Ultrashort, intense X-ray bursts have been used in the past few years to glean structural information from streams of protein nanocrystals, soot, water droplets, and even single molecules. The same approach can also yield structures of short-lived protein reaction intermediates at near-atomic resolution, reports a team led by Marius Schmidt of the University of Wisconsin, Milwaukee (Science 2014, DOI: 10.1126/science.1259357). Researchers have obtained time-resolved crystal structures using traditional synchrotron X-rays, but that approach requires large crystals and the time resolution is limited to about 100 picoseconds. Using SLAC National Accelerator Laboratory’s Linac Coherent Light Source, Schmidt and his colleagues studied light-initiated reactions of a bacterial photoreceptor called photoactive yellow protein, in which laser excitation induces a chromophore switch from a trans to a cis configuration that is associated with protein conformational changes. The scientists were able to combine X-ray pulses with protein microcrystals to determine structures to 1.6 Å and detect previously unidentified structural changes. With further development, the technique likely can produce high-quality structures with subpicosecond time resolution for a variety of biologically and pharmaceutically interesting proteins, the researchers say.