Sample‐minimizing co‐flow cell for time‐resolved pump–probe X‐ray solution scattering

A fundamental problem in biological sciences is understanding how macromolecular machines work and how the structural changes of a molecule are connected to its function. Time‐resolved techniques are vital in this regard and essential for understanding the structural dynamics of biomolecules. Time‐resolved small‐ and wide‐angle X‐ray solution scattering has the capability to provide a multitude of information about the kinetics and global structural changes of molecules under their physiological conditions. However, standard protocols for such time‐resolved measurements often require significant amounts of sample, which frequently render time‐resolved measurements impossible. A cytometry‐type sheath co‐flow cell, developed at the BioCARS 14‐ID beamline at the Advanced Photon Source, USA, allows time‐resolved pump–probe X‐ray solution scattering measurements to be conducted with sample consumption reduced by more than ten times compared with standard sample cells and protocols. The comparative capabilities of the standard and co‐flow experimental setups were demonstrated by studying time‐resolved signals in photoactive yellow protein. For irreversible or slow reactions studies by time‐resolved pump–probe X‐ray solution scattering, the standard sample cell and protocol for data collection at the BioCARS 14‐ID beamline potentially require up to hundreds of milligrams of sample. A sheath co‐flow cell, described here, allows such solution scattering measurements to be conducted with the sample consumption reduced by more than ten times compared with standard protocols at this beamline.