In a multi-way collaboration during the past year, we were the first to employ the ultrabright X-rays available from the free electron laser to perform a time-resolved binding study on an RNA. Our work on ligand binding by the adenine riboswitch captured, in atomic detail, the intermediates through which the RNA transits in order to achieve its ligand-bound, signaling conformation. This information will open the way to the structure-guided discovery and optimization of small molecules that target not the ground state of an RNA, but excited states. In this first study, ligand was simply diffused into the RNA nanocrystals employed for diffraction studies. Because of this, the time resolution of the experiment is of the order of several seconds. Future studies, utilizing photo-caged compounds, will allow time resolutions in the millisecond to microsecond range. For these studies, the ultrabright, femto-second resolution X-ray pulses available from the free electron laser will be essential.
