What is BioSAXS?

The basic idea behind small angle X-ray solution scattering is simple: shine a beam of X-rays on a droplet of protein solution and measure how much the rays get deflected.

BioSAXS

Most X-rays travel right through matter without changing direction, but approximately one out of every million gets deflected by a small angle! The very faint “small angle" scattering pattern on an X-ray detector looks a lot like a sunset with the undeflected beam being the like the sun: there is a bright glow nearest the beam that fades to dark at higher angles. To protect our sensitive detector, we block out the direct beam with a small metal rectangle called a beamstop. With modern algorithms, the simple pattern of light to dark on the X-ray detector can yield a surprising amount of valuable structural information about biomolecules. In fact, BioSAXS is becoming an indispensable tool in biomedical science. Not only can researchers tell the mass and size of a biomolecule in solution, but they can reconstruct its basic shape, tell if it is rigid or flexible, and even figure out how multiple molecules fit together to form complex molecular machines. Increasingly, advances in medical research depend upon gaining a clear understanding of how biomolecules function and interact within the living cell. BioSAXS is one of the few techniques that can yield structural information on how biomolecules behave under conditions very similar to the living cell.

What parameters can BioSAXS determine?

  • radius of gyration (typically 2% accuracy)
  • molecular weight (typically 10% accuracy)
  • maximum intra-particle distance
  • low-resolution electron density
  • degree of folding, denaturation, or disorder
  • comparisons with models
  • interparticle interaction potentials

What are people able to do with BioSAXS data?

  • determining physiological oligomeric state
  • validating proposed models of complexes
  • building complexes from monomers or known fragments
  • studying protein-protein interaction under different solution conditions
  • modeling missing loops and domains
  • refining homology models
  • categorizing discrete folded and unfolded states
  • finding volume fractions in mixtures

Will BioSAXS work on my samples?

In crystallography, poor crystals and overlapping spots are a frequent cause of failure. BioSAXS can have problems too. Just because it does not require crystals does not guarantee success. In crystallography, it is easier to detect bad data: you can't index or integrate the diffraction spots. With BioSAXS, however, you can process bad data with very few indications that anything is wrong. It is therefore very important to understand how to recognize bad data and to diagnose the possible causes.