Small‐angle X‐ray scattering for macromolecules in solution is now widely used in structural biology to complement high‐resolution structure determination obtained by X‐ray crystallography or NMR. In the context of third‐generation synchrotron sources, this increasing interest leads to developments in sample environments and automation. The presence of an online purification system is justified by the need for sample monodispersity. A combined system including an auto‐sampler robot and online high‐performance liquid chromatography (HPLC) has been developed and optimized at the SWING beamline of Synchrotron SOLEIL (Gif‐sur‐Yvette, France). In the sample changer mode, a few microlitres of sample can be injected between two air bubbles and circulated at a controlled speed of typically 40 µl min−1. A maximum of 14 samples per hour could be measured in this mode by remote controlling the sample injections. In the HPLC mode, an initially polydisperse sample can be separated into each of its components before immediate data acquisition. The sample cell is thermostated, and offers a visualization control and online UV–Vis absorption monitoring.
Microbial glycolipids are a class
of well-known compounds, but
their self-assembly behavior is still not well understood. While the
free carboxylic acid end group makes some of them interesting stimuli-responsive
compounds, the sugar hydrophilic group and the nature of the fatty
acid chain make the understanding of their self-assembly behavior
in water not easy and highly unpredictable. Using cryo-transmission
electron microscopy (cryo-TEM) and both pH-dependent in situ and ex
situ small angle X-ray scattering (SAXS), we demonstrate that the
aqueous self-assembly at room temperature (RT) of a family of β-d-glucose microbial glycolipids bearing a saturated and monounsaturated
C18 fatty acid chain cannot be explained on the simple basis of the
well-known packing parameter. Using the “pH-jump” process,
we find that the molecules bearing a monosaturated fatty acid forms
vesicles below pH 6.2, as expected, but the derivative with a saturated
fatty acid forms infinite bilayer sheets below pH 7.8, instead of
vesicles. We show that this behavior can be explained on the different
bilayer membrane elasticity as a function of temperature. Membranes
are either flexible or stiff for experiments performed at a temperature
respectively above or below the typical melting point, T
M, of the lipidic part of each compound. Finally, we also
show that the disaccharide-containing acidic cellobioselipid forms
a majority of chiral fibers, instead of the expected micelles.
Myoglobin and lysozyme picosecond internal dynamics in solution is compared to that in hydrated powders by quasielastic incoherent neutron scattering. This technique is sensitive to the motions of the nonexchangeable hydrogen atoms in a sample. Because these are homogeneously distributed throughout the protein structure, the average dynamics of the protein is described. We first propose an original data treatment to deal with the protein global motions in the case of solution samples. The validity of this treatment is checked by comparison with classical measurements of the diffusion constants. The evolution with the scattering vector of the width and relative contribution of the quasielastic component was then used to derive information on the amount of local diffusive motions and their characteristic average relaxation time. From dry powder to coverage by one water layer, the surface side chains progressively acquire the possibility to diffuse locally. On subsequent hydration, the main effect of water is to improve the rate of these diffusive motions. Motions with higher average amplitude occur in solution, about three times more than for a hydrated powder at complete coverage, with a shorter average relaxation time, approximately 4.5 ps compared to 9.4 ps for one water monolayer.
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