Visualizing molecular structures of reaction intermediates using time-resolved x-ray solution scattering

Abstract

It is essential to figure out the structure of the molecule for understanding the properties of molecules and related chemical reactions. As the chemical reaction proceeds, the three-dimensional structure of the molecule changes through metastable states on the reaction coordinates, called reaction intermediates. The structure of the reaction intermediate plays a key role in determining the pathway and product of the reaction, so investigating the molecular structures of the reaction intermediates is fundamental to understand and control the reaction. Time-resolved x-ray solution scattering (TRXSS), also known as time-resolved x-ray liquidography (TRXL), is a relevant tool for probing the ultrafast structural dynamics with high spatio-temporal resolution. Based on the pump-probe method, an optical laser pulse (pump) initiates a reaction and a time-delayed hard x-ray pulse (probe) tracks the progress of the reaction as the scattering pattern. The scattering pattern is sensitive to the global structure of the intermediate as the scattering signal arises from all atomic pairs in the molecule and thus TRXSS can be a complementary tool to the time-resolved spectroscopy tools that are sensitive to specific energy states or chromophores. The time resolution of TRXSS can be achieved by about 100 ps with the 3rd-generation synchrotron radiation source. We performed TRXSS at synchrotron on the I114F mutant of HbI and compared the results of the I114F mutant with those of the wild type to reveal the relationship between ligand migration and the structural changes. With the advent of x-ray free electron lasers (XFEL), the time resolution of TRXSS can be improved to sub-picoseconds. With the higher time resolution aided by XFEL, we observed the in-cage isomer formation mechanism of the CH2I2 and successfully determined the structure of the transiently formed precursor of the isomer.