ON THE PREPARATION AND X-RAY DATA-COLLECTION OF ISOMORPHOUS XENON DERIVATIVES

A simple method for the preparation of isomorphous xenon derivatives is presented. A device has been designed that allows diffraction studies on protein crystals under xenon gas pressures up to 50 x 10(5) Pa. Crystal mounting and X-ray data collection do not significantly differ from standard techniques. Tests carried out on crystals of the protein porcine pancreatic elastase reveal a single xenon binding site with high occupancy at a pressure of 8 x 10(5) Pa. Xenon binding to several other crystallized proteins has also been investigated and results indicate that the method is generally applicable. Time-resolved studies show that, at 297 K, xenon binding is essentially completed within a few minutes. At pressures above 10(6) Pa, successful data collection is hampered by X-ray absorption and by the formation of xenon hydrate. Absorption can be reduced by using short-wavelength radiation and by mounting crystals in small capillaries. To circumvent xenon hydrate formation, higher working temperatures and the use of cryoprotective mother liquors are advocated.