Exploration of Methods for Serial Microcrystallography at Storage Ring X-ray Sources

Abstract

Protein crystallography has made the largest contribution to our knowledge of protein structure. However, it is well known that many biologically important proteins do not readily form large enough crystals for traditional crystallography. Successful serial microcrystallography (SMX) studies have been performed at X-ray Free-Electron Lasers, but they are limited in experimental availability. We look to more accessible light sources, such as storage ring sources, for the development of SMX. However, improvements to the conventional experiment are required to make SMX viable at storage rings. Here, we explore several devices and techniques designed given this consideration. To isolate crystal diffraction, the sample environment should contribute zero background scatter outside the crystal, since excess scatter obscures the weak microcrystal signal. We will show the feasibility of using atomically-thin, gas-tight graphene to reduce background scatter as a crystal mount and suggest using it as a window material for SMX. We will also explore two microcrystal delivery devices, a microfluidic chip and a viscous jet injector, for use in SMX. While both of these devices show promise for optimizing various aspects of the crystal delivery system, both contribute more background scatter than is acceptable for an ideal SMX experiment at a storage ring source. Merging diffraction from multiple microcrystals is necessary when a complete data set cannot be determined from a single microcrystal. When microcrystal diffraction is weak enough that Bragg peaks are no longer visible, merging through conventional techniques fails since crystal orientation cannot be obtained through Bragg peak indexing. We will explore proof-of-principle experiments which show that indexing data frames on a per-frame basis is unnecessary for a structure solution, when reciprocal space intensities can be reconstructed using the EMC algorithm. In principle, serial microcrystallography is feasible at storage ring sources if improvements in beamline setups, sample chamber construction and microcrystal diffraction analysis evolve to optimize the diffraction of microcrystals.