Use of an open-flow helium cryostat for macromolecular cryocrystallography

A helium cryostat developed at the University of Toledo has recently been described by Hardie, Kirschbaum, Martin & Pinkerton [J. Appl. Cryst. (1998), 31, 815-817]. This helium cooling system has now been tested on macromolecules, using crystals of chicken egg lysozyme and sperm whale myoglobin. Phase changes in terbium vanadate crystals indicate that temperatures delivered by the cryostat were less than 33 K. Unit-cell contraction in the protein crystals is consistent with the previously reported He data. Large crystals approaching suitability for neutron diffraction studies were successfully flash-cooled and a large crystal of lysozyme was rescued for data collection after undergoing a cycle of macro­molecular crystal annealing (MCA) following poor flash-cooling. Comparison of diffraction data from sperm whale myoglobin crystals collected in both He and N₂ showed a 23% lower overall B factor for the He data. The results of these studies show that this device might be an especially useful adjunct both for neutron diffraction studies and at high-intensity synchrotron X-ray sources, in addition to its use in the standard macromolecular crystallography laboratory.