Isomorphous replacement in fiber diffraction using limited numbers of heavy-atom derivatives

Multi-dimensional isomorphous replacement, analogous to isomorphous replacement in protein crystallography, can be used in fiber diffraction analysis to overcome the problems caused by the cylindrical averaging of the intensity data. Large numbers of heavy-atom derivatives are needed, however. A method is presented by which molecular structures may be determined using data from only one or two derivatives, similar to crystallographic single isomorphous replacement. Partial structure information may also be incorporated. Examples are given using data from oriented gels of tobacco mosaic virus, and possibilities for further applications are discussed.