Multiwavelength anomalous diffraction as a direct phasing vehicle in macromolecular crystallography.

Abstract

The possibility for definitive phase determination from diffraction measurements made at multiple wavelengths from crystals that contain anomalous scatterers has long been recognized (Okaya and Pepinsky, 1956). This potential is easy to appreciate since multiwavelength anomalous diffraction (MAD) experiments can be thought of as in situ multiple isomorphous replacements (MIR) arising from the variations in scattering factors that accompany changes of wavelengths. These variations, known as anomalous scattering, result from the resonance that occurs between the oscillations of atomic orbitals and x-ray-induced electronic vibrations. With synchrotron radiation such experiments are now quite feasible, and with recently developed methods for analyzing MAD measurements, accurate phases can be obtained for macromolecular crystal structures. Anomalous scattering centers appropriate for MAD experiments can be introduced as for conventional heavy atom derivatives or they may occur naturally in metalloproteins. In addition, selenomethionyl proteins produced biologically are suitable for MAD phasing. Since a single crystalline species (and often a single crystal) suffices for MAD analysis of such molecules, the MAD method serves as a vehicle for direct structure determination.