Determination of triplet phases of macromolecular structures by means of ψ -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 Pendellösung 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°. First experiments to study the feasibility of phase determination of large proteins, e. g. of catalase (unit cell volume 1200 nm 3 ), are presented.

A first adaptive x-ray mirror prototype was built in 1992 and is now installed on the ESRF beamline 2 (Materials Science). This system consists of a water-cooled 1 m long Pt-coated Si mirror, supported by two rows of 11 piezoelectric actuators. The shape of the mirror is continuously monitored by an optical analyzer; its readout being used by a feedback loop in a real-time algorithm. A workstation associated to real-time electronics calculates the relevant parameters for driving the actuators as a function of both the desired shape and the possible thermal deformation due to the incident beam. Preliminary experiments carried out on the wiggler beamline showed that the system runs correctly: the feedback loop permits a control of the mirror shape in the microrad range at a frequency of 10 Hz. Various shapes (cylinder, parabola, or ellipse) can be produced at will to within the above accuracy. The different components of the device are described and the alignment and calibration procedures are discussed. The first results dealing with the properties of the reflected beam (size, divergence, and shape) versus both the calibration quality and the incoming power are presented.

The materials science beamline (BL2) at the European Synchrotron Radiation Facility (ESRF) can provide monochromatic or white radiation in the energy range from 7 to 100 keV. The source is a multipole wiggler with a maximum critical energy of 29 keV, producing a radiant power of up to 5 kW. The main optical elements are an adaptive mirror, a monochromator with a liquid nitrogen cooled first crystal followed by a sagittally bent second crystal, and a second mirror including a bender for vertical focusing. The general layout as well as the performance of the beamline optics during the commissioning phase of the ESRF are presented.

Abstract. It is shown for the first time that the measurement of triplet phases of three-beam cases where strong structure factors are involved is possible in small protein structures, for example myoglobin. The exploitation of the triplet phase from the interference profile is not affected as long as the structure factors of unavoidable overlapping three-beam cases are small.

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.