Direct Phasing of Macromolecular Structures By Three-Beam Diffraction

Determination of triplet phases of macromolecular structures by means of psi -scan experiments are reported. The ways in which difficulties can be overcome when investigating macromolecular structures (numerous overlap of multiple beam diffraction profiles, overall weak scattering power and radiation damage) are discussed. It is shown theoretically and experimentally that measurable interference effects exist if the crystal dimensions are smaller than the Pendellosung length. The effect of the unavoidable overlapping of weak three-beam cases is negligible if a three-beam case with strong reflections is selected for phase determination. Data recorded at the HASYLAB synchrotron radiation source DORIS III are presented. In case of lysozyme it was possible to measure a set of 80 triplet invariants with a mean phase error of about 17 degrees. First experiments to study the feasibility of phase determination of large proteins, e.g. of catalase (unit cell volume 1200 nm³), are presented.