Minoru Kubo

Abstract 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

Structural information on protein dynamics is a critical factor in fully understanding the protein functions. Pump-probe time-resolved serial femtosecond crystallography (TR-SFX) is a recently established technique for visualizing the structural changes or reactions in proteins that are at work with high spatial and temporal resolution. In the pump-probe method, protein microcrystals are continuously delivered from an injector and exposed to an X-ray free-electron laser (XFEL) pulse after a trigger to initiate a reaction, such as light, chemicals, temperature, and electric field, which affords the structural snapshots of intermediates that occur in the protein. We are in the process of developing the device and techniques for pump-probe TR-SFX while using XFEL produced at SPring-8 Angstrom Compact Free-Electron Laser (SACLA). In this paper, we described our current development details and data collection strategies for the optical pump X-ray probe TR-SFX experiment at SACLA and then reported the techniques of in crystallo TR spectroscopy, which is useful in clarifying the nature of reaction that takes place in crystals in advance.