Probing Protein Structural Dynamics with Pump-Probe X-ray Liquidography and Crystallography

Abstract

Dynamic intermolecular interactions in DNA and proteins, which play crucial roles in their biological functions, often involve transient interactions. Therefore understanding their working mechanisms can be greatly aided by monitoring such structural dynamics with enough temporal and spatial resolutions. In this regard, recent years have witnessed that time-resolved methods based on X-ray diffraction or scattering can serve as excellent tools to study spatiotemporal reaction dynamics of proteins in single crystals and solutions. X-ray crystallography, the major structural tool to determine 3D structures of proteins, can be extended to time-resolved X-ray crystallography (TRXC) with a laser-excitation and X-ray-probe scheme, and all the atomic positions in a protein can be tracked during their biological function. However time-resolved crystallography has been limited to a few model systems with reversible photocycles due to the stringent prerequisites such as highly-ordered and radiation-resistant single crystals and crystal packing constraints might hinder biologically relevant motions. These problems can be overcome by applying time-resolved X-ray diffraction directly to protein solutions rather than protein single crystals. To emphasize that structural information can be obtained from the liquid phase, this time-resolved X-ray solution scattering technique is named time-resolved X-ray liquidography (TRXL) in analogy to time-resolved X-ray crystallography where the structural information of reaction intermediates is obtained from the crystalline phase. In this talk, we will present our representative results obtained by TRXC and TRXL.