Automated gradient equilibration of macromolecular crystals to new solution conditions

We describe a device and a method for changing the ambient solution of a macromolecular crystal. The approach is gentle, automated, inexpensive and open source. Examples are given of the equilibration of three different crystals to new solutions with exchange times ranging from 5 to 180 min. In each case direct transfer of the crystal to the new solution causes cracking, which is eliminated with gradient equilibration using the described device. Crystals equilibrated with the device produce high-quality diffraction that yields refined structures comparable to those determined previously. The device offers a more systematic and labor-saving workflow than current practice both for performing diffraction analysis of macromolecular crystals and for investigating the response of macromolecular crystals to changes in solution composition.