First femtosecond protein pump-probe measurements at SwissFEL

A major milestone in the commissioning of SwissFEL has been reached: the first pump-probe experiments on proteins have been successfully carried out. Crystals of several retinal-binding proteins were delivered in a viscous jet system and a femtosecond laser was used to start the isomerization reaction. Microsecond to sub-picosecond snapshots were then collected, catching the retinal proteins shortly after isomerization of the chromophore. First, several snapshots in the picosecond scale were collected, providing important insights into the early changes in the chromophore and the protein. After commissioning further parts of the timing system, a first sub-picosecond time delay was chosen. At 800 femtoseconds, shortly after the isomerization, a highly distorted retinal could be observed that has not reached the relaxed all-trans geometry. This finding represents a breakthrough for SwissFEL and the photobiology community. Further measurements will shed light on the detailed mechanism of this photocatalytic process. Similar measurements will revolutionize our understanding of enzyme catalysis.