XENON AND KRYPTON AS HEAVY ATOMS AND ANOMALOUS SCATTERERS Marc Schiltz & Thierry Prangé LURE, Université Paris-Sud, Bât. 209d, 91405 - Orsay Cedex, France

Protein complexes with xenon and krypton are highly isomorphous with the native structure and can be used as heavy atom derivatives for phase determination. A general method for the preparation of such derivatives is presented. A device has been designed which allows diffraction studies on protein crystals under gas pressures up to 60 bar. Crystal mounting and X-ray data collection do not significantly differ from standard techniques [Schiltz et al. (1994) J. Appl. Cryst. 27, 950-960].

As a test case, X-ray diffraction data at the high-energy side of the krypton K-edge (0.86 Å) were collected on a crystal of elastase (molecular weight of 25.9 kDa) and put under a gas pressure of 56 bar. Although the overall occupancy of the single krypton atom is approximately 0.5 (giving isomorphous and anomalous scattering strengths of respectively 18 and 1.9 electrons), this derivative could be used successfully for phase determination with the SIRAS method (Single Isomorphous Replacement with Anomalous Scattering). After phase improvement by solvent flattening, the resulting electron density map is of exceptionally high quality and displays a correlation coefficient of 0.85 with a map calculated from the refined native structure. Careful data collection and processing, as well as a proper statistical treatment of isomorphous and anomalous signals have proven to be crucial in the determination of this electron-density map.