Experimental Determination of Reflection Phases by Three-Beam Diffraction and its Applications

Abstract

Information on phase relationships between Bragg reflections can be obtained by the interference of Bragg waves. In the three-beam case, phase difference is given by a structure invariant triplet phase relationship. The intensity variation due to the three-beam interaction can be best measured by a ψ-scan experiment. The resulting ψ-scan diffraction proflles scanning through a three-beam position uniquely depend on the triplet phase relationship involved. In principle each three-beam profile is given by a superposition of a symmetrical phase-independent and a phase-dependent profile. Thus, triplet phases can be determined experimentally with an accuracy of about 45 degrees. The ψ-scan method is discussed and some examples of applications to the determination of the absolute structure as well as to the structure determination by combination of measured triplet phases with direct methods are given.

Direct phasing of macromolecular structures by three-beam diffraction

“…In recent papers it was shown (Chang 1992;Hummer et al 1992) th a t physical triplet-phase determ ination of small molecule structures by three-beam diffraction is now almost routine work. This can be used for the determ ination of the absolute configuration of light atom structures where anomalous scattering effects are too weak for evident deviations from Friedel's law (Hummer & Billy 1986).…”