation exhibited by nanocrystals, and show that low-temperature annealing is sufficient to return amorphized material to the crystalline form.

Structural and vibrational properties of Co nanoparticles formed by ion implantation

This work was financially supported by the Australian Synchrotron and the Australian Research Council. ChemMatCARS Sector 15 is principally supported by the National Science Foundation/Department of Energy under Grant No. NSF/CHE-0822838. Use of the Advanced Photon Source was supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Contract No. DE-AC02-06CH11357.

Leather structure determination by small-angle X-ray scattering (SAXS): cross sections of ovine and bovine leather.

SAXS has been applied to structural determination in leather. The SAXS beamline at the Australian Synchrotron provides 6 orders of magnitude dynamic range, enabling a rich source of structural information from scattering patterns of leather sections. SAXS patterns were recorded for q from 0.004 to 0.223 A(-1). Collagen d spacing varied across ovine leather sections from 63.8 nm in parts of the corium up to 64.6 nm in parts of the grain. The intensity of the collagen peak at q = 0.06 A(-1) varied by 1 order of magnitude across ovine leather sections with the high-intensity region in the corium and the low intensity in the grain. The degree of fiber orientation and the dispersion of the orientation has been quantified in leather. It is shown how the technique provides a wealth of useful information that may be used to characterize and compare leathers, skin, and connective tissue.

Dynamic micromapping of CO2 sorption in coal.

We have applied X-ray and neutron small-angle scattering techniques (SAXS, SANS, and USANS) to study the interaction between fluids and porous media in the particular case of subcritical CO2 sorption in coal. These techniques are demonstrated to give unique, pore-size-specific insights into the kinetics of CO2 sorption in a wide range of coal pores (nano to meso) and to provide data that may be used to determine the density of the sorbed CO2. We observed densification of the adsorbed CO2 by a factor up to five compared to the free fluid at the same (p, T) conditions. Our results indicate that details of CO2 sorption into coal pores differ greatly between different coals and depend on the amount of mineral matter dispersed in the coal matrix: a purely organic matrix absorbs more CO2 per unit volume than one containing mineral matter, but mineral matter markedly accelerates the sorption kinetics. Small pores are filled preferentially by the invading CO2 fluid and the apparent diffusion coefficients have been estimated to vary in the range from 5x10(-7) cm2/min to more than 10(-4) cm2/min, depending on the CO2 pressure and location on the sample.

Swift heavy-ion irradiation-induced shape and structural transformation in cobalt nanoparticles

This work was financially supported by the Australian Synchrotron and the Australian Research Council with access to equipment provided by the Australian Nanofabrication Facility. ChemMatCARS Sector 15 is principally supported by the NSF/ DOE under Grant No. NSF/CHE–0822838.

Nanowetting of rough superhydrophobic surfaces

Small angle x-ray scattering has been used to investigate the in situ immersive wetting of ultrarough surfaces which exhibit superhydrophobicity with extreme water contact angle (θA=169°). Reduced scattering contrast observed from rough surfaces when partially or totally wetted reveals significant physical differences between superhydrophobic surfaces not otherwise apparent from conventional contact angle measurements.