Left: Solution X-ray scattering enables high-throughput structural characterizations of gene products in solution. The technique can distinguish between aggregated and unfolded proteins, define global structural parameters and oligomeric states for most samples, identify shapes and structures similar to unknown structures, and determine protein envelopes. Right: SAXS reveals how protein-mediated reorganization of microbial DNA adapts E. coli to a changing environment. Experimental SAXS curves (left) indicate that HU, a histone-like nucleoid-associated protein (blue and cyan structures at right), organizes the structure of the nucleoid (yellow) differently at a high salt concentration (300mM) than at lower concentrations (50-150mM). This, in turn, affects global gene regulation in the cell. [Left: From Hura, G. L., et al. 2009. “Robust, High-Throughput Solution Structural Analyses by Small Angle X-Ray Scattering (SAXS),” Nature Methods 6, 606–12. DOI: 10.1038/NMETH.1353.] Right: Reprinted under a Creative Commons Attribution 4.0 International (CC BY 4.0) license from Remesh, S. G., et. al. 2020. "Nucleoid remodeling during environmental adaptation is regulated by HU-dependent DNA bundling," Nature Communications 11, 2905. DOI: 10.1038/s41467-020-16724-5.]
Solution X-ray scattering provides structural information about any solution sample. This type of X-ray scattering is often referred to as small angle X-ray scattering (SAXS) because information of interest is most often detected at small angles relative to the probing X-ray beam. Most investigators use SAXS to characterize the behavior of macromolecules in solution at the nanometer scale. Because sample preparation is relatively simple and in solution, and data collection is fast (milliseconds), SAXS can be high throughput and performed during sample purification or while the sample is undergoing dynamic change. The more structurally homogeneous the sample is, the more information can be extracted.
An advantage to performing SAXS at a synchrotron experimental station is that data can be collected during sample elution from attached purification columns. This enables unique access to transient complexes that rapidly assemble and disassemble and observations of the effects of varied solution conditions on stabilizing complexes or processes. Furthermore, in an era where mutations are easily introduced or sequence variants are contrasted, high-throughput screening allows hundreds of constructs to be characterized within hours. SAXS is therefore an ideal assessment tool for use in the iterative design cycle of macromolecular engineering. Scattering from heterogeneous materials can also be used to characterize changes in the degree of ordering used to monitor, for example, the degradation of cellulose.
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