A miniature airlock system to aid the cryo-cooling of protein crystals grown under anoxic conditions

Crystallographic studies of structures and mechanisms of di­oxy­gen-employing enzymes or oxygen-sensitive proteins usually require these molecules to be prepared and crystallized under anoxic conditions in gloveboxes. Handling, cryo-cooling and extraction of these crystals from gloveboxes for X-ray data collection can be a tricky operation, during which samples might be ruined if they are accidentally exposed to atmospheric oxygen. This work reports the design of a new miniature airlock system and describes the associated method in order to simplify these tasks. The system allows flash-cooling of crystals harvested in a glovebox directly into an external liquid nitro­gen container via a miniature airlock while maintaining strict anoxic conditions. The method was assessed using hen egg-white lysozyme crystals soaked in di­hydro­resorufin (an O₂ indicator) and with the reduced tRNA-hy­droxy­lase enzyme MiaE.