Tracking Protein Motions using Serial Femtosecond Crystallography with X-Ray Free-Electron Laser.

Since the birth of biochemistry, researchers have investigated the structure-function relationship of a wide variety of proteins. However, until recently, when X-ray free-electron lasers (XFELs) became available, it was not possible to visualize the motion of proteins from moment to moment with excellent temporal and spatial resolution. Here, we introduce practical methods to visualize protein motions at room temperature using serial femtosecond crystallography (SFX) using XFELs. With the development of this technology, it will be possible to visualize the entire reaction mechanism of many proteins in the future. We first outline a streamlined microcrystallization workflow for hen egg-white lysozyme, enabling rapid detector calibration and data-collection optimization. Next, we present a rotational seeding approach refined on copper-containing nitrite reductase that yields homogeneous microcrystals suitable for high-resolution SFX and readily adaptable to other challenging targets. Finally, we describe a time-resolved strategy combining microcrystals of fungal nitric-oxide reductase with photolabile caged substrates and synchronized UV triggering, capturing catalytic intermediates on the millisecond timescale. Together, these procedures enable investigators to progress from preparing samples to capturing dynamic structural snapshots. © 2025 The Author(s). Current Protocols published by Wiley Periodicals LLC. Basic Protocol 1: Microcrystallization of lysozyme Basic Protocol 2: Microcrystallization of copper-containing nitrite reductase Basic Protocol 3: Time-resolved serial femtosecond crystallography.