What is BioSAXS and why it is important?

SAXS (Small-angle X-ray Scattering) is a powerful technique to probe relatively large-scale structure of the sample by x-ray. BioSAXS (Biological SAXS) focuses on biological macromolecules like protein, DNA, RNA, lipid, virus particles and so no. BioSAXS can play a crucial role in understanding how molecules look like, how they move, and how they function in cells, leading to drug design and discovery.
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 What is BioSAXS?

Q. Why BioSAXS is important for us?
Many human diseases are caused by dysfunctions of molecules in cells. For example, many genetic disorders inherit disease-causing mutations in protein. They lead to changes in its structure and/or function. That may result in its malfunction or overresponse in cells due to the alternation of its property (e.g. affinity of intermolecular interactions). Such structural insights could have significant implications for drug design and development. BioSAXS is a powerful tool to investigate the details of biological macro-molecules at up to nearly atomic resolution.
Q. What is X-ray and why BioSAXS needs it?
X-ray are a type of electromagnetic radiation whose wavelength (0.1-10nm) is much smaller than that of visible light (380-
740nm). X-ray are used for the experiment to probe the sample at high (up to atomic) resolution since the theoretical attainable resolution is associated with the wavelength of the light used. In particular, BioSAXS requires smaller X-ray wavelength (so-called, hard X-ray) not only to obtain high-resolution information but also to minimize radiation damage of the sample. Most biological samples are stable in water. However higher wavelength X-ray (so-called, soft X-ray) generates radicals in water, causing radiation damages to the sample. The X-ray wavelength used at SSRL BL4-2 is typically 0.1-2.0 nm.
As shown in the right figures (see next Q&A for more details about BioSAXS experiments), weaker X-ray scattering signals need to be recorded during BioSAXS experiment. Strong, high quality and tunable X-ray beam is required for it. Synchrotron radiation (of SSRL!) is an ideal X-ray source to perform BioSAXS experiments.
Q. How is BioSAXS experiment performed?
The figures above are general pictures of BioSAXS experiment and analysis. The experimental setup like X-ray wavelength and sample-to-detector distance are decided in advance based on the size of your interest (e.g. overall size of the sample or local dimension of interest in or between the sample). In general, BioSAXS experiment requires a highly purified sample (technically speaking, monomodal and monodisperse sample). The sample applies into the sample cell and then is exposed to an X-ray beam. Scattered X-ray at small-angle (< 10 degrees) are recorded by the 2D detector and usually, a bunch of images is taken for averaging. Depending on the type of experiment, the same things are conducted on the blank (e.g. water or buffer solution without target molecule) for the background subtraction. Once the scattering curve is obtained from the images, you can bring it to further computational analysis (e.g. 3D structure modeling). BioSAXS is quite powerful especially by combining with additional information from other biochemical and biophysical experiments like crystallography.

Representative BioSAXS experiments at SSRL BL4-2
High-throughput SAXS Autosampler
Fully automatic system for BioSAXS data collections and analysis. This is designed for the solution sample. 100 data collections and analyses can be done in 4-5 hrs.
SEC-SAXS (Size-Exclusion Chromatography SAXS) is a powerful tool for unstable and difficult sample. The sample can be exposed to X-ray right after the SEC column. The latest HPLC (High-Performance Liquid Chromatography) is employed at BL4-2.
TR-SAXS using SF mixer
TR-SAXS (Time-resolved SAXS) is a unique experiment to pursue the fast changes during the reaction (e.g. conformational changes of the sample at msec to min time region). The SF (Stopped-flow) mixer is a device that can trigger the reaction by fast mixing.