Anomalous small angle scattering

Abstract

Isomorphous replacement methods have been revolutionary in macromolecular structure research. This has been most clearly demonstrated in protein crystallography. Specific binding of heavy atoms to protein molecules in the crystalline state provides the necessary reference for the phase determination of each Bragg reflexion and opens the way to the direct structure determination to atomic resolution from X-ray diffraction.Are isomorphous replacement methods applicable to the investigation of non-crystalline structures as well? And if so, would there be any reasonable advantage?An answer has been given by neutron scattering. Macromolecular assemblies of nucleic acids, proteins and lipids are most easily studied in H2O /D2O mixtures, as the scattering density of the solvent can be adjusted to any of the chemically different partial structures (Stuhrmann, 1974).