Direct cryocooling of naked crystals: are cryoprotection agents always necessary?

Over the last 20 years cryocrystallography has revolutionized the field of macromolecular crystallography, greatly reducing radiation damage and allowing the collection of complete data sets at synchrotron sources. However, in order to cool crystals to 100 K cryoprotective agents must usually be added to prevent the formation of crystalline ice, which disrupts the macromolecular crystal lattice and often results in a degradation of diffraction quality. This process can involve the extensive testing of solution compositions and soaking protocols to find suitable conditions that maintain diffraction quality. In this study, it is demonstrated that when some crystals of macromolecules are mounted in the complete absence of surrounding liquid no crystalline ice is formed and the diffraction resolution, merging R factors and mosaic spread values are comparable to those of crystals cryocooled in the presence of a cryoprotectant. This potentially removes one of the most onerous manual steps in the structure-solution pipeline and could alleviate some of the foreseen difficulties in the automation of crystal mounting.