ENVELOPE DETERMINATION IN MACRO-MOLECULAR CRYSTALLOGRAPHY BY THE MASC METHOD. M. Ramin, W. Shepard, R. Kahn^*, R. Fourme, I.M. Li de La Sierra, G. Grübel⁺, A. Thompson⁺, A. Gonzales⁺ & M. S. Lehmann^+$, LURE, Université Paris-Sud, Bât. 209d, 91405 Orsay Cedex, France, ^*IBS J.-P. Ebel, 41 Avenue des Martyrs, 38027 Grenoble, France, ⁺ESRF, BP220, 38043 Grenoble Cedex, France, ^$ILL, Avenue des Martyrs, 38042 Grenoble Cedex, France

The Multiple wavelength Anomalous Solvent Contrast (MASC) is a way to produce a physical contrast variation in a macromolecular crystal. This variation is obtained by tuning the X-ray wavelength near an absorption edge of an anomalous scattering species randomly dispersed in the mother liquor. MASC is, in principle, applicable to the determination of the molecular envelope and low resolution phases [Fourme et al . (1995) J. Synchrotron Rad . 2, 36-48]

Ammonium selenate was added to the mother liquor of crystals of two proteins (P64k from the outer membrane of Neisseria meningitidis) and xylose isomerase).Data at 3-4 wavelengths near the selenium K-edge were collected from cryocooled crystals, using undulator radiation at the ESRF ('Troika' beam line) and an imaging plate detector. Results regarding the extraction of the moduli of the Fourier coefficients of the macromolecular envelope {|G(h)|} and their phasing will be presented. Problems encountered during the set up of this new method will be discussed.