We demonstrate the use of an X-ray free electron laser synchronized with an 
optical pump laser to obtain X-ray diffraction snapshots from the photoactivated 
states of large membrane protein complexes in the form of nanocrystals flowing 
in a liquid jet. Light-induced changes of Photosystem I-Ferredoxin co-crystals 
were observed at time delays of 5 to 10 µs after excitation. The result 
correlates with the microsecond kinetics of electron transfer from Photosystem I 
to ferredoxin. The undocking process that follows the electron transfer leads to 
large rearrangements in the crystals that will terminally lead to the 
disintegration of the crystals. We describe the experimental setup and obtain 
the first time-resolved femtosecond serial X-ray crystallography results from an 
irreversible photo-chemical reaction at the Linac Coherent Light Source. This 
technique opens the door to time-resolved structural studies of reaction 
dynamics in biological systems.