Soaking of crystals for macromolecular crystallography in a capillary

A method for soaking macromolecular crystals in ligand solutions is described in which a crystal is placed in a capillary. The length of the capillary is used to form a gradient of the ligand concentration. This set up reduces the crystal susceptibility to cracking. This procedure is essentially a modification of the crystal-mounting technique used in many laboratories. We have carried out tests with 1 × 10-3 M permanganate ions diffusing through 50% saturated ammonium sulfate solution in a 1 mm capillary with a head-to-crystal distance of about 45 mm. The characteristic coloring of permanganate ions was noticeable at the crystal, at the bottom of the capillary, after about two days. After another two days, the concentration of permanganate ions within the capillary had equilibrated and its gradient was not apparent. Using this approach, we were successful in soaking crystals of enolase, which otherwise cracked in ligand solutions.