SFX

Serial Femtosecond Crystallography (SFX) is a technique developed for crystallography experiments at x-ray FELs that has since been adopted for use at synchrotrons and called Serial Macromolecular Crystallography (SMX).

A little bit of background

To arrive at an underlying structure using traditional macromolecular crystallography, one must measure the intesities of Bragg reflections generated from diffraction from a crystal and convert these into structure factors (contibutors to the actual structure contained within the crystal that can be made into a Fourier series to reveal and electron density map). It is important to note that it is the integrated intensity of the unique Bragg peak that is related to the corresponding structure factor so measurements or methods that allow for integrated peak intensities are needed. In conventional crystallography this is accomplished via a rotation series where the relative orientation of the crystal to the X-ray beam is altered to allow an integrated measurement.

So what is serial crystallography?

Serial crystallography is a method developed to allow us to get the necessary integrated intensities for crystal samples from LCLS data. The same  properties of x-ray FELs that allow it to make unique measurements of biomolecules and their dynamics make adopting the methods of traditional crystallography highly ineffective to impossible. The peak intensity of an x-ray FEL pulse destroys the sample after the measurement and makes multiple measurements on any biomolecule crystal challenging to impossible. So each measurement from a crystal will only give data on a small subset of the un-integrated peak intensities of the unique reflections. However, in order to reconstruct an electron density map for the crystallized molecule we still need to measure the integrated peak intensities for all of the unique reflections. The need to collect the integrated data from all of the unique reflections when each measurement destroys the biomolecule crystal birthed the SFX technique. In SFX, the integrated data is accumulated by independent measurements of thousands of separate biomolecule crystals that interact with the x-ray FEL beam--by indexing these data on a per pattern basis and merging/integrating the data for unique reflections that appear on multiple independent measurements.

 

Consequently, for microcrystals, new sample introduction methods to the x-ray beam have been developed as well as novel data analysis methods that allow for accurate determination of the structure factor amplitudes from millions of random single crystal diffraction snapshots.