Figure 1. EF-X principles and implementation.

a, A sampling of charged residues (in CPK colours) in LNX2^PDZ2 (Protein Data Bank (PDB) accession 2VWR), exemplifying potential actuators for applied electric fields (E, in red). Ligand is in yellow. b, EF-X involves stimulation of motions in protein crystals by an applied electric field (E) of duration τ (the ‘pump’), and readout by much faster X-ray pulses (the ‘probe’). c, An LNX2^PDZ2 crystal mounted across the orifice of a glass capillary, filled with crystallization solution and a metal electrode. The crystal is sealed onto the capillary by an electrically insulating glue. d, The crystal is mounted on the bottom electrode and the high voltage is delivered from a top electrode through a liquid junction composed of crystallization solution. Controlled back pressure on a reservoir of solution in the top electrode keeps the crystal continuously hydrated. e, A view of the assembled experimental apparatus.