A drug-discovery-oriented non-invasive protocol for protein crystal cryoprotection by dehydration, with application for crystallization screening.

In X-ray macromolecular crystallography, cryoprotection of crystals mounted on harvesting loops is achieved when the water in the sample solvent transitions to vitreous ice before crystalline ice forms. This is achieved by rapid cooling in liquid nitro-gen or propane. Protocols for protein crystal cryoprotection are based on either increasing the environmental pressure or reducing the water fraction in the solvent. This study presents a new protocol for cryoprotecting crystals. It is based on vapour diffusion dehydration of the crystal drop to reduce the water fraction in the solvent by adding a highly concentrated salt solution, 13  potassium formate (KF13), directly to the reservoir. Several salt solutions were screened to identify KF13 as optimal. Cryoprotection using the KF13 protocol is non-invasive to the crystal, high throughput and easy to implement, can benefit diffraction resolution and ligand binding, and is very useful in cases with high redundancy such as drug-discovery projects which use very large compound or fragment libraries. An application of KF13 to discover new crystal hits from clear drops of equilibrated crystallization screening plates is also shown.