Optimized anomalous dispersion in crystallography: a synchrotron X-ray polychromatic simultaneous profile method

Abstract

In principle, it is possible to phase the X-ray reflections from a macromolecular crystal which contains one heavy atom per molecule, without having recourse to the conventional method of multiple isomorphous replacement, by making use of anomalous dispersion effects alone¹. To do this, it is necessary to measure each reflection over a narrow range of wavelengths centred on an absorption edge of the heavy atom, where f′ and f″, the real and imaginary anomalous components of the scattering factor, are varying rapidly. The availability of powerful synchrotron radiation sources with a continuous energy spectrum has stimulated interest in such phasing methods, but previously experiments¹ have been limited to making measurements on only one reflection at a time and/or to measuring at only a few discrete wavelengths near the absorption edge. We are developing a method at the Daresbury Synchrotron Radiation Source (SRS) which produces an energy profile along an axis of each and every diffraction spot in a screenless oscillation photograph. We show here that the approach is feasible using a single crystal o heptahydrido bis (diisopropylphenyl)phosphine rhenium using the L_III absorption edge of rhenium at a wavelength of 1.1772 Å.